Food Microbiology- Industrial importance of Yeast- genera

1. MICROBIOLOGY QUICK LEARN
FOOD MICROBIOLOGY
Saajida Sultaana Mahusook

2. Industrial importance of Yeast
• The most widely used yeast of industrial importance is the genus
Saccharomyces. Wild yeast refers to any yeast other than the one being used
or encouraged. A yeast used in a particular process can be wild yeast in
another process. Most troublesome wild yeast is false yeast.
• Genus Schizosaccharomyces: These yeasts reproduce asexually by fission
and are found in tropical fruits, molasses, soil, honey. Example: S. pombe.
• Genus Saccharomyces: These yeasts are round, ovate or elongated and may
form pseudomycelium. Reproduction takes place by formation of ascospores
or multipolar budding. The leading species S. cerevisiae is employed in
many food industries with special strains for leavening of bread, top yeast for
ales, alcohol, wine and glycerol. Top yeasts are active fermenters growing at
20°C. The clumping of cells and rapid evolution of CO2 sweep the cells to
the surface, hence called as top yeast. Bottom yeast does not clump, grows
slowly and are active fermenters at 10 to 15°C. These conditions make the
yeast to settle at bottom, hence bottom yeast. Example: S. uvarum is used in
the making of beer.
• S. cerevisiae var. ellipsoideus is a high alcohol yielding variety used to
produce industrial alcohol, wines and distilled liquors. S. fragilis and S. lactis
can ferment lactose and are important in milk or milk products.

4. • Genus Zygosaccharomyces: These yeasts grow in high concentrations of
sugar (osmophilic) and involved in the spoilage of molasses, syrup, honey
and fermentation of soy sauce and wines. Example: S. nussbaumeri grows in
honey.
• Genus Pichia: The oval to cylindrical shaped yeasts may form
pseudomycelium. A pellicle is formed on liquids. Example: P.
membranaefaciens grows as a pellicle on beer and wine.
• Genus Debaryomyces: These round or oval yeasts form pellicle on meat
brines. D. koeckeri grows on cheese and sausage.

5. False Yeasts:
• Genus Torulopsis: These are round or oval fermentative yeasts that spoil
various foods and troublesome in breweries. T. sphaerica ferment lactose and
may spoil milk products. Other species can spoil sweetened condensed milk,
fruit- juice concentrates and acid foods.
• Genus Candida: These yeasts can form pseudohyphae or true hyphae, with
abundant budding cells or blastospores, and may form chlamydospores.
Many form films and can spoil foods high in acid and salt. C. utilis is grown
for food and feed. C. krusei is grown with diary starter cultures to maintain
the activity and increase the longevity of lactic acid bacteria.
• Genus Trichosporon: These yeasts undergo budding and form ascospores.
They grow best at low temperatures and are found in breweries and chilled
beef. Example: T. pullulans.
• Genus Rhodotorula: They are red, pink or yellow and cause discoloration on
foods. Example: colored spots on meats or pink areas in sauerkraut.
• Genus Brettanomyces: These yeasts are ogive- or arch- shaped capable of
producing high amounts of acid and are involved in the late fermentation of
Belgian Lambic beer and English beers. They are also found in French
wines. Example: B. bruxellansis and B. lambicus.

6. Reference
• Food Microbiology by Martin R. Adams and Maurice O. Moss,
3rd e.
• Food Microbiology by William C. Frazier and Dannis C.
Westhoff, 5th e.