Yeast general characterstics

YEAST CHARACTERISTICS

1. YeAST- GENERAL CHARACTERISTICS

2. Yeast Yeasts are True fungi that have essentially single celled morphology reproducing asexually by budding. Simple in morphology and rather than primitive fungi. Their habitat is nutritionally rich environment such as nectares,plant exudates,decaying fruits and body fluids of animals. Molds also grow on yeast under high nutRients,low oxygen and high carbondioxide.

3. A major taxonomic study of the yeasts by kreger-van-rij(1984) describes about 500 species divided into60 genera.of these 33 are considered to be ascomycetes,10 basidiomycetes and 17 deuteromycetes. Mojority of yeasts grow effectively under aerobically . Only a relatively small number are commonly associated with production of fermented foods. They are all either ascomycetes members or members of imperfecti fungi. All strains ferment glucose and plant carbohydrates

4. General characteristics Morphological characteristics form and structure : yeasts may be spherical to ovoid,lemon shaped,pear shaped,cylindrical,triangular or elongate. structure are cellwall,cytoplasm,water vacuoles,fat globules and granules.

5. Reproduction most yeasts reproduce asexually by multilateral budding. sexual reproduction of true yeasts result in production of ascospores. false yeasts produce no ascospores or other sexual spores belong to fungi imperfecti. Cultural characteristics Growth as film on surface of liquid media and production of pignent indicates the rhodotorula. Yeasts are oxidative, fermentative or both. Fermentative yeasts grow on liquid and produce carbondioxide

6. Physiological characteristics Most yeasts grow best with a plentiful supply of available moisture.but since many yeasts grow in the presence of greater concentrations of solutes(sugars or salts) than most bacteria. On the basis of water activity or aw yeasts may be classified as ordinary if they do not grow in high concentrations of solutes,i.E.,in a low aw and osmophilic if they do. In general sugars are the best source of energy for yeasts although oxidative yeasts eg.the film yeasts ,oxidise organic acids and alcohol.. The yeasts also aid in the production of flavours or bouquet in wines.

7. Classification and identification of yeasts The true yeasts are in the subdivision and false yeasts in fungi imperfecti or Deuteromycotina. The principle bases of identification are : Whether spores are formed Apperance of vegetative cells :shape ,size,color,inclusions. Method of asexual reproduction a.budding

8. B.fission c.combined fission and budding d.arthrospores 4.Production of mycelium,pseudomycelium and no mycelium 5.Growth as a film over surface of a liquid. 6.Colour of macroscopic growth. 7.Physiological characteristics.

9. Yeasts of industrial importance Most yeasts used industrially are in the genus Saccharomyces.the yerm wild yeast is applied to any other than the one being used or encouraged. Genus Schizosaccharomyces These yeasts have been found in tropical fruits,molasses,soil,honey.a common species is S.Pombe.

10. Genus saccharomyces top yeasts : Active fermenters and grow rapidly at 20c. bottom yeasts : donot clump,grow more slowly and are best fermenters at 10

11. Genus kluyveromyces these yeasts reproduce by multilateral budding and ascospores are liberated upon maturity.

12. Genus Zygosaccharomyces able to grow in high concentrations of sugar. ex: Zygosaccharomyces nussba

13. Genus pichia these cylindrical to oval yeasts may form pseudomycelia. ex.p.membranaefaciens

14. Genus hansenula these yeasts resemble pichia in appearance although some species form pellicles. Ascospores are hat or saturn shaped. Genus debaromyces these yeasts form pellicles on meat brines.

15. REFERENCE BOOK ~ FOOD MICROBIOLOGY BY MR.ADAMS

16. BY B.UDAY KIRAN 111718012041 NAG THE END