Effect of preliminary processing and method of preservation on the content of glucosinolates in kale (Brassica oleracea L. var. acephala) leaves

The aim of this work was to evaluate the level of total glucosinolates (GLS) in fresh kale leaves and leaves after processing (blanching, cooking), as well as in frozen, canned and dried products after 1-year storage. Fresh kale leaves contained 26.87 μmol GLS per 1 g dry weight, of which aliphatic GLS comprised 63%. GLS levels in kale leaves were affected by blanching and cooking; losses were highest in leaves preserved by sterilization and lowest in frozen leaves. After 12 months of storage, mean amounts of total GLS in frozen and canned products were 15.59 μmol and 10.73 μmol respectively per 1 g. In dried products, mean total GLS levels ranged from 9.09 to 13.81 μmol, being significantly (on average 13%) higher in freeze-dried products than in those dried by hot air. Moreover, dried products obtained from blanched leaves showed higher mean amounts of total GLS than those obtained from unblanched leaves. Frozen products from blanched material contained the highest amounts of GLS; total GLS were average 20% higher than in frozen products obtained from cooked raw material, 58%, higher than in canned products, and 47% higher than in dried products. Aliphatic glucosinolates were usually more stable than indole GLSs.