骏枣变温干燥工艺优化及品质评价

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

    • 摘要: 红枣因具有较高的营养价值而备受消费者欢迎,通常新鲜的红枣采后极易腐烂变质,为了在延长其货架期的同时,也能保持其品质特性,该研究采用变温干燥法,探究了变温干燥过程中不同阶段温度和水分转换点对骏枣糖酸比和褐变度的影响,结合响应面法,建立二次多项式回归方程模型,对骏枣变温干燥工艺进行优化;将此工艺与恒温(60 ℃)烘制工艺进行对比,探究其对骏枣品质指标的影响。结果表明,骏枣变温干燥优化工艺为:前期温度44 ℃,前期水分转换点19.5%,中期温度65 ℃,中期水分转换点17.0%,后期温度49 ℃,与60 ℃恒温烘制相比,其糖酸比增加了7.4%,内部褐变度减少了23%,此外,变温烘制还缩短了骏枣烘制的时间,减少了表皮色差、咀嚼度和硬度,增加了抗氧化物质含量和抗氧化能力。研究结果将有助于骏枣变温烘制技术的推广与应用。

       

      Abstract: 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.

       

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