Vitamin Compendium

The Physiological Importance
of the Vitamins in Man and Animals

Essential dietary components
Human and animal organisms require, for the proper operation of all of their physiological functions, a regular intake of some forty different dietary components. If only one of these is missing or its supply is in adequate, deficiency symptoms appear which if prolonged, can be fatal. All these components of the dietare therefore indispensable for life ("essential" ); they comprise the actual energy-yielding and body-building substances (proteins, fats, carbohydrates, amino-acids, mineral salts) and also the "micro"-nutrients (vitamins, trace elements).

Vitamins: essential dietary components
The vitamins are therefore active substances essential for life. As a group, they are recognised by two characteristic properties:

1. The daily requirement for each vitamin for an individual is very small, usually measured in microgrammes or milligrammes; in this respect they differ from the "macro"-nutrients which are required in at least 1000 times larger amounts. Vitamins, on the other hand, are mediators of synthetic and degradative processes without serving as building substances themselves.

2. Vitamins are organic compounds, differing in this respect from the trace elements such as iron, iodine, manganese and zinc which, however, are also essential compounds.

To-day, 13 vitamins are known, each of which represents a group of related compounds with the same qualitative activity. The provitamin A group is also of great practical importance; this comprises compounds which are partly transformed into vitamin A in the organism. The Table (p. 8/9) surveys the compounds with vitamin properties.

In addition to these 13 vitamins, there are other substances which have been classed with the vitamins although their vitamin character has not yet been established. Examples are: orotic acid (vitamin BI3); inositol or Bios I; lipoic acid or thioctic acid; rutin (vitamin P); xanthopterin (vitamin BI4); carnitine (vitamin BT); pangamic acid (vitamin BI5); and ubiquinone (coenzyme Q).

Unidentified growth factors
In this connection, the so-called "unidentified growth factors" (U.G.F.) growth factors which, under certain conditions, can favourably affect the growth and productivity of livestock, should be mentioned. They are present, for example, in residues of alcohol fermentation, in fish-solubles, grass sap, whey and egg-yolk. Nothing is yet known of the chemical nature of these unidentified growth factors. They may perhaps not be single entities but mixtures of known essential nutritional factors with mutually potentiating effects (synergism).


Vitamins in metabolism
Unlike the nutrients which serve as building materials and storage substances in the growth of an organism, the vitamins exercise catalytic functions. They facilitate the synthesis and degradation of the principle nutrients, thereby controlling metabolism. Research into these biochemical processes is still active. The key functions of the vitamins of the B-complex in particular, have been extensively clarified. Vitamins B15 B2, B6, Niacin, B12, pantothenic acid, folic acid and biotin, and also, in part, their metabolites are incorporated into enzymes which are indispensable for normal metabolism of carbohydrate, lipid and protein. In these processes the vitamins play no part as building substances and this also explains why the daily requirement is small, compared with that of the main nutrients.

Vitamin deficiency: avitaminoses, hypovitaminoses
If one or more vitamins are either not available at all to an organism, or only in inadequate amounts, certain metabolic processes are impaired, leading to disturbances of productivity, growth inhibition and disease. Vitamin deficiency also causes disorders of fertility in male and female animals as well as increased liability to infectious and parasitic disease. The functions of the individual vitamins in metabolism are very specific, so that, in deficiency, one or more defined biochemical reactions in certain organs can be adversely affected. These disturbances of metabolism can therefore give rise to very characteristic deficiency symptoms; frequently, however, the pattern of disturbed health is confused, for example, when the vitamin is required for several metabolic reactions, or when other nutrients or active substances are lacking simultaneously.

In countries where diets are unbalanced and inadequate, or where there are particular dietary customs, certain commonly observed typical disease patterns have been shown to be due to vitamin deficiency. Knowledge of the cause has led to the remedy, so that these avitaminoses, which also occurred quite frequently in Europe even during the last century, are now no longer of importance. In the developing countries, however, they are still significant to-day.

Examples of the most common avitaminoses are:
Xerophthalmia, Keratomalacia (Vitamin A-deficiency)
Beri-beri (Vitamin B1-deficiency)
Pellagra (Niacin-deficiency)
Scurvy (Vitamin C-deficiency)
Rickets (Vitamin D-deficiency)

In addition to the avitaminoses which are characterised by clearly defined symptoms, there are also hypovitaminoses - unspecific states - which are brought about by the inadequate provision of one, or more frequently several, vitamins. They appear in the form of ill-defined symptoms such as skin changes, reduced vitality, lowered resistance to infections, etc.

Finally, latent hypovitaminoses are also known; these are states which, under normal environmental conditions, are not recognisable through deficiency symptoms, but which can immediately induce symptoms under sudden stress. The relationship between stress and vitamin requirements in animal maintenance is discussed in detail in the section on "Vitamins and Carotenoids in Animal Nutrition."

Anti-vitamin, vitamin antagonists
In addition to the hypovitaminoses and avitaminoses, which are due to Anti-vitamins, inadequate provision of vitamins, deficiency states occur occasionally vitamin when the function of a vitamin is disturbed or inhibited. This can occur antagonists in the presence of specific substances which are designated as antivitamins or vitamin antagonists. They reach the digestive tract as natural dietary constituents or as additives (medicaments), either immediately inhibiting resorption of the vitamin or disturbing its specific biochemical action. Thus, in raw and dried egg-white, for example, the substance avidin occurs which forms a complex with biotin in the gastro-intestinal tract, thereby preventing resorption of this vitamin. Raw fish, especially fresh-water fish, and various bacteria contain thiaminase, an enzyme that destroys vitamin B1. Avidin and thiaminase are destroyed by cooking.

Vitamin antagonists also occur in bracken, various foodplants and in certain varieties of vegetables. Thus, lucerne contains one or more substances which markedly reduce the efficacy of the vitamin E present in lucerne, and significantly increase its faecal excretion. Antagonists of vitamin E have also been demonstrated in beans. Linatin - a substance identified in linseed has been shown to be an antagonist of vitamin B6.

Nutrient constituents that raise vitamin requirements
The mechanism of action of typical antivitamins is based on the similar chemical structures of the vitamin and antivitamin which enables the latter to displace the vitamin from its site of action. The antagonistic activity of certain medicaments may also be explained in this way.
bacteriostatics (sulphonamides, and coccidiostats) and antibiotics which, on prolonged administration in high dosage, destroy intestinal flora and so eliminate a source of vitamins for a host organism.

Disease, parasites, bodily stress
When the defence mechanisms of the body are mobilised and demands made on reserve substances there is an associated increase in the activity of enzyme systems. Thus, every increase in metabolic activity leads unavoidably to an increased usage of anabolic and catabolic enzymes and increased vitamin requirement. This is true not only in illness but also for physical exertion and increased production.

In addition infective organisms and parasites themselves require vitamins; they compete with their host organism. Intestinal parasites attack. the mucous membranes and interfere with vitamin resorption. Viruses and bacteria leave toxins in the body, the degradation and excretion of which require greater enzyme activity.

Vitamin requirements
Every individual is constantly exposed to changing influences and environmental pressures. Therefore the metabolic rate must be adapted to the immediate necessity, so that vitamin requirements are also subject to continuous variation between certain limits. It follows that only approximate quantitative estimates of vitamin requirements can be expressed. Even in extended animal studies the results are only valid under the special circumstances of a particular experiment. For example, in studying vitamin requirements, different results may be expected when different criteria are followed - e.g. the maintenance of body weight in indoor rearing, or intensive fattening, work output, milk yield or egg production.

Numerous studies have been made of the vitamin requirements of livestock under different conditions, leading to an approximate estimate of vitamin requirement of the various animal species. It is also known under what conditions increases in requirements can be expected.

In man, however, figures for vitamin requirements cannot be given with the same reliability as for animals. It is of course possible to determine minimum amounts which must be supplied daily to avoid severe deficiency diseases. But observations made on domestic animals show that the vitamin requirements for maximum production are many times greater than such minimum amounts.

The daily supply of vitamins can only be estimated within certain limits from the contents of a diet. In addition to the external influences already mentioned, which affect the utilisation of the vitamins supplied, other factors play a part; the analytical determination of the vitamin contents of foods is costly and time-consuming, and may be subject to errors of 10% or more, according to the concentrations of the vitamins. Some vitamins can only be assayed by biological or microbiological methods, which are also subject to systematic errors.

A further problem in the calculation of the vitamin supply is the degree of utilisation of the vitamins from different foods and animal feeds. Thus, for example, -carotene and vitamin B2, as components of food of vegetable origin, are only incompletely utilised. It is assumed that these compounds are bound very firmly to certain parts of the plant-cells and that the enzymes of the gastric juice are incapable of liberating them completely in the gastrointestinal tract. Accurate data on the degree of utilisation of naturally occurring vitamins are available in only a few cases.

Consideration of all the factors influencing vitamin requirements leads to the following conclusions relating to practical human and animal nutrition:

Data on the minimum vitamin requirements are available, based on numerous experimental results; in view of biological variations and experimental errors these can, however, only be regarded as approximate values. In particular cases account must be taken of appreciable variations since, on the one hand the utilisation of administered vitamins is impaired by many factors and, on the other, vitamin requirements under conditions of physical stress or increased production can rise to a degree that is difficult to estimate.

To cover vitamin requirements fully it is therefore necessary to add a supplement to the diet so that even in unfavourable cases adequate supplies are assured.

Contrary to other active substances, such as the hormones, vitamins can be absorbed in large quantities without ill-effects. Only when the supply exceeds a certain upper limit (more than a hundred times the requirement for most vitamins) and, for a prolonged period, can symptoms of so-called hypervitaminoses develop. So far, hypervitaminosis in man has only been observed in the case of excessive administration of vitamin D.


Vitamin Compendium. The Properties of the Vitamins and their Importance in Human and Animal Nutrition. Vitamin and Chemicals Department. F. Hoffmann-La Roche & Co. Ltd, Basle. Switzerland

Vitamin Compendium

Technological Properties and Formulations of Vitamins

The application of vitamins in pharmaceuticals and for the fortification of food and animal feeds can of course be achieved to a degree with vitamin-rich natural materials such as yeast, wheat-germ etc., or with concentrates or extracts prepared from such products. These, however, are now seldom able to meet the critical requirements of the modern processing industries. Most of such materials have rather low and also varying contents of the vitamins, and they often contain additional substances which adversely affect the organoleptic properties and storage stability of the products in which they are included; further, they frequently occur in forms unsuitable for many applications.

Advances in organic chemistry and the development of new techniques have now made possible the economic synthesis of most vitamins on an industrial scale, and the use of synthetic vitamins is now the predominant practice in human and animal nutrition as well as in medicinal products. Many experimental studies have shown that the synthetic compounds are identical in all biological properties with the naturally occurring vitamins, and the identity of the activity of the synthetic and the corresponding natural vitamins has been well established.

Application forms
The industrial production of vitamins with a high degree of purity, has largely solved the organoleptic problem as well as the difficulty of providing accurate, known quantities. A further problem, however, was that many vitamins are very sensitive substances, unstable in adverse environments, so that the development of more stable forms became necessary for their extended application in the food and animal feed industries. Certain other properties of some pure vitamins, such as their solubility, physical state, concentration etc., also restrict their usage possibilities, so that special forms had to be created, more suitable for specific applications.

Formulations of the vitamins suitable for the most varied purposes can be prepared by the following methods:
The synthesis of stable derivatives.
The addition of stabilisers (antioxidants).
Standardisation with suitable fillers.
Coating with suitable carrier substances.
The transformation of water-soluble vitamins into fat-soluble derivatives.
The transformation of fat-soluble vitamins into "water-soluble derivatives or water-dispersible formulations.

All these methods are used in the manufacture of commercial vitamin formulations. The simultaneous use of several of these procedures is often involved, the method of choice being determined by the desired relationship between physical properties and biological activity.

The most important properties relevant to the use of the various vitamins, and the usual commercial forms, are as follows.

Fat-soluble vitamins
Vitamin A is extremely sensitive to oxidation. Its destruction by atmospheric oxygen is accelerated catalytically by light, especially by ultraviolet light, and also by metal salts, peroxides, and heat, particularly in the presence of moisture. Destruction of the vitamin is also facilitated by the finely divided state which is necessary to ensure homogeneity and optimal absorption.

The instability of vitamin A-alcohol, the essential form of vitamin A, has led to the industrial preparation of its somewhat more stable esters, the acetate and palmitate. Additional stability can be achieved by dissolving these in vegetable oils; and further stabilisation obtained by the addition of anti-oxidants, which can also be combined with synergists and complexing agents. Such forms of vitamin A can be used directly in fats and oils.

The oily forms of vitamin A however, are unsuitable for processing in dry preparations such as animal feeds. For such purposes dry powder preparations have been developed, in which the vitamin A is deposited in a carrier substance. The most important carrier for stabilising vitamin A is gelatine, in which the vitamin A must be present as extremely finely divided droplets to ensure rapid absorption. The particle size of such powders should be between 150 and 500 , (diameter), and the concentration approx. 500,000 IU vitamin A per gramme, in order to ensure satisfactory dispersion in compound feeds and similar products.

The criteria of particle size and concentration necessitate compromise solutions. Larger particles, having relatively smaller surface areas, are more stable, but they result in more irregular distribution of the vitamin A in a mix. Smaller particles are less stable because of their relatively larger surface areas. Highly concentrated powders are thus unfavourable for the distribution of vitamin A in a feed; on the other hand lower concentrations lead to increased costs.

In addition to these dry powder forms, liquid water-miscible formulations have also been developed. The use of appropriate emulsifiers produces aqueous dispersions which are suitable for the preparation of solutions and syrups for human medicine, and for veterinary use as injections and for the enrichment of drinking water.

Vitamin D (D2 and D3) is also sensitive to oxidising agents, light, and acids. Since the considerations of stability and application for vitamin D closely resemble those for vitamin A, similar commercial forms have been developed (oily solutions, stabilised powders and aqueous dispersions).

Vitamin E is used mainly in the form of the relatively stable -tocopheryl acetate, since the unesterified tocopherols are rapidly oxidised and darkened by atmospheric oxygen. In the presence of moisture the vitamin E-ester is hydrolysed by acids or alkalis. Vitamin E is also therefore processed to more stable forms, and adsorbates, granulates and water-miscible preparations are now commercially available.

Vitamin K1 is slowly degraded by atmospheric oxygen, but is very rapidly affected by light and alkalis; it is however, relatively stable to heat.

Water-soluble vitamins
The water-soluble vitamins are generally stable in the pure state. In aqueous solution, however, they are more sensitive to a number of factors.

Thiamine hydrochloride, the most important commercial form of vitamin B1 is stable if protected from light and moisture. In aqueous solution the stability of vitamin B1 is markedly pH-dependent.

Stability is optimal at pH 3.0, and is still good at pH 4.5. In neutral or alkaline solution however, particularly in the presence of oxidising or reducing agents, or if heated, vitamin B1 is unstable and is converted to inactive compounds; heavy metals accelerate this destruction.

In the presence of vitamin B2, vitamin B, is oxidised in aqueous solution to thiochrome. This process is facilitated by increasing concentration of vitamin B2 and by the presence of atmospheric oxygen.

In dry preparations the degree of humidity is important for stability. When conditions are such that the hydrolysis and oxidative decomposition of thiamine are likely, it is advantageous to use the less sensitive thiamine mononitrate instead of the more usual hydrochloride. Thiamine mono-nitrate is also prepared in coated forms to improve its stability, and to reduce its odour and taste.

Vitamin B2 (Riboflavin) is unstable to powerful reducing agents, alkalis and light. It is only sparingly soluble in water; to increase the solubility of riboflavin, solubilisers such as nicotinamide or salicylic acid are used. Optimal stability of solutions is at pH 3.5-4.0.

The sodium salt of riboflavin-5'-phosphate is considerably more water-soluble. Its stability characteristics are similar to those of riboflavin, but it reacts with heavy metal ions and especially with calcium to form insoluble metal salts; the addition of chelating agents can prevent this reaction.

In aqueous solutions, riboflavin acts as an oxidising agent to vitamins B1, C and folic acid. It also acts as a photosensitiser and hydrogen acceptor in the light-induced oxidation of folic acid and vitamin C.

Riboflavin has an unpleasant, lasting bitter taste, but dry preparations of this vitamin are also available in coated form.

Vitamin B6 (Pyridoxine) in the usual commercial form of the hydro-chloride is generally stable to heat and oxygen. It is degraded by light in neutral or alkaline solutions, and to a lesser degree in acid solution; optimal stability is in the region of pH 3.0-5.0.

A coated preparation of pyridoxine hydrochloride is also commercially available for special applications.

Pure crystalline vitamin B12 (cyanocobalamin) is relatively stable to air in the dry state and in neutral to weakly acid solutions; optimal stability is at pH 4.5-5.0. Destruction of vitamin BI2 only occurs at elevated temperatures, but this destruction is increased in the presence of vitamin B1 and, more particularly, nicotinamide. In solution vitamin B12 is also sensitive to light, especially ultraviolet light. Vitamin C and its oxidation products (e.g. dehydroascorbic acid), in the presence of copper, manganese, molybdate or fluoride ions, as well as alkalis or reducing agents, degrade vitamin B12. Vitamins B2, B6 and panthenol, however, are compatible with vitamin B12.

Dry powder dilutions containing from 0.05% to 1.0% of vitamin B12 on a base of, e.g. mannitol or dicalcium phosphate; stabilised adducts on ion-exchangers; and preparations with a gelatine base are now commercially available. For animal feed supplementation, fermentation concentrates are commonly used; these have contents of up to i per cent vitamin B12.

Biotin is stable to oxygen, daylight and heat in the dry crystalline state. Ultraviolet light or powerful oxidising agents can destroy biotin. In strongly acid or alkaline solutions its biological activity falls rapidly. For the enrichment of animal feeds, a diluted product with a standardised content of 1% biotin is available.

Pure, crystalline folic acid is stable to air and heat, bat is degraded by light, especially ultra-violet light. It is most stable in neutral to weakly alkaline media. It is destroyed by acids, strong alkalis, metal salts and by reducing and oxidising agents. Vitamin B, is slightly - and vitamin B2 markedly - destructive to folic acid. Panthenol, nicotinamide and vitamin B6 however, are compatible with folic acid.

Nicotinic acid amide, in the pure anhydrous form and also in aqueous solution, is stable to air, daylight and heat. Ultraviolet light slowly destroys nicotinamide. Strong acids, strong alkalis and also some heavy metals reduce its biological activity.

Nicotinamide is also available in a coated form.

D-Pantothenic acid, as a compound, is very sensitive to many factors, and it is therefore produced commercially as the calcium and sodium salts which possess good stability providing moisture is excluded.

Aqueous solutions of pantothenic acid salts are only stable to a degree at pH 5.0-7.0; they are sensitive to heat and tend to hydrolyse, especially in the presence of acids and alkalis.

For liquid preparations, D-Panthenol (the alcohol corresponding to D-Pantothenic acid) - has been developed; its aqueous solutions at pH 4.0-7.0 are significantly more stable and can be heat-sterilised.

Crystalline Vitamin C (ascorbic acid) is relatively stable in air under anhydrous conditions; the sodium salt however, tends to turn yellow with time. Aqueous solutions of ascorbic acid are sensitive to oxidising agents, decomposition being accelerated by alkalis and traces of heavy metal ions (particularly Cu); to minimise oxidation, solutions are treated with metal (Cu)-complexing agents at a pH below 6.0. In multi-vitamin solutions and syrups the stability of ascorbic acid falls as the content of water present increases. The vitamins B1, B2 and nicotinamide adversely affect the stability of ascorbic acid, but thiamine is compatible. Vitamin B2 absorbs blue light and, in the presence of air, can catalyse the photo-oxidation of ascorbic acid. The destructive effects of vitamin C and vitamin B2 are reciprocal; vitamin C is also destructive to folic acid and vitamin B12.

Ascorbic acid is also available commercially as its sodium and calcium salts and in coated preparations.



Vitamin Compendium. The Properties of the Vitamins and their Importance in Human and Animal Nutrition. Vitamin and Chemicals Department. F. Hoffmann-La Roche & Co. Ltd, Basle. Switzerland

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.