Abstract
Fish provides a major source of proteins and polyunsaturated fatty acids which are known to prevent coronary heart diseases. However, inherent characteristics such as colour and texture prevent the effective utilisation of some fish, especially fatty pelagics for human consumption. Instead, they are used for fish oil and fish meal production. For this reason and for enhancing the utilisation of valuable resources, the physico-chemical properties, which may be species specific, were investigated in relation to two Namibian species, namely, horse mackerel (Trachurus capensis) and snoek (Thyrsites atun). The results showed that horse mackerel (Trachurus capensis) and snoek (Thyrsites atun) contain high contents of palmitic acids (29%), oleic acids (25%), and the omega-3-polyunsaturated fatty acids, such as docosahexaenoic acids (DHA) (15%) and eicosapentaenoic acids (EPA) (9%). The studies also showed that optimum protein extractability was obtained with 5% NaCl at pH 7.2 and the highest gel strength at pH 6.5-7.0. CaCO3 had detrimental effect on the water-holding capacity when used above 50mg.l-1 in washing water. Agar and sodium alginate extracted from Namibian Gracilaria verrucosa (red seaweed) and Laminaria schinzii (brown seaweed) respectively, were also characterised for their physico-chemical properties. Both the nontreated and treated agars were observed by colorimetric method to contain low levels of sulphate, an observation also confirmed by NMR and Raman FTIR spectroscopy. Sodium alginate was found to contain high content of guluronic acid (75%) by NMR, a good index of strong gelling ability. The good functionalities obtained as a result of optimising the physicochemical properties of fish proteins and seaweed polysaccharides enabled an investigation of the interactions of the two components. Studies on the interactions of the both horse mackerel and snoek with either sodium alginate or agar were undertaken by rheology, differential scanning calorimetry and phase contrast microscopy. Large deformation and small deformation rheological studies showed a decrease in the gel strength of fish proteins with the addition of either polysaccharide at all combinations. The DSC results indicated a lowering of the denaturation temperature with the addition ratios of increasing amounts of sodium alginate to fish proteins. Although the mixture of fish proteins and agar resulted in the lowering of the denaturation temperature, the shift; was not as pronounced as that obtained with sodium alginate. The reason for the lowering of gel strength and G' values was attributed to phase separation of fish proteins and polysaccharides which was reflected by phase contrast microscopy studies. This investigation has resulted in the characterisation of valuable fish and seaweed polysaccharide resources from Namibia which could be used in combination for the development of new food products.