Lipids in Cereal Technology || Wheat Germ Oil
19 Wheat Germ Oil P. J. BARNES The Lord Rank Research Centre, High Wycombe, Bucks, U.K 1 Introduction 389 II Production of Wheat Germ 389 III Production of Wheat Germ Oil 390 IV Applications of Wheat Germ Oil 391 V Composition and Properties 392 A Yield of oil 392 Physicochemical Properties . . . . . . 392 C Fatty acid composition 393 D Acyl lipids 394 Non-saponifiable lipids 394 Acknowledgement 399 References 399 I INTRODUCTION Wheat germ oil is a speciality product manufactured in much smaller quantities than the common edible oils such as corn oil. This is due primarily to the relatively low yields of wheat germ and wheat germ oil. Typical yields of commerical wheat germ from flour milling are less than 1% of the grain weight, and the oil content is usually not more than 10% of the germ dry weight, in contrast to 30 -50% of oil for maize (corn) germ. Although obtained in low yield, and therefore expensive, wheat germ oil finds a ready market in the health and cosmetic sectors. These applications are related to the high vitamin activity and, more recently, to the content of octacosanol. II PRODUCTION OF WHEAT GERM The part of the wheat grain described as the germ consists of two major parts, the embryo axis and the scutellum (see Chapter 1), which, on aver-389 " Lipids in Cereal Technology" Copyright 1983 by Academic Press, London ISBN 0-12-079020-3 All rights of reproduction in any form reserved 390 P. J. BARNES age, represent 1.2% and 1.5% of the grain dry weight respectively (Hinton, 1944). As described in Chapter 7, the scutellum is relatively friable and difficult to separate from the other milling fractions. In contrast, the embryo axis passes with coarse semolina (first and second break coarse middlings) to the reduction rolls where it is flaked and may be separated by sieving (see Chapter 7). Commercial wheat germ thus consists predominantly of embryo axis. The commerical germ fraction is not pure wheat germ but also contains variable proportions of bran and endosperm. Oil is pressed out of the germ to some extent during flaking and is transferred to the flour. The lipid composition of commercial germ is thus different from that of the original embryo axis. If commercial germ is to be used in foodstuffs, it must be stabilized against hydrolytic and oxidative rancidity. This may be partly achieved by drying the germ to 4% moisture content, cooking or defatting. Ill PRODUCTION OF WHEAT GERM OIL Oil is separated from wheat germ commercially either by pressure expelling or solvent extraction. Although expelling avoids the dangers of inflammable solvents, it recovers only half of the available oil and yields germ that requires further stabilization against rancidity. Solvent defatting is more efficient and the residual oil content of the extracted germ is less than l O g k g 1 . Solvent defatted wheat germ is more valuable than the corresponding full-fat germ because of its stability, and therefore the production of wheat germ oil must always consider the quality and quantity of the defatted germ product. Only germ having very low levels of bran contamination can be successfully pressed to yield oil; at higher bran contents the overall oil content of the germ is too low (bran contains much less oil than does germ). Solvent extraction is less influenced by the bran content because the yield of oil is greater, but the purity of germ used for extraction is governed by the specifications set for the oil and defatted germ products. High temperatures should be avoided during manufacture to minimize destruction of nutritionally important components. Most manufacturers fail to provide full details of the temperatures which the oil has been subjected to during processing, but one company claims to carry out solvent extractions at temperatures not exceeding 38 C without any subsequent refining (Vio-bin Corp., U.S.A.) . Hexane, 1,2-dichloroethane and ethanol are the preferred solvents for wheat germ oil extraction. 19 WHEAT GERM OIL 391 Most edible oils are refined to remove phospholipids, free fatty acids (FFA), pigments and volatile compounds. However, these treatments may not be necessary for wheat germ oil and may even be considered undesirable for a product that is marketed as "natural" and "unprocessed". Lecithin (phosphatidylcholine), colour and flavour are positive attributes in a product sold in the "health food" market. FFA have been removed from commerical wheat germ oil by alkali treatment but this also results in substantial losses of oil and tocols. An alternative approach is the use of molecular distillation to remove FFA and also to produce a tocol concentrate (Singh and Rice, 1979). IV APPLICATIONS OF WHEAT GERM OIL The market for wheat germ oil is based mainly on the high vitamin content and, more recently, octacosanol. Wheat germ oil is the richest, readily available, natural source of -tocopherol, the tocol with greatest vitamin activity, and it is retailed in bottles or in gelatin capsules. It is also used in admixture with other materials such as lecithin and cod liver oil and as a component of shampoos and cosmetics. Wheat germ oil preparations are sold as diet supplements for farm animals, racehorses, pets and mink. Although synthetic -tocopherol is readily available, the natural vitamin is often preferred and tocol concentates from wheat germ oil are used to fortify other preparations. Vitamin is regarded as a nutrient essential for human health but no clinical deficiency syndrome has been identified in adult man in the absence of other disease conditions or malnutrition. The Food and Nutrition Board of the United States National Research Council believes that "in as much as there is no clinical or biochemical evidence that vitamin status is inadequate in normal individuals ingesting balanced diets in the United States, the vitamin activity in average diets is considered satisfactory". (Committee on Dietary Allowances, 1980.) Similarly, in the U.K. the Department of Health and Social Security notes that vitamin occurs in sufficient quantity in a large number of foods, and, in the light of present knowledge and the context of the U.K. diet, does not recommend a dietary allowance for this vitamin (Committee on Medical Aspects of Food Policy, 1979). Regardless of these pronouncements, wheat germ oil and a-tocopherol still find a ready market and frequent claims are made for the role of -tocopherol, not only as a vitamin but as a treatment for certain diseases. A further boost to the sales of wheat germ oil was given by reports 392 P. J. BARNES indicating that the oil increased human physical fitness (Cureton, 1972). This effect is claimed to be due to the presence of long-chain n-alkanols (particularly octacosanol) in the oil and not by vitamin (Levin etal., 1962). V COMPOSITION AND PROPERTIES A comprehensive review of the composition of wheat germ oil has been published recently (Barnes, 1982) and only a summary will be presented here. A Yield of Oil Typical values for the gross composition of commercial wheat germ are given in Table 19.1. The oil content is influenced by the extent of contamination with bran and endosperm, both of which have much lower oil contents than do the embryo axis, and by loss of oil during flaking. The most pure commercial germ usually yields about 100 g k g 1 (dry weight) of oil by extraction with hexane or light petroleum, whereas unpurified germ may yield only 60 g kg" 1. In contrast, the oil content of dissected embryo axis is approximately 150 g k g 1 . Table 19.1 Gross composition of partly-dried commercial wheat germt Component Protein Starch Sugars Fat Water Ash Crude fibre Others fBarnes, 1982. Physicochemical Properties Content (g k g - 1 ) 260 200 160 100 60 40 30 150 Wheat germ oil typically has a refractive index of from 1.4700 to 1.4800, specific gravity from 0.9000 to 0.9300, iodine value from 120 to 130 and saponification value from 184 to 185. The content of free fatty acids (FFA) 19 WHEAT GERM OIL 393 is usually less than 60 g k g 1 but may be higher if the germ has been stored without stabilization. Solvent-extracted oil has a lower FFA content than oil produced by pressure-expelling, but the FFA content of the finished oil will depend on whether refining has been carried out. The content of non-saponifiable matter is greater than found in most other edible oils and ranges from 15 to 78 g k g - 1 . C Fatty Acid Composition Wheat germ oil is rich in polyunsaturated fatty acids. The major fatty acid (FA) is 18 : 2 (linoleic acid), which accounts for about 60% of the total. Most of the saturated FA is represented by 16 : 0 (palmitic acid) and the content of 18 : 0 (stearic acid) is usually below 2%. The FA compositions shown in Table 19.2 were all determined in the same study and are tabulated in full to illustrate the range of variation in the commercial oils. The laboratory-extracted oils, derived from four diverse samples of commercial wheat germ, showed much less variation. It is possible that some of the variation in the commercial oils is caused by adulteration with other edible oils, as discussed below. This is particularly the case for an oil listed in Table 19.2 with undetectable 1 8 : 3 (linolenic acid) and a relatively high proportion of 18 : 0. The high proportion of polyunsaturated FA in wheat germ oil is an important factor in the "health food" market but the high content of 18 : 3 makes the oil susceptible to oxidative rancidity. The major triglycerides of wheat germ oil are l-palmito-2,3-dilinolein (29%), trilinolein (16%) and l-palmito-2-linoleo-3-olein (12%) (Tamakier al., 1971). Table 19.2 Fatty acid composition of wheat germ oilst Fatty acid composition (7o) Oil sample 16 : 0 18 : 0 18 : 1 18 : 2 18 : 3 Laboratory 16.5 0.5 15.5 58.1 9.4 extracted 17.4 0.9 12.3 58.0 11.4 17.5 0.6 12.3 58.7 10.9 17.5 0.5 13.8 59.3 8.8 Commercial 12.3 2.0 19.3 61.2 5.2 13.7 1.5 21.8 57.9 5.1 15.5 1.3 22.2 57.3 7.0 21.0 1.0 18.8 52.2 3.7 7.1 4.1 22.7 66.1 < 1.0 fBarnes and Taylor, 1980. 394 P. J. BARNES D Acyl Lipids Most of the published data for wheat germ acyl lipid composition concerns lipids extracted with polar solvents rather than commercial pressure-expelled or hexane-extracted oils. Five commercial oils consisted almost completely of non-polar lipids with the total polar lipid content only 0.2% to 1.8% of the oil (Barnes and Taylor, 1980); by comparison, oils extracted with hexane in soxhlet contained from 3.6 to 10.1% of polar lipids. In dissected wheat germ the polar lipids consist mainly of phospholipids with only very low concentrations of glycolipids (Hargin and Morrison, 1980) but there is no comparative information for wheat germ oil. Table 19.3 shows the percentage composition of non-polar acyl lipids in commercial and laboratory-extracted wheat germ oils. Triglycerides are the major component in all the oil samples, with variable proportions of FFA as a result of differences in the extent of hydrolytic rancidity, and possibly due to refining in the case of commercial oils. Table 19.3 Composition of non-polar acyl lipids in wheat germ oilt Composition (7o of total) Lipid class Laboratory-extracted oils Commercial oils Steryl esters 5.1-5.8 1.9-5.7 Triglycerides 63.9-88.5 76.3-94.9 Free fatty acids 0.6-22.0 0.7-8.1 Diglycerides 1.8-6.9 2.6-10.9 Monoglycerides 0.3-1.1 0.1-0.7 f Barnes and Taylor, 1980. Non-saponifiable Lipids The non-saponifiable fraction from wheat germ oil consists predominantly of 4-demethyl sterols, accompanied by much smaller quantities of methyl sterols, triterpenols, tocols, n-alkanols, carotenoids and hydrocarbons (Itoh et al., 1973; Lercker et al., 1977). Although quantitatively minor components of the non-saponifiable fraction, the tocols and n-alkanols are commercially the most important. 1 Tocols As explained in Chapter 3, the a-tocopherol (a-T) and ^-tocopherol (-) of the wheat grain are located in the germ; no other tocols are detectable in pure dissected wheat germ. Thus, the major tocols of wheat germ oil are a-T and -, but small amounts of -tocotrienol (a-T-3) and 0-tocotrienol 19 WHEAT GERM OIL 395 (--3) may also be found in the oil and are derived from bran and endosperm that contaminate the wheat germ (Table 19.4). The y- and 396 Table 19.4 Tocol composition of wheat germ oilst Composition of tocols (% of total) Total tocolsOil sample --acetate a-T - - ---3 --3 (gkg"1) Laboratory-extracted from: Commercially-purified germ: - 71 25 1 - - 3 3.2 69 25 2 - - 3 3.1 Unpurified germ: - 61 16 1 - 4 18 1.8 60 21 2 - 3 14 2.6 Commercial oils: 1 60 34 - - - 5 3.5 63 23 4 - 2 8 1.8 66 26 8 2.8 15 44 28 10 1 - 2 3.0 17 41 23 14 1 - 4 2.0 52 16 28 3 - 2.5 46 18 32 5 - - 2.8 99 - 1 4.1 fBarnes and Taylor, 1980; Taylor and Barnes, 1981. 397 Table 19.5 Composition of sterols, 4-methyl sterols and triterpenols in commercial wheat germ oilt Sterols % 4-Methyl sterols %Triterpenols %Sitosterol 60 Gramisterol 25 24-Methyl cycloartanol 33 Campesterol 19 Citrostadienol 30 Cycloartenol 25 A5 -avenasterol 7 Obtusifoliol 14 -amyrin 12 A7 -avenasterol 2 Cycloeucalenol 6 a-amyrin 7 A7 -stigmasterol 2 24-ethyl lophenol 5 Cyclobranol 2 Stigmasterol 4 Others 20 24-methylene-5cr-lanost-8-en-30-ol 7 Cholesterol trace Others 14 Brassicasterol - Others 5 fModified from Kornfeldt and Croon, 1981. ^Percentage of each fraction.Not detected. 398 P. J. BARNES octacosan-l-ol and triacontan-l-ol is claimed to have been separated from a wheat germ oil non-saponifiable fraction (Levin et al., 1962). Data provided by a manufacturer indicates that unrefined, unadulterated, solvent-extracted wheat germ oil contains 8 0 m g k g _ 1 of octacosan-l-ol (Viobin Corporation Inc., Monticello, Illinois, USA, 1982), and that this concentration is more than eight times greater than found in a sample of partly-refined oil and a sample of pressure-expelled oil. With the current interest in the physiological effects of octacosanol, it can be expected that future studies of wheat germ oil composition will include analysis of the alkanols. 3 Sterols, 4-Methyl Sterols and Triterpenols Sitosterol is the major sterol in wheat germ oil ( 60 -70% of total sterols) accompanied by campesterol (20 -30% of total). The compositions of the sterol, 4-methyl sterol and triterpenol fractions are shown in Table 19.5. These results were obtained using a capillary column gas chromatograph coupled to a mass spectrometer (Kornfeldt and Croon, 1981) and confirm the results of earlier studies (Itoh et al., 1973a, 1973b; Lercker et al., 1977). The percentage compositions of these fractions are similar to the corresponding fractions of many of the other seed oils examined, but wheat germ and maize germ oil characteristically have higher proportions of gramisterol. It is possible that the two oils could be distinguished from one another by means of the much higher proportion of 0-amyrin in the wheat germ oil (Kornfeldt and Croon, 1981). 4 Hydrocarbons There have been two detailed analyses of the hydrocarbons in wheat germ oil. In one study, 50% of the hydrocarbon fraction consisted of squalene and the remainder comprised alkanes with n-C29 as the major component (Kuksis, 1964). In the other study, squalene was not detected and the carbon number distribution of the alkanes was more widely spread with a maximum at n-C24 (Lercker et al., 1977). The hydrocarbons are minor components of wheat germ oil and the composition could be considerably influenced by contamination with trace amounts of mineral oil during processing of the germ and germ oil. 5 Pigments In 1933, Bowden and Moore crystallized the pigment responsible for the colour of wheat germ oil and obtained an absorption spectrum typical of a xanthophyll. The pigment was present in the oil at a concentration of 60 mg kg" 1. It was later shown that the xanthophylls lutein and cryptoxanthin 19 WHEAT GERM OIL 399 were present in wheat germ oil (Drummond et al., 1935). No further information on the carotenoid composition of the oil is available, but it is known that the carotenoids of wheat germ consist of 71 to 88% xanthophylls, 2 to 17% xanthophyll esters and 10 to 12% carotene (Chen and Geddes, 1945). In a recent investigation of the total carotenoid content of wheat germ oils by measurement of absorbance value at 440 nm, oils extracted in the laboratory all had much higher carotenoid content than did the samples of commercial oils (Barnes and Taylor, 1980). The low carotenoid contents of the commercial oils may result from oxidation during pressure-expelling or from refining treatments carried out on the oils. In addition to carotenoids, flavanoid glycosides also contribute to the yellow colour of wheat germ (King, 1962) but there is no published evidence for the presence of these compounds in wheat germ oil. ACKNOWLEDGEMENT I wish to thank the President of the Viobin Corporation, Monticello, Illinois, U.S. for providing information on wheat germ oil. REFERENCES Abe, K., Yuguchi, Y. and Katsui, G. (1975)./ . Nutr. Sci. Vitaminol. 21, 183-188. Barnes, P. J. (1982). Fette. Seifen. Anst. 84, 256-269. Barnes, P. J. and Taylor, P. W. (1980). J. Sci. Food Agric. 31, 997-1006. 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