Fatty acid analysis, TAG equivalents as net fat value, and nutritional attributes of fish and fish products

In this paper our quantitative analytical system for fatty acids in foods (Hyvonen et al., J. Food Comp. Anal. 6, 24-40, 1993) was applied to 33 fresh and processed fish, fish liver, and roe commodities sampled with our conventional system for average composition in a spring season from the Helsinki area. On the basis of total fatty acids, triacylglycerol (TAG) equivalents were calculated for the quantitation of net fat. Seven nutritional attributes for fish fat are also presented based on the fatty acid composition and physiological functions of fatty acids. The analytical system was readily applicable to fish and fish products and at most 34 different fatty acids were present at 0.1 wt% or more. Linear regression analysis of the crude fat (CF) (g/100 g of the fresh or frozen fish) determined with a chloroform-methanol (1:1) solvent extraction and the net fat value as TAG equivalents (g/100 g) gave the equation CF = 1.09 TAG + 1.79 (R(2) = 0.988). The equation demonstrates the risk for overestimation of the fatty acid contents, if the determination is based on analysis of relative fatty acid composition of the crude fat instead of direct quantitative fatty acid analysis. The net fat content of the various species of fresh fish varied from 0.7 to 8.9 g per 100 g of edible portion of fish. Liver and roe samples were about 6 g and processed products from 0.2 to 13.5 g of TAG equivalents. The proportion of the saturated fatty acids in fresh fish varied from 20 to 28 wt%, the range of the C12-C16 variable being 17 to 27 wt%. The stearic acid content was low (1.5-4.9 wt%). The cis monoenic fatty acid content in the fish fat varied greatly, from 16 to 51 wt%. The range of the total of all cis di- and polyenes was 25 to 61 wt%. The most characteristic nutritional attribute of fish fat, n-3 fatty acids, ranged from 24 to 58 wt%. The influence of vegetable oils/margarine on the nutritional attributes of fish fat was evident in such items as tuna and sardines in oil and frozen prefried fish sticks ("fish fingers"). In general the n-3/n-6 ratio of the fish species analyzed proved to be large, although significant differences (2.8-31.4) were found between species in this respect. A clear tendency toward higher n-3/n-6 ratios was determined in the fish species with lower net fat contents. The data support the recommendation to apply a quantitative fatty acid analysis also for fish and fish products for quantifying the net fat as TAG equivalents as well as for quantifying various nutritional attributes of the fish fats.