Modified atmosphere packaging using CO, was demonstrated to be effective in retarding the growth of microorganisms during storage of fresh fish from the Gulf of Mexico. At 4"C, there was at least a log difference in bacterial counts at 2, 4, and 6 days between fish stored in CO, as compared to control fish stored without CO,. While the CO, atmosphere was shown to inhibit the growth of common spoilage types of bacteria such as gram-negative rods (Pseudomonas), stimulation of gram-positive bacteria such as Lactobacillus was also demonstrated. Fish stored at 4°C for 2-8 days in a modified atmosphere containing CO, had lower TVN values than fish stored at the same temperature without CO,. The results indicate that an extension of the shelf-life of fresh fish can be obtained by packaging and storing the fish in a CO, atmosphere.

Swordfish (Xiphias gladius) steaks were held in retail packages containing 100% CO2 and in mixtures of 40% and 70% CO2 in combination with either oxygen or nitrogen. Controls were stored in air. Samples were removed for chemical and microbiological analyses after 2–22 d of storage at 3.5°C. The inhibitory effect of CO2 on psychrotrophic, aerobic gram-negative spoilage bacteria was proportional to the CO2 tension in the packages. Maximum inhibition of growth was achieved with 100% CO2. Except for steaks stored in 40% CO2:60% O2 heterofermentative Lactobacillus spp. became a dominant part of the microflora of steaks stored in CO2-enriched atmospheres. Pseudomonas spp. continued to be a major part of the microflora of steaks stored in 40% CO2:60% O2. During the first 2 d of storage, there was a decrease in the surface pH of the swordfish steaks proportional to the CO2 tension in the packages. Swordfish steaks stored in CO2-enriched atmospheres had lower total volatile nitrogen (TVN), trimethylamine (TMA) and total volatile acid (TVA) values than steaks stored in air. Oxidative rancidity was not a flavor problem of fish in any of the atmospheres after 20 d of refrigerated storage.

The use of modified atmospheres containing C02 was effective in retarding microbial growth during refrigerated storage of retail packaged fresh brown shrimp (Penaeus aztecus). The inhibitory effcct seemed proportional to the C02 tension. The surface pH of shrimp stored in air increased rapidly during the initial storage period while a decrease in pH occurred in all the samples stored in C02enriched atmospheres. Shrimp stored in modified atmospheres also had significantly lower total volatile nitrogen (TVN) values as compared to shrimp stored in air. Changes in the head-space composition were observed in all the atmospheres tested throughout the storage period, and were largest for shrimp stored in air.

Sheepshead (Archosargus probatocephalus) fillets were stored in air and in modified gas atmospheres consisting of: 100% CO2, 80% CO2:20% O2, 60% CO2:40% O2, 30% C02:60% O2, 20% CO2:80% O2, 40% CO2:60% N2 and 44% CO2:36% O2:20% N2 At regular intervals during refrigerated storage, numbers and types of microorganisms and total volatile nitrogen (TVN) were determined. Increases in aerobic plate counts of fish fillets held in air and in 20% C02:80% O2 were greater than those for fillets stored in the other gas atmospheres. The most effective combinations of gas for limiting bacterial growth were 100% CO2 and 40% CO2:60% N2. Total volatile nitrogen values of samples stored in air and in 20% CO2:80% O2 increased similarly to those of fish held on ice. At higher CO2 concentrations, however, increases in TVN were slow and the rate of TVN production appeared inversely proportional to CO2 tension.

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