Although fats and oils have an unhealthy connation to the public, they are necessary for human growth and health. In the past, edible oil quality was mainly defined by organoleptic parameters such as taste, odor, and color. Today, more emphasis is put on the retention of nutritional compounds. Conventional refining processes may reduce the nutritive value of edible oils. However, nutritionally beneficial compounds naturally present in the edible oils can be enriched by developing new refining techniques or by modifying the operational parameters of conventional refining methods. This article will discuss the benefits of biologically active compounds in fats and oils and advances in the processing of fats and oils to improve their nutritional value.
Lipids and Health
Edible oils and fats are essential nutrients. However, growing public health and fitness awareness and advances in nutrition research have raised the level of debate on the pros and cons of various dietary fats and oils. Examples of health issues discussed in public during the past several decades include the role of saturated and trans fatty acids in cardiovascular diseases; the negative campaign against tropical oils in the United States; and attacks on oil processing techniques as being “chemical” and therefore “unhealthy.” In 1994, these attacks were taken to an extreme during public demonstrations in Sweden and the United Kingdom, when the hydrogenation process was declared as “only a shade less dangerous than the hydrogen bomb” (Wesdorp, 1996).
Dietary guidelines were originally introduced to deal with malnutrition, but today countries develop guidelines in an effort to prevent chronic diseases such as heart disease, obesity, and diabetes. They always include recommendations on fat consumption. Krawczyk (2001) provides an excellent review of dietary guidelines around the world.
The United States dietary guidelines have been revised every five years to reflect the findings of health research. The most recent version (USDA/HHS, 2000) recommends limiting the total fat intake to 30% of total calories and 10% saturated fat, which is the same as the previous version. However, the wording of the new version has been slightly revised as follows: “Choose a diet that is low in saturated fat and cholesterol and moderate in total fat,” whereas the previous version suggested a diet “low in fat, saturated fat and cholesterol” [italics added]. This change reflects the emphasis on reducing “saturated fat” intake more than “total fat” intake in the human diet.
Fats and oils are also related to obesity, certain cancers, and high levels of cholesterol in blood with consequent cardiovascular disease. The “evils” of fat consumption have been continuously reinforced by physicians, nutritionists, media, health organizations, and consumer advocacy groups during the past several decades. As a result of all this negative campaign against dietary fat, the anti-fat movement evolved to a point that, as David Kritchevsky of the Wistar Institute in Philadelphia put it, “In America, we no longer fear God or the communists, but we fear fat” (Taubes, 2001).
The science behind the effects of dietary fat on human health is so complex that there are no simple and straightforward answers to the questions in the field today. A captivating article by Taubes (2001) elegantly portrays the history and controversy among government, scientific, private, and nonprofit research institutions regarding the recommendations on dietary fat consumption. The majority of the harmful effects of fats and oils are related to their excessive consumption. All the press about the negative effects of fat and oil consumption has shifted today’s focus toward elimination of fat from the human diet. This is unfortunate, because when they are consumed as a part of a balanced diet, fats and oils have numerous health benefits.
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Lipids and lipid-related compounds play essential roles in disease prevention and growth. Fats and oils are the most concentrated source of food energy, supplying about 9 kcal/g of energy, compared to about 4 kcal/g from protein and carbohydrates. Fats serve as thermal insulation for the body and protect internal organs against external shocks. Lipids affect the texture, flavor, and palatability of food products. Fats and oils are the major sources of the fat-soluble vitamins A, D, E, and K, and ingestion of fat and oils improves the absorption of these vitamins. Fats are vital in the human diet for providing the essential fatty acids linoleic and linolenic acids.
Lipids also have several functions in the body. They are vital and major components of all cell membranes. Phospholipids, glycolipids, and cholesterol play a structural role in cell membranes. Lipids have important functions in biochemical regulatory systems as well. They can serve as precursors for beneficial biologically active compounds such as prostaglandins, steroid hormones, and bile acids.
Lipid-Related Bioactive Compounds
The composition of edible oils and fats is very complex. Numerous components are present, such as mono-, di-, and triglycerides (TGs), free fatty acids (FFAs), phospholipids, pigmented compounds, and waxes, as well as several nutritionally beneficial bioactive compounds.
Long-chain polyunsaturated fatty acids (PUFAs) are fatty acids with more than one double bond. Several PUFAs are recognized as “essential fatty acids” in the human diet for preventing nutrition-related illnesses. The human body is not capable of synthesizing linoleic acid and linolenic acid. These fatty acids must be provided in the diet; hence they are classified as essential fatty acids (WHO/FAO, 1977).
Linoleic acid is an 18-carbon molecule that contains double bonds in the cis-9 and cis-12 configurations. Conjugated linoleic acid (CLA) refers to a group of geometrical and positional isomers of linoleic acid that possess conjugated double bonds in the cis or trans configurations at positions 9 and 11 or at positions 10 and 12. Since 1978, when Michael Pariza and his colleagues at the University of Wisconsin first recognized the anticarcinogenic properties of CLA isolated from grilled ground beef, CLA has been reported to have diverse biological effects such as inhibiting tumor growth, reducing atherosclerotic risk, and reducing body fat (Ha et al., 1989).
It is believed that cis-9, trans-11 and trans-10, cis-12 isomers of CLA are the most active forms. Milkfat, natural and processed cheeses, meat products, and plant oils are dietary sources of CLA. Animal sources are richer in CLA than plant sources. In general, foods from ruminants contain more CLA than those from nonruminants; i.e., beef, lamb, and veal contain 3–6 mg of CLA/g of fat, while pork, chicken, and turkey contain less than 1 mg of CLA/g of fat. CLA is commercially available in oil, soft gel, capsule, and powder forms as dietary supplements in the U.S.
Gamma-linolenic acid (GLA) has been shown during the past decade to be important in disease prevention. It has been claimed that GLA is effective in reducing inflammation and treating diabetic neuropathy, atopic eczema, and certain cancers (McDonald and Fitzpatrick, 1998). Primary natural sources of GLA are evening primrose and borage oils. Hempseed oil is a new addition as a GLA-rich source. There is scientific evidence that evening primrose is effective in the treatment of age-related diseases, alcoholism, hyperactivity, cardiovascular disease, and gastrointestinal, gynecological, neurological, and immunological disorders (Broadhurst and Winther, 2000).
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Alpha-linolenic acid (ALA) is an isomer of linolenic acid. Health benefits of flaxseed oil consumption are attributed primarily to its high ALA content. ALA has been reported to inhibit the production of eicosanoids, alter the production of several prostanoids, reduce blood pressure in hypertensives, and lower TGs and cholesterol (Oomah and Mazza, 1999). According to Johnston (1995), dietary ALA retards tumor growth. It has been suggested that ALA in the diet is essential for optimal neurological development in humans during fetal and postnatal growth periods (Cunnane, 1995). Flaxseed, rapeseed, and walnuts are rich in ALA.
Omega-3 fatty acids have been shown to possess various health benefits, such as preventing coronary heart disease, hypertension, type 2 diabetes, renal disease, rheumatoid arthritis, ulcerative colitis, and chronic obstructive pulmonary disease and aiding brain development and growth (Simopoulos,1999). Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are two w-3 PUFAs that have been investigated for their role in human health. However, large gaps exist in the present understanding of specific issues related to ω-3/ω-6 PUFA balancing, quantitative dose-response, and mechanism of dietary ω-3 PUFA involvement in growth, development, and disease resistance.
The main sources of ω-3 PUFAs are fish and microalgae. BASF, Merck, and Hoffmann-La Roche market fish oil–based products, often packaged with proprietary microencapsulation technology. OmegaTech and Martek market DHA-rich oils produced by a fermentation process. In May 2001, the Food and Drug Administration issued a favorable review of Martek’s generally recognized as safe (GRAS) notification regarding the use of Martek’s DHA oil (DHASCO ®) in infant formula (Anonymous, 2001). This favorable review opens the door for U.S. infant formula manufacturers to add microalgae-derived DHA to domestic infant formula.
Tocopherols and tocotrienols are two groups of closely related fat-soluble compounds, for which the term “vitamin E” is used as a generic name. Although Evans and Bishop (1922) discovered vitamin E as an essential factor for reproduction in 1922, the Food and Nutrition Board did not recognize the essential nature of vitamin E until 1968. Tocopherols and tocotrienols have a very similar chemical structure. They both consist of a chroman backbone (two rings: one phenolic and other heterocyclic) and an isoprenoid C-16 side chain with 3 chiral centers.
Tocopherols have a saturated side chain. Depending on the number and position of the methyl groups on the side chain, these compounds are designated as α, β, γ, and δ tocopherols. Tocopherols display antioxidant activities in vivo and in vitro, α-tocopherol having the highest vitamin E activity (Azzi and Stocker, 2000.) There is also growing research interest in the role of these compounds beyond their antioxidative functions. Although their actions are not yet clearly understood, there is strong evidence that tocopherols play a role in the prevention of some chronic diseases such as heart disease and some cancers (Traber and Packer, 1995). Tocopherols are present in oilseeds, leaves, and other green parts of higher plants.
The main structural difference between tocopherols and tocotrienols is in the saturation of the side chain. Tocotrienols have a triply unsaturated side chain, whereas tocopherols have a saturated side chain. Recently it has been suggested that tocotrienols are better antioxidants than tocopherols. Reported hypocholesterolemic, antithrombotic, and antitumor properties of tocotrienols suggest that these compounds may serve as effective dietary agents in the prevention and/or treatment of many diseases (Theriault et al., 1999). In contrast to the tocopherols, tocotrienols are found not in the green parts of the plant but rather in the germ and bran fraction of certain seeds and cereals.
The recommended dietary allowance for vitamin E is 30 mg/day, equivalent to about 2 tablespoons of tocopherol-rich oil such as soybean or palm oil. Wheat germ oil is another vitamin E–rich dietary source.
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Plant sterols, also known as phytosterols, are minor components of all vegetable oils, constituting major portions of the unsaponifiable fraction of the oil. Phytosterols differ in chemical structure from cholesterol by an ethyl or methyl group in their side chain. They may be present in the oil in both free and esterified forms. They may be esterified with glucosides, ferulic acid, or fatty acids. Ferulic acid–esterified sterol is commonly known as oryzanol and has been reported to have diverse health benefits, including antioxidant and hypolipidemic effects and stimulation of growth and hypothalamus activity (Nicolosi et al., 1992). Rice bran oil (RBO) is an excellent source of nutritionally beneficial compounds, such as phytosterols, tocopherols, and tocotrienols. Diverse health benefits have been attributed to its high oryzanol content.
Hypocholesteremic properties of phytosterols have been studied extensively (Peterson, 1951; Miettinen and Gylling, 1997; Jones and Ntanios, 1998). Phytosterol-enriched margarines were first introduced in Finland in 1995. Later, two cholesterol-lowering functional foods containing plant stanol and sterol esters were introduced in the U.S.: Benecol™ from McNeil Consumer Healthcare and Take Control from Lipton (Hollingsworth, 2001; Hicks and Moreau, 2001). In addition, Forbes Medi-Tech Inc. in Canada produces a phytosterol product named Phytrol™ and has signed an agreement with Novartis Consumer Health to market Phytrol as Reducol™ (Challener, 2000). Novartis will reportedly sell Phytrol to third parties, and Altus Foods, a joint venture between Novartis and Quaker Oats, will offer functional foods containing the phytosterols. Cargill and Archer Daniels Midland have also announced that they will begin producing soybean-derived sterols to be used as food ingredients.
Phytosterols have received FDA clearance as GRAS substances. In the U.S., foods containing plant sterol esters can carry health claims. The claim must specify that the daily dietary intake of plant sterol or stanol esters should be consumed in two servings eaten at different times of the day as a part of a diet low in saturated fat and cholesterol. To qualify for the claim, a food must contain at least 0.65 g of plant sterol or 1.7 g of stanol esters per serving (Chapman, 2000).
The National Institutes of Health, through its National Cholesterol Education Program, has issued guidelines regarding treatment of high blood cholesterol (NIH, 2001; Anonymous, 2000). The guidelines recommend plant sterols and stanols as “therapeutic dietary options to enhance lowering of LDL (low-density lipoprotein) cholesterol”; 2 g of sterols or stanols per day, along with 10– 25 g of soluble fiber, was recommended for significant cholesterol reduction.
Squalene and its hydrogenated form squalane are naturally occurring terpenoid hydrocarbons in fish liver oil. They are relatively high-value compounds with many applications in the pharmaceutical and cosmetics industries. They are involved in cholesterol biosynthesis. Antitumor, antibacterial, and anticarcinoma properties of squalene have also been reported. However, the mechanism of squalene involvement in disease prevention remains unknown (Nakamura et al., 1997).
Squalene is used as a health food called shinkaizame-ekisu (deep-sea shark liver oil) in Japan and also as a folk medicine for chronic skin and liver diseases in South East Asia (Nakamura et al., 1997). The average daily intake of squalene has been estimated to be about 30 mg/day/person in the U.S. The daily intake of squalene can be as high as 200 mg/day if olive oil is the sole source of dietary fat (Liu et al., 1976). Although the primary source of natural squalene has been marine animals, international concerns regarding the protection of marine animals have heightened interest in vegetable sources. Olive oil and amaranth seed oil are two squalene-rich plant sources.
Phospholipids (PLs) usually contain only two fatty acid groups per molecule, as opposed to triglycerides, in which all three OH groups of glycerol are esterified to fatty acids. The third OH group on the glycerol backbone is linked to aliphatic compounds containing phosphoric acid and nitrogen residues. PLs are important natural emulsifiers used in foods, feed, pharmaceuticals, and industrial products. They are essential constituents of all living cells and occur in abundance in egg yolk (8–10%), butter (0.5–1.2%), and vegetable oils (0.5–3.7%). Their main commercial source is soybean lecithin, which is a mixture of PLs and TGs.
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Phosphatidylcholine (PC) has been shown to possess therapeutic effects on various metabolic disorders, such as lowering cholesterol levels, treating neurological disorders, and improving learning and memory in humans and animals (Hanin, 1979).
Medium-chain triglycerides (MCTs) contain fatty acids of primarily 8- and 10-carbon chain length. They have been used in the treatment of fat malabsorption–related diseases and as a significant source of energy for preterm infants (Willis and Marangoni, 1999). Natural sources of medium-chain fatty acids (MCFAs) include coconut and palm kernel oils.
Commercial fats and oils from both plant and animal sources are highly processed commodities. The goal of commercial oil/fat processing is to maximize the oil yield while maintaining high quality and producing highly stable oils by eliminating undesirable compounds.
Fats from animal sources are rendered by heat or dry steam to separate oil and water found in the tissue. This process destroys any endogenous enzymes and most vitamins in the tissue. Processing is more complex for oils from plant sources. The first step is pretreatment of the oilseeds, which involves seed cleaning, drying, dehulling, grinding, and flaking. Then, for seeds with high oil content such as canola, the traditional treatment involves mechanical extraction followed by extraction with an organic solvent. Hexane is most commonly used for edible oil extraction. Low-oil seeds such as soybeans are directly extracted with hexane. The use of other solvents such as methanol, isopropyl alcohol, and supercritical fluids and aqueous extraction using enzymes as a processing aid has also been explored.
The next step is degumming and deacidification. PLs are removed from the crude oil by degumming because they precipitate from the oil during storage and are therefore not desired in the refined product. FFAs are removed from the crude oil by alkali refining because they have adverse effects on the shelf life of the oil. Degummed and deacidified oil is then treated with natural or acid-activated bleaching clay to reduce the color pigments.
The last step in traditional edible oil refining is deodorization using steam to remove any remaining volatile materials.
Some of these processing steps may reduce the nutritive value of the oil or fat by removing or chemically altering oil components. Gutfinger and Letan (1974) showed that a significant portion of the phytosterols present in the crude soybean oil is lost during deacidification. The refined soybean oil contained 25–32% less phytosterols than the crude oil. Although crude RBO is quite rich in oryzanol (~1.5%), no oryzanol was detected in commercially refined regular RBO purchased at a local health store (Dunford and King, 2000a). Phytosterols removed during the refining process end up in the soap stock and deodorizer distillate (DD), by-products of the edible oil processing industry.
Phytosterol-enriched products are made with phytosterols isolated from DD. The conventional sterol isolation process involves a series of energy-intensive and complex unit operations such as liquid/liquid extraction, trans- and interesterification, molecular distillation, and crystallization. DD contains free phytosterols, and the solubility of free sterols in food matrixes is quite low. Therefore, after isolation from DD, free sterols are converted to their ester forms and then added back to the oil for phytosterol enrichment.
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Dunford and King (2000a, 2001) developed a phytosterol-enrichment process utilizing supercritical fluid fractionation (SFF) technology. Enrichment of the phytosterol esters was achieved during oil processing rather than by isolation from the by-products and readdition to the oil. Using a high-pressure packed fractionation column, the researchers were able to obtain RBO fractions with a similar phytosterol ester content to that in commercially available phytosterol-enriched margarines. Commercial phytosterol-enriched margarines contain mainly fatty acid esters of phytosterols. However, the SFF product contained both fatty acid esters of phytosterols and oryzanol. Higher oryzanol content of the SFF-processed oil is an additional feature of the SFF process. Hexane-extracted RBO was used for this study; however, oil extracted with supercritical carbon dioxide (SC-CO2) can also be used as a starting material.
When corn fiber, which is also rich in phytosterol esters, especially oryzanol, is extracted with SC-CO2 at high pressures, the extract contains phytosterol content similar to that of hexane-extracted oil (Dunford and King, 2000b). The researchers also showed that it is possible to obtain phytosterol-enriched TG fractions (>15% phytosterol content) from SC-CO2–extracted corn fiber oil. Utilization of SC-CO2–extracted oil as a starting material further simplifies the oil refining process, since the extracted oil contains very small amounts of PLs and degumming is therefore not required.
Gutfinger and Letan (1974) studied the effect of conventional edible oil processing on the tocopherols naturally present in the oils and found that the tocopherol content of the oil may be significantly reduced during deodorization. About 32% of the tocopherols present in the oil was lost during the refining because of their volatility. Processing lowered the total tocopherol content but did not affect the tocopherol composition significantly—the relative composition of α, β, ɣ, and δ isomers stayed constant throughout the whole refining process. However, it has been also reported that the relative retention of tocopherol isomers varies significantly with different processing parameters. For example, retention of α-tocopherol was 10–20% higher than the retention for δ-tocopherol during steam processing (Jung et al., 1989).
Tocopherols lost during the refining process are concentrated in the DD, which is an important feedstock for industrial production of natural vitamin E. In Europe, physical refining is preferred for the retention of tocopherols, whereas alkali refining is used in the U.S. Dual Temp deodorizer and short path distillation are two relatively new techniques designed for maximum tocopherol retention in the refined oil (De Greyt et al., 1999; Cmolik and Pokorny, 2000.
Biotechnology presents new alternatives to traditional lipid manufacturing methods. Three rapidly expanding areas of lipid biotechnology are genetic engineering of oilseeds and oil-bearing plants for improved agronomic properties and altered fatty acid and lipid composition; use of microorganisms for the production of oils and fats from nonlipid-and lipid-containing carbon sources; and biotransformation of fats and oils into fat-based value-added products via whole microbial cells or pure enzymes.
CLA production is an excellent example of the challenges facing food ingredient companies when making sourcing decisions. For example, CLA can be manufactured through organic synthesis, microbial fermentation, enzymatic isomerization, or genetic engineering/bioengineering. Traditional organic synthesis is highly capital intensive and results in an isomeric mixture of CLAs. The production of an isomeric mix may lead to scrutiny by government agencies during the regulatory approval process. Bioengineering can be used to make CLA the predominant fatty acid fraction produced in a plant’s oil. This method of CLA production, once established, would be the least-capital-intensive route. Disadvantages of bioengineered products would be the length of time required for discovery of desired genetic traits and the high risk of rejection by consumers who are wary of genetic modifications.
Pariza and Yang (1999) reported that microorganisms containing linoleic acid isomerase are capable of converting linoleic acid to CLA. Hence, CLA production by fermentation and utilizing microorganisms containing linolate isomerase may be a viable alternative to organic synthesis. It is also possible that pure enzymes containing isomerase or conjugase activity can be used to manufacture CLA. A great advantage of biocatalysis by whole microorganisms or purified enzymes is that there is high specificity for the isomers that are produced.
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Another application of biotechnology to lipid-based functional foods and nutraceuticals is the production of MCTs. Natural MCFA-rich sources contain a low concentration of essential fatty acids and PUFAs. Modified lipids containing increased concentration of MCFAs, PUFAs, and other essential fatty acids are commercially produced by chemical hydrolysis of the natural oils and random re- or interesterification. Chemical interesterification catalyzed by alkali metals or alkali metal alkylates requires high temperatures and a long processing time. Furthermore, chemical reactions often result in randomized esterification and side reactions.
Physical blending of MCTs and long-chain TGs is another way of enhancing MCFA content of oils and fats. However, this procedure does not improve the absorption characteristics of the final product upon human consumption, since individual TGs retain their original absorption rates. The use of enzyme biotechnology in producing designer TGs with desired fatty acids occupying specific positions of the glycerol has been extensively investigated (Akoh, 1995). Lipases catalyze esterification and transesterification under mild reaction conditions, with minimal side products. The MCT produced via an enzymatic approach would satisfy consumer demand for “all-natural” lipids for medical and nutritional needs.
Innovative Products and Processes Will Improve Nutraceutical Acceptance
The production of high-purity products such as nutraceuticals requires special processing techniques which eliminate solvent residues in the end product and do not cause degradation of bioactive compounds during processing. Supercritical fluid technology is an ideal technique for this purpose and presents tremendous opportunities for the production of nutraceuticals and value-added functional lipid-based food products.
Food and food ingredient companies predict that bioengineered foods and food ingredients will offer numerous opportunities in the next century. Certainly, consumer acceptance and the economics of production will be the determining factors for the success of biotechnology in the food area.
We believe that development of innovative lipid-based functional foods and nutraceuticals will improve consumer perception, economic value, and market share of these products.
by Nurhan Turgut Dunford
The author is Assistant Professor, Dept. of Plant and Soil Sciences and Oklahoma Food and Agricultural Products Research and Technology Center, Oklahoma State University, Stillwater, OK 74078.
Based on a paper presented at the Annual Meeting of the Institute of Food Technologists, New Orleans, La., June 23–27, 2001.
Edited by Neil H. Mermelstein, Editor
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