Food Biochemistry and Food Processing

27 Bakery and Cereal Products 621

sium are added to promote growth. The production
of the yeast biomass for the baking industry is mul-
tistage and takes about 10–13 hours at 30°C. S. cere-
visiaeshows the Crabtree effect, as its metabolism
favors fermentative metabolism at high levels of
energy-giving substrate, thus resulting in a low pro-
duction of biomass (Walker 1998a). To avoid this,
molasses is added incrementally towards the end of
the production of yeast biomass, and the mixture is
vigorously aerated in order to promote respiration
and avoid fermentative metabolism. At the end of
the production, the yeast is allowed to “ripen” by
aeration in the absence of nutrients. This step syn-
chronizes the yeast cells into the stationary growth
phase and also promotes an increase in the storage
sugar trehalose in the cells, thus improving their via-
bility and activity.
When the fermentation is complete, the amount of
yeast is about 3.5–4% w/v. The biomass is separated
and concentrated by centrifugation and filtering. The
yeast cream is then processed into pressed yeast or
is dried. The most usual types of commercial yeast
preparations are (Stear 1990) the following:

• Cream yeastis a near liquid form of baker’s
  yeast that must be kept at refrigerated
  temperatures. It may be added directly to the
  bakery product being made.


• Compressed yeastis formed by filtering cream
  yeast under pressure to give approximately 30%
  solids. It has a refrigerated shelf life of 3 weeks.


• Active dry yeast(ADY) is produced by extruding
  compressed yeast through a perforated steel
  plate. The resulting thin strands are dried and
  then broken into short lengths to give a free-
  flowing granular product after further drying.
  Depending on the subsequent treatment and
  packaging, ADY may have a shelf life of over a
  year. However, ADY requires rehydration before
  application in dough, and this can be a labor-
  intensive operation in a large bakery. The product
  rehydrates best using steam or in water with
  added sugar at 40°C. Rehydration in pure water
  promotes leaching of cell contents and a
  reduction in the activity of the yeast.


• High activity dry yeast(HADY) (instant active
  dry yeast, IADY) is a similar product, where
  improved drying techniques are used to give a
  product with smaller particle size that does not
  need to be rehydrated before use and can

therefore be incorporated directly into bread

dough without prior treatment.

Desirable Properties of Baker’s Yeast

Yeast plays a critically important role in leavened

bread production, and over the decades of commer-

cial production, strains have been selected that give

improved performance. Desirable characteristics in-

clude

• High CO 2 production during the dough
  fermentation due to high glycolytic rate;


• The ability to quickly commence maltose
  utilization when the glucose in the flour is
  depleted;


• The ability to store high concentrations of
  trehalose, which gives tolerance to freezing and
  to high sugar and salt concentrations;


• Tolerance to bread preservatives such as
  propionate; and


• Viability and retained activity during various
  storage conditions.

In the future, strains will probably be developed

with even more useful properties. In particular, the

flavor-forming properties will receive special atten-

tion (Walker 1998b).

The Role of Yeast in Leavened Bread

When yeast is incorporated into the dough, condi-

tions allow a resumption of metabolic activity, al-

though there is little actual multiplication of the

yeast during shorter bread-making processes such as

the straight dough and Chorleywood processes (Fig.

27.2). The yeast has been produced under aerobic

(respiratory) conditions and is therefore adapted to

this metabolism, but conditions very quickly be-

come anaerobic in bread dough since the oxygen

incorporated in the dough is soon depleted. The sug-

ars are metabolized to pyruvate by glycolysis; pyru-

vate is then decarboxylated to acetaldehyde, thus

producing carbon dioxide; and then ethanol is

formed by reduction of acetaldehyde by NADH 2

(Fig. 27.3). For each molecule of glucose (or half

molecule of maltose) that is metabolized, two mole-

cules each of ethanol and carbon dioxide are pro-

duced. This fermentative metabolism is the preva-

lent pathway in S. cerevisiaein dough due both to