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

598 Part 5: Fruits, Vegetables, and Cereals

The Development of Dough Structure

When wheat flour and water are mixed together in an approx-

imately 3:1 ratio and kneaded, a viscoelastic dough is formed

that can entrap the gas formed during the subsequent fermen-

tation. The amount of water absorbed by the flour is dependent

upon, and therefore must be adjusted to, the integrity of the starch

granules and the amount of protein present. A high proportion of

damaged granules, as found in hard wheat flour, results in greater

water absorption. The unique elastic property of the dough is due

to the nature of the gluten proteins. Hydrated gliadin is sticky

and extensible, whereas glutenin is cohesive and plastic. When

hydrated during the mixing process, the gluten proteins unfold

and bond with each other by forming a complex (gluten) as

kneading proceeds, with an increasing number of cross-linkages

between the protein molecules as they become aligned. Disul-

phide bonds (-S-S-) break and re-form within and between the

protein molecules during mixing.

Gluten does not form spontaneously when flour and water

are mixed; energy must be provided (i.e., in the actual mixing

process) in order for the molecular bonds to break and re-form

as the gluten structure. At this point, the dough stiffens and

becomes smooth and shiny. The gluten is now composed of

protein sheets in which the starch granules are embedded. In

addition, free polar lipids and glycolipids are incorporated in the

complex by hydrophobic and hydrogen bonds. The properties of

the dough are determined by the amount of protein present and

by the relative proportions of the gluten proteins.

Another important part of the dough formation is the incorpo-

ration of air, in particular nitrogen. This forms insoluble bubbles

in the dough that become weak points where carbon dioxide

collects during the subsequent fermentation step. In the Chor-

leywood bread process, the dough is mixed under partial vacuum

so that the incorporated bubbles expand and are then split into

many small ones as mixing continues, thus giving a fine-pored

loaf crumb after baking.

Dough Fermentation

During the fermentation step, several processes happen simulta-

neously, and in order to produce a bread of the required quality

characteristics, each of these processes must be optimized to

that end.

Yeasts have been used to leaven bread for thousands of

years, but only in comparatively recent times have pure cul-

tures of the yeastSaccharomyces cerevisiaebeen added to the

bread dough as a leavening agent. The commercial production

of baker’s yeast follows procedures similar to those used in

the production of brewing, wine making, and distilling strains

of this same species. Indeed, the baking industry was orig-

inally supplied with yeast waste from the brewing industry

until about 1860 (Ponte and Tsen 1987). However, commer-

cial production of yeast biomass specifically for the baking

industry developed alongside an increasingly expanding man-

ufacture of bread in commercial bakeries and the development

of the technology that provided the great volumes required by

the industry.

Commercial Production of Baker’s Yeast

S. cerevisiaewas originally produced commercially using grain

mash as a growth substrate, but for economic reasons, it is now

grown on sucrose-rich molasses, a by-product from the sugar

cane or sugar beet refining industry. Nitrogen, phosphorous,

and essential mineral ions such as magnesium are added to

promote growth. The production of the yeast biomass for the

baking industry is multistage and takes about 10–13 hours at

30 ◦C.S. cerevisiaeshows the Crabtree effect, as its metabolism

favors fermentative metabolism at high levels of energy-giving

substrate, thus resulting in a low production of biomass (Walker

1998a). To avoid this, molasses is added incrementally toward

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 synchronizes the yeast cells into the stationary growth phase

and also promotes an increase in the storage sugar trehalose in

the cells, thus improving their viability 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 pro-

cessed 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 refriger-

ated shelf life of 3 weeks.


 Active dry yeast(ADY) is produced by extruding com-

pressed 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 ADY, 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 incor-

porated directly into bread dough without prior treatment.

Desirable Properties of Baker’s Yeast

Yeast plays a critically important role in leavened bread pro-

duction, and over the decades of commercial production, strains

have been selected that give improved performance. Desirable

characteristics include the following:


 High CO 2 production during the dough fermentation due to

high glycolytic rate.