Processing and physiochemical studies of underutilised fish species (Boops boops L. and Trachurus trachurus L.) in Ghana

Abstract

The two pelagic species, the common bogue (Boops boops or Box boops. L.) and the Atlantic horse mackerel (Trachurus trachurus. L.), used are undervalued arid hence underutilised. The proximate composition was determined and the proteins of the species were characterised by amino acid compositions and electrophoretic patterns and fatty acid profiles. The interaction of the proteins with soya flour proteins were studied by observing the rheological properties of the mixtures as well as by Differential Scanning Calorimetry (DSC) and phase contrast microscopy. These studied were designed to provide information on the structure-function relationships of the mixed proteins and help optimise mixtures in product development and new protein applications. Products incorporating mince of the horse mackerel with soya flour and either maize flour or cassava flour were developed by the techniques of ftuidised bed drying and extrusion processing. The quality of the products was assessed by their chemical composition. Shelf life studies of the products packaged in either polyethylene or aluminium foil pouches were also undertaken. The Atlantic horse mackerel (Trachurus trachurus. L.), and the common bogue (Boops boops) had high protein contents of 17.7% and 18.3% (wtw}-and fat contents were 5.3% and 8.3% respectively. The major amino acids present in the Atlantic horse mackerel were Histidine (86.83 ~g/mg) and that of the common bogue was tyrosine (86.53~g/mg). However, like most fishes, the content of lysine and sulfur amino acids was high. The SDS - gel profiles of the Atlantic horse mackerel and the common bogue protein were quite distinguishable especially around the bands whose molecular weights are estimated to be 18,400 and 14,300 daltons. The fatty acid composition indicated that the two fish species were valuable sources of unsaturated fatty acids especially omega-3 polyunsaturated fatty acids. Small and large deformation results showed that gels formed as a result of the interaction of the fish protein and soya flour mixtures were weak. This could be attributed to negative interaction leading to the formation of incompatible gel structures. The Differential Scanning Calorimetry (DSC) studies also suggested that some form of interactions occurred in the common bogue mince/soya flour as well as the Atlantic horse mackerel mince/soya flour. This could be attributed to co-operative binding in a synergistic gelation. Phase separated micrographs showed transformations from continuous fish mince matrix to light scattering agglomerates of soya flour and the inclusion of mince within the soy agglomerates in the micrographs, which suggested a possible form of an interaction. This phenomenon was ascribed to macromolecues interacting in favour of self- association. However, fish mince and soya flour in the ratio of 75:25 appeared to form the best gel. Extruded fish snack pellets and fluidized bed dried products incorporating minced fish:soya (75:25) with varying amounts of either maize or cassava flour were successfully developed. Fluidized bed dried products and extruded snack pellets stored in aluminium pouches were more protected in terms of chemical changes in free fatty acids and to some extent TBA than polyethylene ones. The quality characteristics of fish products stored in the aluminium foil pouchs were fairly unchanged at the end of a 10-month storage period. Vacuum packaging due to the exclusion of oxygen tended to slow down lipid oxidation in the products