Properties of South African fish oils: A review

Abstract

This review records original research done at the Fishing Industry Research Institute (FIRI) Programme on crude fish oils since 1985. This work has led to a better understanding of all the variables that determine fish oil quality. It has been shown that while high free fatty acid (FFA) contents in fish oils are due to spoilage of the fish prior to processing, escalation of FFA levels during oil storage is due to ineffective polishing of the oil. Unpolished or poorly polished oils have high phospholipid (P) and protein (N) contents and as a result also high water levels. These three variables cause FFA escalation during oil storage. Statistically derived expressions have been developed which allow prediction of the daily increase in FFA content during storage of oil from the original FFA content of the oil and its water, P and N levels. These expressions have been used to implement a more equitable pricing system for locally produced fish oils. The primary cause, however, for FFA escalation of fish oil is contamination of the oil by bacteria belonging to the genus Alcaligenes. These aerobic bacteria thrive at the oil‐water interface at the bottom of oil storage tanks, and convert mainly phospholipids into oil‐soluble FFA and water‐soluble phosphate esters. The oil‐soluble FFA, which have a considerable lower density than the oil from which they are derived, then diffuse upwards and so contaminate the oil. Total cholesterol levels in fish oil show a seasonal variation: low levels are found in summer months when the pelagic fish is fat, and high levels in winter when the fish is lean. The cholesterol consists of free and esterified cholesterol. The latter, which is difficult to remove in the refining process, increases during spoilage of the fish. The levels of chlorophyll in anchovy (Engraulis japonicus) and pilchard (Sarditiops ocellata) oils vary widely and are unpredictable. High chlorophyll levels in these oils may give rise to “green margarine” if insufficient bleaching is carried out on the oil during production of margarine. South African fish oils, especially anchovy and pilchard oils, are very rich in the two poly‐unsaturated fatty acids eicosapentaenoic (EPA) and docosahexaenoic acid (DHA), which are thought to be beneficial for various aspects of human and animal health. It was found that a high level of EPA corresponded to a low level of DHA and vice versa. This appears to be due to the fact that EPA and DHA are in competition for position two on the glycerol molecule. EPA and DHA levels of anchovy oils ranged from 14.3‐23.3% and 4.6‐12.5% respectively.