Study On The Efficient Drying Technology And The Effects On Characteristics Of Vegetable Yogurt Melts Based On Kale

Kale（Brassica oleracea var.Acephala）is a nutritious vegetable.And it is often made into high value-added food because it is perishable.Yogurt melts are proposed as the novel snacks which are healthy and delicious.However,at present,the flavor of yogurt melts on the market is single and the production cost is relatively high.This paper mainly studied the effects of combined infrared freeze-drying with microwave vacuum drying（IRFD-MVD）and infrared pulse-spouted freeze-drying（IRPSFD）on the quality and drying efficiency of kale yogurt melts.The characteristics and quality stability of kale yogurt melts prepared by high efficiency drying methods were also studied.First of all,the preliminary preparation was mainly for the drying processing of kale yogurt melts.By investigating the stability of chlorophyll in kale,it was learned that the heating temperature（T≤95°C）and pH（6.00～9.00）should be controlled during processing,and the storage of samples should be conducted under the condition of oxygen isolation and light avoidance.The pretreatment method of kale by blanching 80 s with hot steam at 95°C could keep the content of VC,total chlorophyll and carotenoids better,and it also can soften the leaf tissue effectively,facilitating subsequent beating treatment.After determining the formula of kale yogurt melts,the eutectic point and co-melting point of the kale yogurt melts pulp were measured,which were-19.26°C and-6.42°C,respectively.In addition,through the analysis of the infrared absorption spectra of kale yogurt melts,it was found that kale yogurt melts showed strong absorption in the infrared spectral range at 3384 cm^-1,2926 cm^-1,1650cm^-1,etc.,and the infrared absorption of the melts was mainly determined by water.Secondly,the effects of IRFD-MVD at different water transfer points on the drying characteristics and quality of kale yogurt melts were studied.FD samples and infrared freeze-drying（IRFD）samples were used as control group.The results indicated that among the IRFD-MVD samples,IRFD5h-MVD samples had the best quality and the high drying efficiency.And compared with FD,IRFD5h-MVD could save 32%drying time and 39%energy consumption.In addition,IRFD5h-MVD samples’texture properties（hardness,fracturability,bulk density and rehydration time）were the closest to those of FD samples.The IRFD5h-MVD yogurt melts were bright green,and the yellowness index value was not significantly different from that of the FD samples（P>0.05）.Moreover,the retention rates of total chlorophyll and carotenoids were also high,at 84.03%and 80.62%,respectively.In terms of nutritional quality,the total phenol contents of the IRFD5h-MVD samples were relatively high,and the ABTS and DPPH radical scavenging rates were only second to the FD and IRFD samples,at 75.40%and 78.02%,respectively.The electronic nose,electronic tongue analysis and sensory evaluation of samples revealed that the IRFD5h-MVD samples had similar odor,taste to the FD samples and the sensory scores were high.Then,the effects of infrared pulse-spouted freeze-drying（IRPSFD）on the efficient and uniform drying of kale yogurt melts were studied.FD samples and infrared static freeze-drying（IRSTFD）samples were used as control group.The results showed that after IRSTFD for 3 h,the proportion of non-flowing water in the samples increased to 73.53%.And the structure of samples was stable and the outer layer was not sticky,which was suitable for pulse-spouted operation.Compared with FD,IRPSFD20 samples had the higher drying efficiency,reducing the drying time by 15%and saving the unit energy consumption by 30%.When compared with IRSTFD samples,the RSD values of temperature,moisture content,bulk density,and total color difference of IRPSFD20 samples were reduced by 2.11%,6.37%,5.61%and 1.97%,respectively,indicating the drying uniformity was improved.The retention rates of total chlorophyll and carotenoid of IRPSFD20 samples were high,at 88.06%and90.02%,respectively.And its total phenol content was only second to that of FD samples.The ABTS and DPPH radical scavenging rates of IRPSFD20 samples were also high,at 88.46%and 85.76%,respectively.The sensory scores of IRPSFD20 yogurt melts were high,and the hardness and fracturability of IRPSFD20 samples were not significantly different（P>0.05）from that of FD samples.Finally,the moisture absorption,rapid wettability characteristics and quality stability of IRFD5h-MVD and IRPSFD20 yogurt melts were studied.The FD samples were used as the control.The moisture absorption of IRFD5h-MVD and IRPSFD20 samples was slightly less than that of FD samples,and it was more obvious when RH>70%.Compared with FD samples,the moisture content of IRFD5h-MVD and IRPSFD20 samples increased slowly at25±1°C during storage,remaining below 5.00%（w.b.）during the storage period.After analyzing the water binding force,dispersibility,wetting efficiency and pore structure of samples found that FD,IRFD5h-MVD and IRPSFD20 samples all have the rapid wettability and dispersibility,but their performance was different,which was relatively prominent in FD and IRPSFD20 samples.Based on the chlorophyll loss rates of samples,the models of shelf life prediction were established.It was predicted that the shelf life of IRFD5h-MVD and IRPSFD20 samples was 195 days and 194 days at 25±1℃,meeting the 6 months shelf life requirement.After storage of 25±1℃for 180 days,the total number of colonies and coliforms of IRFD5h-MVD and IRPSFD20 samples did not exceed the standard,and both had higher sensory scores and higher acceptance than FD samples.The GC-MS test found that after 180days’storage at 25±1℃,the relative content changes of the major flavor contributions in the IRFD5h-MVD and IRPSFD20 samples were less than those of FD samples,suggesting that the flavor stability of these two samples was better.