THE POLYMER

Basic Terms and Definitions

A set of key terms and clear definitions is an important requisite for the understanding of any science; and this is especially true in the case of a science as new as high polymers. Since the basic terms and definitions that are the cornerstones of the "language" of high polymers are a logical starting point for the beginner, we are placing them at the beginning of the book. A little time spent now in acquiring a familiarity with these key definitions will make the science of high polymers much easier to assimilate as well as more fascinating.

The nomenclature of high polymers has been assembled in alphabetical order so that it will be convenient to refer back to specific definitions. As an understanding of the language characteristics of high polymer science is acquired, the knowledge behind myriad useful high polymer products that are playing an increasingly important role in your daily life will unfold.

Crystallite (see Fibril, Micelle)
When two or more long-chain molecules come close enough together laterally to lock in any way, the nucleus of a crystallite is born and a crystallite begins to grow. A crystallite consists of a cluster of associated long-chain molecules that make up a more or less regular structural unit, or building block, out of which a particular high polymer is constructed. In other words, it is the basic micro-unit of the geometric architecture of high polymers. The size varies over a wide range—from the order of tens to hundreds of Angstrom units (see Figure 1). Crystallites can increase or decrease in size without a significant change in their chemical make-up. The long-chain molecules in crystallites are so tightly packed that even beams of x-rays directed at them are readily diffracted. It is usually
by means of this structural characteristic that the size of crystallites is determined experimentally.

Degree of Polymerization (D.P.)
The degree of polymerization (D.P.) is the average number of repeating units in a linear macromolecule, if such a macromolecule consists of regularly repeating units; or, the average number of mers (monomeric units) per macromolecule if such a long chain molecule is built up of identical monomers. The D.P. is determined by dividing the (average) molecular weight of the monomer into the molecular weight of the macromolecule.

Elastomer (or Rubber)
Elastomer is a term that refers to non-crystalline high polymers or rubbers that have a three-dimensional space-network structure (e.g., vulcanization) which imparts stability or resistance to plastic deformation. Normally, elastomers exhibit long-range elasticity (rubber band effect) at ordinary room temperatures.

Fiber
A fiber is a thread or thread-like structure composed of fine strings or filaments of linear macromolecules that are intertwined or associated in such a manner as to give rise to an assemblage of molecules having a high ratio of length to width. The dual structure of ordered and disordered regions coupled with the orientation of these regions is known as "fiber structure."

Fibril (see Crystallite, Micelle)
A fibril consists essentially of an aggregate of micelles or crystallites that is large enough to appear like a very fine fiber under a high-power microscope. In the extreme case, of course, a fibril and a large micelle or crystallite may be considered synonymous. Normally, however, a fibril is an aggregate of many crystallites connected by long chain molecules that may run continuously be¬tween several of the component crystallites (see Figure 1).

In modern times, two main schools of thought have grown up regarding the basic nature of fibrils. On the one hand, there are those who believe that fibrils are "preformed" to a characteristic size or dimension. The other group argues that fibrils are a natural con¬sequence of the aggregation of long-chain molecules. If the formation of micelles—i.e., more or less discrete aggregates of molecules —is a reflection of an optimum thermodynamic state, then it may be said that both schools of thought are belaboring the same basic point. A third point of view is worth mentioning. It is true that the size of "micelles" or "fibrils" found after destructive chemical attack on a polymer is influenced by the severity of the Treatment. If these ultimate units of fiber or film structures reduce to finer and finer strands with increasingly severe chemical treatment, then it is conceivable, at least, that the true ultimate fibril is a single long-chain molecule.

Film
A film is a relatively thin skin, membrane, or pellicle less than 10 mils thick which usually is transparent or translucent.

Glass
In the broadest sense, a solid mass comprised of long-chain molecules that is transparent or translucent is termed a glass. In essence, glasses are considered to be supercooled liquids in which the long chain or macromolecules exhibit local regularities in structure only over relatively short ranges.

High Polymer
By convention, the term high polymer includes all materials whose chemical and physical structures depend on the arrangement in sequence of many monomers (identical or similar groups of atoms) connected by primary chemical bonds to form long chains or macromolecules. Aggregations of these long chains yield useful products which have more or less distinctive characteristics such as tensile strength, extensibility, elastic recovery, and many others.

The macromolecules that make up a high polymer consist of multiples of lower molecular weight units. The repeating units (monomers or mers) in the long chain macromolecule do not all have to be of the same size, nor do they have to possess exactly the same composition or chemical structure. Differences in composition and chemical structure in the units in the long chains of a high polymer arise from occasional branches, the presence of end groups, and other irregularities.

By way of clarifying different terms used by English speaking scientists and those in the continental European countries, it should be pointed out that the terms "high polymer" and "macromolecular substance" have the same meaning. Staudinger coined the word "macromolecule" to identify long chain molecules prior to 1930, and it has enjoyed wide usage on the Continent ever since. The term "high polymer" is more commonly used in Great Britain and the United States.

High polymer substances in general may be subdivided into several major types and classes:

Block Copolymer. In a block copolymer the repeating units con¬sist of segments or blocks of similar monomers tied together along the macromolecular chain. This differs somewhat from copolymers (see below) where the repeating unit consists of two or more different single monomers.
Branched High Polymer. This polymer is one in which the long chain molecule is not uniformly straight like a pencil, but has branches extending from its trunk as shown in Figure 2. The long-
chain molecule, despite these branches, remains unattached to other similar molecules surrounding it.

Copolymer. This is the term applied to a long-chain molecule comprised of at least two different monomers joined together in irregular sequence. A typical example of a copolymer is the textile fiber "Vinyon." This fiber consists of a series of vinyl acetate and vinyl chloride molecules repetitively joined together in a hand-in-glove arrangement. In contrast to this, a physical mixture of poly¬ethylene and polypropylene macromolecules, for example, is not a copolymer, but simply a mixture of homopolymers.
Segments of two separate "Vinyon" molecules are compared in Figure 3. Attention is called to the acetate group projecting from
the side of the molecule in the right hand formula; steric hindrance would prevent the chain segment from packing closely with the chain segment represented by the left hand formula.

Cross-linked High Polymers. These polymers are those in which the long-chain molecules — either straight or branched — have ladder-rungs or cross bridges binding them together as shown in Figure 4. In this type of high polymer, all or most of the component long chain molecules are rigidly locked to each other laterally by primary linkages (e.g., wool, Figure 4). This is different from "space-network" polymers, where the cross-bonding between chains proceeds along three-dimensional rather than by two-dimensional or lateral planes. In practice, however, the term
"cross-linked" polymer is sometimes applied to both (see Space-network High Polymer below).

Derived High Polymer. When a primary high polymer or a natural high polymer is altered chemically (so as to produce a derivative) it is called a derived high polymer.

Graft Copolymer. When a given kind of monomer is polymerized and, subsequently, another kind of monomer is polymerized onto the primary high polymer chain, a graft copolymer results.

High Polymeric (or Macromolecular) Compound. By convention, this term is more specific than the broad category of "high polymer." It is used to describe a substance consisting of a composite of long-chain molecules that are alike in composition, chemical structure, and size.
Primary High Polymer. This type of polymer is produced by the polymerization of chemically identical monomers into long chains, without subsequently altering the chemical nature of the resulting macromolecules.

Space-network High Polymer. When there are two or more reactive functional groups in the monomer or mer building block, the growth of the polymer in three dimensions is possible during the course of the polymerization. Such a process gives rise to a space-network high polymer. A good illustration is the reaction between glycerol and phthalic anhydride, which yields a three-dimensional network polymer. Other examples are thermosetting plastics such as the phenol-formaldehyde and the urea-formaldehyde types, respectively.

Stereoregular High Polymers: Atactic Polymer. When the R-groups or substituent groups are positioned on all sides of the main backbone of a long-chain molecule in a completely random manner, an "atactic" polymer results, as shown in Figure 5. Such
molecules cannot pack tightly together because of steric hindrance, and result in soft, non-crystalline and rather gummy products.

Isotactic Polymer. An isotactic polymer is one in which the R-groups or substituent groups all lie either above or below the main backbone of the long-chain molecule. Such an arrangement (sometimes referred to as stereoregular) makes possible a very highly ordered or compact high polymer, one that crystallizes readily (see Figure 5).

Syndiotactic (or Syndyotactic) Polymer. When the R-groups or substituent groups occupy positions that alternate regularly and in sequence above and below the main backbone of a long-chain molecule, a syndiotactic polymer results (see Figure 5). Such an arrangement permits relatively easy packing of the long-chain molecules and gives rise to substances whose properties lie between those of an isotactic and an atactic polymer, respectively.

Macromolecule. See discussion under High Polymer Micelle (see also Fibril, Crystallite)
A micelle is an aggregation of crystallites of colloidal dimensions that exist either in the solid state or in solution. It represents a reasonably reproducible dimension as the result of uniform chemical or mechanical treatments. It is sometimes used synonymously with the term "crystallite." It is very probable that a particular polymer micelle is an aggregation of long chains, reflecting the most stable state (thermodynamically) as the individual molecules pack from a solution or melt to form a solid product under a given environment.

Molecule (see also Monomeric Unit)
In the classic sense, a molecule is the smallest part of a substance that can exist separately and still retain a unique identity. This, in essence, defines a molecule as two or more atoms that are held together by primary or atom-to-atom bonds to produce a specific compound. On the basis of this definition, therefore, a diamond or piece of quartz might exist as a single molecule, just as could the simple gas oxygen (O2).

Monomeric Unit or Mer
All high polymers are formed by the joining together of many molecular units (as in polyethylene) or of groups of molecular units as illustrated in the case of cellulose. The monomeric unit or mer of a linear high polymer is the unit of the molecule which contains the same kinds and numbers of atoms as the real or hypothetical repeating unit.

The monomeric unit (or mer) of a linear high polymer is not, therefore, necessarily a "molecule." The repeating unit of many linear polvmers is a distinct segment of the molecular chain. The complete macromolecule (neglecting minor irregularities at the ends, branch junctions, etc.) might conceivably consist of a large number of these units, oriented in the longitudinal axis of the chain.

Polymer
A polymer in its broadest sense is a product formed by the combination of the same elements in the same proportions, but differing from the original "building blocks" in molecular weight. For example, cyanuric acid, C3N3O3H3, is a polymer of cyanic acid, CNOH, three molecules of CNOH combining to form C3N3O3H3. Similarly, paraformaldehyde (CH2O)n is a polymer of formaldehyde CH2O, in which n molecules of (CH2O) combine to give a new product or polymer (CH2O)n. (See High Polymer.)

Polymerization
The term "polymerization" refers to the process of formation of large molecules from smaller molecules, with or without the simultaneous formation of other products, such as water. (See Polymer.)

Some kinds of molecules, like ethylene (CH2=CH2), can react with themselves to form uniform long chain molecules, for example, —CH2—CH2—CH2—CH2—CH2— (polyethylene). In other cases Iwo kinds oi monomers react, forming copolymers.

Polyaddition. Polyaddition occurs when small molecules join each other, under the stimulus of a catalyst or a free radical mechanism to form linear polymers, usually without the coincident formation of by-product molecules. The formation of polyethylene from ethylene is a classic example.

Poly condensation. Polycondensation is a special type of polymerization commonly called C-polymerization. It refers to the union of monomers involving a chemical change, namely, release, or "splitting off" of simple molecules (such as H2O or NH3) coincident with the formation of macromolecules. The polycondensation of glucose by bacteria to produce long chain cellulose molecules with loss of a molecule of water is a typical example of this kind of polymerization. The formation of nylon by the polycondensation of hexamethyldiamine and adipic acid is a classic example of a synthetic high polymer produced by means of this type of polymerization.

Resin
A resin is a high polymeric compound which will not crystallize, is insoluble in water but soluble in some organic solvents, softens with heat, and may be a very viscous liquid or a solid at room temperature. Solid resins and thermoplastics are very similar, with physical state and properties at room temperature serving as the differentiating basis.

Spherulite
The term spherulite is being used in high polymer science in a sense paralleling its original definition, which refers to spherical crystalline bodies made up of radiating crystalline patterns like those found in mixtures of quartz and feldspar. A spherulite results from the aggregation of partly oriented crystallites that radiate outward from a common point to produce a pattern like butterfly wings. One characteristic of a spherulite structure is that the crystallographic axis of each of the component crystallites points outward somewhat like the spokes in a wheel.



Battista O. A,. 1958. Fundamentals Of High Polymers. Maruzen Asian Edition. Reinhold Publishing Corporation. New York

THE LEMON FRUIT

COMPOSITION AND PHYSIOLOGY

Specific Gravity
AN IMPORTANT FACTOR in determining the quality of a fruit is its specific gravity. The specific gravity of a fruit or its juice may increase, as in the orange, or it may remain practically the same, as shown for the juice in the lemon (table 3), from the time the fruit is very small until it is mature. Where there is an increase in specific gravity the increase may be due largely to a single substance, for example, sugar in the orange. On the other hand, there may be an increase of a single substance as the fruit grows to maturity, such as citric acid in the lemon, but there may be a commensurate decrease in other substances so that the specific gravity remains practically constant. The following are the results of investigations that have been made to determine the specific gravity of whole lemon fruits and of lemon juice.

Whole fruit.—Young (1915) picked and tested ten lots of lemons at intervals during the period from January 14 to March ll and found their specific gravity to range from 0.862 to 0.890 (av., 0.876). In general, the longer the fruits remained on the trees the lower their specific gravity. On January 13 he picked a large amount of fruit and placed it in storage. Nine samples of the stored fruit were tested at four-day intervals between January 14 and February 15. In these samples the specific gravity steadily increased up to the time of the last determination. The range was from 0.890 on January 14 to 0.968 on February 15, with an average of 0.938. In the case of frozen fruit, he found that there was a noticeable decrease in the specific gravity. This would be expected, due to the loss of water without a corresponding loss in volume. On the other hand, in spite of the loss of water, there was no marked change in the specific gravity of the pulp juice of frozen fruits. Any tendency toward an increase in specific gravity of the juice, due to the loss of water, was apparently overcome by the decrease in total soluble solids, as a result of respiration. (For data and a discussion of the losses of sugars and acids in the pulp juice of frozen lemons, see "Sugars," p. 64, and "Organic Acids," p. 74.)

The data presented from Young (1915) were later published as a portion of a University of California bulletin under the joint authorship of Thomas, Young and Smith (1919).

Data similar to those presented by Young on stored unfrozen and frozen lemons were published by Bailey and Wilson (1916). The frozen lemons were picked early on the morning following the freeze and stored at 45° to 50° F so that they would thaw slowly. They found that unfrozen lemons placed in storage and tested at intervals between January 21 and May 2 (16 determinations) had an average specific gravity of 0.943. Similar determinations showed that stored frozen lemons over the same period had an average specific gravity of 0.896 while frozen lemons which were picked from the trees at the regular testing intervals had an average specific gravity of only 0.740. The results show that the frozen fruit which thawed slowly in storage maintained a higher specific gravity than the frozen fruit that thawed more rapidly in the field.

Chace, Wilson and Church (1921) determined the specific gravity of mature Eureka lemons from seven areas and of mature Lisbon lemons from five areas in southern California. Samples were collected and tested once a month for 10 to 13 months in each area, entailing a total of 203 determinations which showed average specific gravities as follows: By applying statistical methods to their data these investigators found that the odds were 78 to 1 that the specific gravity of the Eureka lemons was significantly higher than that of the Lisbon lemons. They also report that the specific gravities were highest in midsummer, lowest during the winter, and, in general, lower in lemons with thick peels than in those with thin peels.

Turrell and Slack (1948) have recently determined the specific gravities of whole freshly picked lemon fruits grown in different areas in southern California. Those grown in the interior had a specific gravity of 0.911, those in the coastal area 0.953, and those in the intermediate area 0.934. The results suggest an actual difference in the specific gravities of the fruits grown in the different areas. These authors point out, however, that although the results are suggestive they do not prove to be statistically significant.

Juice.—As would be expected, the specific gravity of lemon juice is greater than that of the whole fruit. Bailey and Wilson (1916) stored lemons in January. At intervals between then and May they determined the specific gravity of the juice of 16 samples of the stored lemons. The values ranged from 1.031 to 1.046, with an average of 1.042.

The values immediately following this paragraph show the results of three recent determinations of the specific gravity of lemon juice. Sinclair and Eny (1945) determined the specific gravity of the juice of freshly picked mature lemons and that of the juice of lemons that had been kept in storage from one to three months. Bartholomew and Sinclair (unpublished) made similar determinations on the juice of the stem- and stylar-end halves of 15 lots of commercially mature lemons from 12 areas in southern California. Each lot of 50 lemons was taken directly from the trees, or from the packinghouse floor on the day they were picked. All fruits were green to silver in color. Sinclair and Eny (unpublished) also determined the specific gravity of the juice of lemons of different ages from the time they were about 2 cm in diameter until they were mature. That the fruits of a given size in these last tests were all of the same age was known because all fruits concerned were measured and tagged when they were very small.

The results of these three sets of determinations were as follows: These results show that the specific gravity of the juice of lemons remains reasonably constant regardless of age, effects of storage, and the half of the fruit (stem or stylar) from which it comes. As would be expected, there was some variation in different lots of a given group. The stored fruits showed the greatest difference between individual lots, a difference of 0.0099 in specific gravity which would mean a change of 2.40 per cent in soluble solids. The averages of the freshly picked fruit and the stored fruit, however, showed a difference equal to only 0.20 per cent soluble solids. The differences between the averages of the stem and stylar halves of the fruit and those between the young fruit and the more mature fruit were equal to only 0.05 and 0.26 per cent soluble solids, respectively.

Color in Peel
The lemon fruit owes its color principally to two groups of fat-soluble green and yellow plastid pigments, the chlorophylls and carotenoids. There is some evidence that under certain conditions the flavonoids also may contribute at least a small amount of yellow color to the lemon peel from which the chlorophyll has disappeared. For example, Weatherby and Chang (1943) found 1.66 mgs of quercetin equivalent (flavones and their derivatives) per gram of fresh lemon peel. The values for oranges and grapefruit were noticeably lower—0.076 mg and 0.036 mg, respectively (see also Wilson, Weatherby and Bock, 1942). It should be remembered, however, that although such substances as quercetin are yellow in pure crystalline form they usually combine with sugars to form glycosides (Braverman, 1949) which are colorless or almost colorless.

As long as any portion of the fruit remains green in color, it, like the green leaves, continues to manufacture carbohydrates, some of which are stored in the plastids in the form of starch grains. When the fruit is very young, the plastids, even in the innermost tissues, contain chlorophyll and starch. As the fruit enlarges, the chlorophyll and starch disappear progressively out¬ward until the entire fruit becomes yellow.

Dufrenoy (1929) found that the normal change in color of the fruit from green to yellow is caused by the simultaneous loss of chlorophyll and the digestion and disappearance of the starch from the plastids. As the chlorophyll and starch disappear, fat bodies form in the parts of the plastids formerly occupied by the starch grains. The fat-soluble carotenoids are then absorbed from other portions of the plastids by these fat bodies, thus giving the fruit its natural yellow color. This process can be hastened artificially by exposing the green-colored fruit to ethylene gas.

Miller and Winston (1939) found that in limes, lemons, and grapefruits, a portion of the carotenoids tends to disappear while the chlorophyll is disappearing. For example, they found 9.15 parts per million total carotenoids in the peel of mature green Villafranca lemons, but only 3.35 ppm in similar lemons that had remained on the trees until they were yellow. This may at least partially explain why the mature color of these fruits is lighter than the color of mature oranges and tangerines, in which the carotenoids increase in amount as the chlorophyll disappears.

Carotenoid-bearing plastids being present in all parts of the fruit, they are present in the extracted juice. Swift (1946) found that reamed juice contained 50 per cent more carotenoids than pressed juice.

The chlorophylls (a and b) are all-important in the process of forming carbohydrates from water and carbon dioxide. While the carotenoids (carotene and xanthophyll), which are responsible for the yellow color in the lemon fruits, may serve as a source for vitamin A, their physiological functions are not definitely known. They are closely associated with the chlorophylls, however, and it is probable that they too play an important part in the manu¬facture of carbohydrates. The greater the ratio of xanthophyll to carotene in the juice or peel of citrus fruits the deeper orange their color. For the most recent general discussion of carotenoids, see Karrer and Jucker, 1950.
A green-colored lemon that turns "silver" (intermediate be¬tween green and yellow) during cool weather may sometimes become slightly greener as it continues to grow during the warmer months that follow, but it never regains its original green color. Limited observation has indicated also that silver lemons may become slightly greener following the application of zinc spray as a corrective for mottle-leaf. Unlike the orange, however, there appears to be very little tendency for the lemon fruit to turn green again after it once becomes yellow, regardless of how long it remains on the tree.

Lipids
Miller (1938) defines lipids as the esters of fatty acids and alcohols and their hydrolytic products which are solu¬ble in a fat solvent, and he states that "the lipids are said to be the most abundant, the most widely distributed throughout the plant, and apparently the most important from a biological standpoint, of the fatty substances that occur in plants." They are probably present in some form in every living cell. Since the lipids are especially abundant in rapidly growing cells, it is thought that they play an important part in the activities of protoplasm. The findings of different investigators have led to the suggestions that the lipids may be concerned with vital processes such as absorp¬tion, secretion, respiration, and transpiration.

Elbert T. Bartholomew and Walton B. Sinclair: THE LEMON FRUIT Its Composition and Product. University of California Press. Barkeley and Los Angeles. 1951

THE LEMON FRUIT

GENERAL INFORMATION

Origin and History

THE LEMON has existed for so long that its origin is not known. Swingle (see Webber and Batchelor, 1943, p. 399) has the following to say with reference to its origin: "... Probably the lemon should be considered as a satellite species [a species of doubtful validity] of the citron; possibly it may prove to be of hybrid origin, perhaps having the citron and lime for parent species. As is true with the grapefruit, it is difficult to explain the origin of the lemon as a hybrid, as it crosses readily with other species of Citrus and yet, when self-pollinated, reproduces itself from seed with only very small variations. . . ."

That the first-known habitat of the lemon was Southeastern Asia, probably Southern China and Northern Burma, appears to have become fairly well established. It was introduced by the Arabs into Persia and Palestine where it was widely grown by the beginning of the twelfth century. From these countries it was apparently taken into Spain, North Africa and the Canary Islands. The lemon entered the United States, probably indirectly, from the island of Haiti where it had grown from seeds brought from the island of Gomera, one of the Canary Islands group, by Columbus on his second voyage, 1493. A much more detailed discussion of the interesting historical story of the spread of the lemon from its natural habitat into other parts of the world is given by Webber and Batchelor, 1943 (see especially pages 6, 7—9, 10, 20).

Distribution and Production
Lemons are grown in limited amounts for home use in almost every area where citrus can be grown but their commercial production is confined principally to three countries, Italy, Spain, and the United States.

Since 1938 the United States has been the world's largest producer of lemons. Their production in the United States is confined almost exclusively to California. According to the California Fruit Growers Exchange (1947, 1950), the five-year-average number of boxes of lemons produced in California and Arizona between 1924 and 1944 has been as follows:
Since 1944 the annual yields of California and Arizona lem¬ons, in round numbers, have been as follows: During the last three of these years, approximately two-thirds of the crop was consumed as fresh fruit. The standard shipping weight of a box of California or Arizona lemons is 79 pounds; previous to 1943 it was 76 pounds.

The lemon acreage in Arizona is small but is increasing. According to a recently published survey (June 2,1949) the present plantings total 814 acres. Whole groves of lemon trees in Arizona are rather rare. Most of the trees are planted in single or double rows in or bordering groves of other varieties of citrus (for reference see "Literature Cited," under "Arizona Citrus"). The State of Texas also produces a limited supply of lemons, most of which are consumed locally.

Variety
The Eureka is the principal variety of lemon grown in California. Of the total acreage of lemons in California in 1946 approximately 88 per cent consisted of Eurekas, 8 per cent of Lisbons, 2 per cent of Villafrancas, and 2 per cent of all other varieties. The Eureka variety gained predominance because of its superior quality and productiveness in most areas, and because it matures a considerable quantity of its fruit during late spring and in the summer when demand and prices are at a maximum. The Lisbon variety is preferred in south central California and some new plantings of this variety have been made in Arizona. Having a relatively dense foliage its fruit is less susceptible to such factors as sunburn and freeze injury.

As a result of their studies on the relative susceptibility of different species and varieties of citrus to freeze injury in California during the winters of 1947-48 and 1948-49, Hodgson and Wright (1950) found that the Eureka lemon was more susceptible to freezing temperatures than either the Lisbon or the Villafranca. Because of its other predominant favorable qualities it is not probable, however, that the Eureka's greater susceptibility to freeze injury will prevent it from being the principal choice for future plantings.

Fruit Set and Harvest
The lemon tree in California blooms and sets its fruit more or less continuously the year around, but the heaviest sets occur in the spring and fall. In southern California pickings are usually made every six to eight weeks throughout the year, but in the south central California and Arizona areas the pickings are mostly limited to a period of only four months, October to January.

Maturity Defined
In most species or varieties of fruits it is not difficult to distinguish by looks or taste the differences between those that are immature and those that are mature. As applied to lemon fruits, however, the terms "immature" and "mature" are rather vague. The generally accepted maturity test is based on the availability of extractible juice. According to State regulations in California and Arizona a lemon that contains a minimum of 25 per cent of juice by volume is considered to be mature, regardless of size or color. Supply and demand would not make it profitable to pick all fruits just as soon as they contain that amount of juice; they are therefore picked according to size. The rings used to deter¬mine the size of the fruit range from 22/32" to 29/32" in diameter, but the ones most commonly used are 22 6/32", 22 7/32" and 22 8/32" in diameter. The name of the sizing ring is indicated by its number of 32nds of an inch in excess of 2 inches, thus the five rings just mentioned would be called Nos. 2,9, 6, 7 and 8 picking rings. The size of the ring used for any one picking depends on local conditions and on the market's demand for fruit. When brought to the packinghouse the fruits are segregated according to color. The color designations may include all or part of the following: dark green, green, light green, silver, light silver, light yellow, yellow, and dark yellow (tree ripe). A lemon is said to be "tree ripe" when it has remained on the tree for at least several weeks after it has changed from light yellow to dark yellow in color. Some fruits are picked before they have reached the usual picking size. They are those that have turned light silver to dark yellow and have stopped or practically stopped growing. Sizes smaller than will pack 588 fruits (1.750 inches in diam.) to the box are usually sent to the products plant.

All lemon fruits used in the experiments described on the fol¬lowing pages were commercially mature, unless specially designated as immature, young, very young, or very small.

Storage
The nature of the lemon fruit is such that it can be safely stored much longer than other commercial species of citrus. Since lemons are usually picked according to size rather than color they may be green when put into the storage rooms. The demands of the market usually determine the length of time that they are kept in storage. If the demand is strong, a portion or all of the green lemons picked and taken to the packinghouses are treated at once with ethylene gas which causes them to change from green to yellow in color within a few days, generally 5 to 7. They are then ready to be placed on the market. If there is no immediate demand, the lemons are placed in storage rooms where, after several weeks, they become yellow without the ethylene treatment. Citrus fruits themselves give off a small amount of ethylene while in storage but probably not enough to cause degreening of the fruit (see section on "Respiration," p. 46).

When lemon fruits are kept in storage over extended periods, sometimes 3 to 4 months or longer, every precaution must be taken to keep them in as healthy a condition as possible. After picking, and especially if they are green, they are allowed to remain in the open on the packinghouse floor for at least 2 or 3 days until they have lost some of their turgidity, to avoid the swelling and breaking of the oil glands. They are then washed in warm or hot water containing substances which remove dirt and which kill a large portion of the surface molds and other fungi. They are then dried and placed in standard wooden packing boxes which are stacked 9 to 11 high. The standard packing box contains two compartments, each of which is 13" x 12 1/2" x 9 7/8" deep, inside measurements. Spaces are left between the stacks to permit good ventilation. The best storage rooms are equipped for producing forced-air circulation and for controlled temperature (56° to 60° F) and relative humidity (86 to 88 per cent). The ideal storage temperature for lemons varies slightly, depending on locality and age of fruit, but, in general, if it goes below 56° F it will cause such troubles as bronzing of the peel and darkening of the segment membranes. Temperatures above 60° F are conducive to other physiological disorders and to the growth of decay-producing organisms. Sufficient fresh air is introduced to approximate pure air conditions. C02 content of air should not be allowed to rise above 0.10 per cent. See MacRill, Nedvidek and Nixon (1946) for a recent description of the most favorable storage conditions of citrus fruit.

Even under the most favorable conditions, storage of lemons over long periods gives rise to many problems of fruit decay and to external and internal physiological breakdown. Most of these do not fall within the scope of this discussion. Some of the storage disorders, however, that are directly or indirectly related to respiration will be presented in the section on "Respiration." For a detailed discussion of citrus storage maladies see Fawcett (1936) and Fawcett and Klotz (1948).

Structure
The gross anatomical structure of the lemon fruit is so well known that it will not be discussed here in detail. In general, it may be said to consist of the peel and the pulp or edible portion. In the peel of the mature fruit, the first few layers of cells under the epidermis are usually called the "flavedo"; the remainder of the peel, the white portion, is called the "albedo." The oil glands which contain the essential oil of commerce are embedded primarily in the flavedo, but the larger glands may extend into the outer layers of the albedo.

The pulp, the edible portion of the fruit, is composed of segments (locules) and the central axis (sometimes called the "core"). The number of segments per fruit may vary but there are usually 9 or 10. Each segment has a membrane which covers the juice sacs (vesicles) and the seeds. The interior of the juice sac is composed of numerous thin-walled cells which contain the juice of the fruit. Each juice sac terminates at its basal end in a slender, almost threadlike, long, medium or short stalk by means of which it is attached to the segment wall. The length of the stalk depends on the position of the juice sac in the segment. All stalks are attached to the inner wall of the outer portion of the segment, the portion that is in contact with the peel.

The vascular bundles which conduct water, food materials and foods are confined to the peel and central axis. Unlike in most other fruits, no vascular bundles penetrate the fleshy portion of the lemon except where branches from the bundles in the central axis connect with the seeds in the segments. This presents a rather unique physiological situation under which liquids and substances must be transferred for relatively great distances without the aid of the usual conducting tissues.
Collectively, the tissues of the segment walls and of the central axis, in commercial terms, are called the "rag" of the fruit. A high percentage of rag is undesirable.

Citrus fruits are harvested by being cut from the branch with a "clipper." The cut is made at the distal end of the pedicel so that the calyx, receptacle and disc remain attached to the base of the fruit. Collectively these three organs constitute what, in commercial terms, is called the "button" of the fruit. A lemon fruit with the button attached is apt to survive storage conditions longer than one from which the button has been removed. (For detailed structure of citrus fruits in general, refer to Bartholomew and Reed, 1943 and, particularly of the lemon fruit, to the work of Ford, 1942-43.)

Relative Proportions of Peel and Pulp
The peel of the lemon varies a great deal in thickness, as influenced by variety, rootstock, vigor of growth, and environ¬mental factors. If the fruit is allowed to remain on the tree until it begins to change in color from green to yellow the peel slowly commences to decrease in thickness, and continues to do so as long as the fruit remains on the tree. The decrease in thickness proceeds more rapidly, of course, when the fruit is picked and placed in storage.
Chace, Wilson and Church (1921) measured the thickness of a large number of peels within a short time after the lemons were picked. They considered the peel to be thin if it measured less than 3 mm, medium if 3 to 5 mm, and thick if over 5 mm. They seldom found a peel that was more than 7 mm thick.
Instead of measuring its thickness, several investigators weighed the peel separately and expressed its weight as a percentage of the weight of the whole fruit. They report the follow¬ing minimum and maximum percentages on a fresh-weight basis: In working with Eureka lemons grown at Homestead, Florida, Stahl (1935) found that the peel weighed 57 per cent of the weight of the whole fruit.

The preceding values subtracted from 100 give the percentages in terms of pulp.

Age vs. Size
In making biochemical and physiological studies of lemon fruits, both their age and their size should be considered. Because of differences in the twigs to which they are attached, the month in which the fruit is set (Reed, 1919; Bartholomew, 1923;
Furr and Taylor, 1939), and many other factors, the length of time it takes the individual fruits of a given set to attain picking size may vary from 7 to 14 months; some may never reach standard picking size (2 1/16" diam., or larger). Reed's results are shown in table 1 and those of Furr and Taylor in table 2.
Joslyn and Marsh (1937), Harding and Lewis (1941), Sinclair and Bartholomew (1944), and others have found that small oranges contain a higher concentration of soluble solids than large oranges. Some recent unpublished data of the authors show that the same holds true for lemon fruits. For these reasons, the true biochemical and physiological characteristics of lemon fruits cannot be determined unless both their age and their size are taken into consideration.

By tagging a large number of fruits soon after they are set, samples can be obtained at intervals in which the fruits are not only all of the same given size but which are also all of the same age. By this method reliable results are assured.


Growth-Promoting Substances and Fruit Size
Recently interest has centered around the possibility that the size of citrus fruits may be increased by spraying the trees at the proper time with certain growth-promoting substances. While trying to prevent abnormal amounts of leaf and fruit drop, Stewart and Hield (1950) found that spraying a plot of lemon trees with the isobutyl ester of trichlorophenoxyacetic acid (2,4,5-T) caused a 31.9 per cent increase in yield of fruit. In another plot of lemon trees, dichlorophenoxyacetic acid (2,4-D), applied as the triethanolamine salt, caused a similar increase in yield, 24.4 per cent. These increases in yield were found to be due to an increase in fruit size. They also found that an increase in size could be obtained even though there was no increase in yield., The isopropyl ester of 2,4-D failed to cause a significant increase suits appear to support the findings of Stewart and Hield.

Chemical Changes
The chemical changes which occur in lemon fruits during growth and maturation may be very different from those occurring in the orange under similar growth conditions. The lemon fruit, unlike the orange, does not show an increase in soluble carbohydrates with the advance of the season. The organic acids are the chief soluble constituents of the juice of mature lemons but the sugars predominate in the juice of mature oranges. As lemons mature, the free acids of the juice increase and the pH decreases, but as oranges mature, the free acids in the juice decrease and the pH increases. These are some of the commonly encountered physiological differences between the two species.

Interpretation of Data
The degree of accuracy with which biochemical data can be interpreted depends to a marked degree on the terms in which the results are expressed. For example, a value may be significant if expressed on a fresh-weight basis but may not be if expressed on a dry-weight basis. Likewise, neither of these values necessarily reveals the progressive changes in the amounts of a given constituent during the processes of growth and maturation. In the carbohydrate fractions of plants, an increase in the actual amount of one constituent may so affect the dry weight as to cause an apparent, but not an actual decrease in another constituent.

Before drawing general conclusions from limited data, one should bear in mind that different environmental factors in various localities and annual variations may noticeably affect the actual and relative quantities of the chemical constituents of the fruit. For example, lemon juice contains a maximum per cent of citric acid when lemons are grown on non-retentive soil having a relatively low moisture content. As a further illustration, Cheema (1927) found the following variations in the amounts of sugars and acids in the juice of lemon fruits grown in different provinces of Italy. The water content of the lemon fruit consists of more than 70 per cent of its total weight, and a large portion of the total solids is water soluble. The total soluble constituents of the juice, there¬fore, are reported as the percentages of the fresh weight of the juice. The ash, mineral constituents, and water and alcohol-soluble carbohydrates of the peel can be satisfactorily reported as percentages of its dry weight. Calcium pectate, uronic acid, and other alcohol-insoluble components can be best expressed as percentages of the total alcohol-insoluble fraction.

If such precautions as those just indicated are taken into consideration, a more nearly accurate evaluation of analytical results is ensured.

Elbert T. Bartholomew and Walton B. Sinclair: THE LEMON FRUIT Its Composition and Product. University of California Press. Barkeley and Los Angeles. 1951

Chemical and Physical Properties of Fats and Oils

INTRODUCTION
Fats and oils make up one of three major classes of food materials, the others being carbohydrates and proteins. Fats and oils have been known since ancient times as they were easily isolated from their source. They found utility because of their unique physical properties. Fatty tissues from animal sources liberate free-floating fats on being boiled. Olives and sesame seeds yield oil on being pressed. Such fats and oils add flavor and lubricity to foods prepared with them

The very commonness of fats and oils and the familiarity most people have with them have led to an ironic situation. The average user, whether a housewife, chef, baker, or food manufacturer, often has little real understanding of the characcer of fatty products. There are times when selection of the proper fat for a use situation can be very critical but is done incorrectly. Other times the selection need not be critical but a costly choice is made due to prejudice or to lack of adequate knowledge of the subject.

Much of the terminology used in fat and oil work was developed over the years by processors who had only practical knowledge. Even the early chemists were hampered by incomplete understanding of their field of study. Therefore, a fat was defined as the oleaginous material that was solid at room temperature and the liquid form was called an oil.
The process of separating a fatty mass into a more liquid and a more solid fraction resulted in calling the former the oil and the latter the stearino portion. This led to the anomalous situation that oleo oil from beef fat was fairly solid at room temperature while the stearine separated from cottonseed oil/could be liquid at a somewhat elevated room temperature.

COMPOSITION OF FATTY MATERIALS
Edible fats and oils are esters of the three carbon trihydric alcohol, glycerin, and various straight chained monocarboxylic acids known as fatty acids. The fatty acids of natural fats have 4 to 24 carbon atoms and, with minor exceptions, have an even number of such atoms. Figure 1 shows the structure of glycerin, fatty acids, and a fat derived from them.

The fatty acids may be saturated, monounsaturated, or polyunsaturated. Saturated acids have all of the hydrogen that the carbon chain
will hold. Table 1 lists them by chemical name, common name, and source. Monounsaturated acids have two hydrogen atoms missing, one from each of two adjoining carbon atoms, giving rise to one carbon-carbon double bond, Polyunsaturated refers to a fatty acid with two or more double bonds. Specifically from a nutritionist's point of view, the polyunsaturates should be the cis,cis-methylene interrupted isomers. These isomers can be distinguished from trans or conjugated isomers by use of the lipoxidase enzyme analysis using a spectrophotometer to determine the oxidation products. These products are conjugated diene hydroperoxides with an absorption wave length of 234 nm.

Natural fatty acids usually exist in specific isomeric forms. Chemi¬cal isomers are compounds that occur in two or more forms although they have the same number of carbon, hydrogen, and oxygen atoms. Unsaturated fatty acids can have positional isomers in relation to where the double bonds appear in the carbon chain. Natural fatty acids most frequently have the first double bond between carbons 9 and 10. the second between C-12 and C-13, the third between C-15 and.C-16.

Table 2 lists the various unsaturated fattv acids, their common names, and sources. The most commonly occurring unsaturated fatty acids are oleic, linoleic, and linolenic.

Another type of isomer is the geometric isomer. Here the carbon chain is bent into a fixed position at each double bond. The carbon chain sections are either bent towards or away from each other. The former is called cis (meaning same side), the latter, trans (meaning across). Figure 2 illustrates these isomeric forms.
The natural acids are found most frequently in the cis form. Trans isomers are usually formed during chemical reactions, e.g., hydrogena-tion or oxidation. Cis acids have a considerably lower melting point than trans acids of the same chain length. Thus, oleic acid (cis-octa-decenoic) melts at 16°C (61°F) and is therefore liquid at room tempera¬ture. Elaidic acid Urans-octadecenoic) has a melting point of 44°C (111°F) and is solid at room temperature. Infrared spectroscopy is used for analysis of trans isomers in fats.

The length of the carbon chain also affects melting point. For satu¬rated fatty acids, melting point increases as the chain becomes longer. Unsaturation lowers melting point of fatty acids of the same chain length with cis acids leaving a considerably lower melting point than trans acids. It is interesting to note that stearic acid, the saturated IS carbon acid, melts at 69°C (156°F). Oleic acid has a melting point close to that of caprylic acid. The former has 18 carbons and one cis double bond. The latter is saturated with 8 carbons. Elaidic acid has 18 car¬bons and one trans double bond. It has a melting point close to that of 1 auric acid with a 12 carbon chain.

The fatty acids are found in natural fats as complex mixtures. There are three positions available on the glycerin molecule for the es-terification of the fatty acids. The physical characteristics of the vari¬ous triglyceride fats depend on the type, quantity, and distribution of the acids on the glycerin molecule. A simple illustration of this was demonstrated by Norris and Mattil (1946). They randomized mixtures of triolein and tripaimitin by interesterification and analyzed the glyceride composition of the final products.

Figure 3 shows the theoretical composition of one random mixture. It should be noted that a distinction has been made between the 1- and 2-positions on the glycerin molecule. The characteristics of a triglycer¬ide depend on which position each fatty acid occurs. This is most important in the sharpness of melting of cocoa butter and in the crystal structure of lard. It is of lesser importance with other typee of facs and oils.

The melting point of fatty mixtures changes on randomization. A mixture of 30% tripaimitin and 70% triolein had a melting point of 58°C (136°F) before interesterification and 48CC (119°F) after the reaction (Norris and Mattil 1946).

There are also a number of minor components incorporated in natu¬ral fats. Simplest of these are mono- and diglycerides. Glycerin is es-terified with only one or two fatty acids, leaving free hydrophilic hydroxyl groups. This makes the monoglyceride an important emulsifier.
Phospholipids are also important as emulsifiers. They are com¬pounds containing fatty acids and phosphoric acid among other chemi¬cal entities. Lecithin is a glycerin ester of two fatty acids and phos¬phoric acid combined with choline, a nitrogenous compound. Cephalin is similar with ethanolamine, a different nitrogen-containing com¬pound, replacing choline. Inositol phosphatides are compounds of inositol, phosphoric acid, fatty acids, ethanolamine, tartaric acid, and sugars.

Soybean oil phospholipids consist of 29% lecithin, 31% cephalin, and 40% inositol phosphatides. They are commonly called "lecithin." Commerical lecithin is obtained primarily from soybean oil. Some are also available from corn oil and egg yolk.

Sterols are complex polycyclic alcohols. They are found as a class of compounds in all oils. Cholesterol is found only in animal fats. Plant sterols are similar in composition but sufficiently different to suppress absorption of dietary cholesterol by the body or to suppress formation of natural cholesterol in the body (Jandacek 1975). Sterols may have some effect as emulsifiers. Joffe (1942) has postulated that cholesterol in egg yolk aids in the formation of the mayonnaise emulsion although lecithin and lecithoproteins are considered to be the major emulsifiers in yolk.

Tocopherols are natural antioxidants found in vegetable oils. They are also known as vitamin E. They are cyclic compounds with a hydroxyl group and heterocyclic oxygen. Tocopherols are easily oxidized to quinones. They, therefore, have natural antioxidant activity.

YEASTS AND YEASTLIKE FUNGI

Yeast are as difficult to define as molds. In Henrici's Molds, Yeasts, and Actinomycetes, yeasts are defined as "true fungi whose usual and dominant growth form is unicellular." Some of the molds in their conidial stage are like budding yeasts have a mycelial stage. An example of genus that sometimes is listed with the molds and sometimes with the yeasts is Geotrichum.

Yeasts may be useful or harmfull in foods. Yeast fermentations are involved in the manufacture of foods like bread, beer, wines, vinegar, and surface-ripened cheese, and yeasts are grown for enzymes and for food. Yeasts are undesirable when they cause spoilage of sauerkraut, fruit juices, sirups, molasses, honey, jellies, meats, wines, beer, and other foods.

Little attempt will be made to describe genera of yeasts so that they can be identified. Instead the general characteristics of yeasts will be discussed, the important genera listed, and yeasts of industrial importance mentioned.

GENERAL CHARACTERISTICS OF YEASTS
Like most plants yeasts are classified botanically chiefly on their morphological characteristics, although the physiological characteristics are more important to the food bacteriologist.

Morphological Characteristics
The morphological characteristics of yeasts are determined by microscopic examination.

Form and Structure. The shape of yeasts varies from spherical to ovoid, lemon-shaped, pear-shaped, cylindrical (see Figure 2-1), or even elongated into a false or true mycelium. Parts of the structure that can be seen are the cell wall, cytoplasm, vacuoles of water or fat, and granules which may be metachromatic, albuminous, or starchy. Special staining is necessary to demonstrate the nucleus.

Reproduction. Most yeasts reproduce asexually by budding (Figure 2-3), a process in which some of the protoplasm bulges out the cell wall; the bulge grows in size and finally walls off as a new yeast cell. In some yests, notably some of the film yeasts, the bud appears to grow from a tubelike projection from the mother cell. Nuclear is divided between the mother and doughter cell. A few species of yeasts reproduce by fission and one by a combination of fission and budding.

Sexual reproduction of "true" yeasts (Ascomycetes)is by means of ascospores, the yeast cell serving as the ascus. The formation of ascospores follows conjugation of two cells in some species of yeats. Other yeasts produce ascospores without conjugation, but later ascospores or small daughter cells may conjugate. The usual number of ascospores per ascus is characteristic of a species. Differences in methods of conjugation and ascospore formation are used in the classification of yeasts.

"False" yeasts, which produce no ascospores, belong to the Fungi Imperfecti. Cells of some yeasts become chlamydospores by formation of a thick wall about the cell.

Cultural Characteristics
The appearance of massed yeast growth is not, for the most part, useful in the identification of yeasts, although growth as a film on the surface of liquid media would suggest a film yeast, and production of carotinoid pigment would indicate the genus Rhodotorula. However, the appearance of the growth is important when it causes colored spots on foods. It is difficult to tell yeast colonies from bacterial ones on agar plates; the only certain way to identify a yeast colony is by means of a microscopic examination. Most young yeast colonies are moist and somewhat slimy, but may appear mealy, and most colonies are whitish, but some are cream-colored or pink. Some colonies change little with age, but others become dry and wrinkled.

Yeasts are divided into two groups on the basis of the type of growth in or on liquid media. So-called film yeasts, or oxidative yeasts, grow as a film or scum on the surface, while the others, the fermentative yeasts, grow throughout the liquid.

Physiological Characteristics
Although species of yeasts may differ considerably in their physiology, those of industrial importance have enough physiological characteristics in common to permit generalizations, provided that it is kept in mind that there will be exceptions to every statemant made.

Most of the commonly encountered yeasts grow best with a plentiful supply of available moisture. But since many yeasts grow in the presence of greater concentrations of solutes, like sugar or salt, than do most bacteria, it can be concluded that these yeasts require less moisture than the majority of bacteria. Most yeasts require more moisture than do molds, however. On the basis of water activity (aw)supporting growth, yeasts may be classified as ordinary if they do not grow in high concentrations of solutes, that is, a low aw, and as osmophilic if they do. Lower limit of aw for ordinary yeasts tested thus far range from 0.88 to 0.94. Specific examples of minimal aw are 0.94 for beer yeast, 0.94 for yeast from condensed milk, and 0.905 for a bakers' yeast. By contrast, osmophilic yeasts have been found growing slowly in media with an aw as low as 0.62 to 0.65 in sirups, although some osmophilic yeasts are stopped at about 0.78 in both salt brine and sugar sirup. Each yeast has its own optimum aw and range aw for growth for a given combination of environmental conditions. These aw values will vary with the nutritive properties of the substrate, pH, temperature, availability of oxygen, and presence of inhibitory substances.

The range of temperature for growth of most yeasts is, in general, similar to that for molds, with the optimum around 25 to 30°C and the maximum about 35 to 37°C. Some kinds can grow at 0°C or less. The growth of most yeasts is favored by an acid reaction in the vicinity of pH 4 to 4.5, and they will not grow well in an alkaline medium unless adapted to it. Yeasts grow best under aerobic conditions, but the fermentative types can grow anaerobically, although slowly.

In general, sugars are the best food for energy for yeasts, although oxidative yeasts, e.g., the film yeasts, oxidize organic acids and alcohol. Carbon dioxide produced by bread yeasts accomplishes the leavening of bread; and alcohol made by the fermentative yeasts is the main product in the manufacture of wines, beer, industrial alcohol, and other products. The yeasts also aid in the production of flavors or "bouquet" in wines.

Nitrogenous foods utilized vary from simple compounds like ammonia and urea to amino acids and polypeptides. In addition, yeasts require accessory growth factors.

Yeasts may change in their physiological characteristics, especially the true, or ascospore-forming, yeasts, which have a sexual method of re¬production. These yeasts can be bred for certain characteristics, or may mutate to new forms. Most yeasts can be adapted to conditions which previously would not support good growth. Illustrative of different characteristics within a species is the large number of strains of Saccharomyces cerevisiae suited to different uses, e.g., bread strains, beer strains, wine strains, and high-alcohol-producing strains or varieties.

CLASSIFICATION AND IDENTIFICATION OF YEASTS
Yeasts belong to the division Fungi and phylum Eumycophyta (Figure 2-2). True yeasts are in the class Ascomycetes (a few are Basidio-mycetes), and the false, or asporogenous, yeasts are in the class Fungi Imperfecti. Further subdivisions are shown in Figure 2-2.

The principal bases for the identification and classification of genera of yeasts are as follows:
1. Whether or not ascospores are formed.
2. If they are spore-forming—
a. The method of production of ascospores:
(1) Produced without conjugation of yeast cells (parthenoge-netically). Spore formation may be followed by (a) conjugation of ascospores or (b) conjugation of small daughter cells.
(2) Produced after isogamic conjugation (conjugating cells ap¬pear similar).
(3) Produced by heterogamic conjugation (conjugating cells differ in appearance).
b. Appearance of ascospores: shape, jsize, and color. Most spores are spheroidal or ovoid, but some have odd shapes, such as those of most species of Hansenula, which look like derby hats (Figure 2-l,F.).
c. The usual number of ascospores per ascus: one, two, four, or eight.
3. Appearance of vegetative cells: shape, size, color, inclusions.
4. Method of asexual reproduction:
a. Budding.
b. Fission.
c. Combined budding and fission.
d. Arthrospores (oidia).
5. Production of a mycelium, pseudomycelium, or no mycelium.
6. Growth as a film over surface of a liquid (film yeasts) or growth throughout medium.
7. Color of massed growth.
8. Physiological characteristics (used primarily to differentiate species or strains within a species):
a. Nitrogen and carbon sources.
b. Oxidative or fermentative: film yeasts are considered oxidative, and other yeasts fermentative.

YEASTS OF INDUSTRIAL IMPORTANCE
True Yeasts (Class Ascomycetes)
Most yeasts used industrially are ascomycetes, and most are in the genus Saccharomyces. The term "wild yeast" is applied to any yeast other than the one being used or encouraged. Thus a yeast employed in one process could be a wild yeast in another. Many of the troublesome wild yeasts are asporogenous, or false, yeasts.

Genus Endomyces. E. vernalis is one of the yeasts that has been used during war periods for the synthesis of fats. E. fibuliger has been grown for its amylase content.

Genus Schizosaccharomyces. These yeasts, which reproduce asexually by fission, have been found in tropical fruits, molasses, soil, insects, and elsewhere.

Genus Saccharomyces. Only a few species of this important genus will be mentioned. The leading species, S. cerevisiae, is employed in many food industries, with special strains being used for the leavening of bread (Figure 2-3), as top or bottom yeasts for ale or beer, for wines, and for the production of alcohol, glycerol, or invertase. Top yeasts clump together during growth, collect carbon dioxide, and are buoyed up to the surface of the fermenting liquid, from which they can be skimmed. Bottom yeasts do not clump but settle to the bottom of the liquid following the period of active growth. S. cerevisiae var. ellipsoi-deus is a high-alcohol-yielding variety used to produce industrial alco¬hol, wines, or distilled liquors. S. carlsbergensis is a brewers' yeast, and S. fragilis, because of its ability to ferment lactose, may be important in milk or milk products.
Genus Zygosaccharomyces. Some workers consider this a subgenus of Saccharomyces. These yeasts are notable for their ability to grow in high concentrations of sugar (hence are termed osmophilic) and are involved in the spoilage of honey, sirups, and molasses and in the fermentation of some wines and of soy sauce. Z. nussbaumeri is a species found growing in honey.

Genera of Film Yeasts. The genera Pichia, Hansenula, and Debaryo-myces (as well as Mycoderma and Candida of the false, or asporogenous, yeasts) are film yeasts which grow on the surface of acid products like sauerkraut and pickles (Figure 2-4), oxidize the organic acids, and en¬able less acid-tolerant organisms to continue the spoilage. Hansenula and Pichia tolerate high amounts of alcohol and may oxidize it in alcoholic beverages. Pichia species are encouraged to grow on Jerez and Arbois wine, to which they are supposed to impart distinctive flavors of esters. Debaryomyces is very salt-tolerant and can grow on cheese brines with as much as 24 percent salt. The film yeasts produce little or no alcohol from sugars.

Genera of Apiculate Yeasts. The genera Saccharomycodes and Han-seniaspora (and the false yeast Kloeckera) are apiculate yeasts, so called because cells are lemon-shaped, with a nipplelike projection at each end. These yeasts are considered objectionable in wine fermentations be¬cause they give off-flavors and low yields of alcohol.

False Yeasts (Class Fungi Imperfect)
The film yeasts and apiculate yeasts of this class have been mentioned in the discussion of true yeasts.

Genus Cryptococcus (Torula, Torulopsis}. Within this important genus are C. utilis, grown as a food yeast, and C. sphaericus and C-kefyr, lactose-fermenting yeasts.

Genus Mycoderma (Candida mycoderma). Species of this film-forming yeast grow on wine, beer, cheese, pickles, sauerkraut, and other fermented products and take part in their spoilage. These probably are asporogenous species of Pichia or Hansenula.

Genus Candida. These also are film yeasts. C. knisei is grown with dairy starter cultures to maintain activity and increase longevity of the lactic acid bacteria. Some species, which have been grouped under the genus name Brettanomyces, are important in the alcoholic afterfermenta-tion of English stock beers (porter, stout, pale ale) and Belgian Iambic beers.

Genus Geotrichum. This genus has been included with the molds and described there. G. candidum has been grown for food and for fat production, and, as has been stated, is important on dairy products.
Genus Rhodotorula. These pigmented yeasts may cause discolorations on foods, for example, colored spots on meats or pink areas in sauerkraut.

General Groups of Yeasts
From the foregoing discussion it can be gathered that yeasts often are placed in the following groups that have industrial significance but little relationship to botanical classification: the alcohol yeasts, apiculate yeasts, film yeasts, osmophilic yeasts, food yeasts, lactose-fermenting yeasts, etc.



Frazier W.C.: Food Microbilogy, McGraw-Hill Book Company, Inc., New York, 1958

ALAT PELINDUNG DIRI

Alat Pelindung Diri (APD) merupakan peralatan pelindung yang digunakan oleh seorang pekerja untuk melindungi dirinya dari kontaminasi lingkungan. APD dalam bahasa Inggris dikenal dengan sebutan Personal Protective Equipment (PPE). Dengan melihat kata "personal" pada kata PPE terebut, maka setiap peralatan yang dikenakan harus mampu memperoteksi si pemakainya. Sebagai contoh, proteksi telinga (hearing protection) yang melindungi telinga pemakainya dari transmisi kebisingan, masker dengan filter yang menyerap dan menyaring kontaminasi udara, dan jas laboratorium yang memberikan perlindungan pemakainya dari kontaminisasi bahan kimia. APD dapat berkisar dari yang sederhana hingga relatif lengkap, seperti baju yang menutup seluruh tubuh pemakai yang dilengkapi dengan masker khusus dan alat bantu pernafasan yang dikenakan dikala menangani tumpahan bahan kimia yang sangat berbahaya. APD yang sering dipakai a.I., proteksi kepala (mis., helm), proteksi mata dan wajah (mis., pelindung muka, kacamata pelindung), respirator (mis., masker dengan filter), pakaian pelindung (mis., baju atau jas yang tahan terhadap bahan kimia), dan proteksi kaki (mis., sepatu tahan bahan kimia yang menutupi kaki hingga mata kaki).

PERLINDUNGAN MATA DAN WAJAH
Proteksi mata dan wajah merupakan persyaratan yang mutlak yang harus dikenakan oleh pemakai dikala bekerja dengan bahan kimia. Hal ini dimaksud untuk melindungi mata dan wajah dari kecelakaan sebagai akibat dari tumpahan bahan kimia, uap kimia, dan radiasi. Secara umum perlindungan mata terdiri dari :
• Kacamata pelindung
• Goggle
• Pelindung wajah
• Pelindung mata special (goggle yang menyatu dengan masker khusus untuk melindungi mata dan wajah dari radiasi dan bahaya laser). Walaupun telah banyak model, jenis, dan bahan dari perlindungan mata tersebar di pasaran hingga saat ini, Anda tetap harus berhati-hati dalam memilihnya, karena bisa saja tidak cocok dan tidak cukup aman melindungi mata dan wajah Anda dari kontaminasi bahan kimia yang berbahaya.

PERLINDUNGAN BADAN
Baju yang dikenakan selama bekerja di laboratorium, yang dikenal dengan sebutan jas laboratorium ini, merupakan suatu perlengkapan yang wajib dikenakan sebelum memasuki laboratorium. Jas laboratorium yang kerap sekali dikenal oleh masyarakat pengguna bahan kimia ini terbuat dari katun dan bahan sintetik. Ada beberapa hal yang perlu diperhatikan ketika Anda menggunakan jas laboratorium, a.l.,kancing jas laboratorium tidak boleh dikenakan dalam kondisi tidak terpasang dan ukuran dari jas laboratorium pas dengan ukuran badan pemakainya. Jas laboratorium merupakan pelindung badan Anda dari tumpahan bahan kimia dan api sebelum mengenai kulit pemakainya. Jika jas laboratorium Anda terkontaminasi oleh tumpahan bahan kimia, lepaslah jas tersebut secepatnya.

Selain jas laboratorium, perlindungan badan lainnya adalah Apron dan Jumpsuits. Apron sering kali digunakan untuk memproteksi diri dari cairan yang bersifat korosif dan mengiritasi. Perlengkapan yang berbentuk seperti celemek ini biasanya terbuat dari karet atau plastik.Untuk apron yang terbuat dari plastik, perlu digarisbawahi, bahwa tidak dikenakan pada area larutan yang mudah terbakar dan bahan-bahan kimia yang dapat terbakar yang dipicu oleh elektrik statis, karena apron jenis ini dapat mengakumulasi loncatan listrik statis.

Jumpsuits atau dikenal dengan sebutan baju parasut ini direkomendasikan untuk dipakai pada kondisi beresiko tinggi (mis., ketika menangani bahan kimia yang bersifat karsinogenik dalam jumlah yang sangat banyak). Baju parasut ini terbuat dari material yang dapat didaur ulang.
Bahan dari peralatan perlindungan badan ini haruslah mampu memberi perlindungan kepada pekerja laboratorium dari percikan bahan kimia, panas, dingin, uap lembab, dan radiasi.

PERLINDUNGAN TANGAN
Kontak pada kulit tangan merupakan permasalahan yang sangat penting apabila Anda terpapar bahan kimia yang korosif dan beracun. Sarung tangan menjadi solusi bagi Anda. Tidak hanya melindungi tangan terhadap karakteristik bahaya bahan kimia tersebut, sarung tangan juga dapat memberi perlindungan dari peralatan gelas yang pecan atau rusak, permukaan benda yang kasar atau tajam, dan material yang panas atau dingin.


Bahan kimia dapat dengan cepat merusak sarung tangan yang Anda pakai jika tidak dipilih bahannya dengan benar berdasarkan bahan kimia yang ditangani. Selain itu, kriteria yang lain adalah berdasarkan pada ketebalan dan rata-rata daya tembus atau terobos bahan kimia ke kulit tangan. Sarung tangan harus secara periodik diganti berdasarkan frekuensi pemakaian dan permeabilitas bahan kimia yang ditangani. Jenis sarung tangan yang sering dipakai di laboratorium, diantaranya, terbuat dari bahan karet, kulit dan pengisolasi (asbestos) untuk temperatur tinggi. Jenis karet yang digunakan pada sarung tangan, diantaranya adalah karet butil atau alam, neoprene, nitril, dan PVC (Polivinil klorida). Semua jenis sarung tangan tersebut dipilih berdasarkan bahan kimia yang akan ditangani. Sebagai contoh, sarung tangan yang terbuat dari karet alam baik apabila Anda bekerja dengan Ammonium hidroxida, tetapi tidak baik bila bekerja dengan Dietil eter.

PERLINDUNGAN PERNAFASAN
Kontaminasi bahan kimia yang paling sering masuk ke dalam tubuh manusia adalah lewat pernafasan. Banyak sekali partikel-partikel udara, debu, uap dan gas yang dapat membahayakan pernafasan. Laboratorium merupakan salah satu tempat kerja dengan bahan kimia yang memberikan efek kontaminasi tersebut. Oleh karena itu, para pekerjanya harus memakai perlindungan pernafasan, atau yang lebih dikenal dengan sebutan masker, yang sesuai. Pemilihan masker yang sesuai didasarkan pada jenis kontaminasi, kosentrasi, dan batas paparan. Beberapa jenis perlindungan pernafasan dilengkapi dengan filter pernafasan yang berfungsi untuk menyaring udara yang masuk. Filter masker tersebut memiliki masa pakai. Apabila tidak dapat menyaring udara yang terkontaminasi lagi, maka filter tersebut harus diganti.


Dari informasi mengenai beberapa APD diatas, maka setiap pengguna bahan kimia haruslah mengerti pentingnya memakai APD yang sesuai sebelum bekerja dengan bahan kimia. Selain itu, setiap APD yang dipakai harus sesuai dengan jenis bahan kimia yang ditangani. Semua hal tersebut tentunya mempunyai dasar, yaitu kesehatan dan keselamatan kerja di laboratorium. Ungkapan mengatakan bahwa "Lebih baik mencegah daripada mengobati". APD merupakan solusi pencegahan yang paling mendasar dari segala macam kontaminasi dan bahaya akibat bahan kimia. Jadi, tunggu apa lagi. Gunakanlah APD sebelum bekerja dengan bahan kimia.

Referensi:
Stephen K. Hall. Chemical Safety in the Laboratory. CRC Press, Inc., 1994.
Robert J. Alaimo. Handbook of Chemical Health and Safety. American Chemical Society, Oxford University Press. 2001.
Merck KGaA. Fundamentals of Laboratory Safety. Git feffag GmbH. 2001.

TESTING MILK FOR TOTAL SOLIDS

The Importance of the Total Solids Test. Almost every state in the union, as well as the federal government, has standards defining law¬ful milk. These standards have been adopted to protect the public against milk of low butterfat and solids content brought about by watering or skimming the milk, and against milk from very low-test¬ing cows. When a producer's milk tests abnormally low in butterfat a milk company also will wish to run a test for adulteration. Bed the invention of the Babcock test and in the absence of any simpk test for determining the total solids content of milk, adulteration was very commonly practiced; today it is not common.

The milk standard is usually stated in terms of per cent butterfat. per cent S.N.F. and per cent T.S. The Federal requirements are 3.25 per cent butterfat, 8.5 per cent S.N.F., and 11.75 per cent T.S. The federal standard applies only to milk sold in the District of Columbia and milk shipped from one state to another. For example, 3 per cent milk is legal in New York State, but if a New York farmer ships milk into Massachusetts it must not only come up to the federal standard but also to the Massachusetts standard, which in this case happens to be higher than the federal standard.

The enforcement of these state standards usually rests with the state board of health, the state board of agriculture, or a dairy and food commission. City milk inspectors also enforce standards for the cities. In general, inspectors are hired to travel about the state, picking up samples from milk trucks, trains, stores, and so on. These samples are taken to laboratories and tested, and if they are
found to be below the standard, a warning is issued. If a sample taken after the warning shows no improvement, the offender is taken into court.

Methods of Determining the Total Solids in Milk. The only abso¬lutely reliable method, and the one that is likely to stand in court, is the gravimetric test. In brief, this test involves weighing out a small sample of the milk into a small dish on a chemical balance and then evaporating the moisture in an oven or over a hot-water bath, until trial weights show that the residue has reached constant weight. The percentage of total solids is then calculated. Incidentally, this is the only method that can be used for determining the total solids in milk products, such as cream, milk powder, ice cream, etc.

For making approximate tests on milk, there is an instrument on tte market known as the lactometer. From the lactometer reading and the percentage of butterfat, the percentage of S.N.F. or T.S. can be calculated with the aid of a formula.

Principle Upon Which fhe Lactometer Operates. The use of the lac¬tometer rests on the principle that a body floating in a liquid dis¬places an amount of the liquid equal to the weight of the floating body. In other words, if a glass cylinder is filled to the brim with milk and the lactometer is floated in it, the weight of the milk that runs over is equal to the weight of the lactometer.

Description of the Instrument. Two types of lactometers will be described. Until 1956, the Quevenne lactometer was the one in com¬mon use. At the bottom of the Quevenne is a large bulb filled with shot or mercury to keep the lactometer in an upright position when floating. Just above this is the bulb of a thermometer, the paper scale for which is located at the top of the instrument. Above this liulb is a large air chamber for floating the lactometer, and just above tin's air chamber is a paper lactometer scale, reading in lactometer degrees from 15 at the top down to 40.

In April 1956, Paul D. Watson of the Eastern Utilization Research Branch of the U.S. Department of Agriculture published his findings with a modified Quevenne lactometer, designed to give more accur¬ate results. This instrument works on the same principle and reads to 0.2 of a degree.

Common Factors Affecting the Weight of a Given Volume of Milk. TEMPERATURE. Heat causes milk to expand, and thus makes a given volume of it weigh less. Cold, on the other hand, causes the milk to contract and makes a given volume weigh more. This difference is plainly shown on the farm when milk is cooled. A can of warm night’s milk, put into the cooling tank at night, is no longer full in the morning and has to be “topped out” before shipping.

THE PERCENTAGE OF TOTAL SOLIDS NORMALLY PRESENT IN MILK. With the increase of the percentage of butterfat in milk the solids-not-fat also increase, so that a quart of 5 per cent milk, for example, will weigh slightly more than a quart of 3 per cent milk.

ADDITION OF WATER. Water is lighter than milk, and therefore a given volume of milk weighs less if it contains some extraneous water.
THE ADDITION OF SKIM MILK OH THE REMOVAL OF CREAM. Skim milk is heavier than milk; hence, the addition of skim milk to milk, or the removal of some of the cream, increases the weight of a given volume of milk.


Operation of the Test when Using the Quevenne Lactometer. The sample must be representative and must be thoroughly mixed as for the Babcock test. The temperature of the milk must be between 50° and 70°F. A glass cylinder is filled with the milk and set in some receptacle, such as a sink or a pan. The lactometer is then slowly lowered into it.

In a few seconds the lactometer becomes stationary, and the lac¬tometer scale is read at the point to which the milk rises on the lactometer scale (the meniscus formed by the milk on the lactometer scale is included). This reading is recorded, and the temperature noted and recorded. Because temperature affects the weight of a given volume of milk and hence the lactometer reading, obviously the readings must be taken at some definite temperature, or must be corrected to that temperature. The lactometer gives a correct read¬ing directly only when the milk is at 60°F. If the milk is some other temperature between 50° and 70°F, a correction factor can be used to give a corrected reading. This factor is 0.1 of a lactometer degree added to the lactometer reading for each degree of temper¬ature above 60° F, and subtracted for each degree below 60° F. Thus, if the lactometer reads 30 at 68° F, the correct reading is 30.8; and if it reads 29.5 at 53°F, the correct reading is 28.8. The lower the correction factor used, that is, the nearer the temperature is to 60°F, the more accurate the results will be.

The operation of the modified lactometer is the same except the milk must be at a temperature of 102°F ± 2° F. For each degree above 102° F add 0.2 to the reading, and for each degree below 102° F, subtract 0.2. The lactometer should be warmed to 102° F before use, and the milk temperature should be checked with an accurate thermometer.

Calculation of the Specific Gravity of Milk. The rule is to divide the Quevenne lactometer reading by 1,000, multiply by 100, and add 1 to result. Thus if the lactometer reading at 60°F is 32, 32  1,000 equal 0.0032  100 = 0.032 + 1 equal 1.032 specific gravity.

Calculation of the Per Cent S.N.F. and T.S. in Milk. To find the solids in milk, both lactometer and Babcock tests must be made and this formula must be used with the Quevenne lactometer:
¼ L + (0.2  per cent butterfat) = per cent S.N.F.
L is the corrected lactometer reading in degrees. For example, if the lactometer reading is 32.5 at 55°F, at butterfat test is 3.9 per cent, find the per cent S.N.F. and T.S.


The solids-not-fat may be found by using 0.33 F instead of 1.33 F, or by subtraction of the butterfat percentage from the total solids percentage. The total solids of skim milk samples may be calculated by dropping the constant ( 0.40) from the formula.

F. J. Henry and A. K. Harry: “Milk Production And Processing,” John Wiley & Sons Inc., New York, 1960