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.