While cold storage and freezing are the normal methods for fish preservation, they do not completely inhibit the its deterioration in quality.
Oily fish, such as salmon, mackerel, and sardine, are rich in omega-3 fatty acids. As the significant structural components of the phospholipids of cellular membranes, omega-3 fatty acids, particularly α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic 11 acid (DHA), are very important for human health. EPA and DHA play vital roles in prevention of cardiovascular diseases and some types of cancers, including colon, breast, and prostate. DHA is essential for brain functioning in both infants and adults.
The inclusion of plentiful DHA in the diet not only prevents brain aging and Alzheimer's disease, but also improves the recovery from certain visual dysfunctions. The American Heart Association (AHA) recommends the intake of 1g omega-3 fatty acids per day as a safe and effective way to obtain the heart health benefits.
Omega-3 fatty acids are found mainly in fat-rich fishes, but many lean fishes, such as catfish, cod, flounder, grouper, haddock, hake, perch, and swordfish, contain very low content (<0.5 g/100 g portion) of this nutraceutical compound.
hitosan, a cationic polysaccharide obtained from crustacean shells, is a well-known film-forming biopolymer with a broad antimicrobial activity against bacteria and fungi. Chitosan-based films and coatings have been applied as a microbial hurdle in a variety of food, including fruits and vegetables, eggs, cheeses, and meat, for improving overall food quality and prolonging storage life. Chitosan has also been applied as an edible invisible film for preserving fresh fillets of Atlantic cod and herring, and its preservative efficacy has been exhibited by the reduced moisture loss, lipid oxidation, and growth of microorganisms in the tested fishes.
Work by a group at Oregon State University (OSU) in 2004 found that chitosan was a good film-forming material for developing mineral and vitamin E fortified edible films and coatings.
At that stage, however, the application of omega-3 fatty acids fortified chitosan films and coatings to improve seafood quality had not been reported.
The same group, under OSU food science professor Yanyun Zhao, has returned to the topic with a new paper published in Food Chemistry.
In it the group study lingcod, a popular eating fish unique to the west coast of North America and one economically important in coastal and estuarine environments from Alaska to California.
Although a good source of protein and high in vitamin B12 and selenium, its fat content is very low, with 1.06 per cent total lipid and 0.3 per cent total polyunsaturated fatty acids being reported in the US Food and Drug Administration (USDA) nutrient database.
This new study aimed to evaluate the application of fish oil incorporated chitosan coatings for improving physicochemical and microbial qualities and enhancing the omega-3-fatty acids content of lingcod fish during cold and frozen storages.
By dipping the lingcod fillets into this edible, protective coating enriched with fish oil, the researchers have extended their shelf-life.
"With this coating, you can easily keep the fillets in the display case for two to three more days," said Zhao.
What's unusual about the OSU study is that fish oil was added to the chitosan coating, which wasn't visible once it dried. After the coating was applied, some fillets were refrigerated for three weeks while others were frozen for three months.
The study found that the coating tripled the omega-3 fatty acids in the refrigerated and frozen fish when compared against the uncoated fish (Fig. 1).
In addition to increasing the omega-3 levels in the lingcod, the OSU study also found that the coating reduced lipid oxidation, which causes rancidity, in the refrigerated and frozen samples when compared with the uncoated fillets.
The coating also kept the fish moister than the uncoated samples as the frozen ones were thawing. Additionally, the coating delayed the growth of microorganisms in the fresh fillets, and it prevented their growth in the frozen ones. The coating did not affect the colour of the fillets.