Quality improvement of freeze-dried carrots as affected by sugar-osmotic and hot-air pre-treatments

To improve the visual-sensory and nutritional quality of freeze-dried carrots (FDC), the effects of sugar-osmotic dehydration (SO, 40 degrees Brix, 3 h) and hot-air pre-drying (HAPD, 80 degrees C, 10 min) on the FDC quality were investigated. Overall, SO&HAPD significantly improved the redness (8.26%-11.67%), yellowness (1.24%-5.82%) and ABTS radical scavenging activity (1.40%-16.00%) of FDC. FDC pre-treated by sucrose showed the highest a* value (33.31), which was related to the beta-carotene content. The influence order of independent variables on the redness was ABTS radical scavenging activity > total sugar content > beta-carotene content. As for the carotenoid retention, the sample only treated by HAPD had the highest contents of beta-carotene (119.97 mu g/g) and alpha-carotene (93.73 mu g/g). These results indicated that SO&HAPD and HAPD were effective pre-treatments for improving the color and carotenoid contents of FDC, respectively. This work provides an experimental-theoretical basis for the quality enhancement of carrot products. Practical applications Freeze drying (FD) preserves the nutrients of the fresh material to the greatest extent; however, it undesirably decreases the redness/yellowness level of yellow vegetables/fruits. This discoloration reduces the edible quality of freeze-dried products and affects the consumers' acceptance. Sugar osmotic dehydration (SOD) is widely used in the food production of high moisture materials to prevent the color damage and to improve the redness of samples. The positive effects of the sugar-osmotic combined with hot-air pre-drying treatments (SO&HAPD) on the physio-chemical quality improvement of freeze-dried carrots (FDC) were explored. SO&HAPD was an effective way to improve the redness, yellowness and ABTS radical scavenging activity of FDC. The core indices (ABTS radical scavenging activity, total sugar content, beta-carotene content) selected through the stepwise regression analysis may help control the color quality of FDC for industrial scale-up. Moreover, this study can also provide an experimental-theoretical basis for the FD industry in producing yellow vegetables/fruits with original colors and high nutritional retention.