MEASURING PROTEIN-QUALITY

An alternative to the time-consuming and expensive PER assay for measuring food protein quality is needed by the food industry. Many biological and chemical-based assays for measuring protein quality have been described in the literature. Most of these are still too complicated, time-consuming, or too narrow in the range of foods they will test for daily quality control use. In the past five years, rapid methods have been developed that employ chemical assays for essential amino acid composition and availability or biological assays that measure protein digestibility and growth on food proteins. Most of these assays can be completed in five days or less and are applicable to a broad range of foods. These developments have brightened the prospects for the eventual development of a rapid assay that the food industry routinely can use to monitor protein quality. This paper has discussed two assays that were tested with a wide variety of foods and that take less than 72 hr to complete. The C-PER assay, uses data on the in vitro protein digestibility and EAA composition of a food protein to predict its protein quality in terms of PER. The C-PER technique is not limited by the protein, fat, additive or spice levels in the food to be tested, and is therefore applicable to a wide range of food ingredients and processed foods. The second assay is based on the growth of the protozoan Tetrahymena thermophila WH14 on a proteolytic enzyme hydrolyzed food sample along with in vitro protein digestibility data to predict protein quality in terms of T-PER. Because the Tetrahymena are more difficult to control on a day to day basis, the error of the T-PER estimate is greater than that for the C-PER estimate. Also, since Tetrahymena growth is greatly affected by various food additives and spices, caution should be used when this assay is used to measure protein quality in foods where the composition is not definitely known. The T-PER assay is best suited for assaying protein quality in protein-containing food ingredients, such as meats, flours, protein concentrates and isolates, or on foods where the exact composition is known. © 1979 The American Oil Chemists' Society.