Subramaniam Sathivel, Huaixia Yin, Witoon Prinyawiwatkul, Joan King and Zhimin Xu
During the past 20 years, interest has increased in dietary effects of omega-3 fatty acids because they play a major role in human health. Natural fish oils are thought to help maintain heart and vascular health in humans. Producing and purifying oil from catfish for the growing fish oil market can benefit the Louisiana catfish industry.
Internal organs – or viscera – from channel catfish (Ictalurus punctatus
) are an abundant and underused byproduct that can be a unique source of edible oil. The four major commercial U.S. catfish-producing states – Alabama, Arkansas, Louisiana and Mississippi – in 2005 produced more than 300,000 tons of catfish with a stable monthly production of about 25,000 tons. The byproducts of catfish processing consist of heads, bones, skin and viscera, which often end up in landfills or rendering plants. A processed catfish yields around 45 percent whole fillets and about 55 percent waste. The average weight of viscera is about 9.35 ounces, which is about 10 percent of live weight.
Much of the oil in catfish is in the viscera, which contain approximately 33 percent fats – or lipids – which could be converted into edible oil or biodiesel products. With a growing world market demand for fish oil, the catfish industry could handle the byproducts from fish-processing operations with care and be able to sell these as raw materials for producing fish meal and oil.
Small fish-oil processors and entrepreneurs may be interested in a small-scale, cost-effective oil extraction, clarification and stabilization process for potential human consumption. Fish oil can be extracted using a number of methods, including rendering, enzymatic hydrolysis, chemical extraction, mechanical pressing and centrifugal force.
The conventional method of extracting oil from fish is by rendering, a process in which high heat is used to extract fat or oil mainly from animal tissues. Almost all animal fats are recovered by rendering, whereas vegetable fats are obtained by crushing or solvent extraction or both. In general, rendering can be wet or dry. Wet rendering is carried out with large amounts of water. The fat cell walls are broken down by steam under pressure until they are partially liquefied and the released fat floats to the surface of the water. Separated fat may be removed by skimming or by centrifugal methods.
Fish oil’s current high value in the marketplace is due primarily to its long-chain, polyunsaturated, omega-3 fatty acids. These fatty acids are very sensitive to oxidation, especially caused by heat. LSU AgCenter researchers have developed a low-temperature fish oil-extraction process that will minimize fatty acid oxidation and purification costs.
Unpurified fish oil contains free fatty acids – primary oxidation products, minerals, pigments, moisture, phospholipids and insoluble impurities – that reduce oil quality. The amount of these impurities present in the oil may depend on the extraction techniques. The longer these components remain in the oil, the greater their negative effect on oil quality; therefore, these components need to be removed before the oil will be acceptable in many markets. Conventional fish oil refining steps are degumming, neutralizing, bleaching and deodorizing. Degumming removes phospholipids, proteins and other some compounds; neutralization removes free fatty acids, which are precipitated as soaps; bleaching uses clays to adsorp pigments; and deodorization removes oxidized components.
LSU AgCenter researchers have developed a low-cost adsorption process to remove non triglyceride materials. Chitosan from crawfish and shrimp, activated carbon from agricultural byproducts, fish bones or other low-cost materials can be used as adsorbents to remove the non-triglycerides. Compared with conventional refining methods, adsorption technology offers lower refining losses, less lipid oxidation and less flavor reversion in the refined oil. Adsorption technology can potentially provide a simplified process for refining catfish oil for human consumption.
The number of food products that are fortified with long-chain omega-3 fatty acids is growing dramatically. However, attempts to incorporate fish oil into food formulations has had limited success because of "fishy" flavors in the finished products – the main problem of food enrichment with omega-3 acids. One technology for overcoming these problems and also reducing oxidation of omega-3 fatty acids is microencapsulation of fish oil. The microencapsulation process makes it possible to transform the oil into a powder in which small droplets of oil are surrounded by a "shell" of proteins and/or carbohydrate. The result is small, dry granules that have powder-like flow characteristics. The microencapsulation process improves the oxidative stability of fish oil and extends its shelf life to 12-24 months when stored in a dry cool environment.
Microencapsulation of fish oil can provide many benefits, such as an oxygen barrier that extends shelf life, a taste profile barrier that eliminated fish-oil taste and odor, and high nutritional density and nutritional availability. The process also provides a protective barrier when oils are incorporated into food products.
Microencapsulated fish oil powders are commercially available and used in baked goods, beverages and milk products. Microencapsulated catfish oil also could be used as an enrichment in a wide assortment of foods such as milk, bakery products, salad dressings, juice drinks and other nutraceutical products. In addition, microencapsulated fish oil powders may have appeal to broader groups of consumers when used in snack foods, breads, cookies, pizza toppings, chicken nuggets and sausages. Pet foods can be another lucrative market for owners who may want the health benefits of long-chain omega-3 fatty acids for their pets.
Extracting and making use of oil from catfish viscera could bring more revenue to catfish processors in Louisiana. Incorporating adsorption technology for fish-oil refining could provide a simple, cost-effective technology for purifying catfish oil for use as a food ingredient. Microencapsulation can add value to oil produced from catfish by-products and benefit the Louisiana economy. Subramaniam Sathivel, Assistant Professor; Huaixia Yin, Graduate Student; Witoon Prinyawiwatkul, Professor; Joan King, Associate Professor; and Zhimin Xu, Assistant Professor, LSU AgCenter, Department of Food Science, Baton Rouge, La. (This article was published in the summer 2008 issue of Louisiana Agriculture.)