Spoilage microbes’ effect on freshness and IMP degradation in sturgeon fillets during chilled storage

Bacterial spoilage is an important reason causing freshness deterioration of fish during postmortem storage. The aim of this study is mainly to investigate the role of bacteria in freshness and the degradation of inosine 5′-monophosphate (IMP) as well as nucleotide derived values in chilled sturgeon (hybrid, Acipenser schrenckiid × Huso dauricus) fillets using a microbial reduction pretreatment. During storage at 4 or -1 °C, lower total bacterial count and total volatile base nitrogen in the treated samples was observed when compared to the control samples. But very close K, Ki value and IMP was observed in both groups, although higher IMP was recorded at day 6 and 9 at 4 °C. In later storage period, accelerated decrease of inosine (Ino) and increment of hypoxanthine (Hx) occurred in two control groups, resulting in a rapid increase of G and H values, significant higher than the treated groups. Six predominant spoilage bacteria (Acinetobacter johnsonii, Acinetobacter lwoffii, Shewanella baltica, Pseudomonas Fragi, Pseudomonas koreensis and Pseudomonas antarctica) were isolated from the sturgeon fillets during storage. Based on the results of respective co-cultivation of IMP with the six species, only Acinetobacter johnsonii had ability to degrade IMP when reached a high density of 2.84 × 109 CFU/mL. Results suggested that microbial reduction pretreatment could better the freshness of chilled sturgeon fillet. Comparing to endogenous IMPase, spoilage bacteria probably played a small role in IMP degradation, but affected considerably the decomposition of Ino to Hx.