Comparison of the effect of sodium alginate, sodium caseinate and gelatin coatings in combination with thyme essential oil on shrimp shelf life

The most important way to keep shrimp is freezing. But this technique may change the quality of the product at the end of the period, depending on the method of freezing, temperature fluctuations, freezing rate. On the other hand, some consumers tend to consume non-frozen shrimp (refrigerator temperature). Also the use of chemical preservatives raises concerns for consumers because they are harmful. Therefore, it is necessary to study about the use of preservatives that are both natural and able to maintain shrimp quality in a short period of time under non-freezing conditions. The present study was carried out to evaluate and compare the performance of three coating of soduim caseinate- thyme, soduim alginate- thyme and gelatine- thyme in preserving the chemical and microbial properties of Litopenaeus vanamei fillets at refrigerated temperatures (4±1ºC).

Fresh shrimp fillets from Gomishan (Golestan province) was transferred to the laboratory after 3 hours in ice containers and stored under appropriate conditions after being cut, peeled and washed. After preparation of the films, shrimp fillets were coated with Immersion method and were stored at refrigerator temperature in the form of four treatments of 1 (fillet, control), 2 (fillet + sodium alginate 4% - thyme 1.5%), 3 (fillet + sodium caseinate 4% - thyme 1.5%) and 4 (Fillet + gelatin 4% -thyme 1.5%). In order to study the quality properties of shrimp fillets and to compare the antioxidant and antibacterial power of films, PV, TVN-B, FFA, pH, aerobic mesophilic and psychrophilic bacteria were measured for treatment at 0, 3, 6 and 9 days.

The use of these three types of coatings was able to preserve the quality of fillets until the end of the period in refrigeration temperature. But in general and especially in the final days, the fillet coated with sodium alginate-thyme showed significantly lower values in all of chemical and microbial indices than two more covers (p<0.05). In addition, the results indicated that the antioxidant and antibacterial power of sodium caseinate- thyme coating was significantly higher than gelatin- thyme coating, especially in the final days (p<0.05). It should be noted that although the control sample was at standard level in term of qualitive indexes in days 0 and 3, but the most of chemical and microbial indices of this treatment exceeded the allowed limit in the end of maintenance days.

According to the results, all three types of coatings used in the study were able to maintain the chemical and microbial properties of the fillets at the standard level until the end of the storage period. However, the combination of sodium alginate and thyme was more potent and efficient in maintaining shrimp fillets quality than caseinate-thyme and gelatin-thyme coatings (in less than 10 days at refrigerated temperature).