Unravelling the functionality of biopolymers and related water mobility during 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 In this process, water is withdrawn from the gluten network and immobilized in the starch network.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 provide a proper background for investigating the functionality of starch and its interaction with water during wheat bread making, an overview of the literature on the characteristics of wheat starch and its role during bread making and storage is presented in a first part of this doctoral dissertation. Useful analytical approaches are also described. Starch transformations and water redistribution during bread making and storage can accurately be analyzed with time domain proton nuclear magnetic resonance (TD 1H NMR). This technique can be used in a temperature-controlled fashion to study the starch transitions and water dynamics in bread while it is being baked and subsequently cooled. Although changes in the starch fraction during cooling are of major importance for the quality of fresh bread, this part of the bread making process has not been investigated with temperature-controlled TD 1H NMR. Furthermore, NMR measurements in bread dough have mainly been performed during stepwise heating, which is not representative for how bread is baked. Such NMR measurements yield a complex set of data. To the best of our knowledge, these data have never been interpreted in combination with different techniques that monitor starch crystallinity, swelling and leaching phenomena.
Against this background, the aim of this doctoral dissertation was to develop an innovative toolbox primarily based on temperature-controlled TD 1H NMR for studying starch transitions and water dynamics in dough and bread during heating and cooling processes relevant for bread making. In a first part of the experimental section, the combined use of temperature-controlled TD 1H NMR, time-resolved wide angle X-ray diffraction, differential scanning calorimetry (DSC) and colorimetric and gravimetric analyses of starch properties proved to be powerful when investigating changes in the starch fraction and the related water mobility in bread during baking and cooling. This way, an integrated view on gelatinization and gelation phenomena at different length scales during bread making was created.
Temperature-controlled NMR was then used to specifically look into AM and AP functionality during bread making. In this context, wheat flours containing unique starches and a successful antifirming amylase, i.e. the maltogenic α-amylase from Bacillus stearothermophilus (BStA), were excellent research tools.
Some of the flours used were from near-isogenic wheat lines (NILs). NIL 5-5 flour contained less AM and higher levels short AP chains than the control, i.e. NIL 1-1 flour. In addition, wheat flour of which the starch has a very low AM level was used. Incorporation of these flours in a bread recipe resulted in altered temperature ranges of gelatinization and gelation. It was found that the stability of AP crystals was higher when starch contained little AM such that the temperature range over which gelatinization occurred during baking shifted to higher temperatures. In contrast, AP crystal stability was lower when starch contained higher levels of AP branch chains that are too short to form stable crystals such that gelatinization temperatures decreased.
Although starch hydrolysis by BStA also resulted in increased levels of short AP branch chains, no such shift in gelatinization temperature was detected when baking bread prepared with use of BStA. It follows that BStA only significantly hydrolyzes starch after the onset of gelatinization. Later during baking, a slightly weakened starch network with more mobile AM molecules due to hydrolysis of AM by BStA was obtained. This pointed to the structure-building role of AM in bread crumb, as also described in literature, which was further validated. When starch 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. Protons in lower AM starch networks were more mobile, which was indicative for softer fresh bread crumb.
To study the impact of AM and AP characteristics on fresh and stored bread, bread was prepared from unique wheat flours or by incorporating BStA in the recipe. The breads were stored at room temperature. Crumb firming was monitored by TD 1H NMR, DSC and texture measurements. When the starch AM content was lower, a less extended AM network was formed during cooling of freshly baked bread. As a result, the crumb in the fresh, cool bread was too soft to be sliceable. However, after 7 days of storage, crumb firmness readings of bread containing little if any AM were more than twice those of control bread. Since very low starch AM levels imply very high AP levels, it is evident that the extent of AP recrystallization during storage substantially increased. In contrast, the amount of recrystallized AP and the crumb firmness after 7 days of storage were half those of control bread when bread was prepared with use of BStA or from NIL 5-5 flour containing starch with a higher level of AP branch chains that are too short to crystallize.
In the last part of this doctoral dissertation, not the bread recipe but its production 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. The initial crumb resilience increased with partial baking time as a result of more extended starch and gluten networks. With prolonged partial baking, more AM leached and was available for crystallization during cooling. Gluten protein cross-linking was enhanced as well.
During intermediate storage after the first, partial baking, 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 that serve as nuclei for AP retrogradation was formed during cooling. Moreover, the resultant higher concentration of AP in the outer zones of the gelatinized granule remnants and the presence of leached AP molecules in the extragranular space enhanced both intragranular and intergranular AP retrogradation during intermediate storage. In addition, the crust moisture content (MC) of fresh partially baked bread was lower when baking times had been longer such that moisture redistribution between crumb and crust occurred to greater extent during intermediate storage.
During final baking after intermediate storage, the recrystallized AP melted. The moisture redistribution that had occurred during the preceding intermediate storage phase, however, was not heat-reversible. In bread prepared from 100 g of flour, crumb MC decreased typically by 8% during 7 days of storage due to crumb to crust moisture migration. In bread prepared from 270 g flour, stored for the same time, crumb MC decreased only by 1% because of their high crumb to crust ratio. Since the crumb MC of the latter breads remained high during intermediate storage, melting of the recrystallized AP was sufficient to refresh the crumb and to restore firmness and resilience readings to their initial values in fresh bread. In contrast to what is generally believed, the refreshed bread loaves did not firm faster than those produced by conventional, one-step baking. The size of bread loaves is crucial in this context. Indeed, bread loaves prepared from only 100 g of flour were subject to substantial crumb dehydration during storage. Since the latter was not heat-reversible, we assume that smaller bread loaves, prepared by partial and later final baking after intermediate storage, do firm more rapidly than their conventionally baked counterparts.
In conclusion, starch transitions and water redistribution greatly determine fresh and stored bread properties. AP characteristics mainly determine the timing of gelatinization during baking and the extent to which AP recrystallizes during storage. AM characteristics impact on the timing and extent of AM crystallization and the formation of the AM network during cooling. Redistribution of water is an integral part of crumb firming, especially in the case of partial and later final baking, since moisture redistribution during intermediate storage at room temperature is not heat-reversible. 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 (DP 6-10).