Improving the safety and quality of raw tuna fillets by X-ray irradiation

In this study, raw tuna fillet (25 g) samples were inoculated by immersing in 0.1% peptone water that contained 108−9 CFU ml−1 of a three-strain mixture of Salmonella enterica for 1 min. The samples were then air dried at 22 °C for 30 min (to allow bacterial attachment) in the biosafety cabinet and were packaged separately in sterilized bags prior to X-ray treatments (0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 kGy). The surviving Salmonella populations on raw tuna fillets samples were evaluated using a nonselective medium (tryptic soy agar) for 6 h with xylose lysine desoxycholate (XLD) selective medium overlay. The plates were then incubated for an additional 18 h at 37 °C. Finally, the colonies were counted and the results were expressed as log CFU g−1. Furthermore, un-inoculated tuna samples (25 g) were packaged separately in sterilized bags and exposed to the lowest and highest X-ray doses (0.0 and 6.0 kGy), then stored at 5 °C, 10 °C or 25 °C for 25, 15 or 5 days, respectively. On the testing-dates, samples were withdrawn and microflora (psychrotrophic and mesophilic) counts, quality [color (using Hunter colorimeter) and texture (using Instron machine)] were evaluated. The results indicated that more than a 6 log CFU reduction of Salmonella population being achieved with 0.6 kGy X-ray treatment. Furthermore, treatment with X-ray significantly reduced the initial inherent microbiota on raw tuna fillets and inherent levels were significantly (p < 0.05) lower than the control samples throughout the shelf-life storage at 5, 10 or 25 °C for 25, 15 or 5 days, respectively. There was a significant effect of X-ray treatment on tuna color after treatment (day 0). However, no significant differences (p > 0.05) in color or texture of control and treated samples were observed after (day 0). These results indicated that X-ray is a good preservative technology for seafood products intended to be consumed raw.