While wheat (Triticum aestivum L.) flour contains only low levels of lipids (2.0% to 3.0%), they tremendously affect fresh bread quality. They are either starch (30% to 40%) or nonstarch (60% to 70%) lipids. While the former are important in bread staling, they affect neither bread loaf volume nor crumb structure as they are only set free at the very end of the baking process and prior to that they are not available because of their location inside starch granules. This review mainly focuses on wheat nonstarch lipids and how they impact on bread quality. Traditional approaches for investigating their role in bread making have been to use flours varying in bread making quality, to defat and reconstitute flour with (fractions of) the extracted lipids and/or to supplement flour with lipids from wheat or other sources. More recently, lipases have been successfully applied to investigate how wheat lipids affect bread making. It is generally accepted that their impact on bread loaf volume and crumb structure largely if not entirely relates to that on bread dough gas cells stability. However, today there are still different views and hypotheses on the mechanism(s) whereby they impact fresh bread quality. This review first defines and introduces the key terms, concepts, and theories related to lipids, lipases, and bread making. Next, the effects that wheat endogenous lipids and their enzymatically released hydrolysis products have on fresh bread properties and the mechanisms whereby they exert these effects are reviewed. 2 LIPIDS 2.1 Definition and classification Lipids are an extremely diverse group of natural compounds for which no widely accepted definition exists. They exhibit greater structural variety than other classes of biological molecules (such as proteins and polysaccharides) and are similar only in that they are largely hydrophobic and only sparingly soluble in water (Voet,

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Lipids are only minor wheat flour constituents but play major roles in bread making (BM). Here, the importance of a well-balanced lipid population in BM was studied by applying a lipase from Fusarium oxysporum in the process. Monogalactosyldiacylglycerols and N-acyl phosphatidylethanolamines were the most accessible lipase substrates. Hydrolysis thereof into their corresponding lysolipids was largely if not entirely responsible for loaf volume increases upon lipase application. Degradation of endogenously present lipids and enzymatically released lysolipids caused loaf volume to decrease, confirming that an appropriate balance between different types of lipids is crucial in BM. For optimal dough gas cell stability, the level of lipids promoting lamellar mesophases and, thus, liquid condensed monolayers needs to be maximal while maintaining an appropriate balance between lipids promoting hexagonal I phases, nonpolar lipids and lipids promoting hexagonal II or cubic phases.

This work was supported by the Fondazione IRCCS Ca' Granda, Ospedale Maggiore Policlinico: Ricerca Corrente [grant number RC2014/519-02] to M.M. and from ASM onlus 2010-2011 to M.M. The authors declare that they have no conflict of interest.

Three lipases with different hydrolysis specificities were tested in a laboratory-scale dough-batter wheat flour separation process in two concentrations. Lipolase specifically hydrolyzed nonpolar flour lipids. At the highest concentration tested, it significantly improved gluten agglomeration and yield, also when combined with a xylanase with hydrolysis specificity toward water-extractable arabinoxylan. We hypothesize that its action is due to the release of adequate levels of free fatty acids, which, because at least a part of them is dissociated, act as anionic surfactants. Lipolase at the lowest concentration, Lecitase Ultra, hydrolyzing both nonpolar and polar lipids, and YieldMAX, which specifically hydrolyzed phospholipids, had no or a negative impact on gluten agglomeration and yield. In conclusion, this study demonstrated that lipases with hydrolysis specificity toward nonpolar lipids can be used as processing aids in wheat flour separation in the absence or presence of added xylanases to maximize gluten agglomeration and yield.

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Normal-phase high-performance liquid chromatography (HPLC) is widely used in combination with evaporative light scattering detection (ELSD) for separating and detecting lipids in various food samples. ELSD responses of different lipids were evaluated to elucidate the possibilities and challenges associated with quantification by means of HPLC-ELSD. Not only the number and type of polar functional groups but also the chain length and degree of unsaturation of (free or esterified) fatty acids (FAs) had a significant effect on ELSD responses. Tripalmitin and trilinolein yielded notably different ELSD responses, even if their constituting free FAs produced identical responses. How FA structure impacts ELSD responses of free FAs is thus not predictive for those of triacylglycerols and presumably other lipids containing esterified FAs. Because ELSD responses of lipids depend on the identity of the (esterified) FA(s) which they contain, fully accurate lipid quantification with HPLC-ELSD is challenging and time-consuming. Nonetheless, HPLC-ELSD is a good and fast technique to semi-quantitatively compare the levels of different lipid classes between samples of comparable FA composition. In this way, lipid profiles of different flours from near-isogenic wheat lines could be compared.