Optimization of the variable temperature drying process and quality evaluation of Junzao jujube

Abstract: Junzao is one of the plant varieties in the Rhamnaceae family rich in nutrients with a variety of medicinal uses. However, the fresh dates are prone to water loss, softening, and rot during storage and transportation from the primary production area of jujube in Xinjiang in western China. More than 95% of dates are used for drying, due to the fresh loss. However, it is still lacking in the strict technical protocol of the traditional baking house under the uneven production standard, leading to the hardening and browning of Junzao in the drying process. In this study, the multi-stage variable temperature drying was optimized to evaluate the quality of Junzao fruits. Firstly, five models of drying kinetics were selected to fit the drying curve. The statistical parameters (such as coefficient of determination (R2) and root mean square error (RMSE)) were selected to determine the applicability of the model. It was found that the Page model was better fitted. The moisture content of Junzao fruit gradually decreased with the extension of drying time. Secondly, the single-factor experiments were conducted with the pre-temperature, pre-term moisture conversion point, mid-temperature, mid-term moisture conversion point, and post-temperature as the single factors. Good taste and visual sensation, the sugar-acid ratio, and the internal browning were selected as indicators for the single-factor experiment. An excellent taste of jujube was achieved to reduce the browning of the flesh. The test factors were then determined as the pre-temperature (40, 45, and 50 ℃), mid-term moisture conversion point (16.5%, 17%, and 17.5%), and post-temperature (45, 50, and 55 ℃). As such, the response surface test was conducted with the sugar-acid ratio and internal browning as the response values. The results showed that the optimal process parameters were: 45 ℃ for the pre-temperature, 17% for the mid-term moisture conversion point, and 49 ℃ for the post-temperature. A comparison was made with the constant temperature (60 ℃) drying, in order to investigate the effects on the surface color, internal browning, chewiness, hardness, total sugar, titratable acid, sugar-acid ratio, antioxidant substances, and antioxidant properties of Junzao. The drying time was reduced by 13.2% during variable temperature drying, compared with constant one. The total sugar, titratable acid, surface color, internal browning, chewiness, and hardness were reduced by 10.3%, 16.5%, 34.4%, 23%, 4.8%, and 15.1%, respectively, while the sugar-acid ratio, total phenolic, total flavonoids, ascorbic acid content, and antioxidant capacity increased by 7.4%, 1.18 g/kg, 0.08 g/kg, 0.6 g/kg, and 2.6 μmol/g, respectively. The multi-stage variable temperature drying was adopted to make the internal and external temperature uniform, and then greatly reduce the crusting on the surface of Junzao, compared with the constant temperature drying. The drying time was shortened to reduce the nutrient loss caused by high temperature. Therefore, the multi-stage variable temperature drying with low carbon and environmental protection can be expected to greatly improve the drying efficiency and quality. The finding can greatly contribute to promoting the application of variable temperature drying technology for Junzao.