Carbohydrates represent the main fraction of flour, accounting for up to 75% of the total weight. This fraction is largely comprised of starch, although other carbohydrates are also present, including a small amount (about 1%) of simple sugars, cellulose, and fiber.The main carbohydrate component, however, is starch, which consists of amylose, an a-1,4 glucose linear polymer (about 4,000 glucose monomers per molecule) and amylopectin, an a-1,4 and a-1,6 glucose branched polymer (about 100,000 glucose monomers per molecule). About 20% to 25% of the starch fraction is amylose and 70% to 75% is amylopectin. Properties of these two fractions are described in Table 8-2.

In its native state (i.e., before milling), wheat starch exists in the form of starch granules. The amylose and amylopectin are contained within these spherical granules in a rigid, semi-crystalline network. Native starch granules are insoluble and resist water penetration. However, some of the starch granules

(3% to 5%) are damaged during milling, which enhances absorption of water and exposes the amylose and amylopectin to hydrolytic enzymes, such as a-amylase.

Yeast Cultures

The yeast used for bread manufacture is Sac-charomyces cerevisiae, often referred to as simply bakers' yeast.Although ale (Chapter 9), wine (Chapter 10), and various distilled alcoholic beverages are also made using S. cere-visiae, the cultures for each of these products are not interchangeable. The bakers' yeast strains of S. cerevisiae clearly have properties and performance characteristics especially suited for bread manufacture (Table 8-3). For example, bread strains are selected, in part, on their ability to produce CO2, or their gassing rate. It is the CO2, evolved during fermentation, that is responsible for leavening. Yeast strains should also produce good bread flavor. In addition, stability and viability during storage is also important in selecting suitable yeast strains.

Modern industrial production of bakers'yeast starts with a pure stock culture, which is then scaled up through a series of fermentors until a large cell mass is produced (Figure 8-4).The culture is propagated initially in small (1 L to 5 L) seed flasks, and then in progressively larger fer-mentors of about 250 L to 1,000 L. Eventually the culture is inoculated into large (250,000 L) production fermentors. The growth medium usually consists of molasses or another inexpensive source of sugar and various ammonium salts (e.g., ammonium hydroxide, as a cheap source of nitrogen). Other yeast nutrients include ammonium phosphate, magnesium sulfate, calcium sul-

Table 8.2. Physical and chemical properties of amylase and amylopectin.




Monomers per

300 — 1,000

> 5,000


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