Effect of Different Thawing Methods on Quality of Quick-frozen Broccoli

The effects of microwave thawing,steam thawing,natural thawing and still water thawing on total phenolics,glucosinolates,myrosinase,isothiocyanates,carotenoid,ascorbic acidand antioxidant capacity in broccoli were investigated. The results showed that the contents of total phenolics,ascorbic acid,glucosinolates and carotenoids in frozen broccoli were lower than fresh broccoli. However,isothiocyanate formationin frozen broccoli was significantly enhanced by 51.1% compared to fresh broccoli. The four thawing methods affected the quality of broccoli to different degrees. Microwave and steam thawing could shorten thawing time by 97.4% and 94.2%,respectively,compared with natural thawing. There was no significant difference in contents of total phenolics,glucosinolates and carotenoid between microwave thawing and steam thawing. However,steam thawing decreased ascorbic acid contents by 23.7% compared with microwave thawing. And still water thawing resulted in more bioactive compounds lost than natural thawing. After microwave thawing,contents of total phenolics,glucosinolates,ascorbic acid and carotenoids were 1.15,1.20,1.93 and 1.39 times of natural thawing. Thus,antioxidant capacity was higher. In addition,myrosinase activity was reduced after microwave and steam thawing,thus isothiocyanate formation was lower than natural thawing and still water thawing. In conclusion,microwave thawing method was better than other three thawing methods and had less effect on the quality of broccoli.