Abstract: In order to understand the effect of different color protection pretreatments on the vacuum freeze-drying
characteristics of burdock (Arctium lappa L.) slices, comparative experiments of vacuum freeze-drying were performed on
burdock samples pretreated by browning inhibitors, blanching and ultrasound-assisted blanching. The results showed that the
ultrasound-assisted blanching group required the shortest time of 12.20 h to dry, which was reduced by 15.86% and 6.87%
compared with the browning inhibitor and the blanching groups, respectively (P < 0.05). The ultrasound-assisted blanching
group had the highest rehydration ratio of 5.52 g/g, indicating an increase of 18.71% and 12.42% in comparison with the
browning inhibitor and the blanching groups, respectively (P < 0.05). The shrinkage rate in the browning inhibitor group
was 0.93, which was slightly higher than that in the two other groups (P < 0.05). The hardness of freeze-dried products in the
browning inhibitor group was 54.32 N, which was 90.53% and 120.73% higher than that of the blanching and the ultrasoundassisted
blanching groups (P < 0.05), respectively. Compared with the fresh sample, color difference ΔE of dried products in
the browning inhibitor group was the smallest, and a gradual increase was observed for the ultrasonic-assisted blanching and
the blanching groups. However, ΔE of rehydrated products in the browning inhibitor group was significantly higher than that
in the other groups (P < 0.05). Scanning electron micrographs (SEM) revealed more significant cell shrinkage or cell shape
curling in dried products from the blanching and the ultrasound-assisted blanching groups compared with the browning
inhibitor group. Low-field nuclear magnetic resonance (LF-NMR) analysis showed that transverse relaxation time (T2) peaks in
the ultrasound-assisted blanching and the blanching groups presented different sizes of rightward shift in contrast to the browning
inhibitor group, suggesting that the residual moisture in dried products had a greater degree of freedom or lower bound activity.

Key words: Arctium lappa L., color protection, vacuum freeze-drying, characteristics