Food Preservation with Natural Antioxidants
Consumers are spending less time and more money on meals that are quick and convenient to prepare and require less planning. Conversely, producers are looking to preserve the freshness and extend the shelf-life of their products. Color and appearance of food products are major factors in consumer purchasing decisions as they are assumed to be indicators of quality and freshness, and these characteristics have a significant impact on consumers’ expectations of satisfaction.
Oxidation is a common property in biological systems and therefore by extension in foods. While some oxidation reactions are beneficial in foods, most have unfavourable effects such as the degradation of vitamins, pigments, and lipids or fats. In particular oxidation of unsaturated fats leads to the development of off flavors which are perceptible at very low concentrations. In meats this is called warmed-over flavor, and has been described as stale, cardboard-like, painty, and rancid. This characteristic is easily detectable in cooked meats after 48 hours. Lipid oxidation can also be detected in frozen meats, a characteristic known as freezer burn.
In food systems oxidation generally is initiated in two ways. First, it can be initiated by energetic forms of oxygen that are present in the environment (therefore the term oxidation). Second, lipids have a property of auto-oxidation where by reacting with each other they can degrade each other and create the oxidation products which result in the deterioration of foods. This is exaggerated in meats as the iron that is present in animal tissues has the property of accelerating oxidation reactions.
Solving the problem of warmed-over flavor is critical to producing high quality food products, and the addition of antioxidants is almost always the best method of reducing lipid oxidation. Synthetic antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene effectively inhibit warmed-over flavor. However, the growing consumer preference for natural and healthy ingredients has forced the food industry to look for natural methods of extending the product freshness and shelf-life which has sparked the desire for natural alternatives which inhibit the oxidation of foods. In the United States, a food product may be labelled as natural if it contains no artificial ingredients and is no more than minimally processed (frozen, ground, smoked, etc.).
An antioxidant is any substance that delays oxidation by preventing the formation of free radicals or by preventing them from producing more free radicals which can perpetuate oxidation reactions. Antioxidants can scavenge compounds that initiate or perpetuate oxidation, bind metals which participate in oxidation reactions, or decompose the products of lipid oxidation. All of these characteristics can improve the stability, flavor and shelf-life of food products. As such, numerous natural antioxidants have been evaluated for use in the food industry. For a natural antioxidant to be used in the food industry it needs to be effective, not to be cost prohibitive, and not to interfere with the natural flavor profile of the foods to which it is added.
Vitamin C or ascorbic acid is a chelating agent that binds metal ions and prevents them from participating in oxidation reactions. Vitamin C also scavenges free radicals and acts as a reducing agent. At high levels of over 1000 mg/kg it inhibits oxidation, however at lower levels such as under 100 mg/kg it can actually accelerate oxidation reactions. This may help keep the redness of meat as the presence of vitamin C seems to stimulate the oxidation of iron in muscles fibers, and prevents browning. Some studies have also found that vitamin C can extend the shelf-life of ground meats.
Vitamin E (α-tocopherol) is a fat-soluble carotenoid which locates in cell membranes in proximity to lipids where it can prevent oxidative damage. It was found that α-tocopherol is not an effective antioxidant in meat products as it does not target the right lipid fraction, its sibling δ-tocopherol was found to be more effective. Dietary vitamin E can protect against both lipid and pigment oxidation of meat in a dose dependent manner. The higher the initial concentrations of vitamin E (and vitamin A) in the meat from cattle, the better the color stability of steaks during refrigerated display.
Spices and Herbs
The antioxidant effects of spices and herbs come from their high levels of phenolic compounds including phenolic acids (gallic, caffeic, and rosmarinic acid), phenolic diterpenes (carnosic acid, carnosol), flavonoids (catechin, quercetin, kaempferol) and volatile oils (thymol, menthol). These compounds are found in rosemary, oregano, majoram, sage, and thyme. However most research has been conducted on rosemary extracts.
Rosemary extracts have been found to have high levels of various antioxidant compounds and to effectively inhibit the oxidation of foodstuffs. They effectively reduce the oxidation and browning of meats treated with them, thereby extending product display life. It seems that their optimum concentrations is 0.1%, and can be combined with vitamin C and vitamin E for greater effectiveness. However, some of the compounds in rosemary can impart an unwanted rosemary odor to foods, even at low concentrations.
Oregano has the highest antioxidative capacity of all herbs evaluated. Sage also contains high levels of various antioxidant compounds. Both oregano and sage oils are effective in inhibiting the oxidation and color changes associated with meat storage. However, oregano and sage extracts seem to be more effective at preserving the quality of cooked rather than fresh meats.
Green tea has substantial antioxidant activity most of which is associated with its natural flavonoid content. The four major flavonoids in green tea by antioxidant potency are epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC) and epicatechin (EC). However, they are now suitable for use as their addition to beef products results in their discoloration after cooking.