Protein Quality and Energy Density of Leek (Allium porrum L) as Influenced by Water and Nitrogen Supply and Plant Age at Harvest

Leeks were cultivated under conditions differing in level of nitrogen (N) supply (100 kg N ha‐1, 160 kg N ha‐1, 220 kg N ha‐1, 280 kg N ha‐1), level of water supply (normal, low) and time of harvest (September, October, November). Based on chemical analysis and balance studies on rats the nutritive value of the 26 batches of leek was determined. The protein content of the leeks increased progressively from 90 g kg‐1 DM to 163 g kg‐1 DM with N‐supply. This increase in protein was associated with a reduction of all essential amino acids (g per 16 g N) and, subsequently, a reduction of the biological value. Protein and energy digestibilities increased with level of N‐supply. Leeks harvested in September had a higher protein content, but had at the same time the lowest biological value as compared to leeks harvested in October or November. This was due to a lower content of essential amino acids (g per 16 g N) in leeks harvested in September as compared to leeks of later harvest. Only small differences between the two levels of water supply were observed in the chemical composition of the leeks. The content of non‐starch polysaccharides (NSP) was rather high in all samples of leek (approximately 240–280 g kg‐1 DM) and appeared to be unaffected by the growth conditions applied in the investigation. Soluble‐NSP constituted approximately half of the total NSP. © 1997 SCI.