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

Promovendus/a: Mieke Nivelle

Promotor(en): Prof. dr. ir. Jan Delcour

Bread is a staple food in the Western world. Unfortunately, 25% of all bread produced 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.

For bread making, at least wheat flour, water, yeast and salt are needed. Starch is an important flour component and greatly contributes to the properties of fresh and stored bread. Starch appears in granules and is mainly made up of amylopectin (AP) and amylose (AM). When heating starch in sufficient water, such as during bread baking, starch gelatinizes. Gelatinization is accompanied by water absorption and swelling of granules, leaching of AM from the granules and melting of AP crystals. When bread is allowed to cool down after baking, AM gelation occurs during which (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 In this process, water is withdrawn from the gluten network and trapped in the starch network. Together with the transfer of water from crumb to crust this results in dehydration and firming of the networks in bread crumb.

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 component transitions during bread making remain to be elucidated. Furthermore, the impact of these transitions on the crumb firming mechanism during bread storage is poorly known. Starch transformations and water redistribution in bread can accurately be analyzed with proton nuclear magnetic resonance (1H NMR). Against this background, the aim of this doctoral dissertation was to develop an innovative method based on temperature-dependent 1H NMR measurements for studying changes of starch and water in dough and bread during heating and cooling processes that mimic bread making.

In a first part of the dissertation, this temperature-controlled NMR method proved to be powerful when investigating changes in the starch fraction and the related water mobility in bread during baking and cooling. This method was then used to specifically look into the role of AM and AP during bread making and later during storage. Unique wheat flours with different AM and AP characteristics and a successful antifirming amylase (an enzyme that breaks down starch) were excellent research tools in this context.

The timing of gelatinization during baking and the extent to which AP recrystallizes during storage are largely determined by the structure and amount of AP in bread. Higher AP concentrations increased the gelatinization temperature and promoted AP recrystallization. A high portion of AP chains that are too short to crystallize decreased the gelatinization temperature and restricted the extent of AP recrystallization. The amount and structure of AM in bread determine the timing and extent of AM crystallization and the formation of the AM network during cooling. When bread crumb contained less AM, cooling to lower temperatures was required for AM crystallization to start. Eventually, a less extended AM network with less AM crystals was formed. Since AM is a structure-building unit in fresh bread, the resulting crumb was too soft to be sliceable. The formation of a too rigid AM network is inferior to the crumb texture as well. Such AM network was formed when AM chains were shortened and therefore were more apt to crystallize during cooling. During storage of breads containing different AM and AP characteristics, the extent of crumb firming is mainly determined by AP recrystallization.

In the last part of this doctoral dissertation, the bread baking process was altered. Bread was prepared by first partial and, following intermediate storage, final baking. The impact of variations in duration of partial baking on fresh and stored bread was examined.

A longer baking time was beneficial for the crumb texture of fresh, partially baked bread. This was the result of more extended starch and gluten networks. During intermediate storage, the extent of crumb firming was higher when bread had been baked for a longer time. It is believed that leaching of AM and also of AP outside the granules continued during prolonged baking. Consequently, a more extended AM network with more AM crystals was formed during cooling. These AM crystals stimulate AP recrystallization. Moreover, AP recrystallization was enhanced by the higher concentration of AP in the outer zones of the granules (where AM leached out) and the presence of leached AP outside the granules. In addition, the crust moisture content of fresh bread was lower when baking times had been longer such that the transfer of moisture from crumb to crust occurred to greater extent during intermediate storage.

After intermediate storage, bread loaves were fully baked to melt the recrystallized AP. The moisture redistribution that had occurred during the preceding intermediate storage phase, however, was not reversible. In large breads, the crumb moisture content decreased only slightly during intermediate storage because of their high crumb to crust ratio. Consequently, melting of the recrystallized AP was sufficient to refresh the crumb during final baking. In contrast to what is generally believed, the refreshed bread loaves did not firm faster than those produced by one-step baking. In this dissertation it was shown for the first time that the size of bread loaves is crucial in this context. Crumb of small bread loaves did substantially dry out during intermediate storage. Since this cannot be reversed by heat during final baking, we assume that smaller bread loaves do firm more rapidly after the final baking step.

In summary, starch transitions and water redistribution greatly contribute to fresh and stored bread properties. The amount and structure of AM and AP in bread determine the timing and extent of changes in the starch fraction during baking, cooling and storage of bread. The formation of the gluten network and AM network during baking and cooling is responsible for the crumb texture of fresh bread. AP recrystallization during storage makes an important contribution to crumb firming. It was shown that water redistribution as well is an integral part of crumb firming, especially when bread was first partially and later, after intermediate storage, was fully baked. In-depth understanding of the functionality of AM and AP and the related water mobility allows defining an ideal starch in flour for bread making purposes. Such starch has an AM to AP ratio and AM chain length distribution corresponding to that of regular wheat starch and a high portion of outer AP branch chains that are too short to crystallize.