Reducing the presence of fishy flavours and odours in fish oil-fortified processed foods is a major technological hurdle. Xiangqian Zhu, PhD, and Amit Taneja investigate the issues and challenges for food formulators
A group of functional foods ingredients that has been well researched and is well accepted by consumers in recent years is naturally occurring long-chain n-3 polyunsaturated fatty acids (PUFAs). The n-3 PUFAs mainly include alpha-linolenic acid (ALA) and its long-chain metabolites eicosapentaenoic acid (EPA) and docosohexaenoic acid (DHA). Major sources of ALA include oils of flaxseed, canola and walnut, while the primary sources of EPA and DHA are marine products.
Recognition of the health benefits have led to the fortification of bread, dairy products, eggs, pasta, biscuits, margarines and other spreads. One of the limitations of the currently available omega-3 fortified foods is that for an individual to meet the daily target for the omega-3 fatty acids, he or she needs to consume the food in large amounts.1
There is a significant market opportunity to protect omega-3 in such a manner that it can be added to different food matrices so that a total daily requirement for a defined health benefit (or a high proportion of it) can be met in a single daily serving. This presents a major challenge on several fronts, including the price of final products, market positioning, and, more importantly, the taste and flavour of the food product.
There are numerous recommendations concerning the amount of omega-3s (EPA + DHA) needed in the human diet. Two oft-quoted authoritative sources are the International Society for the Study of Fatty Acids and Lipids (ISSFAL), which recommends a total intake of 500mg EPA + DHA per day, and the UK Scientific Advisory Committee on Nutritition (SACN), which recommends 450mg EPA + DHA per day. For some conditions recommendations exceed 1,000mg/day.
The n-3 PUFAs are highly unsaturated fatty acids, which make them susceptible to oxidation, which leads to development of rancidity and off-flavours. The oxidation process occurs by the free radical chain mechanism, involving initiation, propagation, chain branching and termination.2
In the initiation step, free alkoxyl radicals are generated by initiators, which could be metal ions in the lipid or unstable complexes in the lipid generated by heat or light. The free alkoxyl radicals start a new chain reaction with oxygen to form peroxyl radicals and hydroperoxides in a propagation step. One hydroperoxide breaks to form one free alkoxyl and one peroxyl radical in a branching step. Two free radicals can also form a nonradical product by termination. During the chain reaction, the carbon chain of n-3 PUFA is broken, and consequently ketone and aldehyde compounds, which contribute to off-flavours, are generated as secondary products. Important compounds that contribute to off-flavours include 1-penten-3-one; cis-4-heptnal; and trans,cis-3,5-octadien-2-one.3
Generally, it is difficult to completely protect n-3 PUFAs from oxidation. A number of approaches can be used to retard oxidation. Addition of antioxidants, such as alpha-tocopherol, ascorbyl palmitate and BHT (BHA), to PUFA oils can improve stability against oxidation,4 but the effects are rather unpredictable and depend on the composition of fatty acids and minor components in the oil as well as the specific combination of different antioxidants.
The mechanism of oxidation changes when antioxidants are presented in the system. An antioxidant molecule reacts with a free alkoxyl radical to form a lipid molecule and a free antioxidant radical. The free antioxidant radical reacts only with free alkoxyl or peroxyl radicals to terminate the oxidation. Clearly, the antioxidant will be consumed by the free radicals.
Another kind of chemical called a chelating agent can be used to slow down oil oxidation. Chelating agents are able to remove the initiators from the system, hence reducing the possibility of the chain reaction.5 Most common initiators in food systems are metal ions such as iron and copper. Chelating agents such as EDTA and citrate have a strong ability to bind to metal ions and stop them from initiating the chain reaction.6 However, free radicals generated during heat treatment or exposure to light cannot be removed by the chelating agents.
It is important to control the factors that catalyse oxidation during the processing and storage of products rich in PUFAs. High oxygen pressure, large surface, heating and light promote the chain reaction and propagation of the lipid oxidation process. Processing and packaging in an oxygen-free environment will cut off the oxygen supply for the chain reaction, but it is extremely difficult to achieve a totally oxygen-free environment. Trace amounts of oxygen left in the system still lead to the damage of fatty acids. Processors may use plastic containers that are impermeable to oxygen and light.
Microencapsulation of PUFAs is a key technology that has proven practical in delaying or inhibiting oxidation and thus helping to mask undesirable fishy odours and flavours in the final product. A number of companies manufacture and sell microencapsulated fish oil powders for use in food products. These technologies have allowed the fortification of frequently consumed foods such as breads, biscuits, soups, fruit juices and spreads with reasonable consumer acceptability.
However, the levels of incorporation that can be achieved with existing technologies are still low. Further work in this area needs to concentrate on the development of convenience foods, fortified with much larger amounts of n-3 fatty acids per serving, in a palatable format.
The Riddet Centre, Massey University, New Zealand, a centre for research in food and biologicals, has recently developed a patented fish-oil emulsion that reduces the presence of fishy flavours and odours when used in food products. Examples of some of the food products tested to date are shown in the table below. A unique protein-based emulsifier system with inbuilt antioxidant activity has been developed to achieve these characteristics. The emulsion contains 5-50 per cent oil and can be added to food products in relatively large amounts without detection by consumers. The emulsion also prevents unpleasant regurgitation aftertastes normally associated with fish-oil consumption.
Choosing a product
New patents on microencapsulation are being filed regularly, and more food ingredients companies are introducing new omega-3 products, usually in the form of emulsions or powders. They promise better protection against oxidation, but cannot ensure that the final product will be free of fishy flavour and odour. These micro-encapsulated omega-3 products have greater success rates in simple food applications such as low shelf-life products (bakery and certain dairy products).
For challenging food systems, though, simply mixing oils or emulsions into food products isn't enough. The product must be fine-tuned with rigorous testing of flavours and odours over time. Common reasons for the failure of encapsulated products in food applications are pH, shear and presence of minerals.
Knowledge of the final product formulation and the production environment is critical. Usually, changes in recipe and/or processing method of a food product permit low levels of omega-3 additions, but a modification in product packaging and further addition of antioxidants may be required for high levels of addition in order to claim health benefits of omega-3s on the product label.
Although all microencapsulated omega-3 emulsions and powders have added antioxidants along with proteins and carbohydrates, this does not guarantee protection in the final food application. Most omega-3 suppliers now invest time and effort to work with the customer throughout the product development process to overcome processing obstacles, including choosing the right antioxidants and potentially flavour-masking agents for the final product.
Innovative marketing strategies are required to harness the strengths of the omega-3 revolution. Companies that are able to meet the current technical and marketing demands of omega-3 products are sure to reap rewards.
Xiangqian Zhu, PhD, a researcher at Massey University in New Zealand, specialises in omega-3 fatty acid oxidation and antioxidation in emulsion system, flavour-food interaction and flavour release. Amit Taneja specialises in food formulations, product concepts, sensory evaluation and functional foods at Massey.
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1. Garg ML, et al. Means of delivering recommended levels of long chain polyunsaturated fatty acids in human diets. J Food Sci 2006;71(5):r66.
2. Kamal-eldin A, Yanishlieva NY. N-3 fatty acids for human nutrition: stability considerations. Eur J Lipid Sci Technol 2002;104:825-36.
3. Venkateshwarlu G, et al. Modeling the sensory impact of defined combinations of volatile lipid oxidation products on fishy and metallic off-flavours. J Agric Food Chem 2004;52:311-7.
4. Huang D, et al. The chemistry behind antioxidant capability assays. J Agric Food Chem 2005;53:1841-56.
5. Lapidot T, et al. Lipid peroxidation by free iron ions and myoglobin as affected by dietary antioxidants in simulated gastric fluids. J Agric Food Chem 2005;53:3383-90.
6. Let MB, et al. Protection against oxidation of fish-oil-enriched milk emulsion through addition of rapeseed oil or antioxidants. J Agric Food Chem 2005;53:5429-37.