| China is the largest country in production fruits and vegetables. Due to the short shelf life and highly perishable, the majority of fresh fruits and vegetables were saled at low prices at local market. Carrot is one of the most demanded vegetables for human nutrition due to its high vitamin and fiber content. However, there is a challenge to retain the nutritional quality and appearance of fresh carrot. Drying is an effective method to improve the shelf life and retain the nutritional quality of carrot. China produces a large amount of dried carrot slices using hot water blanching and hot air drying for export and domestic consumption. But this method has many disadvantages, including low energy efficiency, long processing time, quality deterioration and environmental problems. The industry is very interested in new blanching and drying technologies for the application to lower production cost and improve the product quality.This study investigated the blanching and drying characteristics of carrot slices under catalytic infrared (CIR) dry-blanching, catalytic infrared drying, hot air (HA) drying and sequential infrared (IR) radiation heating and hot air (HA) drying. A POD inactivation kinetic model, a drying kinetic model and a surface color change kinetic model were successfully developed for describing the dry-blanching and dehydration process of carrot.First, it investigated the drying characteristics and models of carrot slices under catalytic infrared (CIR) heating. Carrot slices with thicknesses of3,5and7mm were dried at radiation distance of26,32and38cm. A two-factor factorial experiment design was conducted to determine the influence of drying parameters on drying rate, time required, surface color change and rehydration ratio. It was observed that the drying rate increased and the times required decreased quite significantly as radiation distance decreased and/or the slice thickness decreased. The best processing parameters for CIR drying with higher drying rates and the best dried carrot quality were radiation distance of32cm, and slice thickness of3-5mm. The obtained correlation coefficient (R2) and root mean square error (RMSE) values indicated that the Midilli model (MR=aexp(-ktn)+bt) was the best for predicting the moisture ratio change kinetics of carrot slices for CIR drying processes. Based on this conclusion, the CIR can be used to drying of carrots.It investigated the drying and quality characteristics of carrot slices under catalytic infrared (CIR) and hot air (HA) heating. Carrot slices with thicknesses of3,5and7mm were dried at temperatures of60,70and80℃with both methods. CIR had higher drying rates, shorter drying times and greater effective diffusivities than HA before the moisture contents reached the range of16%-30%on a wet basis. The total drying times required by CIR to reach the final moisture content was60%less than HA. The effective diffusivities were in the range of2.38-10.30×10-9m2s-1for CIR drying and1.43-5.50×10-9m2s-1for HA drying. Results revealed that carrot slices dried with CIR, particularly at60and70℃, had better rehydration characteristics the samples dried with HA. Thickness had a significant impact on overall color change of carrot slices. The obtained correlation coefficient (R2) and root mean square error (RMSE) values indicated that the Midilli model was the best for predicting the moisture ratio change kinetics of carrot slices for both drying processes. Based on this conclusion, it is highly recommended that the CIR is used in the early stage drying of carrots.It investigated the effects of various processing parameters on carrot slices exposed to infrared (IR) radiation heating for achieving simultaneous infrared dry-blanching and dehydration (SIRDBD). The investigated parameters were product surface temperature, slice thickness and processing time. A three-factor factorial design was conducted to determine the influence of processing parameters on moisture reduction, drying rate, residual peroxidase (POD) activities, surface color change and vitamin C retention. High surface temperature and/or thin slices had faster inactivation of enzymes and quicker moisture removal compared to the low surface temperature and/or thick slices. The process which produced1log reduction in POD activity has resulted in moisture reduction from40.2%to88.8%, overall color change (△E) from3.17to5.13and retention of vitamin C from56.92%to77.34%compared to control. It was concluded that SIRDBD could be used as an alternative to produce high quality blanched and partially dehydrated fruits and vegetables.Based on the data of infrared dry-blanching and dehydration, a POD inactivation kinetic model, a drying kinetic model and a surface color change kinetic model were developed for describing the dry-blanching and dehydration process of carrot. A biphasic model performed well for describing the POD inactivation behavior during the treatment. To apply this mathematical model, it is assumed that only two types of isozymes are present, where, one is heat resistant (ER) and the other is heat labile (EL). In addition, each fraction of the enzyme is assumed to follow first-order kinetics mathematically. Based on the model parameters, the mechanism of POD inactivation was discussed. A Midilli model (MR=aexp(-ktn)+bt) performed well for describing drying behavior during the treatment. The constants and coefficients (a, k, n and b) of the Midilli model were regressed with processing variables including slice thickness (H) and target surface temperature (T) by linear type of equation. MR=(1.01+5.95×10-5,T-7.70×10-4H)exp(-(-1.07×10-3+4.16×10-4T-3.16R10-3H)t(1.47-1.22×10-3T+117×10-3H))+(-2.42×10-3-9.16×10-5T+1.77×10-3H)t. The obtained empirical drying equation demonstrated good predictability with a strong correlation between predicted and experimental values (R2>0.9996,0.0017<RMSE<0.0065). The effective moisture diffusivity and the activation energy represent mass transport of moisture. A quantic model (△E=at5+bt4+ct3+dt2+et+f) fit well for describing the surface color change during the process. The constants and coefficients (a, b, c, d, e and f) of the Quintic model were regressed with processing variables including slice thickness (H) and target surface temperature (7) by linear type of equation.△E=(-3.66×10-4+4.37×10-6T-1.55×10-6H)t5+(1.46×10-2-1.81×10-4T+1.45×10-4H)t4+(-0.16+2.30×10-3T-4.54×10-3H)t3+(0.14-7.27×10-3T1+5.30×10-2H)t2+(3.77-1.51×10-2T-0.18H)t+(-4.05+4.23×10-2r+4.64×10-2H).The blanching and drying characteristics of carrot slices processed with a sequential infrared (IR) radiation heating and hot air (HA) drying were investigated. Carrot slices with5mm thickness were blanched using IR heating and water at90℃. The characteristics of blanched carrot slices, including peroxidase (POD) activities, vitamin C, color and moisture reduction, were evaluated. The IR blanched carrots were dried by separated infrared drying (IRB-IRD), separated hot air drying (IRB-HAD) and sequential infrared and hot air drying (IRB-SIRHAD). Water blanched carrots were dried by hot air (WB-HAD) as control. The drying temperatures were at70℃for both IR. and HA drying. The quality characteristics of dried carrot slices, including vitamin C, rehydration, color, shrinkage and hardness, were evaluated. The blanching results showed that the IR. dry-blanching for15min resulted in a reasonably residual POD activities and moisture reduction, the lower surface color change and higher retention of vitamin C. The drying results showed that IRB-IRD and IRB-SIRHAD have higher drying rate and less drying time. The quality of dried carrots results showed that IRB-SIRHAD can get high-quality carrots. It has been concluded that the best processing method was carrot slices blanched using IR at90℃for15min, then dried using IR drying at70℃until the moisture content (MC) reached30%-40%, and then followed by HA drying at70℃in the latter stage. |
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