| The effect of freeze-thaw treatment on heat pump drying process and productquality was studied with the Tilapia fillets as tests in this project. And the influence ofosmosis–freeze-thaw processing on Tilapia heat pump drying performance wasdiscussed combined with additives. Combining drying curve and drying rate curve,the indicators include rehydration ratio, Ca2+-ATPase activity, whiteness and hardnessas well as glassy transition temperature were studied comprehensively, in order toresearch a new type of atmospheric pressure at low temperature drying process,achieve the purpose of improving the quality of dried tilapia product.Freezing and thawing pretreatment was applied to the tilapia fillets heat pumpdrying process, the effects of freeze-thaw pretreatment on drying characteristics andthe quality of dried products based on four freeze-thaw pretreatment processconditions which include freezing temperature, thawing temperature, freezing andthawing time as variables were studied in this paper using single-factor experimentand response surface analysis. The results showed that: all of those four pretreatmentprocesses of freeze-thaw conditions had effects on Tilapia fillets heat pump dryingperformance in varying degrees. The best freeze-thaw pretreatment process for heatpump drying tilapia fillets was: freezing temperature-32℃, thawing temperature20℃,freezing1.0h, and the thawing time of1.5h. On this condition,10h were spent ondrying the moisture content of drying base to0.30(±0.02)g/g. The Ca2+-ATPaseactivity of dried products was2.33μmolPi/mgprot/hour, the rehydration ratio was36.22%, whiteness value of41.54and the hardness was9.8N. Compared toconventional heat pump drying mode, the drying time was shorted and productsquality was improved benefit from suitably freeze-thaw pretreatment conditions tojoin.After determined the optimal freezing and thawing pretreatment, based on thisprocess, selected three drying time points include1.0h,4.0h and7.0h, freeze-thawconditions placed into drying process one or more times. Results displayed: the dryingrate was improved in a short time after each freeze-thaw placement, but this abilityhas been weakened with the proceeding of drying process. Into once frozen thawprocess respectively at the time points4h and7h the reset, the dry quality improved was most favorable. Through on optimize freeze-thaw conditions, the optimalfreeze-thaw process in combination with heat pump drying conditions were obtained.It was that: freeze-thaw pretreatment (conditions:-32℃freezing1.0h,20℃melting1.5h)---hot pump drying4h---freeze-thaw imbedding (conditions:-40℃freezing1.5h---25℃melting1.0h)---drying3h---freeze-thaw imbedding (conditions:-30℃freezing1.5h,25℃melting0.5h)---heat pump drying to dry moisture0.30(±0.02)g/g. Compared with the control group without freezing and thawing process,the dried products Ca2+-ATPase activity, rehydration ratio and the hardness value wereimproved by a wide margin, composite score has increased by14.58%.Additive dipping pretreatment was applied in the early stage of the process inorder to discuss the influence of osmosis-freeze-throw-heat pump multistage jointdrying process for Tilapia dried performance. It turned out that the best way toachieve the best quality of dried products was immersion treated by the trehalose of15%concentration. And the best optimal was achieved in this paper, it was that:Additive dip (the trehalose of15%concentration)---freeze-thaw pretreatment(conditions:-32℃freezing1.0h,20℃melting1.5h)---hot pump drying4h---freeze-thaw imbedding (conditions:-40℃freezing1.5h,25℃melting1.0h)---drying3h---freeze-thaw imbedding (conditions:-30℃freezing1.5h,25℃melting0.5h)---heat pump drying to dry moisture0.30(±0.02)g/g. Compared with hotair drying and the conventional heat pump drying, the process achieved in this studyhad strong effect on the improvement of the Ca2+-ATPase activity, rehydration ratio,the hardness value and the glass transition temperature. Compared with theconventional heat pump drying, the composite scores of dried products quality wereincreased by26.36%use the best drying craft achieved in to dry fish fillet. Althoughslightly lower than the vacuum freeze drying products, but only4.21%of thedifferences. Which proves that this process parameters can be used to dry highmoisture and high protein content of aquatic products, the quality of the products canbe compared favorably with vacuum freeze drying products. It can be used as a newatmospheric low temperature drying method to replace vacuum freeze drying forfurther research and extension. |
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