INGREDIENT AND PROCESSING VARIABILITY AFFECTING RYE BREAD QUALITY

A. Flour Particle Size

Even though rye flour may be a small component of the flour blend, its particle size can affect bread quality. Rye flours that have a very fine and smooth texture, so-called dead-milled flours, should be avoided. Such flours contain too much soluble matter (starch, pentosans, and proteins). These substances swell easily when mixed into a dough, but the dough quickly loses its stiffness during fermentation. Such flours cannot be used for bread because the dough is too stiff (viscous) during the mixing stage and becomes too fluid for satisfactory dividing and panning. Bread baked from such flours will have a very weak crumb.

Particle size is also important in the case of whole rye meal used for bread production. For example, for Pumpernickel bread the flour must be rigidly uniform in particle size for any one baking procedure. The appropriate particle size must be determined by trial (Drews and Seibel, 1976).

Doose (1964) distinguished eight meals of different baking properties according to granularity and surface characteristics of the particles.

B. Age of Flour

During normal storage conditions, rye flours deteriorate much more quickly than wheat flours. Whole rye flour cannot be stored safely for as long a time as patent or straight-grade rye flours. Abnormal conditions, such as high temperatures or high humidities, accentuate the rate of deterioration during storage.

Storage deterioration of rye flour usually involves hydrolytic and oxidative changes in the flour lipids.

Under some storage conditions, high flour acidity can result from microbial contamination. This type of deterioration usually involves other flour constituents in addition to lipids. It can be detected by measuring the pH of an aqueous flour suspension. The pH should not be lower than 5.5.

If there is any suspicion that a particular flour may not be sound, it should be subjected to detailed sensory evaluation. Freshly milled flour is not odorless and bland, but has a pleasant characteristic odor and taste. The development of an abnormal odor or taste during storage usually indicates that some deterioration has occurred. In some cases, the abnormal flavor disappears during the breadmaking process; in others, it is retained in the bread (Drews and Seibel, 1976).

C. Flour Milling Extraction and Composition

Chemical composition of rye flour can be a good indicator of its baking quality. Differences in composition arise from the fact that different parts of the rye kernel that are converted to flour vary widely in composition. The outer parts of the endosperm yield middlings that are usually contaminated with various amounts of bran. Flours produced by the reduction of these middlings usually are richer in pentosans and proteins than break flours.

Reduction flours, obtained from the outer parts of the endosperm, are not only richer in pentosans but also contain a higher proportion of soluble pentosans. Accordingly, water uptake by the flour and viscosity of flour–

water slurries generally increase with increasing extraction (Drews and Seibel, 1976; Lorenz, 1981).

The most important constituent of rye flour is starch. It plays a major role in the crumb texture of baked bread. Starch is the substance that, in a partially gelatinized form, consolidates the crumb structure and determines its firmness. Therefore, a poor starch quality can have a major effect on crumb characteristics (Seibel et al, 1983; Seibel and Bru¨mmer, 1991).

D. Flour Enzyme Activity

Amylase activity of rye flour greatly affects bread quality. This activity is measured with the amylograph or the falling number apparatus. A minimal gelatinization temperature of 63 8C and viscosity of the paste of 170 AU (amylogram maximum) for commercial rye flours are required. Falling numbers for rye within the range of 120–160 s are generally considered to be satisfactory. This corresponds to amylogram gelatinization temperatures of 64–66 8C. However, there is no strong relationship between falling number and amylogram gelatinization temperature. Therefore, sound evaluation of rye flour is based on amylogram data (Seibel et al, 1983).

Typical sprouted rye flours are characterized by a falling number value of less than 100 s and amylograph starch gelatinization temperatures below 63 8C (Meuser et al, 1994).

Figure 1illustrates the relation of the falling number to the quality of rye bread crumb. Rye flours with a falling number less than 80 invariably produced a loaf of bread with a moist and sticky crumb characteristic of rye bread baked from rye containing a high percentage of sprouted kernels.

Flours possessing a falling number above 100 produced bread with a good dry crumb, with a tendency toward a better loaf of bread with increasing falling number. The relationship was better than that obtained with the amylogram value, although amylogram peak height values of less than 150 were indicative of a sticky crumb (Korkman and Linko, 1966).

E. Absorption and Dough Viscosity

Water absorption of rye flour is affected by many physical and compositional factors (Drews, 1966). It increases with increasing fineness of the flour. Flours with a high content of water solubles will have high water absorption. The pentosans of the soluble fraction have a greater effect on water absorption than the proteins (Drews and Seibel, 1976).

Water absorption of rye doughs does not remain constant, but changes with time at a rate that depends on temperature, pH, and salt content. The proportion of insoluble and soluble constituents is continu-ously changing due to enzymatic action. As the amounts of insoluble constituents are reduced, their contribution to water absorption diminishes rapidly. If enzymatic action is extensive, it will eventually destroy the water binding properties of the water-soluble constituents.

Figure 1 Relationship between falling number and crumb quality of rye bread.

(From Korkman and Linko, 1966, with permission.)

The viscous properties of rye doughs are extremely important to baking quality. This is in contrast to wheat doughs, which must have an optimal balance between elastic and viscous properties for the best baking performance. Viscosity of rye doughs determines dough yield, stability, and bread loaf volume. Doughs of higher viscosity will yield more dough by retaining more water and will have better stability, but will yield lower loaf volumes.

The pentosans play the key role in rye dough viscosity. Proteins are important but not to the same extent as in wheat doughs.

Dough viscosity can change during processing. Vigorous mixing and other types of mechanical handling usually lead to a reduction of viscosity.

Yeasted rye doughs will show a gradual drop in viscosity during fermentation; the rate of this drop depends on pH and salt concentration.

In the sourdough process, there is an additional effect of various degrading enzymes produced by the acid-forming microorganisms. Under some conditions, the drop in viscosity can lead to doughs that are too fluid to process into bread.

Under commercial baking conditions, dough viscosity can be controlled by manipulation of dough temperature, pH, and salt concentra-tion. The correct conditions will depend on the flour, the baking process, and the final product (Drews and Seibel, 1976).

F. Acidification

All doughs containing rye flour should be acidified. Sour conditions have a positive influence the swelling power of the pentosans and mucilages of rye flour and at the same time partly inactivate enzyme activity, particularly amylase activity. The degree of acidification depends on the proportion of rye in the flour mixture.

During dough making, the swelling products absorb the amount of water required for optimal gelatinization of starch during the baking process. Acidification has an influence on the swelling process: A lower pH value (optimal pH value¼ 5) reduces the solubility and swelling power of the swelling products (Seibel and Bru¨mmer, 1991).

Depending on the proportion of rye flour, the following degrees of acidity should be reached for optimal rye bread quality:

Rye bread (at least 90% rye) 8.0–10.0 Mixed rye bread (51–90% rye) 7.0–9.0 Mixed wheat bread (less than 50% rye) 5.0–8.0

The relationship between pH, acidity, and bacteria–acid yeast count is illustrated in Fig. 2. The biochemistry of rye dough fermentation has been thoroughly discussed by Rohrlich (1961) and Spicher and Stephan (1993).

Rye flours with ash contents of greater than 1% produce breads with deficient crumb elasticity, which can be counteracted by using strong dough acidification (Seibel and Bru¨mmer, 1991).

In rye-mixed breads using a sound wheat flour, deficiencies in crumb elasticity are less frequently encountered. If, however, the rye-mixed bread is produced from high-extraction flours with ash contents of 1.0–1.75%, crumb damage may be quite pronounced. The problem can be overcome by using less water, stronger acidification, and a somewhat longer baking time at lower temperature (Seibel et al., 1983).

It is apparent from the foregoing discussion that there are many compositional, recipe, and processing variables that must be controlled in order to produce a dough with optimal physical properties for a specific type of rye bread.

Figure 2 Relationship between pH, acidity, and bacteria and yeast count in a rye dough. (Based on data in Spicher and Stephan, 1993.)

XVII. CHANGES IN ANTINUTRITIONAL FACTORS DURING