Study On Air Drying Characteristics And Water Transfer Of Garlic(Allium Sativum L.)

Garlic is widely used as a condiment and Chinese herbal medicine all over the world.Garlic has a high moisture content,and its quality is prone to deterioration during storage.Dehydration can not only retain the original flavor of garlic,but also inhibit microbial reproduction and some adverse reactions,so as to achieve the goal of extending the shelf life and increasing the added value of the product.Therefore,it is an important processing method for garlic.At present,the traditional drying method of fruits and vegetables is still based on air drying.Air drying is simple and easy to implement,but its energy consumption is high and the product quality is poor.In order to solve this problem,proper treatment of garlic before drying is necessary.Studies have shown that pretreatment will change the structure of garlic and affect the drying process of garlic.However,its drying characteristics are currently unclear.Therefore,it is of great significance to study the air-drying process of pretreated garlic for improving the quality of dehydrated garlic.In this paper,garlic was subjected to three drying pretreatment methods:ultrasonic,freeze-thaw and osmosis,and the ultrasonic pretreatment with short drying time and high product quality was selected.Based on the single factor experiments of ultrasonic power,ultrasonic time and ultrasonic water temperature,the optimal constant temperature ultrasonic assisted drying process was obtained by response surface optimization experiment with garlic quality and drying energy consumption as indexes.The characteristics of ultrasonic assisted air drying of garlic were studied and the drying kinetics model was established.Low field nuclear magnetic resonance(LF-NMR)technique was used to study the moisture distribution and its variation during the drying process of garlic treated with ultrasound.Quantitative and qualitative research on flavor substances was carried out for fresh garlic,constant-temperature air drying garlic,and ultrasonic-assisted air drying garlic.The main results are as follows:(1)Ultrasound,freeze-thaw and penetration are applied to garlic drying pretreatment,combined with multiple indicators(drying time,energy consumption,color,rehydration ratio and allicin content)to analyze the influence of pretreatment on the drying process and product quality.The results showed that the indexes of garlic after ultrasonic pretreatment were better than other pretreatment.The effects of ultrasonic power,ultrasonic time and ultrasonic water temperature on garlic drying were further studied.Taking energy consumption,color,allicin content and rehydration ratio as indexes,the effects of ultrasonic power,ultrasonic time and ultrasonic water temperature on garlic drying were studied.The range of ultrasonic power(250?350 W),ultrasonic time(10?20 min)and water temperature(25?35?)were determined by single factor test.The optimal drying process of ultrasonic garlic slices was obtained by response surface optimization test:ultrasonic power 297.5 W,ultrasonic time 15.06 min and ultrasonic water temperature 31.33?.Under the conditions,the model predicted energy consumption of 2.21 k W·h/kg,color difference of 17.99,rehydration ratio of 2.76,and allicin2.5 mg/g.(2)The hot-air drying of garlic with ultrasonic pretreatment is mainly a deceleration process.Six drying models were selected to fit the drying process,and it was concluded that the Midilli model had the best fitting effect,with the largest R² value,the smallest ?² and RMSE,and fewer dynamic parameters.The effective water diffusion coefficient was calculated by the Fick's second law,which ranged from 2.27×10^-9 to 9.16×10^-9m²/s,and increased with the increase of temperature.The activation energy of ultrasonic garlic in air drying was 48.28 k J/mol.(3)LF-NMR was used to study the moisture migration during ultrasonic garlic drying.There were three states of water in garlic:bound water(0.1-1 ms),non-flowing water(1-10ms)and free water(>10 ms).The free water content was the highest,followed by non-flowing water and bound water is the least.During the drying process at different temperatures,the peak area and relaxation time of the three states of water all change.The higher the temperature,the faster the change.During the whole drying process,the free water content gradually decreased and was removed at 150,90,and 60 min,respectively.Both the non-flowing water and the bound water increased in the early stage of drying and decreased in the later drying period.The time for the bound water to reach the peak lagged behind that of the non-flowing water.At the end,only bound water existed in the dried garlic.(4)The influence of constant temperature and ultrasonic pretreatment on dried garlic flavor substances.The flavor substances in fresh garlic were quite different from the samples after drying.The main flavor substances in fresh garlic,dimethyl disulfide,dimethyl butanol,ethyl 2-methylpropionate and ethyl acetate,had not been detected in the dried samples.Different drying processes had different effects on the flavor substances of garlic.The contents of diallyl disulfide,diallyl trisulfide,methylallyl sulfide and isoeugenol in ultrasonic dried garlic were close to those in fresh garlic,and the types and concentrations of flavor compounds were significantly higher than those in untreated garlic,in which diallyl sulfide,diallyl disulfide,methylallyl sulfide and allyl sulfide were the main components.The contents of diallyl sulfide,diallyl disulfide,methyl allyl sulfide,propyl allyl disulfide and diallyl trisulfide increased by 0.77,1.9,1.48,2.7 and 0.77 times,respectively,compared with untreated ones.