Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 171

the cell-free wheat flour hydrosylate fermented L. sanfranciscensis , C. milleri
released metabolites belonging to the family of lactones. Glutamic acid was sug-
gested as the presumptive precursor of certain g-lactones in yeasts [ 75 ] . D -Lactones
were obtained from 11-hydroxy fatty acids during fermentation with S. cerevisiae ,
Sporobolomyces , Geotrichuum and Candida spp.

6.5 Baker's Yeast in the Bread-Making Industry

The production of baker's yeast biomass represents a highly competitive multi-billion
dollar global industry. The large variety of bread-making processes and recipes used
around the world place considerable demands on baker's yeasts. These demands
translate into technological and economic challenges for baker's yeast industries.
During production and dough fermentation, cells of baker's yeasts are exposed to
environmental stresses such as freeze-thaw, dehydration and oxidation. Damage to
cell macromolecules (e.g. proteins, nucleic acids and membranes) is avoided
through microbial adaptation and tolerance to multiple stress conditions [ 76 ] .

6.5.1 Baker's Yeast Production

The first stage in the production of baker's yeast consists of growing the yeast from
a pure culture in a series of fermentation vessels. Baker's yeast is recovered from the
last step of fermentation through centrifugation to concentrate the biomass.
Subsequently, filtration further concentrates the yeast cells. The collected pellet is
blended in mixers with small amounts of water, emulsifiers, and oil. Finally, the
mixed press cake is extruded, cut and either wrapped for shipment or dried to form
dry yeast.

6.5.1.1 Raw Materials

The species S. cerevisiae is used for producing compressed yeast. Different strains
of S. cerevisiae are required to produce each of the two dry yeast products, Active
Dry Yeast (ADY) and Instant Dry Yeast (IDY). Instant dry yeast is produced from a
faster-reacting yeast strain than that used for ADY. Cane molasses and beet molas-
ses are the principal substrates for yeast growth. Molasses contains 45-55% (wt/wt) of
fermentable carbohydrates (sucrose, glucose and fructose). The amount and type
of cane and beet molasses used depend on the types and costs, and on the presence
of inhibitors and toxins. Usually, a blend consisting of both cane and beet molasses
is used. Once the molasses mixture is blended, the pH is adjusted to 4.5-5.0 to prevent
bacterial growth. The substrate is clarified to remove sludge and is then sterilised