Yeasts may be viewed as being unicellular fungi in contrast to the molds, which are multicellular; however, this is not a precise definition, as many of what are commonly regarded as yeasts actually produce mycelia to varying degrees. Yeasts can be differentiated from bacteria by their larger cell size and their oval, elongate, elliptical, or spherical cell shapes.
Typical yeast cells range from 5 to 8 µm in diameter, with some being even larger. Older yeast cultures tend to have smaller cells. Most of those of importance in foods divide by budding or fission. Yeasts can grow over wide ranges of acid pH and in up to 18% ethanol. Many grow in the presence of 55–60% sucrose. Many colors are produced by yeasts, ranging from creamy, to pink, to red. The asco- and arthrospores of some are quite heat resistant.
(Arthrospores are produced by some yeast-like fungi.) Regarding the taxonomy of yeasts, newer methods have been employed in the past decade or so consisting of 5S rRNA, DNA base composition, and coenzyme Q profiles. Because of the larger genome size of yeasts, 5S rRNA sequence analyses are employed more than for larger RNA fractions.
Many changes have occurred in yeast systematics, due in part of the use of newer methods but also to what appears to be a philosophy toward grouping rather than splitting taxa. One of the most authoritative works on yeast systematics is that edited by Kreger-van Rij and published in 1984.30 In this volume, the former Torulopsis genus has been transferred to the genus Candida, and some of the former Saccharomyces have been transferred to Torulaspora and Zygosaccharomyces.
The teleomorphic or perfect states of more yeasts are now known, and this makes references to the older literature more difficult. The taxonomy of 15 or so foodborne genera is summarized below. For excellent discussions on foodborne yeasts, the publications by Deak and Beuchat,15 Beneke and Stevenson,3 and Pitt and Hocking37 should be consulted. For identification, Deak and Beuchat15 have presented an excellent simplified key to foodborne yeasts.