超高压和超声波预处理对蒜片热风干燥过程及品质的影响

为缩短蒜片热风干燥时间,提高干制蒜片品质,将超高压与超声波技术应用到蒜片干燥前处理,并利用低场核磁共振技术(Low Field Nuclear Magnetic Resonance, LF-NMR)分析预处理对蒜片干燥过程中内部不同状态水分迁移的影响。结果表明:干燥前超声或超高压预处理有利于加快干燥过程,超声、超高压和超高压-超声预处理的干燥时间分别比没有预处理的对照组降低了37.50%、43.75%、62.50%;三种预处理均可以降低干燥能耗,尤以超高压-超声最为显著(P0.05),相比对照降低了32.31%。低场核磁共振测定结果表明,预处理后蒜片中存在三种状态的水分,自由水占的比例最大,不易流动水次之,结合水最少。干燥过程中脱除的主要是自由水,超声和超高压均降低了自由水的脱除时间,超高压-超声预处理脱除自由水的时间最少,为90 min,说明超高压和超声联合更有利于减弱蒜片组织对水分的束缚并增强水分的流动性。通过扫描电镜发现,超高压使蒜片细胞间隙增大,细胞壁破坏严重,而经超声预处理的蒜片组织出现疏松多孔的结构,超高压-超声扩增了蒜片的显微通道,验证了超高压-超声更有利于提高水分的流动性。经超高压或超声预处理后,干制蒜片的色差降低,复水比和蒜素含量显著提高(P0.05),超高压与超声联合预处理得到的干制蒜片品质较优,色差值为16.11,大蒜素含量为2.67 mg/g,复水比为2.90 g/g。研究结果为高效节能、高品质的大蒜干燥技术提供理论参考。

Effects of ultrahigh pressure and ultrasound pretreatments on hot-air drying process and quality of garlic slices

Garlic (Allium sativum L.) is a biennial plant in the Liliaceae family. It is widely used as a condiment in cooking and daily life in the world, as well as Chinese herbal medicine. The garlic is rich in biologically active components, including allicin, polysaccharides, amino acids, and volatile oils, thereby mainly relieving fatigue and improve human immunity. Since the garlic has high water content in general, the quality is prone to deterioration during storage. Therefore, the dried garlic can inhibit the reproduction of spoilage microorganisms and chemical degradation, and further retain the original flavor of garlic, thereby extending the shelf life. At present, a commonly used technology of hot-air drying is characterized by the simple operation and large loading capacity, but the low drying efficiency and the long drying time has resulted in low quality of products after drying. In this study, a feasible pretreatment combined with the ultrahigh pressure (UHP) and ultrasound (US) was proposed to improve the drying process and quality of dried garlic slices. A low field nuclear magnetic resonance (LF-NMR) was selected to analyze the influence of pretreatments on the internal moisture migration during the drying process of garlic slices. The results found that the US or UHP during the hot air drying was beneficial to enhance the internal mass transfer of garlic slices, and thereby speed up the drying process. The US, UHP, and UHPs-US pretreatments resulted in a reduction in the drying time of garlic slices by 37.50%, 43.75%, and 62.50%, respectively, compared to the control. Six mathematical models were selected to fit the drying process of garlic slices. All three pretreatments reduced the energy consumption of drying, especially in the UHP-US, 32.31% lower than that with the untreated. The LF-NMR results showed that there were still three states of water in the garlic slices after pretreatments. Free water accounted for the largest proportion, followed by the non-flowing water, and the bound water was the least. In the drying characteristics, the free water was removed first in the drying process. All the US, UHP, and UHP-US reduced the removal time of free water. The UHP-US pretreatment took the shortest time (90 min) to remove the free water, indicating that the UHP-US was more conducive to weakening the binding of tissue structure to water for the better fluidity of water. Scanning electron microscopy (SEM) images revealed that the intercellular space of dehydrated garlic slices increased, and the cell wall was severely damaged after the UHP. The porous structure appeared in the dried garlic slices after the US and UHP-US pretreatments, where the cell wall contour became clear. The larger pore size indicated that the UHP-US was more contribute to improving the fluidity of water. The color difference of dried garlic slices was reduced after the three pretreatments, and the rehydration ratio and allicin content were significantly increased (P<0.05). Furthermore, the combined UHP-US pretreatment achieved the best quality of the dried garlic slices, with the color difference value of 16.11, while the allicin content of 2.67 mg/g, and the rehydration ratio of 2.90 g/g. The experimental findings can provide a theoretical reference for the potential technology of garlic drying with high efficiency, energy-saving, and high quality of dried garlic slices.