Mechanism of crumb toughening in bread-like products by microwave reheating

Comparing breads reheated in conventional and microwave ovens revealed that the latter considerably toughens the crumb texture when internal boiling is induced. Moisture loss in itself has a relatively minor toughening effect. The major changes, caused by boiling, occur only in systems with starch concentration in excess of a threshold level of about 37% (wet basis). Substantially greater amounts of amylose are leached out of the granules in the case of sustained boiling during microwave heating, as compared to conventional oven heating. The free amylose solution is being "pushed" by the generated steam pressure toward the air-cell wall interface. A rich amylose phase is accumulated at that interface and over the granules. Upon cooling, the amylose undergoes rapid phase changes; thus, toughening is apparent in a relatively short time after heating. Minimizing the textural deleterious effects in microwave reheating of bread-like products should entail (a) preventing or minimizing internal boiling, (b) diluting of the starch concentration below the threshold level, (c) interfering with the amylose phase change by using complex forming agents.