Yeast: Key Role in Food, Alcohol, and Human Health - Prof. Jahan, Slides of Microbiology

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Yeast

Morphology Yeasts are non filamentous, unicellular/ multi cellular fangi that can be oval or spherical or elongated in shape. Yeast size can vary greatly depending on the species, typically measuring 3– 4 μm in diameter, although some yeasts can reach over 40 μm. They have no flagella or other organelles of locomotion. Their cell wall is composed of chitin, cellulose or hemicelluloses. Most yeasts reproduce asexually by mitosis, and many do so by an asymmetric division process called budding.

Physiology of yeast Yeast can withstand certain extreme enviorenmental condition than most other microorganisms. They can grow in a medium containing 4 to 5% suger, can tolerate more acidic condition pH (3.8 to 5.6) that inhibit most microbes. Some yeast are facultitative & can grow under both aerobic & anerobic condtion. Yeast grow over a wide range of temperature 22- 0 C for saprophyte & 30- 0 C for pathogenic. Yeast use organic nitrogen as nutrient.

Pathogenic yeasts Yeast infection or candidiasis is most frequently caused by Candida albicans. Candida is often isolated from warm blooded animals, including human, where it exist as a part of the normal microbial yeast in the mucous membrane. Sometimes it causes disease in the mouth, alimentary tract, serious infection in the heart (endocarditis), in blood (septicemia), in brain (meningitis). As an opportunistic pathogen it produces disease in case of physiological disorder such as obesity, alcoholism and prolong use of antibiotic. Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (Candidemia).

Importance of yeast

1. Bread Making: For the manufacture of bread, yeast is mixed with white flour. Blocks of treated flour are kept at suitable temperature. The process of fermentation starts. Ethyl alcohol and carbon dioxide are formed. The bread swells up and becomes spongy due to the pressure exerted by accumulated carbon dioxide. Flavor and quality of bread is attained by selecting different strains of yeast.
2. Production of alcohol: Ethyl alcohol is manufactured by the fermentation of potatoes, cereals, molasses etc. C 6

H

12

6

• Yeast zymase

2C 2

H

5

OH + 2C

2 (Glucose) Fermentation (ethyl alcohol) + (carbon dioxide) The breakdown of carbohydrates (starch and sugar) by yeast into carbon dioxide and ethyl alcohol in the absence of oxygen is called alcoholic fermentation. The alcohol is the major ingredient in brewing industry.