Figure 1. Model of bread staling. In fresh bread, amylose and amylopectin exist in amorphous or gelled forms. During storage, moisture is lost, the amylose and amylopectin retrograde, and crystalline forms appear, leading to firmness and staling. Adapted from Zobel and Kulp, 1996 and Lallemand Baking Update, Volume 1/Number 6.
The commercially-available a-amylases used as anti-staling agents are derived from either bacterial or fungal sources. Most are produced by species of Bacillus and Aspergillus, and consist of either pure enzymes or mixtures of enzymes.These enzymes have optimum activity within a moderately high temperature range (50°C to 70°C), and are most active during the early stages of baking. However, bacterial a-amylases are generally thermal-resistant and are inactivated only at high baking temperatures (>80°C).
If too much hydrolysis occurs during dough formation and baking, the dough will be gummy and sticky, and the baked bread may also have a similar texture. Depending on the specific application, residual activity may remain in the bread even after baking, resulting in an increase in
Box 8—6. Fresh Ideas for Controlling Staling of Bread (Continued)
starch hydrolysis and free sugars during storage.Thus,there is interest in identifying a-amylases that are less thermal-resistant.Although the fungal-produced a-amylases are more heat-labile (or have intermediate heat tolerance) than bacterial enzymes, they typically have less activity at the baking temperatures normally used.
The manner in which the dough is mixed and fermented has considerable impact on bread staling. In general, bread freshness may be poorly retained during storage if the dough is over-mixed or under-mixed. Likewise, if the fermentation time is too short or too long, bread freshness is reduced. Thus, all other factors being equal, bread made from no-time doughs or long-time (i.e., straight) doughs usually will be less soft than bread made using continuous or sponge and dough processes.
Bowles, L.K. 1996. Amylolytic enzymes, p. 105—129, In Hebeda, R.E., and H. F. Zobel. (ed.). Baked goods freshness. Marcel Dekker, Inc. New York. Gray, J.A., and J.N. Bemiller. 2003. Bread staling: molecular basis and control. Comp. Rev. Food Sci. Food Safety 2:1-21.
Leon,A.E., E. Duran and C.B. de Barber. 2002. Utilization of enzyme mixtures to retard bread crumb firming.
J.Agri. Food Chem. 50:1416-1419. Knightly,W.H. 1996. Surfactants. p. 65—103, In Hebeda, R.E., and H. F. Zobel. (ed.). Baked goods freshness.
Marcel Dekker, Inc. New York. Zobel, H.F., and K. Kulp. 1996.The staling mechanism,p. 1—64,In Hebeda, R.E., and H. F. Zobel. (ed.).Baked goods freshness. Marcel Dekker, Inc. New York.
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