Study On Freeze Drying Fruit Slices And Drying Method Of Peel Powder Of Kiwifruit

The objective of this study was mainly to investigate the effect of varieties and maturities of raw material,process of blanching and freezing temperature on the quality of freeze-dried kiwifruit slices.Besides,the drying methods of processing fruit peel powder using the by-product of freeze-dried kiwifruit slices were also compared.The main experimental results include:(1)The suitable variety and the raw material maturity were choosed by comparing the physcocchemical composition of fresh fruit and the quality of freeze-dried product of three varieties,Qinmei,Xuxiang and Hayward.Results showed that the product of Hayward had a minimum shrinkage with regular surface and desired crisp taste,in addition,it was more suitable for processing kiwifruit slices.Whereas,the product with low maturity kiwi as raw materials appeared serious shrinkage and collapse while the product with higher maturity kiwi as raw materials produced desirable crispiness and porous internal structure as well as less color loss during processing.Considering the color,texture,rehydration ratio as well as microstructure,when the kiwifruit of maturity reached the soluble solid content(13.65±0.07°Brix)and hardness was 2000-4000 g,the raw material was suitable for processing freeze-dried kiwifruit slices.(2)The changes of central temperature,POD activity,color parameter,chlorophyll,Vc content and fruit hardness in the process of blanching were studied as well as the dynamics of several important parameters were discussed.The results indicatedd that the central temperature of kiwifruit slices increased exponentially,with the temperature increasing,the sample center could achieve higer equilibrium temperature and heat transfer rate increased gradually.With the blanching time prolonged,the color parameter L * value decreased fluctuant and the b * value increased fluctuant.At the same blanching time,the temperature had significant effects on the green color-a * value,the higher the temperature,the faster the decline.In the blanching process,the breakdown of chlorophyll a,chlorophyll b and green color-a* values loss followed a first-order reaction,with the increase of temperature,the rate increased constantly.The effect of blanching time and temperature on Vc content and hardness of kiwifruit slices was significant.Moreover,the Vc content and the hardness value decreased with the increasing of blanching temperature and blanching time.(3)The pre-freezing temperature could significantly affect the quality of freeze-dried kiwifruit slices,the lower the pre-freezing temperature,the faster the freezing rate and the production of the largest ice crystal,besidess,the freezing rates in-20,-30,-40,-50 ? were 0.44,0.46,0.86 and 2.0 cm / h,respectively.The results also showed that the higher the pre-freezing temperature,the faster the drying rate,the larger the pore formed in the freeze-dried kiwifruit slices.The lower the pre-freezing temperature,the slower the drying rate,the more regular of the pore structure,and the more homogeneous distribution of the pores.(4)The drying method could significantly influence the quality of kiwifruit peel powder.Vacuum freeze-dried could get the best green color and best retention of Vc and total phenol in kiwifruit peel powder,however,the followed vacuum drying after the hot air drying got the worst quality of kiwifruit peel powder.The moisture absorption curve in all three kinds of drying methods was J-type,at the same a_w,the vacuum freeze-dried fruit powder could get the highest moisture content.