Blanching impact on pigments, glucosinolates, and phenolics of dehydrated broccoli by-products

Because of high water content, the valorisation of broccoli by-products requires dehydration that can preserve bioactive compounds. Blanching pre-treatment has been reported to improve the drying rate of broccoli. As a thermal treatment, it promotes also enzyme inactivation. Therefore, in this study, the impact of pre-dehydration blanching step, freeze-drying, air-drying at 40 °C, and microwave hydrodiffusion and gravity (MHG) dehydration on the levels of pigments, glucosinolates, and phenolics, was evaluated by UHPLC-DAD-ESI/MSn. When compared to freeze-drying, a technique known to preserve compounds, a pre-blanching step increased the extractability of both pigments and phenolics, while air-drying only retained 49% of the pigments and 70% of phenolics, both without affecting glucosinolates. However, when air-drying was preceded by blanching, less than 50% of compounds were retained. On the other hand, MHG dehydration increased the phenolics extractability by 26%, particularly that of kaempferol derivatives while also retaining the amount of the glucosinolate glucoraphanin, when compared to freeze-drying. Nevertheless, only 23% of indole glucosinolates were recovered and pigments were severely reduced, with lutein accounting only for 32% and only chlorophyll b was observed in trace amounts after MHG dehydration. Therefore, to valorise broccoli by-products as ingredients, different drying technologies may be used when targeting different composition richness: freeze-drying is suitable for pigments and glucosinolates, air-drying is suitable for glucosinolates, while MHG promotes the extractability of phenolic compounds.