126 ◾ Fundamental Food Microbiology
genetic improvements are being made to improve their desirable characteristics. A brief discussion
is provided here for some that are used in food fermentation.
Yeasts
Among the many types of yeasts, only a few have been associated with fermentation of foods
and alcohol, production of enzymes for use in food, production of single cell proteins (SCPs),
and as additives to impart desirable flavor in some foods. The most important genus and species
used is Saccharomyces cerevisiae. 8 It has been used to leaven bread 9 and produce beer, wine, distilled liquors, and industrial alcohol; produce invertase (enzyme); and flavor some foods (soups).
However, many strains have been developed to suit specific needs, such as ethanol fermentation
from biomass as biofuel. 10
The cells are round, oval, or elongated. They multiply by multipolar budding or by conjugation and formation of ascospores. The strains are generally grouped as bottom yeasts or top yeasts.
Top yeasts grow very rapidly at 20°C, producing alcohol and CO 2 . They also form clumps that,
because of rapid CO 2 production, float at the surface. In contrast, bottom yeasts grow better at
10°C–15°C, produce CO 2 slowly (they also grow slowly), do not clump, and thus settle at the bottom. Top yeasts and bottom yeasts are used according to the need of a particular fermentation
process.
Candida utilis has been used to produce SCPs. It is a false yeast (Fungi imperfecti) and reproduces by budding (not by conjugation). The cells are oval to elongated and form hyphae with large
numbers of budding cells. They are also involved in food spoilage.
Kluyveromyces marxianus and Klu. marxianus var. lactis can hydrolyze lactose and have been
associated with natural fermentation, along with other yeasts and lactic acid bacteria, of alcoholic
dairy products such as kefir. They have also been associated with spoilage of some dairy products. 6
At present, they are used to produce β-galactosidase (lactase) for commercial use to hydrolyze
lactose. The enzyme is now used to produce low-lactose milk.
Molds
Although most molds are associated with food spoilage and many form mycotoxins while growing
in foods, other species and strains are used in processing of foods and to produce additives and
enzymes for use in foods. 11
In general, molds are multicellular, filamentous fungi. The filaments (hyphae) can be septate or
nonseptate and have nuclei. They divide by elongation at the tip of a hypha (vegetative reproduction) or by forming sexual or asexual spores on a spore-bearing body.
Among many genera, several species from genera Aspergillus and Penicillium and a few from
Rhizopus and Mucor, have been used for beneficial purposes in food. Strains to be used for such
purposes should not produce mycotoxins. It is difficult to identify a nonmycotoxin-producer strain
in the case of natural fermentation, but it should be an important consideration in the selection of
strains for use in controlled fermentation.
Aspergillus oryzae is used in fermentation of several oriental foods, such as sake, soy sauce,
and miso. It is also used as a source of some food enzymes. Asp. niger is used to produce citric
acid and gluconic acid from sucrose. It is also used as a source of the enzymes pectinase and
amylase. Rhizopus oryza is used to make tempe or temphe, murcha, chu, and nuruk. Penicillium
roquefortii is used for ripening of Roquefort, Gorgonzola, and blue cheeses. Some strains can
produce the neurotoxin roquefortin. In the selection and development of strains for use in
genetic improvements are being made to improve their desirable characteristics. A brief discussion
is provided here for some that are used in food fermentation.
Yeasts
Among the many types of yeasts, only a few have been associated with fermentation of foods
and alcohol, production of enzymes for use in food, production of single cell proteins (SCPs),
and as additives to impart desirable flavor in some foods. The most important genus and species
used is Saccharomyces cerevisiae. 8 It has been used to leaven bread 9 and produce beer, wine, distilled liquors, and industrial alcohol; produce invertase (enzyme); and flavor some foods (soups).
However, many strains have been developed to suit specific needs, such as ethanol fermentation
from biomass as biofuel. 10
The cells are round, oval, or elongated. They multiply by multipolar budding or by conjugation and formation of ascospores. The strains are generally grouped as bottom yeasts or top yeasts.
Top yeasts grow very rapidly at 20°C, producing alcohol and CO 2 . They also form clumps that,
because of rapid CO 2 production, float at the surface. In contrast, bottom yeasts grow better at
10°C–15°C, produce CO 2 slowly (they also grow slowly), do not clump, and thus settle at the bottom. Top yeasts and bottom yeasts are used according to the need of a particular fermentation
process.
Candida utilis has been used to produce SCPs. It is a false yeast (Fungi imperfecti) and reproduces by budding (not by conjugation). The cells are oval to elongated and form hyphae with large
numbers of budding cells. They are also involved in food spoilage.
Kluyveromyces marxianus and Klu. marxianus var. lactis can hydrolyze lactose and have been
associated with natural fermentation, along with other yeasts and lactic acid bacteria, of alcoholic
dairy products such as kefir. They have also been associated with spoilage of some dairy products. 6
At present, they are used to produce β-galactosidase (lactase) for commercial use to hydrolyze
lactose. The enzyme is now used to produce low-lactose milk.
Molds
Although most molds are associated with food spoilage and many form mycotoxins while growing
in foods, other species and strains are used in processing of foods and to produce additives and
enzymes for use in foods. 11
In general, molds are multicellular, filamentous fungi. The filaments (hyphae) can be septate or
nonseptate and have nuclei. They divide by elongation at the tip of a hypha (vegetative reproduction) or by forming sexual or asexual spores on a spore-bearing body.
Among many genera, several species from genera Aspergillus and Penicillium and a few from
Rhizopus and Mucor, have been used for beneficial purposes in food. Strains to be used for such
purposes should not produce mycotoxins. It is difficult to identify a nonmycotoxin-producer strain
in the case of natural fermentation, but it should be an important consideration in the selection of
strains for use in controlled fermentation.
Aspergillus oryzae is used in fermentation of several oriental foods, such as sake, soy sauce,
and miso. It is also used as a source of some food enzymes. Asp. niger is used to produce citric
acid and gluconic acid from sucrose. It is also used as a source of the enzymes pectinase and
amylase. Rhizopus oryza is used to make tempe or temphe, murcha, chu, and nuruk. Penicillium
roquefortii is used for ripening of Roquefort, Gorgonzola, and blue cheeses. Some strains can
produce the neurotoxin roquefortin. In the selection and development of strains for use in