Investigation on the discoloration of freeze-dried carrots and the color protection by microwave combined with coating pretreatment

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The color of freeze-dried carrots tends to be lighter than that of fresh ones, and the degree of color fading becomes more evident with longer storage time, thus affecting the sensory quality. To investigate the causes and alleviate the discoloration problem of freeze-dried carrots, this paper discusses various properties in terms of color, moisture content, enzyme activities of polyphenol oxidase, peroxidase and lipoxygenase, the content of color-related substances such as total phenols and total carotenoids, functional groups and microstructure. According to the correlation analysis, the discoloration of freeze-dried carrots was mainly due to enzymes. Polyphenol oxidase, peroxidase and lipoxygenase were significantly correlated with L* and a*. Additionally, total phenols, carotenoids and moisture content also affected color. Freeze-dried carrot slices pretreated by the combination of microwave blanching at 700 W for 60 s and coated with 0.4% (w/w) pectin solution had the largest value of a* (16.44) and the smallest value of L* (77.69), which indicated the best color-protective effect.