Microbiological Safety of Ready-to-Eat Foods in Hospital and University Canteens in Hanoi, Vietnam.

ABSTRACT: The aim of this study was to analyze and document the microbiological safety and quality of ready-to-eat foods in hospital and university canteens in Hanoi, Vietnam. In total, 420 ready-to-eat food products from 21 canteens were sampled in July 2018 and May 2019. The ratio of samples exceeding the unsatisfactory level for total plate count was 31%. Escherichia coli, Listeria, and Staphylococcus aureus were detected in 35 (8.3%), 99 (24%), and 46 (11%) samples, with 3, 10, and 0% exceeding the unsatisfactory level, respectively. The total plate count, Listeria, Bacillus cereus, E. coli, and S. aureus ranged from below detection limit to 5 × 109, 4.6 × 105, 6.2 × 103, 3.4 × 103, and 7.6 × 103 CFU/g, respectively. L. monocytogenes was isolated from 3 (0.7%) of 420 samples. In addition, 21 (5%) of 410 samples were contaminated with Salmonella. Overall, our data indicate frequent problems with the microbiological quality and safety of these canteen foods in Hanoi and provide a baseline measurement that will allow environmental health officers and food microbiologists to develop targeted intervention strategies to reduce the economic and public health risk associated with these foods.

Carvacrol suppresses high pressure high temperature inactivation of Bacillus cereus spores.

The inactivation of bacterial spores generally proceeds faster and at lower temperatures when heat treatments are conducted under high pressure, and high pressure high temperature (HPHT) processing is, therefore, receiving an increased interest from food processors. However, the mechanisms of spore inactivation by HPHT treatment are poorly understood, particularly at moderately elevated temperature. In the current work, we studied inactivation of the spores of Bacillus cereus F4430/73 by HPHT treatment for 5 min at 600MPa in the temperature range of 50-100°C, using temperature increments of 5°C. Additionally, we investigated the effect of the natural antimicrobial carvacrol on spore germination and inactivation under these conditions. Spore inactivation by HPHT was less than about 1 log unit at 50 to 70°C, but gradually increased at higher temperatures up to about 5 log units at 100°C. DPA release and loss of spore refractility in the spore population were higher at moderate (≤65°C) than at high (≥70°C) treatment temperatures, and we propose that moderate conditions induced the normal physiological pathway of spore germination resulting in fully hydrated spores, while at higher temperatures this pathway was suppressed and replaced by another mechanism of pressure-induced dipicolinic acid (DPA) release that results only in partial spore rehydration, probably because spore cortex hydrolysis is inhibited. Carvacrol strongly suppressed DPA release and spore rehydration during HPHT treatment at ≤65°C and also partly inhibited DPA release at ≥65°C. Concomitantly, HPHT spore inactivation was reduced by carvacrol at 65-90°C but unaffected at 95-100°C.

Thermal inactivation parameters of spores from different phylogenetic groups of Bacillus cereus.

The thermal inactivation kinetics of spores from 39 Bacillus cereus strains belonging to six different phylogenetic groups (group II to VII) was studied. Fresh spore suspensions in glass capillaries were heated in an oil bath at three or more different temperatures for five different times. Survival curves and thermal death curves were established and the kinetic parameters DT (decimal reduction time at temperature T) and z (temperature dependence of DT) were derived by linear regression. Most strains (38/39) had survival curves without a pronounced shoulder or tail, as reflected by linear regression coefficients R(2) generally higher than 0.95. The heat resistance of the strains and groups of strains was then compared by determining the temperature (°C) at which logD=0.8 (TlogD=0.8). Spores from group VI strains showed significantly lower heat resistance than all other groups except group II, with TlogD=0.8 ranging between 82.7°C and 92.8°C. Spores from groups III and VII, on the other hand, were generally most heat resistant, with TlogD=0.8 between 91.9°C and 101.8°C. Further analysis revealed a positive correlation between spore heat resistance and both minimal and maximal growth temperatures of the strains. In contrast, the z value was negatively correlated with the minimal and maximal growth temperatures. The availability of genetic group-specific heat resistance data will contribute to a more accurate risk assessment of B. cereus.