基于低场核磁共振的热风干燥猕猴桃切片含水率预测模型

Model for predicting the moisture content of kiwifruit slices during hot air drying based on low-field nuclear magnetic resonance

为研究猕猴桃切片热风干燥过程中水分迁移规律,该试验通过对猕猴桃切片进行热风干燥,考察不同干燥温度(70、80、90℃)、切片厚度(3、4、5 mm)下的干燥特性。试验采用直接干燥法测定含水率,运用低场核磁共振技术(Low-Field Nuclear Magnetic Resonance,LF-NMR)分析热风干燥过程中猕猴桃切片内部水分分布状态与变化规律,建立动力学模型,验证并预测。结果表明:猕猴桃切片热风干燥开始为外部控制,随后属于内部扩散控制,水分有效扩散系数范围为1.58×10^-7~4.18×10^-7 m^2/s,扩散效率随温度升高而增大。升高温度能显著提高猕猴桃干燥速率,可加快结合水、不易流动水以及自由水的迁移。自由水和结合水先于不易流动水发生变化,自由水含量在干燥前期逐渐下降,此过程中不易流动水和结合水含量均表现为先升高后降低的趋势。当自由水被脱除后,不易流动水和结合水含量依次达到最大值;此后,随着干燥的进行,不易流动水逐渐被脱除,此时结合水含量开始下降直至干燥结束。整个干燥过程中,猕猴桃切片部分自由水先转化为不易流动水和结合水,结合水与不易流动水相互转化,循环往复伴随整个干燥过程。以干燥过程中的自由水、结合水、不易流动水的核磁峰值总和、切片厚度和干燥温度为自变量,猕猴桃切片含水率为因变量,进行多元线性回归分析,建立含水率预测模型,模型的拟合优度为0.982。结果表明,低场核磁共振技术结合数学模型可用于描述猕猴桃切片热风干燥过程,可实现对猕猴桃切片干燥过程中含水率的快速、无损检测,研究结果可为猕猴桃热风干燥工艺和过程设计提供理论依据。

An experiment has been designed to investigate the drying features of kiwi slices with the thickness of 3,4,5 mm,during hot-air drying at different drying temperatures(70,80 and 90℃),in order to understand the internal migration of moisture in the drying kiwi slices for the food production.A low-field nuclear magnetic resonance(LF-NMR)method was used to characterize the internal distribution of moisture and its change rules in the drying kiwi slice during hot-air drying.The moisture in kiwi slices was also determined according to the national standard GB5009.3-2016“Determination of moisture in foods”.A dynamic model was then established based on the obtained moisture-time data that verified and predicted in this experiment.The results indicated that the hot-air drying process of kiwi slices started with external control step,and then changed to internal diffusion control.The effective diffusion coefficient of moisture ranged from 1.58×10^-7 to 4.18×10^-7 m^2/s,and the diffusion efficiency increased with the increase of temperature.The rise of temperature can significantly increase the drying rate of kiwi slices,and thereby accelerate the migration of combined water,immobilized water,and free water.Specifically,the free water and combined water changed before the immobilized water,and the content of free water gradually decreased in the early stage of drying.In this process,the contents of immobilized water and combined water both presented a trend of first increase,and then decrease.Upon the removal of the free water,the immobilized water and combined water successively reached the maximum.Thereafter,as the drying continued,the immobilized water was gradually removed,while the content of combined water started to decline until the end.The part of free water in kiwi slices was first converted into the immobilized and the combined water,which could be converted into each other,thereby to form a cycle reciprocates with the entire drying process.A multiple linear regression(MLR)model was established to quantitatively detect the moisture content in kiwi slices during the drying process.In this model,the correlation coefficient of prediction(Rp)and root mean square error of prediction(RMSEP)reached 0.981 and 0.51%respectively.In the NMR data,taking the slice thickness and drying temperature as the independent variables,and the moisture content of the kiwi slice as the dependent variable,a multiple linear regression analysis was carried out to establish a dynamic model of moisture content,where,the goodness of fit of the model was 0.982.The results demonstrated that the low-field NMR combined with mathematical model can be used to clarify the hot-air drying process of kiwi slices,in order to achieve rapid and non-destructive detection of moisture content during the drying process.The finding can provide a sound theoretical basis for the hot-air drying process to effectively improve the production design of kiwi slices.