Influence of different processing methods on quality and shelf life of dried shrimp

Marine shrimp fishery contributes around 55% to the small shrimp production Sri Lanka. In addition to the targeted large shrimp for the export market, small shrimp are marketed locally as value added dried products. This activity has been established as a cottage industry. Development of optimum control conditions of processing is very important for maintaining nutritional quality of the dried shrimp. Present study was conducted to evaluate the effect of different salt concentrations, cooking methods, drying time and drying temperature on sensory properties of processed whole and peeled shrimp with a view to develop improved processing methods. In addition, sensory properties, total volatile nitrogen (TVN) content, trimethylamine (TMA) content and total bacterial count (TBC) of the processed whole and peeled shrimp which had been stored under different packaging systems viz. polythene bags, Styrofoam boxes and open bulk packages were assessed weekly for six weeks to determine the shelf life of the processed shrimp. The results revealed that the whole shrimp salted at 5% (w/w), pressure cooked for 20 minutes, dried at 70 C for 2 hours and the peeled shrimp treated under the same conditions and cooked only for 10 minutes gave the best quality dried shrimp. TVN contents (in mg/100g) in whole dried shrimp packed in Styrofoam box, polythene bags and under bulk packages were 25, 34.5 and 39.5 respectively at the end of six weeks. TMA contents (in mg/100g) in whole dried shrimp stored in three types of packing systems mentioned above were 16.8, 21, and 26.3 respectively whereas


Introduction
In the past, shrimp industry of Sri Lanka depended mainly on the wild shrimp catch.At present cultured shrimp also make a substantial contribution to the shrimp production and more than 60% of shrimp production is exported (Jayasinghe 1995).However, substantial portion of the low value small shrimp varieties such as Metapenaeus dobsoni, Penaeus indicus and Parapenaeopsis coromandelica are used for local consumption as they are not accepted for export market.Small shrimp are mainly used for preparation of salted, value added and dried products.In processing shrimp, adoption of proper handling practices in the production process is important to obtain a high quality nutritional product with higher consumer demand.It would also prolong the shelf life of the value added product.
Present study was conducted to evaluate the effects of different salt concentrations, cooking methods, drying times and drying temperatures on sensory properties of processed whole and peeled shrimp with a view to develop improved processing methods.It was attempted to determine the optimum processing conditions to maintain the natural quality of the whole and peeled processed shrimp.In addition, organoleptic properties, total volatile nitrogen (TVN) content, trimethylamine (TMA) content and total bacterial count (TBC) of the processed whole and peeled shrimp which had been stored under different packaging systems viz.polythene bags, Styrofoam box and open bulk packages were assessed weekly for six weeks to determine the shelf life of the processed shrimp.

Materials and Methods
Several Penaeus indicus samples (3 kg each) were collected from lagoon and coastal trawl fishery in Negombo and transported to the processing plant at National Aquatic Resource Research and Development Agency (NARA) at 0 0 C in ice packs and subsequently stored in a deep freezer at -20 0 C for 24 hours.Shrimp samples were thawed in a flooded water container for one hour.Then samples were washed with chlorinated chilled water until washing water became clear.The samples of shrimp were divided into two parts (1 kg each) and one portion was processed without peeling and other was peeled.The samples from each whole (unpeeled) and peeled shrimp were divided into several sub-portions and subjected to different treatments in triplicates as described below.

Treatment 1 -Determination of optimum salt concentration
Initially an attempt was made for evaluation of most palatable salt content from the four different salt percentages for both types of shrimp.Four different salt percentages (3%, 5%, 7% and 10%, w/w) were added and allowed to season for 30 minutes.Both types of shrimp were washed just after 30 minutes of salting and sensory evaluation was carried out for colour, odour, flavor and texture based on a 5 point hedonic grading scale (5 = Excellent, 4 = good, 3 = Fair , 2 = poor, 1= unacceptable).The best acceptable salt concentration was detected by trained panelists: a panel of seven judges.A score card was prepared to evaluate the quality of shrimp as described by Soudan (1978).Selected salted shrimp samples were stored in the deep freezer after sealing in the polythene bags for the rest of the experiments.

Treatment Π -Determination of optimum cooking time
Whole and peeled shrimp samples treated with best acceptable salt content (5% w/w) from the treatment Ι, was used for the treatment Π. Sub samples of 250g each from the salted whole and peeled samples were used for the cooking process.Four different time durations (10, 15, 20 and 25 minutes) were tested using a pressure cooker.Optimum cooking time was evaluated using the same sensory evaluation procedures as described above.

Treatment 111 -Determination of optimum drying time and drying temperatures
The shrimp samples having the best acceptable salt concentration and best acceptable cooking time resulted from the previous treatments were used in the treatment Ш. Sub-samples of salted and cooked whole and peeled shrimp (250g each) was selected to determine the optimum drying time and drying temperature.During the drying process, three different time periods were tested (1, 2 and 3 hours) at three different temperatures viz.50 0 C, 70 0 C and 90 0 C. The first set of samples were oven dried at 50 0 C, 70 0 C and 90 0 C for one hour and second and third sets of samples were oven dried at two and three hours respectively.Each sample was subjected to sensory evaluation process as described above.

Treatment IV -Evaluation of shelf life under different packaging systems
Considering the results of the above three experiments, samples of peeled and unpeeled shrimp (2 kg of each) were processed with most acceptable salt content, optimum cooking time, optimum drying time and drying temperature.These processed samples were used for evaluation of shelf life under different packaging systems.
Sub-samples of both peeled and unpeeled end product (100g each) were packed using three different packaging systems: polythene bags, Styrofoam boxes and open bulk packages.The packed samples were stored at ambient temperature.Seven samples from each packaging type were analyzed thrice at weekly intervals for the determination of total bacterial counts (TBC), total volatile nitrogen (TVN) and trimethylamine (TMA) contents.TBC of the samples were determined following Speck (1984).TVN and TMN contents in the samples were determined as described by Conway (1968).

Data analysis
Data are presented as mean ± standard deviations of a number of replicates.Statistical analysis was carried out using SAS statistical analysis package system (SAS 1985).Analysis of variance, generalized liner model was used to determine least significant differences between treatments at p< 0.05 level.

Results
Organoleptic scores (saltiness, flavor and texture) given by the trained panel to whole and peeled shrimp treated with different salt concentrations are presented in table 1. Statistical analysis revealed that there are significant differences among the organoleptic scores .The best preference was given to both whole and unpeeled shrimp treated with 5% salt content.
Table 2 indicates the organoleptic preference for salted (5%) whole and peeled shrimp which had been cooked for different time periods.According to the results, 20 minutes cooking time was found to be the best acceptable time for salted whole shrimp.For peeled shrimp, the salted shrimp which had been cooked for 10 minutes was given the maximum preference.Statistical analysis revealed that there are significant differences among organoleptic scores given to the product processed under different cooking times.
Table 3 indicates the organoleptic scores recorded for salted and cooked whole and peeled shrimp dried at different temperatures for different drying times.The drying temperature of 70 0 C was found as the best preferred temperature for both types of shrimp.Organoleptic scores obtained by the product processed under different drying temperatures, 70 0 C was found to be the best acceptable.Considering different drying times, 2 hours drying at 70 0 C was found to be the best.Shrimp stored in Styrofoam boxes (Carton) generally obtained the highest organoleptic scores for both peeled and whole (unpeeled) shrimp in comparison to the shrimp stored in polythene or bulk packages (Table 4).Tables 5 and 6 show the changes in TVN, TMN and TBC for whole and peeled dried shrimp stored at ambient temperature under different packaging systems.The TVN contents of the both bulk packed peeled and unpeeled shrimps were relatively high.This value ranged from 17.8 to 39.5 mg/100g unpeeled shrimp in bulk packages over the six weeks storage period.The TVN values of shrimp stored in Styrofoam packs were significantly lower than those stored in the other packages.
The peeled shrimp in open bulk package, had the highest TMA range (10.6 to 25.2 mg/100g) during six weeks.The lowest TMA ranges (4 to 16.8 and 7.46 to 15 mg/100g) were recorded for the samples packed in Styrofoam and polythene packed samples respectively.For the whole dried shrimp, the lowest TMA ranges (3.73 to 16.8 mg/100g) were recorded for packs in Styrofoam boxes.The highest TMA range (11.2 to 26.3 mg/100g ) was recorded for the samples packed in bulk packages.
TBC values recorded for the unpeeled shrimp stored in Styrofoam boxes and polythene were significantly lower than those values recorded for the samples stored in bulk packaging methods.TBC values of the peeled shrimp stored in Styrofoam boxes were significantly lower when compared to the other packaging methods (Table 5 and 6).However the TBC increased from 1.3 x 10 5 to 1.08 x 10 6 cfu/g in whole shrimp stored in Styrofoam boxes over the six weeks storage period.

Discussion
In the present study, salting at 5% level followed by pressure cooking for 20 minutes and drying for 2 hours at 70 0 C were found to be the preferred method of processing that resulted in the most acceptable end product for whole dried shrimp.For peeled dried shrimp, salting at 5% level, and pressure cooking for 10 minutes followed by drying for 2 hours at 70 0 C were the most acceptable conditions for processing.Zaitsev et al. (1969) stated that the process of peeled dried shrimp consumes less time compared to the whole dried shrimp as indicated in the present study.
The most palatable salt content of the product is 5% for both peeled and whole (unpeeled) shrimp.This is due to enhanced water protein relationships of the product.However, addition of salt disturbed the flavor properties of the raw shrimp.The most acceptable time for cooking for 5% salted, whole shrimp is 20 minutes.The 20 minute heating period appeared to accelerate the nutritional reactions and improve the colour and flavours in the product.The biological reactions have an optimum temperature at which the reaction rate reaches a maximum as pointed out by Price and Schweiger (1971).It also accelerates the amino-sugar reactions and improves the colour and flavour properties in the product.
In the present study, shrimp packed in Styrofoam box showed significantly lower bacterial growth.Styrofoam packages have very much effective barrier properties, resist bacterial contaminations and reduce the bacterial multiplication inside the package.TMA and TVN values are quality indicators of the processed product and the upper recommended values are 12 and 30 mg/100g respectively.For whole shrimps stored in Styrofoam boxes, polythene and packed in bulk had the respective optimum storage times between three and five weeks.Same trend was observed for peeled shrimp as well.TVN concentration in the shrimp did not exceed the recommended levels in all packages.The maximum shelf life of five weeks is recommended for Styrofoam for both types of shrimp packages .
standard deviation of organoleptic scores Means with different superscripts across columns differ significantly.(p < 0.05)-

Table 1 :
Organoleptic quality* of whole and peeled shrimp treated with different salt concentrations (%).

Table 4 :
Variation in organoleptic scores* of salted (5%), cooked dried shrimp stored under different packaging systems for six weeks.