Effect of Freeze-chill Technology and Modified Packaging on the Quality of Whiting (Merlangius merlcmgus), Mackerel (Scomber scombrus) and Salmon (Salmo salar) Portions
Freeze-chilling involves freezing and frozen storage followed by thawing and chilled storage. It offers logistic benefits for fish packers as it enables packaged fillets to be held frozen and then released into the chill chain as required. Trials with whiting, mackerel and salmon fillets/portions indicated no difference in odour scores (raw samples) between freeze-chilled and chilled samples; however, freeze-chilled salmon portions were inferior in terms of odour to chilled. Fresh fillets received the highest acceptability scores (cooked samples) followed by frozen, chilled and freeze-chilled samples. The pattern in the data was the same for each species and there was no statistically significant difference between the freeze-chilled and chilled samples. The total volatile base nitrogen (TVBN), trimethylamine (TMA) and total viable count (TYC) data were the same for the three species in that the chilled and freeze-chilled samples had the highest values and the fresh and frozen the lowest. However, there was no statistically significant difference between the freeze-chilled and chilled samples. Freeze-chilled samples had the highest free fatty acid (FFA) and peroxide values (PV) but the levels were low and did not influence sensory response. Gravity drip was significant in the frozen and freeze-chilled samples but presented no major visual problems and could readily be absorbed by drip pads. The effects of the four treatments on the colour and texture of the raw samples were small in practical terms. Freeze-chilling was then combined with modified atmosphere packaging (MAP) to try to extend the shelf-life even further. The MAP packs for mackerel and salmon (60% N2 /40% CO2), and for whiting (30% N2 / 40% CO2 / 30% O2) maintained their shape during freeze-chilling whereas packs with 100% CO2 were slightly imploded with concave sides. The chosen chilled shelf-life of 5-7 days in the MAP trials was vindicated by the results as the products were near the end of their shelf-life (in acceptability terms) after 5 (whiting and mackerel) and 7 (salmon) days. This compares with shelf lives of 3 and 5 days respectively for freeze-chilled fillets in air. Samples in MAP had lower total viable counts than samples in air for raw fillets/portions of each of the three species. However, MAP did not influence odour or acceptability scores, but had a variable effect (generally small) on fillet colour, springiness, drip loss, total volatile base nitrogen / trimethylamine content, peroxide values and free fatty acid contents. Freeze-chilling appears to be an acceptable method of preserving fresh fish and should have commercial advantages.
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