Fats and oils are recognized as essential nutrients in both human and animal diets. They provide the most concentrated source of energy of any foodstuff, supply essential fatty acids (which are precursors for important hormones, the prostaglandins), contribute greatly to the feeling of satiety after eating, are carriers for fat soluble vitamins, and serve to make foods more palatable. Fats and oils are present in varying amounts in many foods. The principal sources of fat in the diet are meats, dairy products, poultry, fish, nuts, and vegetable fats and oils. Most vegetables and fruits consumed as such contain only small amounts of fat. Recent data from the U.S. Department of Agriculture (1) suggests that actual fat consumption currently is about 33% of total calories. A knowledge of the chemical composition of fats and oils and the sources from which they are obtained is essential in understanding nutrition and biochemistry.
Experimental work in the 1930s in animals and humans demonstrated that certain long chain polyunsaturated fatty acids, linoleic and arachidonic, are essential for growth and good skin and hair quality. Now linoleic and linolenic acids are termed "essential" because they cannot be synthesized by the body and must be supplied in the diet. Arachidonic acid, however, can be synthesized by the body from dietary linoleic acid. Arachidonic acid is considered an essential fatty acid because it is an essential component of membranes and a precursor of a group of hormone-like compounds called eicosanoids including prostaglandins, thromboxanes, and prostacyclins which are important in the regulation of widely diverse physiological processes. Linolenic acid is also a precursor of a special group of prostaglandins. The dietary fatty acids that can function as essential fatty acids must have a particular chemical structure, namely, double bonds in the cis configuration and in specific positions (carbons 9 and 12 or 9, 12, and 15 from the carboxyl carbon atom or carbons 6 and 9 or 3, 6 and 9 from the methyl end of the molecule) on the carbon chain.
The requirement for these essential fatty acids has been demonstrated clearly in infants. While the minimum requirement has not been determined for adults, there is no doubt that they are essential nutrients. The current American diet provides at least the minimum essential fatty acid requirement. According to the Food and Nutrition Boards Recommended Dietary Allowances (2), the amount of dietary linoleic acid necessary to prevent essential fatty acid deficiency in several animal species and also in humans is 1 to 2% of dietary calories. However, for much of the general population, 3% of calories as linoleic acid is considered to be a more satisfactory minimum intake. In the case of linolenic acid, the requirement for humans has been estimated to be 0.5% of calories.
The Committee on Diet and Health of the Food and Nutrition Board (3) has recommended that the average population intake of polyunsaturated fatty acids (primarily linoleic acid) remain at the current level of about 7% of calories and that individual intakes not exceed 10% of calories because of lack of information about the long-term consequences of a higher intake. For many reasons, especially because essential fatty acid deficiency has been observed exclusively in patients with medical problems affecting fat intake or absorption, the Food and Nutrition Board has not established an RDA for omega-3 or omega-6 polyunsaturated fatty acids.
Fats in the diet are often referred to as "visible" or "invisible." Visible fats are those added to the diet in foods such as salad dressings, spreads and processed foods, whereas invisible fats are those that are naturally occurring in foods such as meats and dairy products.
According to the Economic Research Service of USDA, between 1970 and 1997, Americans increased their consumption of total visible fat from 52.6 to 65.6 pounds per person per year (see Table X). On the other hand, if these data are expressed as a percentage of total calories consumed, fat intake from visible and invisible sources would appear to have decreased from about 43% to about 33% of calories. This apparent paradox of increasing amounts of fat consumed per day while decreasing the percentage of fat calories is explained by the fact that while daily fat consumption has been increasing recently, total caloric intake has been increasing at a greater rate (4). This increased level of calorie intake will reduce the calculated percentage of calories from fat even though actual fat consumption has not gone down. The dietary trend of increased caloric consumption is thought to be the result of increased carbohydrate intake, larger food portions being consumed, more eating occasions, and increased soft drink and alcoholic beverage intake (4).
Fat intake is generally measured by two methods: (1) surveys of individuals recalling the amounts of foods consumed over a specific time period and (2) "consumption" estimates from food disappearance data as calculated from available sources. The latter method may overstate actual consumption estimates because food disappearance data do not account for foods wasted or discarded and lost to spoilage, trimming, or cooking. It has been estimated that wastage of deep frying fats used in the food service sector may be as high as 50% (5); therefore, food component estimates from food "disappearance" data, particularly fat, may be overestimated. The accuracy of dietary recall data is also difficult to ensure due to errors of memory in recalling foods eaten previously.
Dietary guidelines were first issued by the American Heart Association in the early 1970s in an attempt to educate Americans as to the importance of a healthful diet. The U.S. Department of Agriculture in conjunction with the U.S. Department of Health and Human Services later developed Dietary Guidelines for Americans in 1980. In 1990 these guidelines first included recommended limitations for fat consumption which remained unchanged in a 1995 update (6) of the guidelines. The guidelines for fat call for a total fat intake of no more than 30% of calories with a saturated fatty acid intake of no more than 10% of calories. Although these dietary guidelines for fat have been established for over 20 years, the relatively small changes in fat intake during this time period reflect the difficulty on a national scale in achieving dietary goals such as reducing fat intake.
Food allergies are caused by the protein components of food. Edible oils in the U.S. undergo extensive processing (sometimes referred to as "fully refined", discussed in section VII Processing) which removes virtually all protein from the oil. Refined edible oils therefore do not cause allergic reactions because they do not contain allergenic protein. Food products containing refined edible oils as ingredients are also non-allergenic unless the food products contain other sources of protein.
Some edible oils may be extracted and processed by procedures that do not remove all protein present. While the vast majority of oils found in the US are refined by processes which remove virtually all protein, mechanical or "cold press" extraction is occasionally used, which may not remove all protein. These cold pressed oils are rarely used domestically and are usually found only in health food or gourmet food stores. Studies using cold pressed soybean oil have shown it to be safe; however, insufficient testing has been done to ensure that all cold pressed oils can be safely consumed by sensitive individuals.
Edible oils have been blamed for causing allergic reactions in people, but there are conflicting views and inadequate scientific evidence regarding their allergenicity. Many reports alleging edible oil allergenicity have been testimonial in nature. Of those reports that have been scientifically recorded, most lack evidence that edible oils were indeed the causative agent or were even ingested. For example, many investigators did not perform tests to confirm an allergic response from the oil in question nor were analyses conducted to determine if protein was present in the oil. Also many reports do not indicate if the oils were cold pressed or not. There is also a lack of scientific data to determine the levels of proteins needed to cause an allergic reaction; therefore such tolerance levels in humans have not been established. Furthermore, the sensitivities of food allergic individuals may vary widely, and not all allergenic foods have the same tolerance level.
While some consumers are convinced they are allergic to edible oils, there are usually alternate explanations for these reactions. For example, foods containing peanuts, a common allergenic food ingredient, may be cooked in peanut oil. An allergic reaction experienced as a result of eating this food may be mistakenly blamed on the oil. Also foods containing inherent allergens may be cooked in edible oils resulting in traces of the allergenic protein being left behind in the oil. Restaurants and food service facilities should therefore exercise caution in cooking techniques and be able to readily identify not only the oils used but also a complete list of all foods cooked in the oil.
The vast preponderance of edible oils consumed in the US are highly refined and processed to the extent that allergenic proteins are not present in detectable amounts. The majority of well-designed and performed scientific studies indicate that refined oils are safe for the food-allergic population to consume (53).
Biotechnology has been defined broadly as the commercial application of biological processes. It includes both hybridization and genetic modification of plants and animals. The goal of biotechnology is to develop new or modified plants or animals with desirable characteristics. The earliest applications of this technology have been in the pharmaceutical, cosmetic and agricultural sectors. Agricultural applications to food crops have resulted in improved "input" agronomic traits, which affect how the plants grow. Such traits include higher production yields, altered maturation periods, and resistance to disease, insects, stressful weather conditions, and herbicides. Currently researchers and seed developers are placing more emphasis on improving "output" quality traits which affect what the plant produces. An example of this application is the custom designing of nutrient profiles of food crops for improved nutrition and reduced allergenic properties.
The more notable biotechnology applications within the oilseed industry include herbicide tolerant soybeans and canola, high and midoleic sunflower, low linolenic/low saturate soybeans, high linoleic flaxseed oil, low linolenic canola, high laurate canola, high oleic canola, and high stearate canola. Exciting opportunities for edible oil crop nutrient content and functionality improvement include reduction in saturated fatty acid content, improved oxidative stability resulting in a reduced need for hydrogenation, reduced calories or bioavailability, creation of specific fatty acid profiles for particular food applications, and creative "functional" foods for the population at large or for medical purposes. Other applications may include increased oil yield, improved extraction of oil from oilseeds through enzyme technology, industrial production of fatty acids, and improved processing methods.
Genetic engineering is a specific application of biotechnology. This technique is also called recombinant DNA technology, gene splicing, or genetic modification, and involves removing, modifying, or adding genes to a living organism. New plant varieties that result from genetic engineering are referred to as transgenic plants. The most recognized examples in the U.S. are herbicide resistant soybeans, corn which is resistant to the European corn borer, and cotton which is resistant to the bollworm. The acceptance and utilization of these and other transgenic food crops in the U.S. have been very rapid. For example, transgenic herbicide resistant soybeans after being first introduced commercially in 1996 on 1 million acres were planted on about 25 million acres in 1998. Genetically modified insect resistant corn was also commercially introduced in 1996, and it occupied about 16 million acres in 1998. Transgenic soybeans and corn are expected to be about 50% of the U.S. total planted acreage for these crops in 1999.
Global plantings of transgenic crops are also increasing at very rapid rates. While almost 7 million acres were planted internationally to transgenic crops in 1996, about 31.5 million acres were planted in 1997 (54). All indications are that worldwide acreage will continue to expand at a rapid pace for the next several years. The most popular crops as a percent of total global acreage planted to transgenic crops are the following: soybeans (40%), corn (25%), tobacco (13%), cotton (11%), and canola (10%). The United States is the leader in agricultural applications of genetically engineered crops representing 64% of the total global acreage devoted to transgenic crops in 1997, followed by China with 14%, Argentina with 11%, Canada with 10% and Australia and Mexico with about 1% each (54).
In the U.S., the Food and Drug Administration has principal regulatory responsibility for approving the introduction of foods and food additives from transgenic plants into the marketplace. The agency has maintained a biotechnology policy since 1992, which states that foods derived from new genetically engineered plant varieties will be regulated essentially the same as foods created by conventional means. Labeling of such foods or food additives is not required unless the nutrient composition is significantly altered, allergenic proteins have been introduced into the new food, or unique issues have been posed which should be communicated to consumers.
While U.S. consumers appear to have accepted biotechnology and recognize its potential benefits (e.g., foods, drugs), Europeans have been less willing to embrace biotechnology due to concerns regarding the safety of genetically modified foods. The European Union requires foods containing genetically altered components to be labeled as such and has been very slow in approving new genetically modified plant varieties as imports. This position has caused much anxiety between Europe and the U.S. from an agricultural trade standpoint. The prospects for resolution of these differences between Europe and the U.S. appear to be improving.
As new varieties of oilseeds are developed to incorporate specific fatty acid components, historical fatty acid profiles for source oil identification will become less useful. A challenge for food manufacturers will be how to identify these food components through food labeling.
The age of biotechnology is here with a vast array of improved plant varieties already commercially available. The future looks bright particularly for the emergence of new oilseed varieties that will have improved agronomic characteristics, nutrient profiles, and functionality in foods and food ingredients as well as in industrial products. It is therefore important for consumers to understand the many benefits of biotechnology and that the application of genetic engineering technology to foods will render them safe, more functional, and nutritious.
Americans have been advised during the past two decades by many health organizations, including the Office of the Surgeon General, American Heart Association, U.S. Department of Agriculture, and Department of Health and Human Services, to reduce total dietary fat intake to less than 30% of calories and saturated fat intake to less than 10% of calories. High intake of total and saturated fat has been associated with increased risk of obesity and coronary heart disease.
Healthy People 2000, a program of the U.S. Public Health Service to promote health and prevention of disease, recommended food manufacturers double the availability of lower fat food products from 1990 to the year 2000. That goal was quickly met by 1995. One of the primary methods employed by the food industry in creating newer food products containing less fat has been the use of fat replacers or fat substitutes. The technology used in the development of these fat replacers allows key sensory and physical attributes and functional characteristics of affected foods to be maintained.
Fat replacers are generally classified into three basic categories: fat-based, protein-based and carbohydrate-based. These categories and the most important examples within them are discussed below:
In response to consumer demand of recent years, food manufacturers have developed a wide variety of reduced fat food products utilizing fat substitutes as a primary method of fat reduction. Since 1990 an average of over 1,000 new low fat foods have entered the marketplace annually bearing nutrient content claims of lowered fat (55). In 1996 the introduction of new low fat foods reached a peak at 2,076 products. Since 1996, however, introductions of new reduced or low fat products have declined to 1,405 products in 1997 and 1,180 in 1998.
The "smoke," "flash," and "fire points" of a fatty material are standard measures of its thermal stability when heated in contact with air. The smoke point is the temperature at which smoke is first detected in a laboratory apparatus protected from drafts and provided with special illumination. The temperature at which the fat smokes freely is usually somewhat higher. The flash point is the temperature at which the volatile products are evolved at such a rate that they are capable of being ignited but not capable of supporting combustion. The fire point is the temperature at which the volatile products will support continued combustion. For typical fats with a free fatty acid content of about 0.05%, the smoke, flash, and fire points are around 420º, 620º, and 670º F, respectively. The degree of unsaturation of an oil has little, if any, effect on its smoke, flash, or fire points. Oils containing fatty acids of low molecular weight such as coconut oil, however, have lower smoke, flash, and fire points than other animal or vegetable fats of comparable free fatty acid content. Oils subjected to extended use will have increased free fatty acid content resulting in a lowering of the smoke, flash and fire points. Accordingly used oil freshened with new oil will show an increased smoke, flash and free points. For additional details see Baileys Industrial Oil and Fat Products (56).
1. Autoxidation. Of particular interest in the food field is the process of oxidation induced by air at room temperature referred to as "autoxidation." Ordinarily, this is a slow process which occurs only to a limited degree. In autoxidation, oxygen reacts with unsaturated fatty acids. Initially, peroxides are formed which in turn break down to hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols. Heavy metals present at low levels in fats and oils can promote autoxidation. Fats and oils often are treated with chelating agents such as citric acid (Also see Tables IV and V to inactivate heavy metals.)
The result of the autoxidation of fats and oils is the development of objectionable flavors and odors characteristic of the condition known as "oxidative rancidity." Some fats resist this change to a remarkable extent while others are more susceptible depending on the degree of unsaturation, the presence of antioxidants, and other factors. The presence of light, for example, increases the rate of oxidation. It is a common practice in the industry to protect fats and oils from oxidation to preserve their acceptable flavor and shelf life.
When rancidity has progressed significantly, it is apparent from the flavor and odor of the oil. Expert tasters are able to detect the development of rancidity in its early stages. The peroxide value determination, if used judiciously, may be helpful in measuring the degree of oxidative rancidity in the fat.
It has been found that oxidatively abused fats can complicate nutritional and biochemical studies in animals because they can affect food consumption under ad libitum feeding conditions and reduce the vitamin content of the food. If the diet has become unpalatable due to excessive oxidation of the fat component and is not accepted by the animal, a lack of growth by the animal could be due to its unwillingness to consume the diet. Thus, the experimental results might be attributed unwittingly to the type of fat or other nutrient being studied rather than to the condition of the ration. Knowing the oxidative condition of unsaturated fats is extremely important in biochemical and nutritional studies with animals.
2. Oxidation at Higher Temperatures. Although the rate of oxidation is greatly accelerated at higher temperatures, oxidative reactions which occur at higher temperatures may not follow precisely the same routes and mechanisms as the reactions at room temperatures. Thus, differences in the stability of fats and oils often become more apparent when the fats are used for frying or slow baking. The more unsaturated the fat or oil, the greater will be its susceptibility to oxidative rancidity. Predominantly unsaturated oils such as soybean, cottonseed, or corn oil are less stable than predominantly saturated oils such as coconut oil. Methylsilicone often is added to institutional frying fats and oils to reduce oxidation tendency and foaming at elevated temperatures. Frequently, partial hydrogenation is employed in the processing of liquid vegetable oil to increase the stability of the oil. Also oxidative stability has been increased in many of the oils developed through biotechnological engineering. The stability of a fat or oil may be predicted to some degree by the oxidative stability index (OSI).
CONSUMPTION (AVAILABILITY) OF VISIBLE AND INVISIBLE FATS (1970-1997)