Research On Pretreatment Technologies To Improve The Drying Efficiency And The Quality Of Vacuum Freeze-Dried Garlic Slices

Garlic(Allium sativum L.)is a perennial vegetable spice and is used worldwide as a flavourer,spice,and herbal cure.Due to their great demand in various applications,dehydrated garlic slices have become the most popular commercial garlic-based products.However,as people’s requirements for quality dehydrated foodstuffs are getting higher and higher,dehydrated garlic products with attractive appearance,rich aroma,and high nutritional content have become the first choice of consumers.Therefore,the vacuum freeze-drying technology(VFD),which can maintain the appearance,flavor,and nutrients of foodstuffs to a greatest extent,has become an important choice for the production of high-quality dehydrated garlic.Thus,this study first comparatively investigated the effects of VFD and thermal drying technologies(hot air drying,infrared hot air drying,relative humidity drying,and pulsed vacuum drying)on the drying time and physicochemical properties of garlic.Results showed that garlic slices treated with VFD had an attractive bright white color and complete appearance,which is the first factor that determines consumers’ buying desire.However,it has the longest drying time,and some quality(flavor,hardness,bioactive component content,etc.)is poorer than thermal drying technology;this is the outstanding problem that needs to be solved urgently in the VFD process of garlic.On these grounds,this study proposes to pretreat garlic before VFD,such as blanching,osmotic dehydration,ultrasound and freeze-thaw cycles pretreatments,to shorten the drying time,improve the drying efficiency and the quality of garlic products,and finally establish a "pretreatment combined with vacuum freeze-drying" technology for efficient production of high-quality dehydrated garlic slices.For blanching pretreatment,catalytic infrared(CIR)heating technology was introduced in this study.Firstly,the effects of different CIR processing parameters on garlic peroxidase(POD)activity and moisture reduction were investigated,to create a catalytic infrared dry-blanching process(CIRDB)with water-saving,pollution-free,and high nutrient retention for garlic.Then,the effect of CIRDB on the VFD drying time and quality properties of garlic was studied.The results showed that CIRDB could passivate 90%POD activity in 120-540 s and remove 15.87%-40.37%moisture.The optimum CIRDB process of garlic:the CIR heating temperature is 115℃,the thickness of garlic slice is 3 mm,and the processing time is 210 s.Furthermore,the application of CIRDB pretreatment can shorten the VFD time of garlic by 3.7%.Additionally,compared with the hot water blanching pretreatment(WB),CIRDB pretreatment is superior to WB in hardness,rehydration,allicin and total phenol retention,antioxidant activity and flavor.For osmotic dehydration(OD)pretreatment,vacuum and multi-frequency mode ultrasound technologies were introduced into the normally osmotic dehydration(NOD)process.Thus it created a novel garlic osmotic dehydration technology-vacuum pretreatment coupled to ultrasound-assisted osmotic dehydration(VUOD).Subsequently,the effect of VUOD on the VFD drying time and quality properties of garlic was investigated.The results indicated that the water loss of the garlic slices treated with VUOD was significantly higher than slices treated with NOD,vacuum pretreatment OD,and ultrasound-assisted OD.The vacuum and ultrasound technology combined effect was attributed to the exhaustion of intercellular air and the increase in the mass transfer area,forming the microscopic pores and the cell rupture.Furthermore,the VFD drying time of garlic was shortened by 20.99%with the application of VUOD pretreatment,which was 1.21 times the NOD.While the hardness,rehydration,allicin,and total phenol content of the product were increased by 28.84%,25.71%,2.04%,and 11.68%,respectively.The antioxidant activity of the product was also enhanced due to the increase of the content of the bioactive component,and the volatile component content was higher than that of the control and the NOD pretreatment samples.For the ultrasound pretreatment,this research innovatively replaced the conventional ultrasound transmission medium "water" with "alcohol" to pretreat garlic,and investigated the influence of ultrasound-assisted alcohol pretreatment(US+Alcohol)on the VFD drying time and the quality properties of garlic.The results showed that,the application of "US+Alcohol" pretreatment could significantly shorten the VFD drying time of garlic slices by 24.69%,which is 1.35 times that of the traditional "US+Water" pretreatment technology.The results of electrochemistry and microstructure analysis showed that after "US+Alcohol" pretreatment,53.58%water was removed from garlic,cell structures were damaged,and the intercellular gas was discharged.All these factors promoted the garlic pretreated by "US+Alcohol"to have more efficient drying efficiency than the garlic pretreated by "US+Water"and the unpretreated samples.Furthermore,the application of "US+Alcohol"pretreatment has maintained the original complete appearance and bright white color.The hardness value has increased by 37.75%,and the product flavor has been significantly improved and higher than fresh garlic.However,in terms of bioactive component content and antioxidant activity of garlic products,"US+alcohol"pretreatment has a negative impact on them.For freeze-thaw cycles pretreatment,one,two,and three freeze-thaw cycles(abbreviated as 1-FTC,2-FTC and 3-FTC,respectively)pretreatments were employed before the VFD of garlic slices,aimed at improving the drying process and the quality of the end product.The results indicated that FTC pretreatment reduced the drying time(22.22%-33.33%)and the energy consumption(14.25%-15.50%),owing to the water loss,the increase in free water,and the formation of porous structures.The more the freeze-thaw cycles,the shorter the VFD drying time.The FTC pretreatments improved thermal stability,flavor,bioactive component content,and antioxidant activity of dried products.The FTC pretreatments had no significant effect on the functional groups of garlic polysaccharides,but reduced the molecular weight of garlic polysaccharides,and thus increased its antioxidant activity.Furthermore,it is important to highlight that in order to obtain higher quality freeze-dried garlic products,the number of cycles of freeze-thaw pretreatment of garlic slices should not be more than 2 cycles.Furthermore,in the process of accelerated storage,the water activity and microbial content of control and dried garlic slices pretreated with CIRDB,VUOD,"US+Alcohol" and 2-FTC increased gradually,while the whiteness value,hardness value and volatile component content decreased gradually.The results of storage test showed that the application of "US+Alcohol",CIRDB and VUOD pretreatments could extend the shelf life of freeze-dried garlic slices.In summary,aiming at the scientific problems such as long drying time and deterioration of some quality indicators of garlic vacuum freeze drying,the application of CIRDB,VUOD,"US+Alcohol" and 2-FTC pretreatment technologies established in this study improved the drying efficiency and the quality of vacuum freeze-dried garlic slices.Additionally,due to the different mechanisms of action of the four pretreatment technologies,there are significant differences in the quality properties of the resulting freeze-dried garlic products.This research provides a novel drying technology-"pretreatment combined with vacuum freeze-drying",to efficiently give rise to a high-quality vacuum freeze-dried garlic slice.