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

600 Part 5: Fruits, Vegetables, and Cereals

bread processes. However, the modern mechanical dough de-

velopment processes, where the fermentation stage has been

radically reduced, produce bread with a flavor that is inferior

to that produced by the traditional straight dough process. This

indicates that the yeast fermentation does make a positive con-

tribution to bread flavor (Stear 1990). Zehentbauer and Grosch

(1998) showed that yeast level and fermentation time and tem-

perature affected aroma in the crust of baguettes, and they identi-

fied the flavor compounds 2-acetyl-1-pyrroline (roasty), methyl

propanal and 2- and 3-methylbutanal (malty), and 1-octene-3-

ol and (E)-2-nonenal (fatty). An increase in fermentation time

allows for a development of flavor, but this trend is not really

noticeable until much longer fermentation times are used, as

in sourdough breads. There is not a clear borderline between

regular bread and sourdough bread.

The production of ethanol and CO 2 is essential for the devel-

opment of the desired bread crumb structure, and several fac-

tors affect both the development of the dough and its leavening

(Fig. 31.4). During fermentation, some of the CO 2 is lost to the

atmosphere, but most either collects in the small pockets of air

incorporated during dough mixing or is dissolved in the dough’s

aqueous phase. The amount that can be dissolved in the aqueous

phase is dependent on temperature, and is greater at lower tem-

peratures. As the aqueous phase is already saturated with CO 2 ,

it cannot escape from the bubbles by diffusion into the dough,

so the bread begins to increase in volume. As the gas collects,

the rheological properties of the dough allow it to expand in

order to equalize the pressure that builds up. Ethanol reacts with

the gluten to slightly soften it, allowing for easier expansion of

the dough. It is important that CO 2 develop immediately after

dough preparation and proceed at an adequate intensity. In ad-

dition, the dough must have the physical properties necessary

to withstand dough manipulation and allow for gas retention, so

that the optimal structure has been obtained for the final proof

and baking (Stear 1990).

The Bread-Baking Process

When the bread has undergone the final proofing and is put in

the oven, the outer surface rapidly starts to form the crust. A

temperature gradient develops due to transfer of heat from the

pan to the loaf, and if the loaf is to achieve optimal properties,

then the heat of the oven and the state of the bread proof need

to be synchronized (Stear 1990). Apart from the outer crust,

no part of the bread ever becomes dry; therefore, despite oven

temperatures of well over 200◦C, the temperature in most of

the loaf will not exceed 100◦C. The primary rise in tempera-

ture increases the activity of the yeast, and its production of

CO 2. At the same time, the solubility of CO 2 decreases, ethanol

and water evaporate, and the gases increase in volume. This re-

sults in a marked increase in the volume of the dough, called

“oven spring.”

As the temperature in the loaf continues to rise, several other

changes take place. The yeast is increasingly inhibited, and its

enzymes are inactivated at about 65◦C. The amylases in the

dough are active until about 65–70◦C is reached and a rapid in-

crease in the amount of soluble carbohydrate takes place. Gela-

tinization of starch occurs at 55–65◦C, and the water that this

requires is taken from the gluten protein network, which then

becomes more rigid, viscous, and elastic until a temperature is

Figure 31.4.Important factors for bread leavening.