Formulators looking to put fish oils into food products are confronted with tough challenges, including processing degradation, oxidation and the all-important concerns about taste. Ernesto Hernandez, PhD, explains
Sources of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are primarily obtained from cold-water fish such as tuna, salmon and mackerel. Another source of omega-3 fatty acid is alpha-linolenic acid (ALA), which can be obtained from flaxseed, walnuts and oilseeds, such as soybean or canola. Recently, Omega Protein?s menhaden oil and Martek?s omega-3 oils from micro algae have been granted GRAS status by the US government, and a wider use of these oils is expected in both supplements and fortified foods.
Regardless of the source, all polyunsaturated oils such as omega-3 oils are inherently unstable and prone to oxidation. This presents problems of rapid deterioration during processing, cooking or storage. In addition, special precautions have to be taken to minimise exposure to high temperatures and air. Furthermore, even if omega-3 oils are properly handled and stored, the residual ?fishy? taste and aroma remains a natural component and presents one of the major obstacles for industry to increase acceptance of omega-3 oil-fortified food products and nutritional supplements.
Even though oils rich in polyunsaturated fatty acids (PUFAs) are inherently susceptible to heat and oxygen, technologies such as encapsulation and formulations containing antioxidants allow their incorporation into foods that require heating, such as baked goods. The bioavailability of PUFAs in fish oil-enriched foods has been reported to be similar to encapsulated n-3 PUFAs. Reports on human feed studies show that micro-encapsulated fish oil is efficiently incorporated and increases plasma docosahexaenoic acid and total omega-3 fatty acids in blood lipids.1
Micro-encapsulation is one of the latest technologies used by the industry to protect sensitive PUFAs against oxidation, thus helping preserve PUFAs during processing and storage. The process can also mask undesirable fishy odours and flavours in the final product, in addition to facilitating handling, incorporation into foods and long-term storage.
Odour and taste issues
In order to remove the fishy odour and taste from omega-3 oils, companies refine and deodorise the oil using processing steps similar to those used for conventional vegetable oils. Basically, the oil is refined to remove mucilaginous materials and acidity and is then subjected to high heat under high vacuum, to strip off the fishy smells and taste. If not done carefully, this process has the potential of damaging the oil.
Once the oil is deodorised, packaging is done under an inert gas like nitrogen to prevent oxidation. Natural and synthetic antioxidants such as tocopherols and TBHQ are also commonly used to help prevent oil deterioration. Micro-encapsulation helps to inhibit oxidation and allows the manufacturer to handle and incorporate the oil into supplements and fortified foods.
However, the issue of flavour stability in either fortified foods or nutritional supplements remains a concern for longer-term storage. In some cases, the fishy smell can be masked with other flavour and odour traits of a variety of foods. For example, non-excessive fishy flavours in foods can easily be masked in products that are sweet or have other intense flavours, such as in instant powdered milk-based formulae concentrates. Products like salad oils require additional precautions against off-flavour generation, such as the addition of effective antioxidants.
Vitamin E, a natural antioxidant, has been used to prevent oxidation in feed fish diets containing high concentrations of lipids. Another example is the use of bland-tasting surimi to manufacture restructured beef steaks rich in omega-3 fatty acids.2 Incorporation of the antioxidant propyl gallate also has effectively been used to formulate manufactured fish steaks with an improved flavour.
The susceptibility of polyunsaturated fats to oxidise and deteriorate is greatly exacerbated by the presence of metals such as copper and iron. Iron is a well known pro-oxidant naturally present in fish muscle. This metal has been identified as most responsible for the generation of ?fishy? notes in omega-3 oils in both short-chain PUFAs (C18:3) from vegetable oils and in longer-chain PUFAs from fish oils.
In general, monounsaturated fats such as oleic acid (18:1) have been identified with cooked beef fat flavour. Saturated and monounsaturated fatty acids have been correlated to pork taste. The products of oxidation of linolenic and other polyunsaturated fatty acids have been positively identified with fishy and metallic flavours.
In one study where pigs were fed a diet high in linolenic acid (18:3), panellists found that the fat in bacon was considered to have a ?fishy? flavour.3 The specific compounds identified as having a fishy off-flavour after oxidation of fish oils were 2-trans, 4-cis and 7-cis-decatrienal.
A report on milk and mayonnaise fortified with fish oil identified more than 60 different volatiles with a strong fishy odour, including alkenals, alkadienals, alkatrienals and vinyl ketones. The most potent odours were identified as 1-penten-3-one, (Z)-4-heptenal, 1-octen-3-one, 1,5-octadien-3-one, (E,E)-2,4-heptadienal and (E,Z)-2,6-nonadienal, with 1-Penten-3-one reported as the major contributor to the unpleasant sharp-fishy off-flavour in fish oil.4
These off-flavours, generally identified as fishy, metallic and rancid, are considered one of the major deterrents for the increased consumption of fish and fish oils.
Omega-3s in food products
There is general consensus on the health benefits of fish oils, and most health institutions recommend increasing the consumption of omega-3 fatty acids.5,6 In fact, a number of fortified foods and supplements are already aimed at the general population, especially at babies and lactating mothers. Examples already on the market include oil capsules, fortified products such as milk and beverages, and prepared frozen fish products. Foods such as fresh fish and pre-cooked or frozen fish dishes like fish cakes, fish balls and sticks are also widely available, though their demand has not increased as desired by nutritionists. These products are successful in such countries as Norway and Japan.
Another way to increase omega-3 content in the diet is to introduce their consumption in conventional diets without asking the consumer for a radical change in eating habits.7 The idea is to enrich foods that are commonly consumed with PUFAs. Some reports have compared consumption of PUFAs through direct sources such as fish oil and flaxseed products versus indirect means such as margarines, lunchmeat, sausage, French onion dip and shelf-stored milk. Their results showed that the participants preferred the fortified conventional foods, such as margarine and lunch meats. This report also showed that participants had large increases in their blood levels of omega-3 fatty acids after two weeks of consuming omega-3 fats.8 This reinforces the notion that fortifying foods normally low in omega-3s with fish oils can be done without affecting consumers? eating habits.
Incorporating PUFAs as a supplement in animal feeds can also boost PUFA content in the animal meat and animal food products. Studies have shown that chicken feed supplemented with fish oils results in PUFAs in the chickens? lipid composition. The resulting broilers fed with 2 or 4 per cent fish oil in their diets had increased levels of PUFAs, compared to the control group. The resulting chicken meat used to manufacture frankfurters also had higher levels of omega-3s without appreciably increased fishy flavour in the chicken frankfurters.9 Similar effects are being obtained with eggs from chickens fed a diet high in omega-3 oils. These omega-3-rich eggs are now commercially available in Europe and the US. The recognition of the importance of omega-3 fatty acids in the human diet is the primary driver for new technologies and developments that will more easily enable formulators to integrate these oils into foods. Issues of stability, taste and processing degradation are being addressed. The concerns over mercury and PCB contamination are also hot-button issues that must be overcome before we see widespread consumer acceptance of fish oils.
Ernesto Hernandez, PhD, is head of the Fats and Oils Processing Program, Food Protein Research and Development Center, at Texas A&M University System.
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