Olive oil remains one of the most popular oils in cooking, due to its flavor and heart-healthy reputation.
Over the last 35 years, dietary lipids have maintained the attention of scientists, regulators, the media and consumers. There have been animal experiments; human subject feeding studies; major national surveys from around the world based on dietary intake; and selected dietary intake using comparative lipid replacement. Studies examining total lipid intake, saturated fat intake, cholesterol and trans fats have resulted in their being labeled as “bad” for one’s health. Other studies have characterized monounsaturated fats, polyunsaturated fats and fish oil (W-3 fatty acids) as “good” fats.
The popular press has transformed the science into good fat and bad fat. Unfortunately, the good vs. bad visualization appears to confuse the public. In addition, the food industry has been wrongly accused as a contributing factor in delivering food high in dietary lipids linked to health problems. The efforts made by the food industry to stay abreast of constantly changing lipid-health guidelines has been challenging, but those efforts are noted throughout this article. The industry has acted responsively, changing lipid formulations to resolve the health issues—once scientific consensus has been reached.
The typical American diet contains 35% of its calories as fat (12% of those calories are saturated fats), 15% protein and 45-60% as carbohydrate. In recent years, the consumption of fat in the U.S. has occurred mainly through ingestion of plant fats in salad oils, cooking oils, frying fats, bakery shortening and margarine, while animal fats included meat, poultry, fish and dairy products (i.e., cheese, butter). Every fat has unique properties that contribute to the quality characteristics of the food product.
The world demand for vegetable oil continues to grow annually at 4-5%, with growing consumption of salad oils, baking and frying fats, margarines and spreads. The role of the physical properties imparted by fats and oils include flavor, mouthfeel, texture induced in fried and baked goods, and aroma. Some of these favorable properties are incorporated into the food, when fats are used as a heat transfer medium.
Lipids are essential to a healthy body, growth and development. Fat is one of the essential components of our balanced diet, delivering essential fatty acids and fat-soluble vitamins (A, D, E and K), as well as energy (9Kcal/g). In addition, lipids are integral to membrane structure, acting to insulate the body from heat loss; as cushions for organs protecting them from shock; participating in blood clotting; the immune response; and the regulation of blood pressure.
Blood cholesterol levels, LDL-Cholesterol (LDL-C, or bad cholesterol) and HDL-Cholesterol (HDL-C, or good cholesterol) are affected differently by dietary saturated and trans fats and differently affect the risk of heart disease. Saturated fats increase both LDL-C (bad) and HDL-C (good), while trans fats raise LDL-C but have a neutral or modestly decreasing effect on the HDL-C. Saturated fats are one of the few dietary components that raise HDL-C. Trans fats also have been shown to cause other harmful effects, such as increased inflammation, while saturated fats increase insulin resistance.
The Dietary Guidelines for Americans (2005) clearly recognize the role of fat in our bodies. Nutrition guidelines recommend no more than 25-30% of total calories as fat, with no more than 10% as saturated fat. It is also recommended that 10-15% occur as monounsaturated fatty acid (MUFA). It should not be expected that food products be designed to individually meet these guidelines. The challenge and decision process remains with the consumer to select food products that together achieve these dietary guidelines. Over the last 30 years or so, recommendations have suggested that diets lower saturated fat and cholesterol intake, yet remain moderate in total fat.
Fatty Acids
Fatty acids are long-chain hydrocarbons with a carboxyl group at one end and a methyl group on the other end. When hydrogen atoms are located on all of the carbon atoms (-CH2-CH2-), the fatty acids are considered saturated. The linear structure of the saturated fatty acid provides unique, tightly packed, stacking properties. If adjacent Cs are each missing a hydrogen atom, each carbon has an unpaired electron, which forms an unsaturated (-HC=CH-) bond. If only one unsaturated bond occurs in the hydrocarbon chain, it is called a MUFA (e.g., oleic acid, 18:1); if two or more unsaturated carbons are formed, it is called a polyunsaturated fatty acid (PUFA) [e.g., linoleic acid (18:2, W-6), a-linolenic acid (18:3, W-3) and arachidonic acid (20:4)]. The human body cannot make linoleic or a-linolenic fatty acid; therefore, they must be provided by dietary sources and are considered essential nutrients. The diet must supply 3% of total calories as linoleic and linolenic fatty acids.Another group of uniquely required fatty acids is the W-3 fatty acids--eicosapentaenoic, EPA (20:5) and docosahexaenoic, DHA (22:6)--identified as long-chain PUFA (LC-PUFA). Although known to be provided by deep ocean fish, the fish actually acquire the LC-PUFAs from plant algae they consume from the water.
Most vegetable oils, extracted from corn, sunflower and safflower, are rich in W-6 PUFAs. The W-6 PUFAs indicate the first unsaturated bond from the methyl end occurs between the 6th and 7th carbons, whereas, the W-3 PUFAs have their first unsaturated bond between the 3rd and 4th carbon from the methyl end. Some vegetable oils contain a-linolenic acid (W-3 PUFA), in which the chemical and biological properties differ from the LC-PUFA found in marine oils, due to the longer carbon chains and additional unsaturated bonds that are present.
The LC-PUFAs are required for vision, affecting the structure of the neuronal sheath, retina and visual cortex during early infant development. It has been recommended that the consumption of the W-3 fatty acids should be 12-18g/day of the total dietary unsaturated fatty acids.
The W-3 PUFA consumption has attracted increased interest because of its potential health benefits. LC-PUFAs are now being taken as supplements, as well as being added to other food products to enhance dietary intake. Consuming fish and fish oils has been the primary source, while more recent sources include W-3 eggs, incorporated into the yolk, when delivered by feeding algae and W-3 oils to laying hens.
Fats and Oils
The chemical characteristics of fats and oils are related to their fatty acid composition. Fats and oils are triacylglycerols (TAG), formed by the attachment of saturated and unsaturated fatty acids to a glycerol backbone. If only one fatty acid is attached, it is called a monoacylglyceride, whereas if two or three fatty acids are attached, it is called a diacylglyceride and triacylglyceride, respectively.The ratio of saturated, unsaturated and polyunsaturated fat present in the TAG determines whether it is a liquid or a solid at room temperature (RT). The melting point, the temperature at which a fat becomes a liquid, varies with the extent of saturation and the molecular size/weight of the fatty acids that constitute the fat.
Saturated fat has a high proportion of saturated fatty acids and tends to solidify at refrigeration temperature; if very saturated, it remains a solid at RT. Fats with a higher proportion of unsaturated fatty acids tend to be liquid at both refrigeration and RT. Animal fats frequently melt when heated at or above body temperature, but may solidify at room temperature (e.g., bacon fat), whereas, most plant seed oils are liquids at room temperature.
With greater degrees of unsaturation, the fatty acids become more susceptible to oxidative damage and development of off-flavors (rancidity). To prevent this effect under cooking conditions, the oil is partially hydrogenated to decrease the number of unsaturated bonds (18:3>18:2). Thus, partial hydrogenation increases stability and provides a product with varying properties, depending on the final extent of hydrogenation.
As unsaturated bonds undergo hydrogenation (insertion of hydrogen atoms using a Ni catalyst), the process causes energy shifts and hydrogen rearrangement, forming trans unsaturated bonds. The normal cis-structure of unsaturated fatty acids causes bends in the fatty acid chains, adding bulk to the stacking properties of the lipid. The trans arrangement causes an increase in linearity of those fatty acids, enabling a tighter stacking. Relative to the cis form, the trans form raises the MP of the fat, making it similar to saturated fats. For example, oleic acid (cis-C18:1d9/n-9) melts at 13°C, while its trans isomer, elaidic acid (trans-C18:1d9/n-9), melts at 44°C, and stearic acid (C18:0), the saturated fatty acid equivalent, melts at 72°C. This structural change provides the physical characteristics of partially hydrogenated fats defining trans fatty acid-containing fats.
The consumption of trans fats increased in the 1980s, when fast-food restaurants and processed food manufacturers switched from animal fat to partially hydrogenated (PH) vegetable oils to meet the current nutrition guidelines. Currently, trans fat intake is estimated to be 2.6% of the daily energy intake (5.8g/day) and comes mostly from products made with PH vegetable oils. Partially hydrogenated vegetable oils make up 85-90% of the trans fatty acids consumed each day. The rest comes from naturally formed trans fats found in meat and dairy products, as a result of ruminant fermentation. By comparison, saturated fat intake is about 12-14% (about 25-30g/day) of the total calorie intake.
It became apparent that the PH fat acted much like saturated fat on serum cholesterol levels, when compared with the non-hydrogenated oils. Controversy reigned, with researchers trying to determine the appropriate dietary reference lipid, and its amount, to evaluate the trans fatty acid effect. Altering reference parameters affected research outcomes and interpretation. The conclusion was that trans fats increase risk for heart disease, causing elevation of LDL-C and a decrease in HDL-C. The trans fats have a greater impact on serum cholesterol distribution than dietary saturated fatty acids.
Cholesterol
Cholesterol becomes a wax, when the cholesterol molecule is conjugated with a fatty acid. Animals and fish carry cholesterol in their fat stores. Plants do not synthesize cholesterol, but do form sterols that are contained within the lipid stores. If present in the digestive tract with cholesterol from animal products, the plant sterols block cholesterol absorption.The liver synthesizes 800-1,500mg of cholesterol each day, which is a much greater contribution to circulating blood cholesterol than that contributed from dietary intake (300mg/day), absorbed at 40% of intake. In an attempt to self-regulate the cholesterol in circulation, dietary cholesterol decreasesde novosynthesis. Dietary saturated fatty acids increase LDL-C in some people.
Cholesterol is an important base material from which the body creates bile acids, sex hormones, adrenal hormones, vitamin D, cell membranes and the myelin sheath. However, too much cholesterol in the blood becomes a major risk factor for coronary heart disease and stroke.
Food Fats and Oils Composition
The fatty acid composition of the most common plant and animal fats and oils is identified in the chart titled “Dietary Fats and Oils.” The fats are categorized into three groups (saturated, monounsaturated and polyunsaturated fatty acids), listed in order of the highest to lowest amount of the primary fatty acid of that group. The food fats do not contain solely saturated or unsaturated fatty acids. As noted above, they contain a random variety of both saturated and unsaturated fatty acids. The ratios of these fatty acids alter the characteristic properties of the fat as a whole.The composition of the fat determines its stability, handling and application properties, including its health properties in the human body. Processed fats are included in this list, to indicate efforts to create fats with altered properties to meet specific food industry needs and dietary concerns.
The saturated fats group has a percent of saturated fatty acids ranging from 42-92%. Most of these fats have MUFA between 29-48% and very low PUFA. Coconut oil has the highest level of saturated fatty acids at 92% and is firm at RT, but when left at RT, the coconut fat still separates and becomes liquid at 27°C. This reflects the degree of short- chain fatty acids within the saturated group. Coconut oil can be used as a spray on cookies and baked goods and incorporated into whipped toppings, icings and glazes.
The frying characteristics of palm oil are similar to beef tallow. Both have 50% saturated fatty acids and proportionally similar MUFAs. Lard is also similar. The net effect is a vegetable oil with similar characteristics to animal fat.
Plant oils are partially hydrogenated to the desired saturation level, and then processed to the desired consistency (e.g., as shortening or margarine). When a variety of different plant fats are blended and processed similarly to the mechanization of converting milk fat into butter, the end product becomes margarine. Both are solid at refrigeration temperature and soften at room temperature, while holding their form. Tub margarine is softer, due in part to a lower total fat (more water added), and a higher proportion of PUFA.
The monounsaturated fats group has a MUFA concentration ranging from 42-79%. The highest levels of MUFAs occur in sunflower oil (high-oleic, 79%), olive oil (74%), NuSun (mid-oleic, 65%) and canola oils (62%). The high-oleic and mid-oleic acid sunflower oil was modified from the original sunflower plant, using general breeding practices to produce higher levels of oleic acid in their seeds. Using general breeding practices, the original sunflower plant (18% MUFAs and 69% PUFAs) was modified so as to produce a mid-oleic acid sunflower oil (65% MUFAs and 26% PUFAs) and a high-oleic oil (79% MUFAs and 11% PUFAs).
Partial hydrogenation of canola oil decreased the MUFA and PUFA fraction proportionately, forming 28% trans fatty acids. Peanut oil, soft margarine and sesame oil are similar in MUFA and PUFA. Peanut oil is relatively stable, but is a more expensive cooking oil. Sesame oil has a unique flavor characteristic and must be used accordingly.
The polyunsaturated fats include safflower, sunflower, flaxseed, soybean, corn and cottonseed oil, having PUFA concentrations from 78-54%, respectively. Flaxseed oil is high in 18:3 (48%), while the others are high in 18:2 (54-78%). Cottonseed oil is the most saturated of this group, at 28%. This group is very prone to oxidative damage and off-flavors, especially during repeated uses as cooking oils.
Examples of two partial hydrogenation fats for soybean and canola oil are provided for comparison. Partial hydrogenation decreases the susceptibility of unsaturated fats to oxidation, primarily decreasing the linolenic acid fraction. The effect on soybean oil was a reduction of linoleic acid by half--from 62% down to 31%--increasing the stability of the oil, but increasing the amount of trans fatty acids. MUFA was little affected. Similarly, PH canola oil decreased the PUFAs significantly, from 32-12%, and MUFAs from 61-52%, with a similar formation of trans fatty acids. As noted earlier, the trans fatty acids are unsaturated in nature, but provide characteristics similar to saturated fatty acids. These results would vary by the specific degree of hydrogenation requested to meet the application needs.
Beef tallow and lard fall between high saturated and monounsaturated fats. Although the lowest saturated content in that category, they compare to the middle of the group of high monounsaturated fatty acids. Thus, they have many shared characteristics.
Unique food fats include wheat germ oil, which has the characteristics of polyunsaturated fats similar to corn oil. Almond and avocado oils are similar to olive oil in being high in MUFAs, and rice bran is similar to peanut oil. These specialty oils would be very expensive to use commercially but can be compared to other food lipids as part of a total diet.
The FDA Modernization Act of 1997 (FDAMA) defined healthy oils as those having less than 20% saturated fatty acids. This would include all of the fats listed under the monounsaturated and polyunsaturated fats. Cottonseed oil just misses this arbitrary line, having 28% saturated fatty acids, while peanut oil (19%) and soft margarine (20%) would be considered healthy. The justification for choosing 20% was not based on clear science. The healthy or unhealthy aspect of a fat depends on the total composition of the fats in the diet and the amounts consumed.
Historical Perspective of Fats and Oils Used in Food Processing
Throughout recorded history, animal fats have been collected and used as lubricants, to provide light and as part of food preparation. As meat preparation moved from open-roasting to pan-roasting and pan-frying, meat juices and fats were collected and reused. In the home, frying fats were collected from the pan after cooling and stored in a container, to be used later, when frying low-fat food items such as eggs, potatoes, etc. The fat enabled lubrication to prevent sticking to the pan, to provide even heat transfer and to add flavor.Carcass fat continues to be collected during slaughter, rendered (cooked at 280°), and the fat skimmed off and collected as tallow. Beef tallow and pork lard are commercial examples of fat sold for use in cooking and food processing.
Margarine was created and patented in 1869 by the French chemist Hippolyte Mege-Mouries, as a result of a contest promoted by the Emperor Napoleon III to produce an inexpensive replacement for butter. Originally, it was an animal by-product (neutral lard, oleo oil from rendering animal fat) mixed with vegetable oil (cottonseed oil) and often churned with pasteurized milk to form a product to replace butter fat.
During this period of time, lard also began to be transformed with the addition of vegetable oil. Processes to extract oil from seeds, especially cotton seed, were improving, and the oil was less expensive than that obtained from animal fat. In addition, the volume of fat needed for commercial uses exceeded the amount of animal fat that was available. Since there was no standard of identity, and labeling laws were non-existent, the addition of plantseed oil was not identified on the product label. Lard content was commonly altered. By 1890, the practice was so common that lard became the minor component, and only enough lard and beef stearin was being added to assure the expected flavor. At this time, the product name was changed to “lard compounds,” a practice which continues today.
In 1890, Paul Sabatier developed the hydrogenation process in France. A short time later, in 1902, Wilhelm Normann patented the process for hydrogenation of liquid oil to specifically produce trans fats. The process maintained an unsaturated fat character, while gaining a more saturated, solid product consistency. This was the first man-made fat to be added to the food supply.
By 1908, new cooking fats became commercially available, which consisted of cottonseed oil hydrogenated to a desired consistency. Three types of new fats were made: a lard compound--beef tallow, beef stearin and vegetable oil (mainly cottonseed oil); a mixture of cottonseed oil hardened by hydrogenation and then blended with cottonseed oil; and a partially hydrogenated cottonseed oil.
The first all-vegetable oil shortening was produced in 1911 by Procter & Gamble (Crisco.com, 2007). It was developed initially to replace animal fat used in candle-making, but was quickly adapted to use as a lard substitute. As such, Crisco (crystallized cottonseed oil) became one of the most-recognized food products worldwide. It was the first product totally made of plant oil and was accepted as appropriate for strict vegetarian diets (lard was not). Crisco was the same product every time it was purchased, providing cooking consistency, stable to oxidation, snow-white in color (suggesting purity), a solid at RT, and it did not smoke when used in frying. The success of Crisco catalyzed the acceptance of hydrogenated fats throughout the food industry. Its characteristics were so good, it was also used as a fat substitute in home cooking and baking. Crisco vegetable oil was introduced in 1960. In 1976, Puritan oil (sunflower oil) was introduced and changed to canola oil in 1988.
The degree of hydrogenation can be modified with temperature, stirring speed, catalyst (Ni) and the hydrogen pressure. Controlling these conditions allows the food industry to produce the desired melting point, hardness, stability and mouthfeel required in specific foods. Total hydrogenation converts C-18 polyunsaturated fatty acids to stearic acid, while partial hydrogenation varies monounsaturated and polyunsaturated fatty acid composition, as well as the levels of trans fatty acids.
Natural oil blends of cottonseed and corn oil, or cottonseed oil blended with soybean or canola oils that are targeted to achieve low-linolenic acid levels of 3% or less, increase the stability of the oil. For example, Crisco now consists of a blend of soybean oil, fully hydrogenated cottonseed oil, and partially hydrogenated soybean and cottonseed oils. The reformulated Crisco has the same cooking properties and flavor as the original version of the product.
From 1937 through WWII, butter became rationed and increased the need for substitutes. The dairy industry lobbyists prevented commercial addition of yellow coloring to margarine, which would have made the product more closely resemble butter. (The margarine was sold like lard, with a separate yellow coloring packet which the consumer blended at home.) Oleo margarine had a distinct odor and flavor as well. In many states through the 1960s, the dairy industry also blocked use of non-dairy fats in the production of cheese and ice cream, legally protecting their standard of identity.
In 1957, The American Heart Association linked total and saturated fat as a causative factor and a major health risk to the nation. The per capita consumption of total fat stalled, but the vegetable oil segment continued to grow. In the 1960s, processed animal fat displaced natural animal fats, specifically decreasing cholesterol consumption. In the late 1950s and early 1960s, soybean oil became more economical--with development of improved mechanization to recover the oil from the soybeans--and processing techniques were applied to meet industry and consumer needs.
The health agencies demanded that dietary saturated fats (beef tallow and tropical oil) and cholesterol (red meat, eggs and cheese) should be avoided, linking them to the risk of atherosclerosis and heart disease. Most food companies switched to hydrogenated vegetable fats to replace animal fats, especially in deep-fat fryers during the 1960s-1970s. At that time, hydrogenated fats containing trans fatty acids were an acceptable alternative to replace saturated animal fats and tropical oils.
Indeed, dietary unsaturated fats were promoted for health, but they were susceptible to oxidative damage and were then linked to initiation of cancer and plaque formation in the arterial wall. Oleic acid and linoleic acid reduced LDL-C (good fatty acids), while linolenic acid became the not-so-good fatty acid. Hydrogenation decreases the 18:3 fatty acids to a greater extent than the 18:2 or 18:1 fatty acids, enhancing the quality of the unsaturated fats remaining. Unfortunately, during hydrogenation, trans fatty acids are formed.
For the last 15 years, concerns have been expressed about whether or not trans fatty acids also increased heart disease risk. The debate focused on the appropriate research design concerning use of the appropriate dietary reference controls: should the plasma lipid changes resulting from dietary trans fats be compared to the unsaturated fats or the saturated fats? The scientific conclusion was that the trans fats were more atherogenic than the saturated fats, causing specific increases in LDL-C (bad cholesterol) and decreases in HDL-C (good cholesterol). Thus, beginning last year, the amount of trans fats present in a food must be listed on food labels immediately below the saturated fat content. The industry response was to immediately reduce and try to eliminate the use of hydrogenated fats that would contain trans fatty acids.
The FDA began notification that trans fats would be added to food labels as early as 1999, and then it passed into law in 2003 and was implemented January 2006. Efforts to reduce trans fats have been ongoing since the early 1990s. For example, margarine formulated from hydrogenated soybean oil demonstrated a decrease in trans fats. When soft tub margarine was first produced in 1992, it contained 20% trans fat. By 1999, it was reduced by 56%--to 9%. By 2006, it reached 0%. Other manufacturers switched oils to achieve the same outcome.
During processing, the formation of trans fats can be eliminated by using lower temperatures during hydrogenation or by using interesterification, whereby the fatty acids are redistributed on the glycerol or across different glycerol molecules. This process can be controlled to reach a specific final product characteristic, by choosing the starting fats and their fatty acid composition. Until recently, the process had been rather expensive, but with new enzyme sources and applications, the cost is now comparable to hydrogenation to achieve the desired outcome.
After 30 years, developers are returning to some of the tropical oils, which are higher in saturated fatty acids, to regain the characteristics needed to produce desired fats. Palm and palm kernel oils are not as saturated as coconut oil and can be used in interesterification reactions to produce products with desired characteristics.
All of these efforts reflect well on the food industry in its efforts to respond to health concerns associated with cooking oils and dietary fats in order to assure a safe and healthy food supply. One area in which the industry has shown reluctance is in keeping the public informed about the food supply. This is a failing that increases public distrust. The only way the industry can protect itself is to publish accurate material about the nutritional content and safety of foods and the means by which the industry delivers a wide variety of quality food products to every economic group at an affordable price.pf
Wayne R. Bidlack, Ph.D., is a member of the department of Human Nutrition and Food Science at California State Polytechnic University--Pomona, Pomona, Calif.
Website Resources and Other References:
Crisco.com/about/history/timeline(2007) -- More information on Crisco Dietary Guidelines for Americans, 2005, Department of Health and Human Services and U.S. Department of Agriculture (www.healthierus.gov/dietary guidelines).Institute of Shortening and Edible Oils, Food Fats & Oils, 9th edition, 2006 (www.iseo.org).
USDA, ARS, National Nutrient Database for Standard Reference 18, 2005 (www.USDA/ARS/nutrientdatabase).
Eckel, R.H., Bora, S., Lichenstein, A.H., Yin-Piazza, S.Y. Understanding the complexity of trans fatty acid reduction in the American diet. American Heart Association Trans Fat Conference, 2006. Circulation, 115:2220-2235, 2007.
O’Brien, RD (1998). Fats & Oils. Formulating & Processing for Application. Technomics Publishing Co. Inc.
Potter, NN and Hotchkiss, JH (1995). Food Science, 5th ed., Chapman Hall, 359-380.
Stier, R.F. Of Fats and Oils. Prepared Foods Sept: 99-100, 103-105, 2007.
Tiffany, T. Oil Options for Deep Fat frying, Food Technology 61: 46-56, 2007.
