| Kiwifruit originated in China and is considered as a valuable health fruit,and has attracted a great deal of attention in recent years.Drying as a common technique is widely applied in agricultural and food processing,which not only plays an important role in extending the shelf-life,but also reduces the cost or difficulty of packing,storage and transport.In this study,hot air drying?HAD?,microwave drying?MWD?and air-impingement jet drying?AIJD?were employed to investigate the drying characteristics and the quality of kiwifruit slices.Besides,the effects of different AIJD conditions on drying characteristics and rehydration kinetics of kiwifruit slices were tested.Meanwhile,the drying and rehydration mathematical model of AIJD kiwifruit slices were established.Moreover,to further improve the quality of kiwifruits and provide a useful theory basis and technology reference of combined drying,the AIJD combined vacuum freezing drying?FD?were also studied.The Main results are as follows:?1?The processing time and energy consumption of drying kiwifruit were respectively decreased by 62%and 25.71%under AIJD due to faster drying rate?DR?and lager effective moisture diffusivity?Deff?of AIJD as compared to HAD.The entire drying process of AIJD and HAD kiwifruit slices occurred in the falling rate period,but the drying rate curve of MWD kiwifruit showed a constant rate period.?2?The Aa value?-3.42?and the AE value?4.22?of AIJD kiwifruits were lower than that of HAD and MWD kiwifruits,indicating that the AIJD kiwifruit was more greenness.In addition,the hardness and chewiness of AIJD kiwifruits were lower than that of HAD and MWD kiwifruits,and AIJD kiwifruits showed more porous structure and better rehydration capacity than HAD and MWD kiwifruits.Meanwhile,AIJD was better than HAD and MWD for reservations of polyphenols,ascorbic acid and antioxidant capacity in kiwifruit slices.?3?The drying characteristics of AIJD kiwifruit slices were effected by drying temperature,air velocity and distance between jet nozzles and material steel wire mesh box?DJM?.The drying rate increased with the increases of drying temperatures and air velocities,and the decrease of DJM.In addition,the values of effective moisture diffusivity?Deff?for AIJD kiwifruit slices varied in the range of 6.75 ×10-10?12.81×10-10 m2·s-1,and the values of Deff increased with the increases of drying temperatures and air velocities,and the decrease of DJM during AIJD process.Modified Page model with the highest R2,and the lowest ?2 and RMSE was shown to be the best model to describe the drying kinetics of kiwifruit slices during AIJD process.The predictive equation of Modified Page model was expressed as:MR=exp{-[?-0.022673+0.000850T+0.001173V-0.0000875H?×t]?1.076942+0.001700T-0.014942 T+0.000538H?}?4?The heating rate of kiwifruit slices inner center during AIJD increased with the increase of drying temperatures and air velocities,but there were no distinct effects of different DJM on heating rate.Moreover,the trinomial regression equation can be used to descript the temperature of inner-center of kiwifruit slices at any time within the range of AIJD process,and the predictive equation was given below:T=?-1.1502+0.0242T+0.0318V+0.0008H?xt+?0.0134-0.000225T-0.00044V-0.000012H?×t2+?-4.611×10-5+7.094×10-7T+1.525×10-6V+5.001×10-8H?×t3+?20.5102+0.1262T+0.1012 V-0.0250H??5?The rehydration capacity of AIJD dried kiwifruit increased with the increases of drying temperature and air velocity,but an increase of DJM caused the decrease of rehydration capacity.Besides,it was found that Peleg,Weibull and Proposed model could all describe the rehydration process of dried kiwifruit slices,but Weibull model had the highest fitness for the experiment data than Peleg and Proposed model.The Weibull model was more suitable for describing the rehydration kinetics of dried kiwifruit slices and the predictive equation of Weibull model was expressed as:MR=exp[-(t/?1.576+0.187T+0.655V-0.036H?0.663-0.0003T+0.001V+0.0003H]?6?The air-impingement jet drying combined vacuum freeze drying?AIJD+FD and FD+AIJD?could effectively reduce energy consumption by 17.39%?31.88%,when compared with FD method.Furthermore,combination-dried kiwifruits has higher sugar-acid ratio,better green colour and rehydration capacity than FD kiwifruits.Moreover,the ascorbic acid content and total phenolics content could be effectively retained by AIJD combined FD,which caused stronger antioxidant activity of kiwifruits than FD method.?7?All the AIJD+FD treatment resulted in better green color,texture and rehydration properties of kiwifruit products than FD+AIJD treatment.In addition,65min AIJD pre-drying combining with the FD finish drying?65AIJD+FD?kiwifruits had the highest sugar-acid ratio?348.81?than the other three combination-dried kiwifruits.The ascorbic acid and total phenolics content of 65AIJD+FD kiwifruits were 85.03 mg·100g-1 and 3.81 mg·g-1,respectively,and they were higher than that of any other AIJD+FD samples. |
PDF Full Text Request