Kale Carotenoids Remain Stable while Flavor Compounds Respond to Changes in Sulfur Fertility

Dietary intake of certain carotenoids has been associated with a reduced risk of disease. Kale (Brassica oleracea L. Acephala Group) has the highest levels of carotenoids lutein and β-carotene, and is an excellent source of minerals among the green leafy vegetable crops. However, Brassica vegetables contain glucosinolate (GS) and S-methylcysteine sulfoxide (MCSO). While these sulfur compounds have medicinal value, they are also responsible for the bitter, acrid flavors that are often regarded as objectionable by consumers. Therefore, the objectives of this study were to investigate the influence of increased S fertility levels on (1) elemental accumulation, (2) GS and MCSO production, and (3) the accumulation patterns of carotenoid pigments in the leaves of three kale cultivars. Winterbor, Redbor, and Toscano kale were greenhouse-grown using nutrient solution culture with S treatment concentrations of 4, 8, 16, 32, and 64 mg of S/L. Decreasing S fertility decreased S leaf content, but increased the levels of Mg and Ca accumulation, two important minerals for human health. Levels of GS and MSCO decreased in response to a decreasing S level in nutrient solution. However, accumulation of lutein and β-carotene was unaffected by S treatment. Lowering the S fertility in the production of kale should decrease the levels of negative flavors associated with high levels of GS and MCSO without affecting carotenoid pigment levels. Understanding the combined impact of fertility on flavor compounds and carotenoid pigments may help improve consumer acceptance of phytonutritionally enhanced vegetable crops.