Handbook of Enology: Volume 1. Pascal Ribéreau-Gayon
Delimitation
Yeast taxonomy (from the Greek taxis: putting in order), and the taxonomy of other microorganisms for that matter, includes classification and identification. Classification groups organisms into taxa according to their similarities and/or their ties to a common ancestor. The basic taxon is species. A species can be defined as a collection of strains having a certain number of morphological, physiological, and genetic characteristics in common. This group of characteristics constitutes the standard description of the species. Identification compares an unknown organism to individuals already classified and named that have similar characteristics.
TABLE 1.1 Classification of Grape and Wine Yeast Genera (Kurtzman et al., 1994)
| Phylum | Subphylum | Order | Families | Genera |
|---|---|---|---|---|
| Ascomycota | Saccharomycotina | Saccharomycetaceae | Saccharomycetales | Saccharomyces Kluyveromyces Torulaspora Zygosaccharomyces Kazachstania Lachancea |
| Pichiaceae | Pichia Brettanomyces Dekkera | |||
| Metschnikowiaceae | Metschnikowia | |||
| Debaryomycetaceae | Debaryomyces Lodderomyces | |||
| Saccharomycodaceae | Hanseniaspora Kloeckera Saccharomycodes | |||
| Saccharomycetales Incertae sedis | Wickerhamo‐mycetaceae | Candida |
FIGURE 1.19 Observation of two winemaking yeast species having an apiculate form
(Source: Photographs from M. Mercier, Department of Electron Microscopy, Université de Bordeaux 1.)
(a) H. uvarum. (b) S. ludwigii.
Taxonomists first delimited yeast species using morphological and physiological criteria. The first classifications were based on the phenotypic differences between yeasts: cell shape and size, spore formation, cultural characteristics, fermentation and assimilation of various sugars, assimilation of nitrates, growth‐factor needs, and resistance to cycloheximide. Ribéreau‐Gayon et al. (1975) described the application of these methods to wine yeasts in detail. Since then, many rapid, ready‐to‐use diagnostic kits have been proposed to determine yeast response to different physiological tests. Lafon‐Lafourcade and Joyeux (1979) and Cuinier and Leveau (1979) designed the API 20 C system for the identification of winemaking yeasts. It contains eight fermentation tests, 10 assimilation tests, and a cycloheximide resistance test. For a more complete identification, the API 50 CH system contains 50 substrates for fermentation (under paraffin) and assimilation tests. Heard and Fleet (1990) developed a system that uses the various tests listed in the work of Barnett et al. (1990).
FIGURE 1.20 Binary fission of S. pombe.
(Source: Photographs from M. Mercier, Department of Electron Microscopy, Université de Bordeaux 1.)
Due to the relatively limited number of yeast species significantly present on grapes or in wine, these phenotypic tests identify winemaking yeast species in certain genera without difficulty. Certain species can be identified by observing growing cells under the microscope alone. Small apiculate cells, with small lemon‐like shapes, are indicative of the species H. uvarum and its imperfect form Kloeckera apiculata (Figure 1.19). Saccharomycodes ludwigii is characterized by much larger (10–20 μm) apiculate cells. Since most yeast multiply by budding, the genus Schizosaccharomyces can be recognized because of its vegetative reproduction by binary fission (Figure 1.20). In modern taxonomy, this genus only contains the species Schizosaccharomyces pombe. Finally, the budding of Candida stellata (formerly known as Torulopsis stellata) occurs in the shape of a characteristic star.
According to Barnett et al. (1990), the physiological characteristics listed in Table 1.2 can be used to distinguish between the principal grape and wine yeasts. Yet some of these characteristics (for example, fermentation profiles of sugars) vary within the species and are even unstable for a given strain during vegetative reproduction. Taxonomists have realized that they cannot differentiate species based solely on phenotypic discontinuity criteria. They have progressively established a delimitation founded on the biological and genetic definition of a species.
In theory, a species can be defined as a collection of interfertile strains whose hybrids are themselves fertile—capable of producing viable spores. This biological definition runs into several problems when applied to yeasts. First of all, many Ascomycetes yeasts are homothallic; hybridization tests are thus especially tedious and difficult for routine identification. Finally, certain wine yeast strains have little or no sporulation aptitude, which makes the use of strain infertility criteria even more difficult.
TABLE 1.2 Physiological Characteristics of the Principal Grape and Wine Yeasts (Barnett et al., 1990)
| Fl D‐Glucose fermentation | F2 D‐Galactose fermentation | F3 Maltose fermentation | F4 α‐D‐glucoside fermentation | F5 Sucrose fermentation | F6 α,α‐Trehalose fermentation | F7 Melibiose fermentation | |
| Candida stellata | + | – | – | – | + | – | – |
| Candida vini | v | – | – | – | – | – | – |
| Candida famata | v | v |
v
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