The oxidative stability of chilled and frozen pilchards used as feed for captive southern bluefin tuna

Abstract

Freshly caught pilchards (Sardinops neopilchardus) were obtained from South Australia and subjected to chilled and frozen storage trials in order to determine their stability. The stability was gauged by visual and sensory assessment of the raw pilchards, by sensory assessment of the odour and flavour of cooked samples and by the chemical measures of peroxide value and levels of free fatty acids. There was considerable variability in fat content between individual fish with average content over all samples analysed in the trial being about 4 to 5%. In chilled storage, the pilchards exhibited obvious deterioration within two days. Substantial peroxide values were found and oxidised odours and flavours were clearly evident after 4 days' chilled storage. In frozen storage, oxidation occurred after only one month at a temperature of −20°C. This could be delayed if the fish were glazed with water. Vacuum packaging in a film of low permeability to oxygen was less effective than glazing and is not recommended due to cost. Pilchards in which oxidation had commenced before freezing continued to oxidise in frozen storage irrespective of whether they were glazed or vacuum packed. It was thus thoroughly demonstrated that the oil in the pilchards is very readily oxidised and careful handling, chilling, freezing and storage procedures need to be adopted to provide a product which is a nutritionally sound feed material for captive tuna. The demerit point scoring system was found to be a rapid evaluative technique useful for estimating the state of the pilchards.