NUTRITIVE-VALUE OF POTATO CRUDE PROTEIN AS INFLUENCED BY MANURING AND AMINO-ACID COMPOSITION

Potatoes were grown in pots and fertilised with varying amounts of nitrogen (as nitrate or ammonium sulphate or urea), phosphorus, potassium and cow manure. The largest yield, 451 g DM per pot, was obtained with nitrogen in the form of urea. N‐applications and P‐ and K‐deficiency in the soil increased the total‐N content, which varied from 1.14–3.07%. Concentrations of nitrate‐N (0.001–0.016%) were negligible. Increasing N‐concentrations were associated with decreases in the crude protein of most amino acids including lysine (6.26–4.17%), threonine (4.08–2.90%), methionine (1.98–1.46%) and tryptophan (1.74–0.86%). The concentrations of aspartic and glutamic acids (probably mainly present as their amides) increased. Differences in amino acid composition between boiled and unboiled potatoes were negligible. From 70 to 90% of the decrease in concentration of most essential amino acids could be accounted for by changes in N‐content of dry matter. Phosphorus and potassium affected the amino acid composition indirectly through their effects on N‐content. In rat feeding experiments increasing N‐concentrations in potatoes increased the true digestibility of the crude protein from 79 to 91 but decreased the biological value from 82 to 59. The net protein utilisation did also decrease (from 67 to 54). These relationships were linear below 2.2% N in dry matter. Changes were mainly dependent on variations in N‐content. Decreases in essential amino acid indices or chemical scores were closely reflected in the results of the feeding experiments. The true amino acid digestibilities were different for individual amino acids and increased with increasing N‐concentration in dry matter. Copyright © 1979 John Wiley & Sons, Ltd