Seasonal variation in protein, mineral and glucosinolate composition of Portuguese cabbages and kale

Abstract

Portuguese cabbage ( Brassica oleracea L. var. tronchuda B., costata DC.) and kale ( Brassica oleracea L. var. acephala DC.) are commonly used for fodder in Portugal and Spain, but little information is available on the mineral and crude protein composition and none on glucosinolate concentration. This paper reports on seasonal variation of mineral and crude protein level in leaves, stems, and heads and glucosinolate in leaves and heads in two Portuguese cabbages, known as ‘Penca de Chaves’ and ‘Troncha de Mirandela’, and one Portuguese kale, ‘Galega’ grown in Spring/Summer (SS) and Summer/Winter (SW). Average concentration of Ca was 34.4 g kg −1 DM in the leaves with a minimum of 7.8 in heads. Sulphur was also higher in the leaves than in stems and heads (8.8 vs. 6.1 and 7.0 g kg −1 DM). The highest Mn concentration was 86.6 mg kg −1 in the leaves, with 25.1 in stems and 32.7 in heads. Levels of K and Zn were highest in stems with 41.8 g kg −1 and 137.3 mg kg −1, respectively, whereas P tended to be higher in the heads with 5.2 g kg −1. In leaves and heads, concentrations of P, K, S, Fe, Mn, and Zn were higher in the SW season than in SS and in stems, P and K were higher in SW. In heads the highest crude protein level was 200 g kg −1 DM in “Troncha” whilst in leaves was 267 g kg −1 in “Galega”. Summer/Winter conditions induced higher amounts of crude protein than SS conditions, in leaves (257), stems (169) and heads (217 g kg −1). Glucosinolate levels were higher in the heads (1919 μmoles 100g −1 DM for “Troncha” and 1991 for “Penca”) than in the leaves with levels in SS seasons 35% higher in the heads and 36% higher in the leaves than in SW seasons.