Improving quality of tilapia fillets freeze-thaw combined with heat pump drying using suitable additive pretreatment

Abstract: In order to obtain the combined effects of additive pretreatment on the quality of freezing-thawing and heat pump drying of tilapia fillets, and to provide an atmospheric cryogenic drying method which can substitute vacuum frozen drying method, a series of tests of freezing-thawing process combined with heat pump drying for tilapia fillets were conducted in the laboratory, and the best experiment processes obtained were as followed: freezing-thawing pretreatment (conditions: freezing at -32℃ for 1.0 h, melting at 20℃ for 1.5 h), hot pump drying (conditions:Temperature 45℃,wind speed 2.5 m/s for 4 h), freeze and thaw imbedding (conditions: freezing at -40℃ for 1.5 h, melting at 25℃ for 1.0 h), hot pump drying (conditions:Temperature 45℃,wind speed 2.5m/s for 3 h), freeze and thaw imbedding (conditions: freezing at -30℃ for 1.5 h, melting at 25℃ for 0.5 h), heat pump drying (conditions:Temperature 45℃,wind speed 2.5 m/s) to dry basis moisture content 0.30±0.02 g/g. In order to further shorten the drying time and improve the quality of the dried products, on the basis of the previous studies, this paper chose three different types of additives including trehalose, sorbitol and maltodextrin to be used on tilapia fillets under the condition of ice-water bath for 1 h before freezing-thawing process combined with heat pump drying. Drying curves and quality indexes variation of different types and concentrations of additive treatments were obtained by the experiments, and the best additive impregnation conditions were obtained through the comprehensive score of dried product's quality index. Contrast tests were carried out using three methods including hot air drying, heat pump drying and vacuum freeze drying under the condition of trehalose solution (mass fraction of 15%) impregnation. The results showed that suitable pretreatment additive and its mass fraction was beneficial to the method of freezing-thawing combined with heat pump drying, and could shorten the drying time and effectively improve the comprehensive quality of tilapia fillets after the process of freezing-thawing and heat pump drying. Within the scope of this study, trehalose solution (mass fraction of 15%) immersion treatment obtained the best results. The comprehensive score of tilapia products' drying quality under the optimum processing conditions increased by 25.41% than that without additive processed, increased by 40.18% and 33.57% respectively compared with hot air drying and conventional heat pump drying, yet was 4.39% lower than vacuum freeze drying, but its drying condition had a greater degree of improvement because of its atmospheric condition compared with vacuum freeze drying. The results can provide a reference for the atmospheric pressure low temperature drying of aquatic products.