The functionality of starch and the related water mobility during wheat bread making

Bread is the main food product prepared from wheat and a staple food in the Western world. It fits well in a balanced diet since it is an excellent source of energy, fiber, vitamins and minerals. Unfortunately, of all bread produced, about 25% is wasted. The loss of bread in a home environment represents the largest share of this waste. Consumers indeed prefer fresh bread with a soft, but sliceable crumb, a crispy crust and a desired flavor. Regrettably, upon storage the crumb firms, the crust loses its crispiness and the flavor characteristics of fresh bread disappear. Storage therefore renders bread unacceptable for consumers. To address the crumb firming component of this problem in an efficient way, a thorough understanding of bread constituent transitions during baking and cooling and their impact on crumb firming is required. Although the impact of different bread components on the properties of fresh and stored bread in general is well known, the exact timing and extent of constituent transitions during bread making remain to be elucidated. For bread making, at least wheat flour, water, yeast and salt are needed. In quantitative terms, starch is the main flour component. It greatly contributes to the properties of fresh and stored bread. Starch is almost exclusively made up of amylopectin (AP) and amylose (AM) molecules which appear in highly ordered, partially crystalline starch granules. Starch crystallinity is mainly attributed to parts of the AP component. When heating starch in sufficient water (such as during bread baking) the order of starch granules is lost and starch gelatinizes. This phenomenon is accompanied by substantial granule swelling due to water absorption, leaching of AM from the granules and melting of AP crystals. At what point and to what extent these phenomena take place during bread baking depends on the level and the structure of AM and AP. During bread cooling, AM gelation occurs. In the process, (leached) AM crystallizes and forms a network throughout the bread crumb. Together with the gluten network formed during baking, this network provides the crumb of fresh bread with its desired texture. During storage, AP recrystallization reinforces the starch network. In this process, water is withdrawn from the gluten network and immobilized in the starch network. Together with water redistribution between crumb and crust this results in dehydration and (further) firming of gluten and starch networks and, thus, of crumb. To provi