Abstract:
This research focuses on studying the effects of freezing, an 'age-old technology', on Escherichia coli (E. coli) in Greenshell™ mussels (Perna canaliculus). Keeping food safety in mind, risk mitigation technologies were designed by using air blast freezing and frozen storage techniques in order to eliminate E. coli from New Zealand Greenshell™ mussels. Greenshell™ mussels were inoculated with a cocktail of ten different strains of E. coli and their survival was monitored over a period of 84 days. Two freezing temperatures (-10°C and -20°C) and two inoculum concentrations, i.e. low (104 colony forming units/g) and high (107 CFU/g) were used. Enumeration of E. coli was carried out at regular intervals using two methods: the conventional modified most probable number (MPN) method and the SimPlate® method. The use of different temperatures that is -10°C and -20°C did not have a major impact on E. coli counts although -10°C was slightly more effective at inactivating E. coli based on the regression analysis. The SimPlate® method correlated well (0.81 for -10°C and 0.75 for 20°C) with the conventional method, however, it was found to be less sensitive than the modified MPN method. A constant trend towards better E. coli recovery from Greenshell™ mussels was observed using the modified MPN method compared to the SimPlate® method although the modified MPN method was time consuming. To reduce the detection time, a modified cost effective rapid screening MPN method was evaluated. The advantage of the modified rapid screening MPN method is that only a fraction of media is required and results can be obtained within 12 hrs compared to the 24-48 hrs required by the modified MPN method. The rapid screening MPN method and the modified MPN method gave comparable recoveries for high inoculum (106 CFU/g). However, for low inoculum (103 CFU/g) of E. coli could not be detected by the rapid screening MPN method. Hence the rapid screening MPN method might not be a reliable analytical technique for its practical use in quality control laboratories. This research also investigated the effect of frozen storage on the two health claimable components eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in Greenshell™ mussels. EPA and DHA concentrations reduced significantly (p < 0.05) after storing Greenshell™ mussels at -10°C for a period of 90 days. However, EPA and DHA levels did not reduce significantly (p > 0.05) in mussels stored at -20°C. Microstructure analysis of the Greenshell™ mussels showed deformation in the tissue structures as a result of freezing.