本文基于水分活度和玻璃化转变理论,分析了枸杞粉的吸附等温线及玻璃化转变温度,通过构建状态图确定枸杞粉具有良好贮藏稳定性的临界水分含量;通过测试不同水分含量的枸杞粉的流动特性表征体系玻璃化转变的变化。结果表明:枸杞粉的吸附等温线呈"J"型,GAB模型为描述其水分吸附特性的适宜模型;枸杞粉的玻璃化转变温度随水分含量的升高而降低,以LB1枸杞粉为例,当干基含水率从0.133 g/g增加到0.530 g/g时,玻璃化转变温度从10.82℃降至-59.71℃。状态图表明,4种枸杞粉(LB1~LB4)干基含水率分别低于0.0833,0.0906,0.0992,0.1059 g/g时,在25℃下可稳定贮藏。宏观流动特性结果表明:随着枸杞粉水分含量的增加,粒径越大,越易发生玻璃化转变,粒径越小越有利于贮藏。
In order to obtain the optimal storage conditions of Goji powders at different particle size distribution,differential scanning calorimetry(DSC)and dynamic vapor sorption(DVS)were used to determine glass transition temperature and moisture sorption isotherm,respectively.Moisture sorption isotherms of different Goji powders were of“J”type,it is typical of products with high-sugar contents to show a gradual increase in sorption pattern at low water activities but present a sharp increase at high water activities.GAB model was found to be the best for fitting the experimental data,with average values of R2(0.9985),χ^2(3.8988×10^-4)and RMSE(6.7153×10^-4).A depression in Tg with increasing water content was observed.For example,when the moisture content(d.b.)of Goji powder(LB1)was increased from 0.133 g/g to 0.530 g/g,the Tg was decreased from 10.82℃to-59.71℃.Gordon-Taylor equation also showed good fit for the relationship between moisture content and glass transition temperature(R2>0.93).State diagrams of different Goji powders were established based on the moisture sorption isotherm and the glass transition curve.Results showed that Goji powders would have good storage stability under the room temperature of 25℃when the moisture content(d.b.)of the LB1~LB4 was below 0.0833 g/g,0.0906 g/g,0.0992 g/g and 0.1059 g/g,respectively.This conclusion was further validated by the macro flowability change of Goji powders at 75%relative humidity(RH)for 1 h.The larger particle size distribution,Goji powder showed more obvious caking phenomenon.After exposure to 75%RH for 1 h,all the Goji powders were of rubber state,thereby structure collapse and caking of Goji powders happened.The smaller particle size distribution of Goji powder,more stable of the storage process would be obtained.The results of the present study would provide useful information for the processing and storage of Goji powders.
Journal of Chinese Institute Of Food Science and TEchnology
water absorption isotherms
glass transition temperature
total flow energy