Study On Pretreatment,Infrared Freeze-Drying And Hygroscopic Properties Of Typical Berries

Berry is a kind of soft and juicy fruit,which is rich in multivitamins,antioxidant active substances,essential amino acids well as water-soluble fibers.However,fresh berries are susceptible to spoilage due to their high water content and are susceptible to microbial infection or mechanical damage.Drying is an effective way to extend the shelf life of berries.With the increasing demand for high-quality dried berries,the research on energy conservation and quality preservation of dried berries is of great significance to the development of berry dehydrated industry.Compared to the other drying methods,freeze-drying(FD)can better retain the color,aroma,taste,shape and nutritional components of dried products.However,low efficiency and high energy consumption limit its application.In this study,the high efficiency infrared freeze drying(IRFD)was applied to the preparation of dried berries,the IRFD characteristics and quality characteristics of berries were studied,and a secondary strategy to improve the drying efficiency and product quality is also proposed.To determine the suitability of IRFD for berry drying,the thermal characteristics and infrared absorption characteristics of three typical berries(blueberry,cranberry,and raspberry)were analyzed.Then,the differences between FD and IRFD were analyzed in terms of chemical composition,quality characteristics,drying time and energy consumption.The results showed that the optical properties of three berries were mainly determined by water,and water exhibits strong absorption and weak scattering characteristics of radiant energy in the whole infrared spectrum.The eutectic points of blueberry,cranberry and raspberry were-22.34°C,-18.03°C and-14.88°C,respectively.The chemical composition of FD and IRFD samples were identical.The total anthocyanins content(TAC)and total phenolics content(TPC)of IRFD samples were significantly lower than those of FD samples,but the structural properties of FD and IRFD samples were similar.In addition,when the three berries were dehydrated to the target moisture content at the same drying temperature,the time and energy consumption of IRFD were significantly lower than that of FD.To enhance the drying efficiency of IRFD for berries,CO₂laser perforation,ultrasonic and freezing-thawing were used to pretreat frozen berries,and the effects on the drying characteristics and quality of the berries were investigated.The results showed that the three pretreatment methods could can effectively strengthen the mass transfer process and shorten the drying time for berry IRFD.The effect of pretreatment on berry quality was related to berry species.CO₂laser perforation could significantly inhibit the deformation of cranberry and raspberry,effectively improve the rehydration ratio of the three IRFD berries,and had no significant effects on TAC and TPC of the three IRFD berries.Ultrasonic and freezing-thawing treatment can effectively improve the rehydration ratio of IRFD blueberry and cranberry,but reduce the rehydration ratio and nutritional value of IRFD raspberries.To improve the uniformity of infrared heating,pulse spouting system was introduced.IRFD drying characteristics and drying uniformity of three berries were studied.The results showed that the Weibull model could characterize the moisture ratio(MR)of the three berries during IRFD,and the Weibull model parameter?values indicated 57%of the total drying time required to remove the remaining 37%of the moisture in samples.The drying uniformity of pulse spouted infrared freeze-drying(PSIRFD)is higher than that of IRFD,and the drying time is shorter than that of IRFD.The drying uniformity of PSIRFD improved with shorter spray interval time.The spray interval of PSIRFD was determined to be 20 min by integrating drying uniformity,energy consumption and nutritional quality.In order to understand the moisture sorption of the three IRFD berries during storage,the moisture sorption characteristics of blueberry,cranberry and raspberry were studied based on dynamic vapor sorption(DVS).The results showed that the isothermal adsorption curves of three IRFD berries were composed of multilayer adsorption zone and capillary condensation zone,and the adsorption curves were J-shaped.According to International Union of Pure and Applied Chemistry classification,the isothermal adsorption curves of three IRFD berries were type III,which was consistent with the moisture adsorption characteristics of high sugar content materials.Among the five isotherm adsorption models,the GAB and MHE models could better simulate the isothermal adsorption desorption curves for the three IRFD berries(R²>0.99).The absolute safe storage moisture content(relative safe storage moisture content)of blueberry,cranberry and raspberry were 20.63%(32.19%),18.58%(28.22%)and 10.92%(17.32%)respectively.The moisture content monolayer of IRFD blueberry,cranberry and raspberry were 10.73%,8.86%and 7.84%on dry basis,respectively.