Protein, Oil and Carbohydrate Composition of Food Crops.

Nutrient management in crop production is focussed on optimizing crop yield and profitability, but can also affect the protein, carbohydrate, and oil composition of crops and the quality of food products they produce.  Protein concentration is determined largely by the amount and timing of N supply relative to the yield potential of the crop. Management tools such as late foliar applications or controlled-release technologies can boost N availability for protein production while keeping losses of surplus N to a minimum. Nitrogen applications increase the protein content and hardness of rice grains, reducing breakage during milling and improving the amino acid nutritional balance. Nitrogen applications increase protein in maize but may reduce its biological value by reducing the levels of the limiting amino acids lysine and tryptophan. However, opaque-2 corn cultivars bred for high quality protein maintain their biological value at higher N levels. A balanced supply of other nutrients, particularly S, is important to optimize protein content of wheat for bread-making and pasta quality. Enhancing S supply to soybeans during seed-filling can improve protein composition by increasing the limiting amino acids methionine and cysteine. Carbohydrate quality is generally less influenced by nutrient management than protein.  Excess N applications should be avoided in crops where high carbohydrate is desirable, since protein and carbohydrate are generally inversely related.  For example, moderate rather than high levels of N optimize starch composition of rice for improved cooking and eating quality. Effects of nutrient management on oil quantity and quality also tend to be minor. Increasing protein concentration with N applications may reduce oil concentration in canola and soybean. Where S limits yield in canola, its application can also boost oil concentration. In general, fertilizing for optimum yields does not differ greatly from fertilizing for optimum quality for most of the world’s major food crops.