Effect of Packaging on the Major Microbial Flora of Irradiated Haddock

Abstract

A NUMBER of investigations has been made to determine the alterations in the microflora of fish caused by irradiation, and in some cases, the subsequent growth patterns of the survivors during refrigerated storage^1–4. These investigations have indicated that radiation drastically reduces the Gram-negative micro-organisms normally associated with fish or with fish spoilage and leaves larger numbers of the more radioresistant Gram-positive bacteria and fungi. The nature of the microbial spectrum surviving irradiation depended on the dosage and the initial level of contamination. Subsequent growth of the radiation survivors was then found to be a function of the storage temperature. One variable which has been relatively neglected is the effect of packaging and the accessibility of air on the growth patterns of irradiated seafoods, although certain investigations have indicated that this could be important for ground beef⁵ and for poultry packed and canned under nitrogen⁶. In order to evaluate the relationships between radiation, refrigerated storage, and the presence of air, haddock fillets were packaged in cans without hermetic sealing but covered with aluminium foil to allow free exchange of air (FP), sealed with a limited air headspace (LP), or vacuum packed (VP). The samples were then exposed to 150 krads and stored at 3° C for 3 weeks. For taxonomic identification of the various organisms the determinative scheme of Shewan⁷ was used for the Gram-negative organisms. Gram-positive organisms were determined by standard morphological and biochemical criteria. Three per cent hydrogen peroxide was used for detection of catalase, and for cytochromes the benzidine test was used⁸. The results indicated that, in addition to a shift in the major microflora caused by radiation, the microbial spectrum is further altered by the presence or absence of air during refrigerated storage.