Evaluation of protein quality: comparison of single-point and multi-point assays

[Advances in possibilities for protein evaluation]

With protein evaluation the changing abilities of food proteins are characterized with regard to their covering the requirements of the organism for protein or amino acids, respectively. The biological value of proteins is influenced by several factors. Measuring the reaction of the whole organism (of man or animal) to the food protein the sum of all influencing factors is covered. Hence, these direct evaluation methods have priority. The general dose-reaction curve demonstrates the direct evaluation methods: The evaluation criteria protein efficiency or minimum protein requirement are presented and the different possibilities of interpretion are compared. Most direct evaluation methods are performed with suboptimal protein supply, therefore values for an optimum protein supply can be derived only with reservation. Improved values for protein evaluation can be obtained from investigations of the protein metabolism. With that the substrat-related evaluation can be more orientated to a organism-related one. Indirect biochemical evaluation methods are less important. That does not concern in-vitro evaluation by amino-acid analysis and/or in-vitro digestibility tests. Possibilities and limits of these methods are discussed.

Experimental studies on the quality of food proteins

1. This paper reviews chemical and biological assays for measuring the quality of food proteins. 2. The availability of amino acids in a protein is determined by digestability and the capacity of the recipient animal to absorb the amino acids. 3. Intracellularly amino acid concentration influences enzyme activities associated with amino acid and nitrogen metabolism. 4. Amino acids utilized for metabolism affect the cells at the gene level: transcription of DNA to RNA and translation of RNA to protein. 5. More accurate measurements of amino acid utilization for metabolism will follow from further understanding of basic cellular metabolic events.