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Bread --- Cooking (Bread)

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Wheat flour, water, yeast and salt are essential bread ingredients. The main biopolymer types of wheat flour are the glucose polymers of starch, amylose and amylopectin, and the gluten proteins. During storage, bread becomes unacceptable for the consumer. It is generally accepted that crystallization of amylose during cooling and recrystallization or retrogradation of amylopectin during storage contribute to bread crumb texture and how it evolves in time, but it is also clear that these structural changes in the starch fraction alone cannot fully explain the crumb firming mechanism. Gluten is somehow involved in bread firming, and changes in water content and distribution due to drying of bread, moisture migration from crumb to crust and/or moisture redistribution between the different bread constituents are believed to further impact crumb firmness. Moreover, starch and gluten are immiscible. Despite its potential contribution to moisture redistribution within bread crumb and, thus, to crumb firmness, this incompatibility has never been investigated in the context of bread storage. Since water related phenomena seem to be involved in crumb firming, the use of time domain (TD) proton Nuclear Magnetic Resonance (1H NMR) to assess bread aging seems promising in this respect. However, prior to the start of this work, no standard 1H NMR methodology existed for studying changes in proton mobility of water and biopolymers in food products, resulting in often confusing interpretations of experimental results.When certain alfa-amylases, i.e. enzymes that hydrolyze starch chains, are added to the bread recipe, crumb firmness is reduced. In addition, shorter baking times, lower baking temperatures or cool storage (at 5 °C), all impact the starch fraction and crumb firmness as well, pointing to an important role of starch reorganization for crumb firmness during storage. However, no consensus exists about the exact interactions between the starch, gluten and water fractions during bread storage that result in crumb firming.Against this background, the aim of the present dissertation was to determine the relation between water dynamics, biopolymer interactions and crumb firmness, and to further unravel the mechanism whereby bread crumb firms. The latter was done by using several alfa-amylases, process and storage conditions. In a first part of this work, a TD 1H NMR method was optimized for interpreting proton distributions of bread. By studying unheated and heated starch, gluten and flour model systems, starch gelatinization and gluten polymerization related changes in proton distributions were identified. Six and two different populations were detected for fresh bread crumb and crust, respectively, and assigned to certain categories of protons based on the results of the model systems. The 1H NMR method was then used in combination with Differential Scanning Calorimetry (DSC), X-ray diffraction and texture studies to further unravel the interactions between starch, gluten and water fractions that cause crumb firming during bread storage. Amylopectin recrystallization resulted in formation of a continuous, rigid, semi-crystalline network that included water in its structure. This water was withdrawn from the amorphous networks, including the gluten network, and became unfreezable. Furthermore, it was found that the thermodynamic immiscibility of starch polymers and gluten resulted in water diffusion from gluten to starch. Together with crumb to crust moisture migration, this water redistribution partially dehydrated the gluten network during bread storage. By combining the obtained results, a profound view on the bread firming mechanism was proposed. In this view, the increase in crumb firmness during the first days of storage is largely attributed to amylopectin recrystallization, while further increase in firmness at later storage times is caused by stiffening of the gluten network when its moisture content (MC) drops below a critical value of 35% for it to be fully plasticized. Although quantitative information on amylopectin recrystallization is more accurately obtained with DSC, 1H NMR provides a good estimate of both the extent of amylopectin crystal formation and changes in water distribution during storage. To further verify the proposed crumb firming mechanism, three alfa-amylases were added to the bread recipe. These enzymes differently impacted the starch (re)crystallization behavior and the resulting network organization in fresh and stored bread crumb. This way, the importance of the starch mesoscale network organization for crumb texture properties was further investigated. The most prominent effect of the maltotetraose forming thermostable alfa-amylase from Pseudomonas saccharophila was an 8% loaf volume increase, which led to a crumb firmness reduction by nearly 20% after 168 h of storage. The starch structure was not extensively impacted and amylopectin retrogradation still occurred when using this enzyme. In contrast, addition of Bacillus subtilis alfa-amylase, an enzyme that hydrolyzes the starch chains internally, resulted in extensive starch degradation and, therefore, in structure collapse and undesirable sticky bread crumb. Its use, though, provided further evidence for a fringed micellar network organization of starch in bread crumb during storage. The best anti-firming results were obtained when adding a Bacillus stearothermophilus maltose forming alfa-amylase to the bread recipe. Despite a 25% decrease in initial crumb resilience, it slightly increased bread volume (by approximately 3%) and prevented amylopectin retrogradation and, thus, formation and reinforcement of a prominent starch network during storage without collapse of the crumb structure. In a final part of this work, changes in the starch fraction were induced by using altered baking and storage conditions. Firstly, reducing the baking time from 24 to 9 min led to a less prominent amylose and gluten network. During storage, amylopectin retrograded less, resulting in a less prominent starch network that included less water. This led to a smaller increase in crumb firmness and a smaller decrease in crumb resilience. Secondly and in contrast, lowering the storage temperature from 23 to 4 °C increased the rate of amylopectin crystal formation. This resulted in a more prominent and rigid starch network that included more water and that was responsible for larger changes in crumb firmness and resilience. In both of the above cases, crumb MC remained high. Crumb firmness was then largely determined by the strength of the starch network, which, in turn, depended on the degree of amylopectin recrystallization. Thirdly, bread was only partially baked or parbaked and then stored at room, refrigerator or subzero temperatures prior to final baking. The properties of the fully baked parbaked (FBPB) bread loaves depended on changes in their parbaked (PB) counterparts. The extent of retrogradation during intermediate storage of PB bread loaves together with crumb MC determined initial proton mobility and resilience of fresh FBPB bread loaves. In addition, amylopectin retrogradation during storage of FBPB bread appeared to be affected by its crumb MC. In spite of ice crystal formation during frozen storage of PB bread which impacted water distribution of the FBPB counterpart, this FBPB bread had a longer shelf life than FBPB bread obtained from PB bread stored at refrigerator or room temperature. In conclusion, the presented view on the relation between starch structure, water redistribution and crumb firmness maintains that besides amylopectin retrogradation, changes in water distribution contribute to the crumb firming mechanism. The bread shelf life can then be improved by using additives, altered processes or other storage conditions that result in changes in starch structure or water distribution.

academic collection --- 664.6 --- Baking. Bread. Flour confectionery --- Theses --- 664.6 Baking. Bread. Flour confectionery