Taxonomy, Role, and Significance of Microorganisms in Foods
31
Basipetospora, Chrysosporium, Eremascus, Polypaecilum, Wallemia, and Xeromyces. These molds
are characterized by the ability to grow below a w (water activity) = 0.85. They are of significance
in foods that owe their preservation to a low a w . Only Wallemia and Xeromyces are discussed further
below.
Wallemia produces deep-brown colonies on culture media and on foods. W. sebi (formerly Sporendonema), the most notable species, can grow at an a w of 0.69. It produces the “dun” mold condition
on dried and salted fish.
Xeromyces has only one species, X. bisporus. It produces colorless cleistothecia with evanescent asci
that contain two ascospores. This organism has the lowest a w growth of any other known organisms.
37
Its a w high is ∼0.97, its optimum is 0.88, and its minimum is 0.61. Its thermal D at 82.2
◦ C is 2.3
minutes. It causes problems in licorice, prunes, chocolate, syrup, and other similar products.
SYNOPSIS OF COMMON GENERA OF FOODBORNE YEASTS
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
Hocking
37 should be consulted. For identification, Deak and Beuchat
15 have presented an excellent
simplified key to foodborne yeasts.
Division: Ascomycotina
Family: Saccharomycetaceae (ascospores and arthrospores formed; vegetative
reproduction by fission or budding)
Subfamily: Nadsonioideae
Genus: Hanseniaspora
31
Basipetospora, Chrysosporium, Eremascus, Polypaecilum, Wallemia, and Xeromyces. These molds
are characterized by the ability to grow below a w (water activity) = 0.85. They are of significance
in foods that owe their preservation to a low a w . Only Wallemia and Xeromyces are discussed further
below.
Wallemia produces deep-brown colonies on culture media and on foods. W. sebi (formerly Sporendonema), the most notable species, can grow at an a w of 0.69. It produces the “dun” mold condition
on dried and salted fish.
Xeromyces has only one species, X. bisporus. It produces colorless cleistothecia with evanescent asci
that contain two ascospores. This organism has the lowest a w growth of any other known organisms.
37
Its a w high is ∼0.97, its optimum is 0.88, and its minimum is 0.61. Its thermal D at 82.2
◦ C is 2.3
minutes. It causes problems in licorice, prunes, chocolate, syrup, and other similar products.
SYNOPSIS OF COMMON GENERA OF FOODBORNE YEASTS
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
Hocking
37 should be consulted. For identification, Deak and Beuchat
15 have presented an excellent
simplified key to foodborne yeasts.
Division: Ascomycotina
Family: Saccharomycetaceae (ascospores and arthrospores formed; vegetative
reproduction by fission or budding)
Subfamily: Nadsonioideae
Genus: Hanseniaspora