Food Biochemistry and Food Processing (2 edition)

BLBS102-c31 BLBS102-Simpson March 21, 2012 14:0 Trim: 276mm X 219mm Printer Name: Yet to Come

31 Bakery and Cereal Products 599

 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.


 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 proper-

ties will receive special attention (Walker 1998b).

The Role of Yeast in Leavened Bread

When yeast is incorporated into the dough, conditions allow a

resumption of metabolic activity, although there is little actual

multiplication of the yeast during shorter bread-making pro-

cesses such as the straight dough and Chorleywood processes

(Fig. 31.2). The yeast has been produced under aerobic (respi-

ratory) conditions and is therefore adapted to this metabolism,

but conditions very quickly become anaerobic in bread dough

since the oxygen incorporated in the dough is soon depleted.

The sugars are metabolized to pyruvate by glycolysis; pyruvate

is then decarboxylated to acetaldehyde, thus producing carbon

dioxide; and then ethanol is formed by reduction of acetaldehyde

by NADH 2 (Fig. 31.3). For each molecule of glucose (or half

molecule of maltose) that is metabolized, two molecules each

of ethanol and carbon dioxide are produced. This fermentative

metabolism is the prevalent pathway inS. cerevisiaein dough

due both to the absence of oxygen and to the nonlimiting supply

of fermentable sugars (Maloney and Foy 2003).

The amount of maltose available is a complex interaction

between the amount of damaged starch, the level of amylases in

the flour and the stage, and length of the fermentation process.

Maltose accumulates during the early stages of the fermentation

because it is generated by amylase but is not metabolized by

the yeast because the presence of glucose represses maltose

utilization.

When readily fermentable sugars (glucose and fructose) are

exhausted, the yeast shows a lag in fermentation and then turns

its metabolism to the maltose produced from the action of

β-amylase on starch. If sucrose has been added in the bread

formulation (e.g., 4%), this is fermented in preference to mal-

tose, and the lag in the fermentation may not be observed. High

amounts of added sucrose (e.g., 20%) significantly retard fer-

mentation due to the high osmotic stress on the yeast (Maloney

and Foy 2003).

The products of yeast metabolism in dough fermentation vary

considerably with pH. In bread, the pH is usually below 6.0,

but above this, end products in addition to ethanol and CO 2

are formed, such as succinate, acetic acid, and glycerol, and less

ethanol and CO 2 are formed.S. cerevisiaeis also able to degrade

proteins and lipids, and several flavor compounds are produced

(Fig. 31.3).

It is generally not considered that the yeast fermentation is

important for bread flavor and aroma development in traditional

Figure 31.3.Biochemical changes during yeast fermentation of bread.