What happens to the microflora of retail sushi in the warm season?

Sushi is a perishable ready to eat product composed by several raw ingredients, and the storage temperature is crucial in the maintenance of satisfactory hygiene. Aim of this study was the microbiological characterization of sushi mimicking a thermal abuse likely occurring in the summer season. Mixed sushi (rolls and nigiri) produced in a small scale factory in Northern Italy was stored for 2 h at 12°C (―transport‖) and subsequently for 4 days at 8°C (―home storage‖) and daily submitted to microbiological analyses coupled with the control of organoleptic quality. Total viable Count was above 5 Log since the production day and was mainly constituted by Pseudomonas spp.; the values increased during storage overcoming the 6 Log level from day 3, and reaching level above 8 Log CFU/g at the last sampling time. From a sensorial point of view, from the second day a decay in odour and colour was observed. LAB showed a gradual increase never overcoming 6 Log CFU/g, while Enterobacteriaceae increased and overcame 4 Log CFU/g after 2 days. Yeasts showed a moderate growth (always <5 Log CFU/g) while Bacillus cereus, Staphylococci and Clostridia were generally below the detection limits. Listeria monocytogenes was never detected. A reduction of shelf-life from 3 to 2 days should be applied especially in particular warm months in order to limit bacterial replication.