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.
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.
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.
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:
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