莲子热风干燥过程对其淀粉热特性及凝胶化的影响

为解决莲子干燥过程中淀粉形态结构变化造成莲子结壳、硬化,不利于干燥以及复水难、易返生问题,该文利用差示扫描量热技术(differential scanning calorimetry,DSC)对新鲜莲子以及不同热风干燥(70、80、90℃)莲子的淀粉热特性与凝胶化过程进行了研究.研究发现,莲子淀粉在低水分环境(42.2%,以质量比计)时存在2个明显的吸热峰,高水分环境(71.1%,以质量比计)时存在1个明显的吸热峰;莲子在干燥过程中不断失水,并伴随着淀粉凝胶化.方差分析(analysis of variance,ANOVA)表明,高温干燥显著影响莲子淀粉的热特性,其淀粉凝胶化温度(峰起温度To、峰顶温度Tp以及峰止温度Te)部分显著升高.相同干燥条件下,莲子淀粉糊化焓ΔH受水分显著影响,但干燥温度、升温速率对其影响不显著(P>0.01).采用Kissinger、Crane方程获得了淀粉凝胶化动力学参数(活化能Ea、指前因子Z以及反应级数n).莲子淀粉的非等温凝胶化反应可近似为一级反应,高温干燥后其Ea值出现增加,并随着水分增加呈现降低趋势.研究结果可为确定莲子高品质干燥工艺以及干莲子、莲子淀粉后续加工过程提供技术支持.

Starch thermal property and gelatinization of lotus seeds during hot air drying

Lotus (Nelumbo nucifera), an aquatic crop, is a type of important economic plant in Asia and Africa. Lotus seeds are popular food ingredients to East Asian cuisine and are used extensively in traditional Chinese medicine and Chinese desserts. There is rich source of C type starch in lotus seeds. Starch gelatinization often occurs when starch granules are heated in an aqueous medium, which is a phase transition from an ordered state to a disordered state. Starch gelatinization affects texture, nutritional value and porosity, which changes moisture diffusivity of porous materials significantly. On the other hand, gelatinized starch tends to an ordered crystalline structure during storage, which is termed as retrogradation that affects acceptability and shelf life of starchy food. Drying is one of the major processes for dried lotus seeds, which could suppress activities of microorganisms, enzymes or ferments and maintain its nutrition content. Hot air drying is a complex process of heat and mass transfer simultaneously. A better understanding of starch thermal property and gelatinization of lotus seeds during hot air drying will help to improve product quality and efficiency of drying process. The gelatinization properties of starch extracted from lotus seeds dried at different temperatures (70, 80 and 90 ℃) were determined at various water contents that varied from 42.2% to 71.1% (mass ratio) and heating rates (5, 10 and 15 ℃/min) by differential scanning calorimetry (DSC) in this study. DSC thermograms were determined from 40 to 120 ℃. DSC analysis revealed that a single endotherm peak denoted as G was observed in lower temperature region (60-80 ℃) when starch was heated in high water content of 71.1%, and 2 combined endothermic peaks (G and M) were exhibited at 42.2% water content. All gelatinization transition temperatures, such as onset temperature, peak temperature and end temperature, increased with drying temperature and heating rate significantly. The gelatinization enthalpy of lotus seed starch increased with water content significantly and the effect of drying temperature and heating rate on gelatinization enthalpy was not significant (P>0.01). The kinetic parameters (reaction order, frequency factor and activation energy) were determined using Kissinger equation and Crane equation at different water content. First-order reaction kinetics described non-isothermal gelatinization process of lotus seed starch well. Activation energy values decreased with water content from 42.2% to 71.1%, and activation energy values of lotus seed starch dried at 70 and 80 ℃ were higher than those of starch extracted from fresh lotus seed dried at 20 ℃. Lotus seed starch gelatinization occurs throughout the hot air drying process, especially at higher drying temperature and at higher water content varying from 63% to 45%. This study will be helpful in optimization of lotus seeds drying process and in application of lotus starch in food industry.