Food microbiology- cultural, morphological and physiological characters of yeast

1. MICROBIOLOGY QUICK LEARN
FOOD MICROBIOLOGY
Saajida Sultaana Mahusook

2. Morphological Characteristics
Form and Structure:
• Microscopically, yeasts may be spherical to ovoid, lemon-shaped, pear-
shaped, cylindrical, triangular or elongated to false or true mycelium. They
also differ in size. The cell wall may be metachromatic, albuminous, or
starchy. Special staining is necessary to demonstrate the nucleus.

3. Reproduction:
• Asexual methods: Most yeasts reproduce asexually by multilateral or polar budding. The cell
wall bulge out and softens in the area by certain enzymes brought by vesicles. The protoplasm
also bulges out as a small protuberance. The parent nucleus divides into two, one of the
daughter nucleus migrates into bud, the cytoplasm of bud and mother remain continuous for
some time. As the bud enlarges, a septum is laid down at the joining of bud with mother cell.
Then bud separates and leads independent life. A few species of yeast reproduce by fission.
• Sexual methods: Sexual reproduction of “true yeast” (Ascomycotina) results in the formation
of ascospores, where the yeast cell serves as ascus. The number of spores per ascus and the
appearance of ascospores are characteristic of the kind of yeast. The ascopore differ in color,
smoothness or roughness of their walls, shape (round, oval, reniform, bean- or sickle- shaped,
Saturn- or hat- shaped, hemispherical, angular, fusiform, or needle- shaped).
• “False yeast” (Fungi Imperfectii) produce no ascospores or sexual spores. Cells of some yeasts
become chlamydospores by the formation of thick wall about the cell. Example: Candida,
Rhodotorula, and Cryptococcus.

4. Cultural Characteristics
• The appearance of massed yeast growth is not useful in the identification of
yeast.
• Yeast growth as a film on liquid surface suggests an oxidative or film yeast,
and production of carotenoid pigment indicates the genus Rhodotorula.
• It is difficult to differentiate the growth of yeast and bacteria on agar plate;
the only certain way is microscopic examination of the organisms.
• Most yeast colonies are moist and slimy but may appear mealy; most
colonies are white, cream- colored or pink. Some colonies change with age,
others become dry and wrinkled.
• Yeasts are oxidative, fermentative or both. Oxidative yeasts may grow as a
film, pellicle or scum on the surface of liquid and are called film yeasts.
Fermentative yeasts usually grow throughout the liquid and produce carbon
dioxide.

5. Physiological Characteristics
• Most common yeasts grow best with a plentiful supply of available
moisture. Many types of yeast grow in the presence of greater
concentrations of solutes (such as sugar and salt) and require less
moisture than a majority of bacteria. Most yeast requires more
moisture than molds. Specific examples of minimal aw are 0.94 for
beer yeast, 0.90 for yeast from condensed milk, and 0.905 for baker’s
yeast. Each yeast has its own characteristic aw which vary with the
nutritive properties of the substrate, pH, temperature, oxygen
availability and presence or absence of inhibitory substances.
• Temperature requirement for the growth of yeast is similar to that of
molds with optimum around 25 to 30° C and the maximum about 35
to 47° C. Some can grow at 0° C or less.
• Most yeast favor an acid reaction in the vicinity of pH 4 to 4.5, and
will not grow well in alkaline medium unless it has adapted to it.
Yeast grows best under aerobic conditions. Fermentative yeasts can
grow anaerobically, although slow.

6. • The best source of energy for yeast is sugars although oxidative
yeasts oxidize organic acids and alcohol.
• Carbon dioxide produced by bread yeasts accomplishes the leavening
of bread, and alcohol made by fermentative yeasts is the main product
in the manufacture of wine, beer, industrial alcohol and other
products. Yeasts also aid in the production of flavors or bouquet in
wines.
• Nitrogenous sources utilized range from simple compounds such as
ammonia and urea to amino acids and polypeptides. Yeasts also
require accessory growth factors.
• Yeast may change its physiological characteristics, especially true
yeast which has a sexual mode of reproduction. They can be bred for
certain characteristics or may mutate to new forms. Most yeast can be
adapted to conditions which previously would not support good
growth.
• Example: Large number of strains of Saccharomyces cerevisiae
within a species suited for different uses such as bread strains, beer
strains, wine strains and high- alcohol- producing strains.

7. 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.