Effect of hot air drying temperature on microstructure of Chinese jujube

Abstract: Chinese jujube is one of the most important dried fruits, and it is popular for its unique flavor, good taste and rich nutrition. Over the years, with the improvement of people’s living standards, more and more people require high-quality jujube products, however the existing processing technology can not satisfy the needs, so improving the processing technology becomes increasingly important. Currently, most researches have focused on the changes in the organizational structure of finally dried jujube, and fewer studies have focused on drying process. In order to study the effect of hot air drying temperature on microstructure of Chinese jujube, jujubes were dried under different temperature (35, 40, 45, 50 and 55℃). Paraffin section and microscopic imaging technology were used to obtain the distribution curve of the flesh cell and the cavity structure parameters during the hot air drying process. Distribution curve skewness and kurtosis (fresh jujube as the reference) were used to describe the changes of microstructure parameters during the hot air drying process, and the fitting equation of microstructure parameters (area ratio, perimeter ratio and equivalent diameter ratio of flesh cell to cavity cross-section) and macro drying parameter (moisture ratio) was established. The results indicated that the distribution curve skewness, kurtosis with various microstructural parameters could better describe the flesh cell and cavity changes trend during the drying process with different temperatures. Due to environmental stress response and differences in the distribution of moisture and heat, the shrinkage of pulp cells and the collapse or expansion of cavity were not with consistency or continuity during drying process of 5 temperatures. Comparing different temperatures, low temperature (35℃) and high temperature (55℃) drying process had great impact on flesh cells’ shape changes, and under high temperature (55℃) the shrinkage degree of pulp cells was more significant. During the hot air drying process, collapse and expansion of cavity both existed, which were more significant under high temperature, and cavity expansion was serious in early and post drying process. In actual production, we should not adopt prolonged high temperature or low temperature drying to avoid excessive shrinkage and cavity’s excessive expansion seriously, which affected the texture of flesh and the mouthfeel of jujubes. In drying process, jujube microstructure parameters did not decrease linearly with the decreasing of moisture content, but presented undulation changes. Therefore, the nonlinear model was used to describe the change of each microscopic parameter with different moisture content, and we found that the nonlinear model could better simulate the effect of cell perimeter ratio (R2>0.8379, standard deviation<2.9943%), however, the simulation effect of the cavity perimeter ratio was poor (R2>0.9229, standard deviation<25.1682%). So when the relationship between macro drying parameter and microstructure parameter during hot air drying process was establish, we could select the cell area ratio, cell perimeter ratio and cavity equivalent diameter ratio as microscopic parameters. Results can provide a reference for the development of new hot air drying technology.