[Determination of the protein quality of food and animal feed]

The estimated value of true digestibility of food and feed proteins is in dependence from the excretion of metabolic faecal nitrogen (MFN). Results of many authors showed that a high fibre content of the diet increase the MFN-excretion and lower the true digestibility of the diet protein. The exact estimation of MFN is only possible with isotopic methods. The labelling of feed proteins with isotopic nitrogen (15N) is possible in experiments with small animals. In big animals (pigs) the utilization of recycled 15N is too high following the long transit time of non digested 15N-labelled feed protein and the parallel 15N-labelling of MFN. The best method for the estimation of true digestibility of proteins is the method with 15N-labelled animals and the differentiation between nonlabelled undigested feed protein and the 15N-labelled MFN in faeces. The estimation of digestibility of essential amino acids at the end of the ileum as a measure for protein quality is not in advantage because the bacterial breakdown and synthesis of amino acids in the small intestine is unknown. The estimation of the biological value (BV) of proteins with the classical method is useful when the exact MFN was determined. The classic formula of BV [formula: see text] is only applicable in experiments with growing animals with nitrogen retention. This result of BV value is in agreement with the method only valid for growing animals with N-retention and not for animals or human beings in maintenance. The measurement of a BV of proteins with animals in maintenance is possible when the animals are labelled with 15N. The 15N-loss of the animal after the feeding of different protein sources is the smallest when the amino acid pattern of the protein is adapted to the need of amino acids for the maintenance metabolism of the animal. It was found that proteins with a high content of glutamic and aspartic acid (proteins of grains) are better proteins for the maintenance metabolism as animal proteins. Measurements with the method of the oxydation rate of essential amino acids showed that the amino acid pattern of wheat protein is in agreement with the need of amino acids for maintenance of the adult men. The evaluation of protein quality in animal nutrition for growing or lactating monogastric animals is in the present time the balance of essential amino acids in the feed protein and the need of this amino acids of the animals.