Interaction of lipids with proteins and carbohydrates in breadmaking

After reviewing the evidence for interact ion in breadmaking of lipids with proteins and carbohydrates, theories on the shortening response are discussed. Recent studies show that the response cannot be explained entirely on the basis of physical phenomena. Both overall breadmaking quality (presumably related to gluten and its components) and the presence of wheat flour lipids are impor tant in the shortening response. I N T R O D U C T I O N While the significance in breadmaking of lipid interaction with proteins and carbohydrates is widely accepted, it has been studied little for several reasons ( I ) . They include the complexi ty of the system, the l imited knowledge of the interacting components , and the pauci ty of methods to s tudy such interactions. In recent years there has been renewed interest in studying the interaction and its significance. This review summarizes briefly previous work from our laboratories, some recent studies conducted by investigators in several laboratories, and some new work from the U.S. Grain Marketing Research Center. THE I N T E R A C T I N G COMPONENTS Wheat flour contains about 70% starch, 12% proteins, 2 % lipids, 2% pentosans, and 12% moisture. Total wheat flour lipids contain about equal amounts of nonpolar and polar components , Wheat flour lipid composition is shown schematically in Figure 1 adapted from data reported by MacMurray and Morrison (2). Triglycerides (TG) are a major component of nonpolar lipids, digalactosyldiglycerides (DGDG) of glycolipids; and lysophosphatidylcholines (LPC) and phosphat idylcholines (PC) are major components of phospholipids (Fig. 1). Differences in solubility provide a convenient and useful means of separating wheat flour lipids in to major categories: free and bound (Fig. 2). Free lipids can be extracted with nonpolar solvents such as ether or petroleum ether (PE). For extraction of bound (mainly to protein) lipids, polar solvents such as water-saturated butanol (WSB), or a mixture of chloroform-methanol-water are required. Lipids extracted by PE are arbitrarily defined as free and those by WSB, following PE extraction, as bound lipids. The free lipids can be fractionated according to their elution from a silicic acid column. About 70% free lipids can be eluted with chloroform, and they form what is arbitrarily called the "nonpola r" fraction containing TG as a major component . The residual 30% free lipids can be eluted from the column with a more polar solvent, such as methanol, and comprise a mixture of free polar lipids. Among the free polar lipids, about two thirds are glycolipids containing DGDG as a major component , and one third are phospholipids with PC as a major componen t (3). Ipresented at the AOCS Meeting, New York, May, 197"7 (Symposium: Interaction of Oxidized Fats with Amino Acids and Carbohydrates). About 0o6-1.0% bound lipids can be extracted from flour with WSB after PE extract ion. Bound lipids contain about 30% nonpolar and 70% polar lipids. Bound polar lipids are rich in phospholipids with LPC as a major phospholipid component (3). As glycolipids are impor tant in breadmaking, it is impor tan t to distinguish clearly between two classes of polar lipids. Although the free polar lipids are richer in glycolipids than the bound polar lipids are, the actual amounts of both glycolipids and phospholipids are higher in bound polar than in free polar lipids (Fig. 2). Recently the lipids of wheat starch have received new attention. Morrison et al. (4) divided flour lipids into those inside starch granules, which are true starch lipids, and all other lipids outside the starch granules, which are called non-starch lipids. Even polar solvents containing alcoholwater mixtures such as WSB extract mainly non-starch lipids at room temperature. Starch lipids can only be extracted efficiently with hot n-butanol-water (65:35) or hot WSB (4-7). Starch lipids are almost exclusively monoacyl lipids, and 86 to 94% of the total starch lipids are lysophospholipids, LPC as the chief constituent. Gluten proteins comprise about 80% of the total flour proteins (8). Gluten proteins can be separated into two, approximate ly equal, fractions of gliadin (a mixture of prolamines soluble in 70% alcohol) and glutenin (a mixture of glutelins soluble in dilute acids and alkali). The starch content of wheat flour is, in general, inversely related to protein content (9). In flours below 80% extract ion, the starch content ranges from about 65 to 70% (on an as-is WHEAT FLOUR LIPIDS I I NONPOLAR (50.9%.! POLAR 149.1%) I l I GLYCOPHOSPHOLIPIDS LIPIDS (26.4%) 122.7%1 I I TG (20.8) DGDG (13.5) LPC (7.1) SE (7.5) MGDG (4.9) PC (5.8) FFA (7.0) AMGDG (3.6) APEA (4.9) 1,2-DG (6.2) SG + CMG (1.8) ALPEA (2.9) 1,3-DG (6.0) ASG (1.6) LPEA (0.9) FS (2.1) DGMG (0.6) PEA (0.8) MG (1.3) MGMG (0.4) PS (0.2) CDG (0.03) Pl (0.I) FIG. 1. Composition of total wheat flour lipids (extracted with water-saturated butanol). The abbreviations are: TG=triglycerides; SE=steryl esters; FFA=free fatty acids; 1,2-DG=l,2-diglycerides; 1,3-DG =l,3-diglycerides; FS=free sterols; MG=monoglycerides; DGDG= digalac tosy ld ig ly cerides; M GDG=monogalactosyldig lycerides; AMGDG=-lYacyl monogalactosyldiglycerides; SG=steryl glucoside; CMG=ceramide monoglycerides; ASG=6-0-acyl steryl glucosides; DGMG=digalactosylmonoglycerides; MGMG=monogalactosylmonoglycerides; CDG=ceramide diglycosides; LPC=lyso phosphat idylchol ines ; PC=phoshatidylcholines; APEA=N-acylph o sph atid yl ethanolamines; ALPEA=N-acyl lysophosphatidylethanolamines; LPEA=lysophosphatidylethanolamines; PEA=phos phatidylethanolamines; PS=phosphatidylsefines; PI=phosphatidylinositolso (Adapted from Ref. 2).