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Title: The role of wheat (Triticum aestivum L.) endogenous lipids in bread making and bread quality
Other Titles: De rol van tarwe (Triticum aestivum L.) endogene lipiden in de broodbereiding en -bewaring
Authors: Gerits, Lien
Issue Date: 5-Dec-2014
Abstract: The four essential ingredients in wheat (Triticum aestivum L.) bread making are flour, water, yeast, and salt. Wheat flour typically makes up more than half of the recipe. Its characteristics and functionality, hence, determine bread quality. Its main constituents are starch, water and protein. Flour also contains low levels of non-starch polysaccharides and lipids. Wheat lipids form a complex group of components with varying polarities. They occur as starch internal lipids or as non-starch lipids, with the latter being analysed as either free or bound based on their extractabilities with solvents of opposing polarities. Despite their low levels in wheat flour, wheat endogenous lipids exert a profound role in bread making and product quality. Several hypotheses have been proposed with regard to their action mechanisms during bread making. These include (i) indirect stabilization of gas cells by strengthening the gluten network, (ii) direct stabilization of gas cells by lipids positioning themselves at gas cell interfaces or (iii) a combination of both. However, the methodologies applied to date do not allow to either confirm or reject these hypotheses. Therefore, in this doctoral work, a lipase-based approach was applied to further elaborate on the role of wheat endogenous lipids in the entire bread making process and to understand how they affect bread quality and its evolution over time. Lipases can in situ selectively modify the lipid population without altering other flour constituents. In a first part of this doctoral work, a method to extract, separate and detect endogenous wheat lipids in a single chromatographic run was optimized. This method was then applied to study the lipid (re)distribution in the transition from flour to bread. Profiles obtained with a high pressure liquid chromatograph coupled to an evaporative light scattering detector of free and bound lipid extracts of wheat flour, its derived dough (both during development and fermentation), and of bread made thereof were analysed and compared. This revealed that dough mixing redistributed lipid from the free to the bound lipid fraction, which has already long ago been referred to as ‘lipid binding’. Surprisingly, major redistribution of lipids occurred already before the gluten network was optimally developed. That more lipids could be extracted in total from dough than from flour, was related to flour particle disintegration and simultaneous release of previously unextractable (polar) lipids during mixing. Fermentation released triacylglycerols into the free lipid fraction, which was probably due to (extensive) stretching of the gluten network. The baking phase increased the levels of free (mainly unsaturated) lysophosphatidylcholine. This was caused by dissociation of amylose-lipid inclusion (AM-L) complexes in which it originally occurred as well as its replacement by (saturated) free fatty acids (FFA) and (saturated) N-acyl lysophosphatidylethanolamine upon formation of new AM-L complexes during cooling.In a second part, a lipase-based approach was applied to study the role of endogenous lipids in bread making more in depth. Lipase use impacted bread loaf volume (LV) to an extent depending on the lipase type, its applied dosage and the scale of bread making. Lipase hydrolysis of galactolipids and to a lesser extent of phospholipids is important for increasing bread LV. The FFA formed partitioned to the free lipid fraction whereas the ‘lyso’lipid component(s) remained in the bound lipid fraction. This suggested that polar lipids interact through their hydrophilic head group with other flour constituents (mainly gluten). Improved gas cell stabilization upon use of selected lipases was related to (i) sufficient amounts of lipids favouring the lamellar phase which stabilizes the interface by forming a condensed monolayer, (ii) an overall decrease in the levels of those lipids promoting hexagonal phase II, and (iii) an increase in the levels of lipids favouring the hexagonal phase I. The latter phase can emulsify deleterious non-polar lipids and prevent their adsorption at gas cell interfaces.To further elaborate on the mechanism(s) by which endogenous lipids enhance bread LV, Lipopan F and Lecitase Ultra lipases and diacetyl tartaric esters of mono- and diacylglycerols (DATEM) were selected. The latter was chosen since lipases have been proposed to replace surfactants in baking applications as they can in situ generate surfactants in dough. Our results indicated that a prerequisite for lipid(-like) components to impact dough rheology is their presence at the start of mixing. Surface active components responsible for gas cell (de)stabilization, such as endogenous lipids, can be isolated together with dough liquor (DL) by ultracentrifugation of dough. DL recovered from dough prepared with lipases contained higher lipid levels than that of control dough, because of higher levels of all polar lipid classes being present. Moreover, the larger the increase in the portion of polar lipids in DL (in the order Lipopan F > Lecitase Ultra > control), the larger the bread LV. Differences in bread LV during baking arose from differences in oven rise. Too high lipase dosages induced earlier oven rise termination due to earlier gas cell coalescence. Remarkably, optimal lipase dosages did not alter the duration of the oven rise. The data demonstrated that oven rise strongly depends on starch gelatinization in the outer baking dough layers, which prevented further expansion. This also explained why lipases exerted different effects in terms of bread LV at the different scales of bread making. Lipase or DATEM use yielded a more uniform and fine crumb structure, which was caused by the phenomena also increasing LV. Next to that, lipase use also changed bread crumb texture. Whether or not differences in mechanical properties result from different (swelling) behaviour of the starch granules and/or differences in gas cell stabilization, and thus crumb structure, however, remains unclear. A third part dealt with the impact of endogenous lipids on changes during storage of bread. Addition of the lipases or sodium stearoyl lactylate (SSL) impacted the evolution of bread crumb texture over time to a similar extent and slowed down both the increase in corrected firmness and stiffness and the decrease in resilience when compared to those in control bread loaves. At a molecular level, the lipase or SSL induced delay in amylopectin retrogradation was clearly noticeable in the free induction decay population A as measured with proton nuclear magnetic resonance. Remarkably, after 7 days of storage only Lipolase addition significantly reduced amylopectin retrogradation. AM-L complex formation upon lipase addition was independent of the type and location of the hydrolysed lipids in dough, and was similar to that for SSL addition. Therefore, differences in amylopectin retrogradation were not solely attributable to AM-L complex formation but also to their location. Indeed, those hydrolysis products having the most impact are present in spherosomes and therefore probably more readily form junction zones within the starch network than those lipid components ‘bound’ to the gluten network. In conclusion, both altering the galactolipid and to a lesser extent the phospholipid fraction as well as the concomitant increases in the levels of lipids promoting the lamellar or hexagonal phase I conformation positively affects bread quality. Improved LV and crumb structure were related to increased gas cell stability which mainly originated from a better direct gas cell stabilisation. In addition, wheat endogenous lipids also affected crumb texture during storage.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Food and Microbial Technology

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