Baked Goods

Although a number of cereal grains may be used in the production of baked goods, the primary source for such products is wheat. Wheat is utilized in the production of baked goods such as breads, cakes, rolls, and pastries. The enzymes of greatest interest for this discussion will be those involved in protein and carbohydrate metabolism, specifically, the amylases, pentosanases, and proteases. Amylases are starch-degrading enzymes that act on the linkages between the glucose monomers making up the starch polymer. In starch, these linkages are either a-1,4 or a-1,6 glucosidic bonds connecting adjacent glucose residues. ^-Amylase (EC 3.22.1.2) is an exo-type hydrolase that is able to hydrolyze only the penultimate a-1,4 bond, each cleavage producing a molecule of maltose. a-Amylase (EC 3.2.1.1) is an endo-type hydrolase that, while specific for the a-1,4 glucosidic bond, is capable of hydrolyzing such bonds anywhere along the internal portion of the glucose chain, producing small amounts of glucose as well as many different glucose oligomers. The difference between the two enzymes is important because they have different effects on the rheology of a starch system. /?-Amylase has relatively little effect on the molecular weight of the starch molecule because it removes only one maltose at a time from starch molecules having molecular weights as large as several million. a-Amylase, however, can drastically alter the molecular weight because it can hydrolyze anywhere along the chain. That is, it can hydrolyze an internal a-1,4 glucosidic bond about in the middle of the glucose chain and immediately reduce the molecular weight by half. This action can greatly affect (decrease) the viscosity of the starch system.

Under ideal harvest conditions, wheat is in a dormant storage stage and contains comparatively few active enzymes. There may be significant amounts of ^-amylase, but a-amylase levels are quite low. Thus, the amount of potentially detrimental endogenous starch-degrading enzyme in the sound wheat kernel is low. However, it is not unusual, particularly in the Pacific Northwest, that harvest conditions are not ideal. The most serious consequence from the standpoint of enzyme content is when harvest is preceded by humid, rainy conditions. In this environment, the seed breaks its dormancy and begins to grow. This phenomenon is commonly referred to as "sprouting" in the grain industry. This growth requires that the stored carbohydrate and protein be utilized for energy and synthesis of new plant parts. Thus, enzymes such as those which hydrolyze carbohydrate and protein material are synthesized and act on the stored material in the plant seed. It is the plant seed that is then processed into a food material such as wheat flour.

Whether the endogenous enzymes are a factor in utilizing the food material depends on a number of factors. First, how extensive was the sprouting? It may range from slight, in which case little enzyme was synthesized, to extensive, in which case a large amount of carbohydrase and proteinase enzymes have probably been formed. Second, what are the processing conditions for the manufacture of the food material? In general, high-moisture products such as breads will be much more susceptible to the action of these hydrolytic enzymes than will low-moisture products such as crackers. In bread processing, the dough may contain approximately 35 to 40% moisture, and the final baked product retains much of that moisture. Since processing involves a lengthy proofing period as well as the bake time, the presence of such high levels of water (most with a water activity [aw] of 0.95-0.98) may lead to extensive hydrolysis if significant levels of endogenous enzymes are present.

In the dough stage, only that portion of the starch (both amylose and amylopectin) that is physically damaged in the milling process is susceptible to amylolytic attack. Depending on the type of wheat, the amount of starch damage may vary from about 3 to 8%. The amylase enzymes can attack the damaged starch and will release some water that was associated with the starch; however, the effect on the dough consistency will be relatively minor. However, the starch will gelatinize during the baking process as the internal temperature of the loaf reaches about 60 to 65°C (104°F). Both amylase enzymes may then rapidly degrade the gelatinized starch. However, it should be noted that endogenous amylase enzymes are relatively unstable above 65°C, and so are less effective as the temperature increases. The overall effect of the starch-degrading enzymes will depend on the amount of endogenous enzyme present and how long it has to act on the gelatinized starch.

It was thought for many years that a-amylase was the enzyme from sprouted wheat that was most responsible for the detrimental effects observed during baking of high-moisture bread products, but this is not the case. The most damaging of the hydrolytic enzymes in such a process are the proteases. Sprouted wheat contains proteolytic enzymes that can act on the protein fraction throughout the dough stage and into the baking stage. The result of excessive hydrolysis by proteolytic enzymes is a loss of gluten structure. This is manifest as a slack dough that has lost the viscoelasticity characteristic of a well-developed wheat flour dough. The finished product will have poor volume and texture characteristics. If hydrolysis proceeds to an extreme, all gluten structure may be lost, resulting in a very sticky, slack semiliquid. Some effects of these enzymes may be observed even when the degree of sprouting is relatively low. An assay for this type of enzyme would serve as a good diagnostic indicator of the quality of the flour used in an industrial food process.

In low-moisture baked products such as crackers where the final moisture content of the product is only about 3%, endogenous enzymes usually have less potential for detrimental effects. Because the processing conditions require less time compared with breads, and water bakeout in the oven during baking is more rapid, starch-degrading enzymes usually have relatively little effect. The rapid loss of water in the initial stage of baking leads to more rapid increases in the temperature of the dough piece, thus tending to inactivate the enzymes faster. In addition, because the available water is driven off rapidly, the starch gelat-inization temperature increases such that relatively little starch is gelatinized in most cracker products. Thus, the amylolytic enzymes have much less of an opportunity to hydrolyze the starch in its most susceptible condition. Proteolytic enzymes may have an effect even in low-moisture products, but this will depend on proof time (long or short), since they will normally be rapidly inactivated in the baking process.

Unsprouted wheat flour also contains endogenous protease enzyme(s). These enzymes are activated only at relatively low pH (<4.0) and so are not normally active in most baked product processes. However, processing of fermented products such as soda crackers utilize long periods during which lactobacilli-based fermentation leads to low pH in the sponge stage. These low-pH endogenous proteases can be an important processing factor under these conditions, softening the dough and helping to "mellow" the final cracker dough-handling characteristics. Many chemically leavened cracker doughs have relatively basic pH values, so the endogenous low-pH proteases will not be activated.

Lipoxygenase enzymes in cereals, especially soybeans and wheat, may have significant consequences, both good and adverse. The desirable effect is that of bleaching the carotenoid pigments, resulting in whiter products made from the flour. This is a desirable attribute in products such as white bread and rolls. The amount of the lipoxygenase enzymes is greater in soy compared with wheat; thus, wheat flour may be supplemented with soy flour to achieve the bleaching effect. The downside to this activity is that too much lipoxygenase activity may lead to oxidation of native flour lipids, producing an undesirable rancid flavor/aroma characteristic. The type I lipoxygenase enzyme acts only on free fatty acids while type II lipoxygenase can act on triglycerides as well.

The pentosan fraction of the flour is an important functional participant in the formation of the dough. Though normally only about 2 to 3% of the flour, it is capable of absorbing up to 25% of the moisture in the dough. Hydrolysis of the pentosan fraction would have a significant effect on dough viscosity. However, at present, while pento-sanase activity has been detected in germinated wheat, the levels appear to be quite low and of relatively little practical significance in terms of processing effects. Exogenous pentosanases, available commercially, may be used to greatly alter dough viscosity by destroying the waterholding capacity of the pentosan network.

Flour may also be used in nonbaking applications. For example, flour is often added to soup as a thickening agent. Endogenous enzymes, which may be present in that flour, are normally inactivated during the retorting step in the manufacture of the soup. If the flour is sound (unsprouted) containing relatively low levels of hydrolytic enzymes, the enzymes have little effect prior to retorting, and the desired thickening effect is maintained in the canned soup for long periods. However, if a sprouted wheat flour is used in this application, the endogenous enzymes can, prior to retorting, hydrolyze both starch and protein components, producing a thin, watery soup consistency.

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