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
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-
" 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
Protein Starch Sugars Fat Water Ash Crude fibre Others
Content (g k g - 1 )
260 200 160 100 60 40 30
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