Shrimp freshness preservation : effects of chemical preservatives and modified atmosphere packaging on shrimp (Penaeus monodon) stored at chilling temperature

Abstract

Shrimps (Penaeus monodon) were caught in the local coastal waters and were kept alive in water tanks on board. Immediately after arrival to pier, the shrimps were sorted by size (12 to 15 cm) and kept alive in aerated tanks during the transportation to the laboratory within half an hour. The shrimps were then chilled to death by crushed ice in the laboratory. For whole shrimps, they were soaked in preservatives for 5 minutes and either stored in plastic boxes (packed with ice cubes) or packed in Nylon/PE bags. For peeled shrimps, they were de-headed, de-gutted and peeled before they were soaked in preservatives. The bags were then filled with gases of different composition by using vacuum packing machine. Samples packed with ice cubes were stored at 0-2 C while those packed in bags were stored at 2-4 C.. Objective tests in the research included Aerobic Plate Count (APC), lightness (L) measurement, Trimethylamine (TMA) determination, and Total Volatile Basic Nitrogen (TVBN) determination. Subjective tests conducted were sensory evaluations in which each panelist of the experienced 10-member panel evaluated 12 raw and 2 cooked shrimps. Cooked samples were made by blanching raw shrimps for 3.5 minutes. Panelists were asked to evaluate samples for odor, texture, integrity (shell-on shrimps only), color, appearance and overall acceptability (OA), using a 1 to 9 hedonic scale; and to evaluate melanosis using a 6 interval (0,2,4,6,8,10)scale. The shell-on shrimps treated with the combination of 1.25% sodium metabisulphite and 100ppm lysozyme could be kept in ice for 8 days which doubled that of the control samples. The shelf life of peeled shrimps treated with the same combination of preservatives stored in 40% CO2 and 60% N2 was at least 46 days while the shell-on control samples can be kept for only about 2 days. APC showed that the bacterial load in treated samples were still acceptable after such storage. TMA values increased generally, but there was no significant difference between different treated samples. The trend for the TVB-N study showed that TVB-N contents in shell-on shrimps were much higher than that in peeled shrimps. The TVB-N value may be used as a freshness indicator for the species because the OA scores correlated well with TVBN values. The TVBN level of samples were determined to be ranged from 44.9 to 58.3 mg/100g with average of 51.6 mg/100g at time of odor rejection. The TVBN levels in two batches of fresh shrimps were 27.3 mg/100g (standard deviation = 0.84) and 13.3 mg/100g (standard deviation = 0.73) respectively. Moreover, experimental data showed that lightness may also be used as a fresh indicator for raw shell-on shrimps because the lightness decreases while the OA score increases. The surface lightness of samples were determined to be ranged from 14.8 to 18.1 with average of 16.5 at time of melanosis rejection. The averaged surface lightness in two batches of fresh shrimps were 17.49 (standard deviation = 1.60) and 21.5 (standard deviation = 1.29) respectively.