In vivo study of spoilage bacteria on polyphenoloxidase activity and melanosis of modified atmosphere packaged Pacific white shrimp.

This study investigated the effect of the three spoilage bacteria (Carnobacterium maltaromaticum, Shewanella putrefaciens and Aeromonas salmonicida) on the development of melanosis by inoculating the bacteria on modified atmosphere packaged shrimp. The three bacteria, which inoculated at about 5 log cfu/g, proliferated to a maximum level of 7.49, 6.86 and 6.89 log cfu/g, respectively at the end of storage. In regards to the effect of bacteria on melanosis, it was found that C. maltaromaticum did not display a significant effect on PPO activity and melanosis, and A. salmonicida showed an inhibitory effect on PPO activity with an activity value of less than 1.5 Units/ml. However, the PPO activity of shrimp inoculated S. putrefaciens was about one time higher than other samples and greater melanosis was displayed in the first 48 h. As melanosis can cause sensory quality loss, the growth of S. putrefaciens should be limited.

Impact of the O2 concentrations on bacterial communities and quality of modified atmosphere packaged Pacific white shrimp (Litopenaeus vannamei).

The importance of spoilage-related bacteria in fresh Pacific white shrimp (Litopenaeus vannamei) under different modified atmospheres (MAs) at 4 °C and the effect of O2 were demonstrated in the current study. The changes of bacterial communities in MA-packed shrimp during cold storage were studied by a combined method of plate counts with isolation and identification. Three gas mixtures were applied: 80% CO2 /5% O2 /15% N2, 80% CO2 /10% O2 /10% N2 and 80% CO2 /20% O2, and unsealed packages of shrimp were used as the control. In addition, the TVB-N, pH, whiteness index, and sensory scores were also determined to evaluate the quality changes of shrimp. MA packaging effectively inhibited the increase of total psychrotrophic bacteria counts and H2 S-producing bacteria counts by about 1.7 and 2.1 log cycles, respectively. The growth of Gram-negative spoilage bacteria in shrimp, including Shewanella spp., Aeromonas spp., and Pseudomonas spp., was inhibited by MA packaging, but the growth rate of Gram-positive bacteria such as lactic acid bacteria (LAB) and Brochothrix spp. were less affected by MA as effectively as Gram-negative bacteria. In comparison with the MA-packaged samples, the counts of H2 S producers in shrimp under a CO2 -enriched atmosphere with 20% O2 were slightly lower than the count in samples under an atmosphere with 5% O2 . However, MA with 20% O2 led to higher concentrations of TVB-N, and lower whiteness values and sensory scores. The shelf life of shrimp under 80% CO2 /10% O2 /10% N2 has been prolonged by > 6 d in comparison with the control according to the sensory scores.