THE ALCOHOL TEXTBOOK THE ALCOHOL TEXTBOOK

Understanding yeast fundamentals 89
· Formation of new bud or
daughter cell
· Divisional age of 0
· Known as a virgin cell
· Mother cell continues
to divide until the Hayflick
Limit is reached
· Once reached, it may not
re-enter the cell cycle
· Note increase in cell size
· Virgin cell reaches critical
size for division
· Mother cell divides to
produce first daughter
· Acquires bud scar
· Mother and daughter cell
separate asymmetrically
· Mother cell is larger and will divide
more rapidly than the daughter
· Mother cell enters the
senescence pathway
· No further divisions are permitted
· Mother cell exhibits 'aged' phenotype
· Senescent mother cell
enters death metabolism
· Culminates in cell lysis
Figure 7. The lifespan of yeast (adapted from Smart, 2000).
When a sample of yeast is analyzed, its
composition will depend greatly on the genotype
of the yeast as well as the growth parameters
and hence the differences in composition
between a yeast propagated for the baking
industry vs a yeast propagated for use in the
fermentation industry. Table 1, compiled from
a number of sources, gives a good indication
as to the constituents of a typical yeast cell. The
exact values will vary with the particular strains
examined, the composition of the media, as well
as many environmental factors. The general
composition of yeast guides us to the levels of
nutrients that the yeast may require in order to grow.
Yeast growth requirements
Specific requirements for yeast growth include:
i) water
ii) a carbon source - fermentable carbohydrates
as an energy source
iii) oxygen/lipids - lipids for membrane
biosynthesis, which can be made by the yeast
if oxygen is present
iv) a nitrogen source - amino acids and peptides
are needed for growth and enzyme synthesis.
Saccharomyces yeast can also use ammonia.
v) growth factors -vitamins
vi) inorganic ions - essential for yeast
metabolism
WATER
Water is a main component of all living material
and most microorganisms need at least 15% water