Effect of Gamma-irradiation on Fish Meal

Abstract

The irradiation effect on components of a commercial fish meal and their storage effect were investigated. The numbers of contaminating microorganisms in the fish meal were as follows : total aerobic bacteria, 3.5×10⁵ ; coliforms, 1.4×10³ ; osmophilic moulds, 4.8×10² ; and fungi, 2.8×10² per gram. The osmophilic moulds, fungi and coliforms could be eliminated below their identification limit at 0.2, 0.3 and 0.5 Mrad, respectively, while the number of total aerobic bacteria could not be eliminated even at 1.0 Mrad. The crude protein content, pepsin digestibility, amino acid compositions and crude lipid content were not changed by irradiation up to 5.0 Mrad, while acid value and peroxide value were slightly increased. The increase in peroxide value by irradiation was depressed in the deoxyganated condition.
The number of osmophilic moulds in unirradiated fish meal, in which the moisture content was adjusted to 15.5%, increased during storage at 30°C. The number of moulds increased after 1 month storage of the 0.5 Mrad irradiated sample but the occurrence of the moulds was inhibited in the 1.0 Mrad irradiated sample during 2 months storage. The crude protein content and pepsin digestibility were unchanged during storage in both the irradiated and unirradiated samples, while the acid values increased. As the number of moulds increased, the moisture content and peroxide value increased, with the crude lipid content decreasing markedly during storage of the unirradiated samples. On the other hand, changes in irradiated samples could be controlled during storage.