Optimization of vacuum pulsed drying process of garlic

Abstract: Garlic is a semiannual spicy herb, which has been used widely as seasoning in our life. Garlic is known to possess a variety of biological functions, such as antimicrobial, anticancer, hypolipidemic, enhancing immune function of the body, which has high medicinal value. These properties are related with the chemical composition of garlic, and more specifically with its content of sulfur-containing compounds. The dormant period of garlic is very short, and it is hard to storage, which makes huge waste. Dehydration operations have been used for decades in processing of agricultural products for efficient long-term preservation. The basic objective for drying products is the removal of water from fresh product, reaching a level at which microbial spoilage is avoided. The drying of garlic is the best way to preservation of garlic. The common drying technologies for garlic are hot air drying, vacuum freeze-drying, microwave drying and infrared drying. Garlic is rich in the content of carbohydrate, especially for the polysaccharide, which makes the garlic have high water retention, and the garlic cloves are covered with a layer of protective film. These cause difficulties for the garlic drying. Vacuum pulsed drying technology is a new type of drying technology, and it has been applied to the drying of carrots, lily, grapes, wolfberry, red dates and other fruits and vegetables. It has been found that the vacuum drying technology has a significant effect on shortening drying time and improving the drying quality. To explore the drying technology and improve the drying quality of garlic, vacuum pulsed drying technology was applied to garlic drying in this paper. The carbon fiber infrared plate has been used in garlic drying as a heating source. The effects of infrared plate temperature (55, 60, 65, 70 ℃), vacuum time (6, 9, 12, 15 min) and sample thickness (2, 3, 4, 5 mm) on the drying characteristics and quality of garlic were explored, and the content of allicin, color and rehydration ratio were used as indicators of drying quality. The orthogonal test was designed based on the result of the single factor experiment. Results indicated that infrared plate temperature, vacuum time and sample thickness all had significant effect on the drying characteristics, and the order of the effect was sample thickness﹥infrared plate temperature﹥vacuum time. It was found that the content of allicin initially decreased and then increased gradually with the increase of infrared plate temperature, and initially increased and then decreased gradually with the increase of sample thickness, and when the vacuum time was 15 min, it had the highest allicin content. The vacuum pulsed drying had significant effect on the color of drying garlic slices, and the color of the drying garlic powder was whiter than the fresh garlic, which had a better color quality. Vacuum time had significant effect on the rehydration ratio of drying garlic slices. The optimum technological parameters were infrared plate temperature of 65 ℃, vacuum time of 15 min, and sample thickness of 2 mm. The findings of the study can provide technical support for applying vacuum pulsed drying technology to garlic drying process.