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Abstract

Survival of Vibrio spp. including inoculated V. cholerae 0139 during heat-treatment of cockles (Anadara granosa).

Abstract

Cockles (Anadara granosa) purchased from markets in Malaysia with an average weight including shells of 8.90±2.45 g were used. In experiment 1 heat penetration of individual cockles was examined. Cockles weighing <8 g (including shell) exhibited maximum internal temperatures of between 50 and 75°C when heated in water at 99°C for 10 s and 71-93°C when heated for 30 s. Cockles weighing >12 g exhibited maximum internal temperatures between 42-58°C when heated in water at 99°C for 10 s and 56-69°C when heated for 30 s. In experiment 2, heat-treatment of 10 cockles treated as a group at 99°C for 10 or 30 s resulted in reduction of levels of intrinsic Vibrio spp. from 5.73 to 3.15 log cfu g-1 or below 1 log cfu g-1, respectively. The levels of Vibrio spp. after heat-treatment decreased with an increase in numbers of cockles grouped together during treatment. In experiment 3 V. cholerae 0139 was inoculated into cockles and subjected to heat-treatment at 99°C for 0, 10, 15, 20, 25, or 30 s. The levels of Vibrio spp. in uninoculated, non-heat-treated cockles was 4.89 log cfu g-1 on a thiosulphate-citrate-bile salt sucrose agar (TCBS), and the predominant species were V. parahaemolyticus and V. alginolyticus. V. cholerae 0139 inoculated into cockles with an average weight of 13.5±1.90 g (including shell) decreased in samples examined immediately after heat-treatment from 6 log cfu g-1 initially to 3.5 log cfu g-1 after 25 s and <1 log cfu g-1 (TCBS) after 30 s of heat-treatment. The most probable number method (MPN) by enrichment in alkaline peptone water gave in general within 1 log unit higher counts than TCBS direct enumeration. TCBS direct enumeration and MPN counts were up to 2.38 or 1.30 log units higher, respectively, for samples heat-treated for 20 s or longer and stored for 6 h at 30°C before examination, than for samples heat-treated for same periods of time and examined immediately. It is concluded that a mild heat-treatment of cockles for up to 25 s is inadequate to ensure a large reduction in numbers of Vibrio spp., including V. cholerae 0139.