Quality Indicators of Northern Shrimp (Pandalus borealis)

Qingzhuof the main spoilage-causing microorganism was restrained by the liquid ice. Similarresults were shown by the bacteria measurements (Figure 19).A comparison of the rates of TMA formation during 6 days of storage revealed that saltwatericed group (S-ICE/-) and iced group (ICE/+), in which TMA values exceeded10 mgN/100g, spoiled earlier than other two groups where TMA level remained below10 mgN/100g until day four of storage. The extent of increase in TVB-N and TMA ofshrimp stored in liquid ice at -1.5 o C were considerably smaller than for sample groupsstored under other conditions (S-ICE/-, ICE/+).5.3 Change in pH of whole shrimpThe pH of shrimp meat gives some valuable information about its quality change.Significant differences were noticed from Figure 9 and the results of statistical analysis.There was a continued increase in pH for all sample groups, probably due metabolism ofmicroorganisms producing alkaline compounds like amines formed by deamination ofamino acids (Huss, 1988, Jackson et al. 1997). The initial post mortem pH varies withspecies, catching ground and season. Usually pH decreases during anaerobic formation oflactic acids during the first hours after death, but microbial metabolism leads to anincrease in pH during storage time. This is in good agreement with Krishnakumar et al.(1985) who showed reduction of total nitrogen in fish stored in ice sea-water and icebecause some compounds contained nitrogen were leached out. The pH changes, showedgood correlation with sensory and microbiological results. The pH changes also reflectedTVB-N and TMA accumulation and indicated the spoilage progress.5.4 CO and NH 3 responses of electronic nose measurementResults from electronic nose measurements indicate that response of NH 3 sensor can beused to evaluate the shrimp quality in a similar way as TVB-N, TMA and TVC (Table 4).The rapid onset of NH 3 production at low bacterial cell densities indicates that autolysismay be causing the production during the first day of storage and the rapid increase in therate of production during following days of storage indicates a bacterial contribution asreported by Lakshmanan et al. (2002), who found that the amine-forming bacterialpopulation in fresh shrimp was slightly higher (10 2 cfu.g -1 ) than in fish. Olafsdottir et al.(1997a, 2002) reported that the NH 3 response of electronic nose measurement gave thebest prediction of TVB in capelin raw material and was similar to the informationprovided by TMA for redfish. The CO sensor showed lower responses than the NH 3sensor, but similar overall trends for all the storage groups except for the group stored inICE/+ on day 6. It can be speculated that the decline in the CO sensor could be explainedby specific spoilage flora utilizing different substrates for their growth and thus formdifferent volatile degradation compounds. It is well known that the development ofmicrobial metabolites changes because of competition of the microflora for availablesubstrates. However, this can not be confirmed because only one measurement was done.The lower responses of the CO sensor indicating lower spoilage level of samples ofshrimp stored in ice is not in accordance with results from other indicators that were TVC,TVB-N and TMA in the trial. This should be studied further in combination withUNU-Fisheries Training Programme 35