Reinventing Oil Stability

When good fats go bad, they arent charged with Murder One; theyre just responsible for assaulting our senses and perhaps our health and battering the finished product quality. The actual culprit is typically oxygen, via oxidation of the fatty acids, the replacement of a hydrogen ion by an oxygen ion, which destabilizes the molecule and lets other, less-desirable compounds attach to the chain. The result is rancidity, an objectionable change in the flavor and odor of the oil or the food it contains.

Factors that accelerate oxidation in fats and oils include: trace metals, especially iron, copper and zinc; salt; heat; visible and ultraviolet (UV) light; moisture; and microbes, such as bacteria and molds that produce lipolytic enzymes. Unsaturated fatty acids are far more susceptible to oxidation reactions than saturated fats.

To prevent or slow fat oxidation, the food industry has employed many strategies, most revolving around modifying a fats composition, making it less susceptible to oxidation, and adding compounds that act as antioxidants. In addition, processing and packaging that minimizes exposure to heat, oxygen and light can also extend shelf life.

Oxidation frustration 

Oxidation not only produces rancid flavors, but research suggests that it can also decrease a products nutritional quality and safety, which may play a role in the development of diseases. Therefore, in terms of stability, preventing or reducing oxidation is at the forefront. Under mild conditions, molecular oxygen reacts with the double bonds of unsaturated fatty acids. Following a free-radical mechanism, autooxidation of the unsaturated fatty acids takes place. This complex process varies in edible oils due to its dependency on oil type and conditions of oxidation. Some oils resist auto-oxidation, while others are more susceptible, depending on the degree of unsaturation, the presence of antioxidants and other factors. For example, because light and thermal stress speed up oxidative reactions, oil oxidation is a major concern in fried products.

When oxygen and unsaturated fatty acids interact, they initially form peroxides, which in turn break down to hydrocarbons, ketones, aldehydes and smaller amounts of epoxides and alcohols. When rancidity has progressed significantly, the flavor and odor of the oil is quite unpleasant. Experts can organoleptically detect the development of rancidity in its early stages. In addition, determining the peroxide value might help in measuring the degree of oxidative rancidity by measuring the level of peroxides, intermediates in the oxidation reaction.

Certain strategies can lessen the impact of oxidation in specific applications. For example, adding methylsilicone, or dimethylpolysiloxane, to institutional frying fats and oils can reduce oxidation and foaming (which introduces additional oxygen) at high temperatures. Since low levels of heavy metals promote oxidative breakdown of fats, those that will be exposed to metals are often treated with chelating agents, such as citric acid or phosphoric acid. Reducing exposure to high temperatures and oxygen, with techniques such as nitrogen blanketing, can be used during production, storage and transportation.

In general, however, the most common strategies to minimize oxidation potential are decreasing the amount of unsaturated bonds in oils used or the addition of natural or synthetic compounds with antioxidant properties.

Transforming the landscape 

Until recently, partial hydrogenation seemed to be the best method for imparting stability¯ with the added benefit of improved functionality to commercial fats without relying on high levels of saturated fatty acids.

Briefly, hydrogenation takes hydrogen atoms and adds them directly to points of unsaturation on fatty acids, eliminating some of the reactive double bonds, as well as reconfiguring some of bonds that remain unsaturated, changing cis to trans isomers. Hydrogenation is an extremely important process for our food supply, as it economically imparts the desired stability and other properties to many edible oil products.

Hydrogenation involves reacting hydrogen gas with an oil at elevated temperature and pressure in the presence of a catalyst, often nickel on an inert carrier that is removed when the process is complete. The process is easily controlled and can be stopped at any point. That is, oil can be hydrogenated to any level, from its natural, unsaturated liquid state to its fully saturated solid state. When an oil is completely hydrogenated, it has no double bonds and is hard and brittle at room temperature, rendering it unusable for most applications. When the process is stopped somewhere in between the oils natural, unsaturated liquid state and its fully saturated solid state, the oil is partially hydrogenated.

Processors have a great deal of latitude in the range of hydrogenation, which affects both the stability and the oils properties. For example, if the hydrogenation of soybean oil is stopped after only a small degree of hydrogenation, the oil remains liquid. Further hydrogenation can produce soft but solid-appearing fats that still contain appreciable amounts of unsaturated fatty acids and are used in solid shortenings and margarines. As hydrogenation progresses, generally a gradual increase in the melting point of the fat or oil occurs.

Depending on the hydrogenation conditions, both positional and geometric (trans) isomers are formed. Hydrogenation creates trans double bonds by moving one hydrogen atom across to the other side of the carbon chain at the point of the double bond. In effect, the two hydrogen atoms balance each other and the fatty acid straightens, creating a packable plastic fat with a much higher melting temperature. Although trans fatty acids are technically unsaturated, they are configured in such a way that the characteristics of unsaturation are lost. It is this rearranging of hydrogen atoms at the double bonds that makes the fatty acid more compact, more solid.

Hydrogenation dates back to the late 1800s. But the emergence of the saturated fat and heart disease relationship put vegetable oil products in the limelight, making more-stable partially hydrogenated oils predominate in the industrial marketplace. The tables started to turn in the 1980s, after word was out that those formerly innocuous trans isomers in partially hydrogenated oils also contributed to heart disease.

Now what are the options? Economics and marketing aside, food scientists expect two things from their fat ingredient: functionality and stability. Good flavor and color are added bonuses. Therefore, they can look at two different routes: add something to the fat or fat-containing products that puts a damper on oxidation, or use a fat that has less-reactive fatty acids, or lower amounts of those that tend to oxidize.

The antioxidant alternative 

Product designers can add an array of natural and synthetic ingredients to formulations to prevent or slow fat oxidation. Some must be labeled as a preservative, while others simply look like they are part of the product formulation. Each source has, in addition to label and regulatory factors governing their use, different effects resulting from characteristics regarding solvent polarity, physical conditions and synergies.

Certain spices contain phenolic compounds, which can delay the onset of oxidation. These compounds help neutralize the oxidation reaction by contributing hydrogen ions from their own hydroxyl groups to unstable free radicals formed during the initiation of oxidation. The phenolic compounds are still transformed into free radicals, but are more stable than the initial free radicals involved in oxidation. Thus, rather than preventing oxidation, they naturally slow its progression.

Rosemary is one of the most widely used spices for its antioxidative properties. Other increasingly popular spices that reduce oxidation are oregano, cinnamon, cloves, sage and thyme. Among the active phenolic compounds are: rosmarinic acid (in rosemary, sage and oregano), carnosol and carnosolic acid (in rosemary and sage), and eugenol and gallic acid (cloves). The real spice can be added to formulations when the flavor complements the finished product. In addition, some suppliers have identified extraction methods that remove many of the characterizing flavors and aromas from spice extracts, rendering them virtually taste-free at levels that provide some protection from oxidation. These levels vary by ingredient and supplier.

Other natural ingredients that may show promise in extending oxidative shelf life include green tea, due to its catechins, grape pomace, and fruits with high antioxidant content, such as tart cherries and dried plums.

In a recent study, researchers from Cairo University, Egypt, showed that olive juice, obtained through the pressing of olive leaves, may act as an antioxidant in sunflower oil, prolonging its frying life (International Journal of Food Science and Technology, 2007; 42:107-115). 

The Egyptian researchers compared the antioxidant performance of 400, 800, 1,600 and 2,400 ppm of olive polyphenols with 200 ppm of the synthetic antioxidant BHT on the stability of sunflower oil under heating conditions (180°C, five hours per day for five days). No significant effect on the oils color at any level of olive polyphenols was recorded. The formation of peroxides in the oil as a result of heating also decreased with increasing olive-juice concentration.

Tocopherols (vitamin E) are important minor constituents of most vegetable oils. They naturally serve as antioxidants to retard rancidity and as sources of the essential nutrient vitamin E. Tocopherols exist in four formsalpha, beta, gamma and deltawith the gamma and delta forms the most active for stabilizing fats. Unfortunately, most of these are removed during modern commercial oil processing. Nevertheless, they can be added back to help prevent rancidity. Ingredients that contain all four forms are referred to collectively as mixed tocopherols. Manufacturers typically add mixed tocopherols to foods for their antioxidant activity. These ingredients do not impart flavor, color or odor to final food applications and are effective at very low concentrations.

FDA allows the use of tocopherols in most food applications. Typical usage levels are 0.01% to 0.02% of a foods total fat content. Food products formulated with tocopherols typically state their inclusion in the ingredient statement as Natural vitamin E added to preserve freshness, or Natural vitamin E added to protect flavor. In addition to vitamin E, ascorbyl palmitate and ascorbic acid, types of vitamin C, can also delay the onset of oxidation. Ascorbic acid acts by chelating metal ions and reducing oxygen, making it less available for participating in fat oxidation.

Synthetic antioxidants have traditionally been used to prevent oils from going rancid. BHA, BHT, propyl gallate and TBHQ are all FDA-approved for specific applications, with many having maximum usage levels, which to make things complicated, vary depending on application, the antioxidant combination used or whether the product falls under UDSA or FDA jurisdiction. See Product Preservation, in the Dec. 1997 issue of Food Product Design (http://www.foodproductdesign.com/articles/466/466_1297DE.html), for a more in-depth look at the use and restrictions of synthetic antioxidants. These antioxidants are considered phenolics and are highly soluble in oils and fats.

They work by delaying the onset of lipid oxidation reactions. These antioxidants are generally added directly to the fat, oil-based ingredient or emulsifier, or oil-containing product matrix, but can also be sprayed on the surface of a finished product, such as a breakfast cereal. The key to their effectiveness in these applications is proper dispersal. In addition, synthetic antioxidants can be added to packaging material to discourage oxidation of the contents.

Fats for the future 

The same technologies that change the functional properties of fats can also be used to increase their stability. The key is to reduce the number and types of fatty acids that are susceptible to oxidation and the more double bonds a particular fatty acid has, the more quickly it will oxidize. The problem for product developers is that not only does the fatty-acid profile need to be resistant to oxidation, it also needs to produce the desired characteristics in the finished product while minimizing the level of unhealthy saturates.

Interesterification involves shifting fatty acids within the oil molecules by chemical or enzymatic processes to modify melting properties and functionality. Chemical interesterification randomly distributes fatty acids across the glycerol backbone of the triglyceride by blending the desired oils, drying them and adding a catalyst such as sodium methoxide. Enzymatic interesterification rearranges the fatty acids on the glycerol backbone using lipases that enable a specific rearrangement.

Although less economical than hydrogenation, this process does not change the composition of the fatty acids from the starting materials, so unsaturated fatty acids do not become saturated and are not moved into trans configurations. However, it enables a fully saturated fat to interchange with a nonhydrogenated, unsaturated oil to give a mix of triglycerides that have a final melting profile tailored to specific applications.

According to a recent study conducted by researchers at Central Food Technological Research Institute, Mysore, India, the process of interesterification had no or marginal adverse effect on the development of oxidative rancidity for select fats with a linoleic content of 25%. Higher linoleic acid (about 50%) made the fat more unstable as a result of interesterification.

Biotechnology, advanced breeding, as well as genetic modification, are also being used to develop new or modified plants that yield edible oils with more oxidation-resistant fatty-acid profiles. Advanced breeding technologies produce canola, soybean and sunflower seeds with low-linolenic fatty-acid contents and/or high oleic levels. Linolenic, an omega-3 with three double bonds, is very unstable, whereas oleic, a monounsaturated omega-9, is much more stable. The morenotable commercialized biotechnology applications within the oilseed industry include high- and mid-oleic sunflower, low-linolenic soybeans, high-linoleic (two double bonds) flaxseed and lowlinolenic and high-oleic canola.

Biotechnology also allows for the creation of oils with specific fatty-acid profiles for particular food applications and vitamin-Efortified oilseeds, which assists with preventing oxidation. Research being conducted by the USDAs Agricultural Research Service Food and Industrial Oil Research Unit, Peoria, IL, showed that oil extracted from high gamma-tocopherol sunflower seeds was significantly more stable to oxidation than sunflower oil with a normal gamma-tocopherol content. The researchers found that quinones, compounds with antioxidant properties, were formed from the breakdown of tocopherols during frying, which explained in part why oils that lose all of their tocopherol content still can inhibit some oxidation.

Some oils are naturally stable more stable than others. Corn and sunflower oils contain less than 1% linolenic acid, a level that renders them more stable than soybean oil, which contains 7% to 8% linolenic acid. Another possibility is rice bran oil, which has been commercially produced in the United States since 1994 by RITO, a partnership between Riceland Foods, Inc., Stuttgart, AR, and Oilseeds International, Ltd., San Francisco. Its most-notable feature is its high level of the antioxidants gammaoryzanol and tocotrienols (vitamin E), both of which contribute to the oils stability.

Traditionally, potato chips, rice crackers and even french fries have been fried in rice bran oil throughout Asia, as many Asian consumers enjoy the flavor of rice bran oil, and manufacturers appreciate its oxidative stability. Rice bran oil has a nutty flavor profile, which has been shown to enhance the flavor of certain fried foods. In addition, rice bran oil gives an ideal color and desirable texture to all types of fried foods.

Palm oil is the most heavily consumed edible oil in the world, after soybean oil. If one were to exclude the United States, where most of the worlds soybean oil is consumed, palm oil would be the most-popular edible oil in the world. Palm oil traditionally has been used for baking, shortenings, margarines and deep-fat frying. While palm contains no trans fatty acids, around half of the fatty acids are saturated, as compared to conventional soy (15%), sunflower (12%) or canola (7%). It is the saturate content that keeps palm oil from being the ideal solution for many product designers.

However, several studies, including those published in the Oct. 2002 issue of Asia Pacific Journal of Clinical Nutrition (11: 394- 437), suggest that dietary palm oil has a potentially more-favorable dietary impact than trans fats, especially as part of a moderate-fat diet. One study (Journal of the American College of Nutrition, 1992; 11:383-390) indicates palm oils effect on blood cholesterol levels might be similar to olive oil. Other researchers point to the antioxidant benefits of palm oils inherent vitamin E content.

Tropical Traditions, West Bend, WI, developed a palm shortening that is palm oil with some of its unsaturated fats removed. The result is a very firm texture and high melting point (97°F), making it very shelf stable. It is not hydrogenated, nor does it contain any trans fats. The company says it is great for deep-fat frying and baking, as it is not prone to oxidative rancidity. Since it has been separated from some of the unsaturated portion of the oil, it is colorless and odorless, and will not affect the taste of foods like virgin palm oil.

Oils pressed from seed without heat, also known as cold-pressed, have been shown to get rancid more slowly than oils extracted from seeds with hexane solvent. Another type of extraction, expeller pressing, is similar to cold pressing, except some heat is applied to the seeds.

A study published in the Journal of the American Oil Chemists Society (May 2006; 83:435-441) evaluated the stability of expeller-pressed oil. Investigators fried potatoes in expeller-pressed soybean oil, standard hexane-extracted soybean oil with an added antioxidant, and hydrogenated soybean oil. They found the quality of the fried food and the frying life of the expeller-pressed oil were similar to that of the oil with the antioxidant, and that the quality of the french fries was better. Researchers noted expeller-pressed soybean oil had slightly more total tocopherols and phytosterols than did the hexane-extracted, refined, bleached, deodorized soybean oil. They also suggested that compounds such as Maillard reaction products in the expeller-pressed oil probably inhibited the loss of tocopherols. This new knowledge will help the oil industry find oils that have natural stability, without the need for synthetic antioxidants or hydrogenation.

Many emerging and already-available technologies can keep fats and oils on the straight and narrow and prevent rancidity. Try them out. The oil you save may be your own. 

Donna Berry, president of Chicago-based Dairy & Food Communications, Inc., a network of professionals in business-to-business technical and trade communications, has been writing about product development and marketing for 13 years. Prior to that, she worked for Kraft Foods in the natural-cheese division. She has a B.S. in Food Science from the University of Illinois in Urbana-Champaign. She can be reached at [email protected].