Yeasts are single-celled fungi that reproduce asexually by budding. They are found in nutrient-rich environments like plant matter and animal fluids. Major yeasts used industrially belong to the genera Saccharomyces, Kluyveromyces, Zygosaccharomyces, Pichia, and Hansenula. Yeasts can be identified based on whether they form spores, their vegetative cell characteristics, and physiological properties like carbon dioxide production during fermentation.

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 :
1. WHETHER SPORES ARE FORMED
2. APPERANCE OF VEGETATIVE CELLS :SHAPE ,SIZE,COLOR,INCLUSIONS.
3. 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.
1. GENUS SCHIZOSACCHAROMYCES
THESE YEASTS HAVE BEEN FOUND IN TROPICAL
FRUITS,MOLASSES,SOIL,HONEY.A COMMON SPECIES IS S.POMBE.

11. • GENUS SACCHAROMYCES
TOP YEASTS : ACTIVE FERMENTERS AND GROW RAPIDLY AT 20C.
BOTTOM YEASTS :DONOT CLUMP,GROW MORE SLOWLY AND ARE BEST
FERMENTERS AT 10

12. • GENUS KLUYVEROMYCES
THESE YEASTS REPRODUCE BY MULTILATERAL BUDDING AND ASCOSPORES
ARE LIBERATED UPON MATURITY.

13. • GENUS ZYGOSACCHAROMYCES
ABLE TO GROW IN HIGH CONCENTRATIONS OF SUGAR.
EX: ZYGOSACCHAROMYCES NUSSBA

14. • GENUS PICHIA
THESE CYLINDRICAL TO OVAL YEASTS MAY FORM PSEUDOMYCELIA.
EX.P.MEMBRANAEFACIENS

15. • 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.

16. REFERENCE
• BOOK ~ FOOD MICROBIOLOGY BY MR.ADAMS

17. BY
B.UDAY KIRAN
111718012041
NAG
THE END