The present invention is directed to chemoprotectant precursor compositions provided from crucifer seeds and sprouts and methods for their preparation. Treatment of aqueous extracts from crucifer seeds or sprouts with adsorbents substantially increases the ratio of certain highly chemoprotectant precursor compounds (alkyl glucosinolates such as glucoraphanin, a.k.a. sulforaphane glucosinolate) to undesirable compounds such as indole glucosinolates (for example 4-hydroxyglucobrassicin). The method provides an extract which has a ratio of glucoraphanin to 4-hydroxyglucobrassicin of about 70 or greater. The resulting extract has improved color and odor and may be dried or used directly as an additive in a variety of foodstuffs.