An in vitro method using a multienzyme system for the estimation of protein digestibility has been developed. The multienzyme system consists of trypsin, chymotrypsin and peptidase. It was found that the pH of a protein suspension immediately after 10 min digestion with the multienzyme solution was highly correlated with the in vivo apparent digestibility of rats. Regression analyses of 23 samples tested showed that the correlation coefficient between pH at 10 min and in vivo apparent digestibility was 0.90 with a standard error of estimate of 2.23. The regression equation was Y = 210.464 -18.103, where "X" was the pH of protein suspension immediately after the 10 min digestion with the multienzyme solution. The most significant advantage of this in vitro method for predicting apparent protein digestibility was that it can be completed within 1 hr and with a high degree sensitivity. The method can detect the effects of trypsin inhibitor, chlorogenic acid, and heat treatment on protein digestibility. Strong buffer salts may affect the measurement of protein digestibility, but the buffering effects found in general food proteins and products tested did not create any problem with the procedure.

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 protozoanTetrahymena 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 theTetrahymena 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, sinceTetrahymena 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.

The apparent and true digestibilities of the same preparations of six proteins (spray dried whole egg, cottage cheese, canned tuna, peanut flour, soy isolate, and wheat gluten) were estimated in four to five men and in rats and compared to estimates of digestibility from three different in vitro enzymic digestion procedures. For all six proteins, the correlation coefficient was 0.46 between true digestibility in humans and in rats; with values for tuna excluded, r = 0.96. With all six proteins, none of the in vitro values was significantly correlated with values from humans or rats. However, with either the three animal proteins alone or the three plant proteins alone, correlations were high (r greater than 0.90) between one or more of the in vitro estimates and the observed true or apparent human and rat digestibilities. The differences in the relationship between enzymic digestion estimates and the human digestibility estimates for plant or animal proteins suggest that for accurate prediction of protein digestibility in humans by these enzymic methods, different equations would have to be used for plant and animal proteins. For protein sources containing both plant and animal protein, use of the in vitro enzymic procedures would give only an approximate estimate of digestibility in humans.

The major protein of Great Northern beans was isolated through salt extraction, ammonium sulfate fractionation, gel filtration and DEAE-cellulose ion exchange chromatography. Physicochemical properties of the major bean protein were determined. Results from heat stability studies showed that the protein was the most stable at pH values between 4 and 6. A complete unfolding of the bean protein was not essential in order to improve its digestibility. The native protein had a compact structure and therefore was resistant to the attack by proteolytic enzymes. A glycopeptide containing a N-glycosidic protein-carbohydrate linkage, isolated from a protease digestion of the major bean protein was also characterized. Results implied that the carbohydrate moeity might have a negative influence on the digestibility of the native protein.

Natural actomyosin (NAM) and "crude" actomyosin formed gels yielding maximum strengths (from back extrusion force) at pH 5.0 and 5.5, respectively. At pH 6.0, NAM gels had a least protein concentration endpoint (LCE) value of 6 mg/ml. Gel strength increased exponentially with an increase of NAM concentration from 3.75-10 mg/ml. With constant time (30 min)-temperature heating, NAM gel forces increased by 20.5% (NS, P>0.05) in the 30-80°C range. Arrhenius plots of NAM interaction in solution and in gelation at pH 6.0 indicated two different reaction mechanisms within the temperature zones above and below approximately 35°C for solutions and 40°C for gels. Similarity of interaction slopes above the 35-40°C region suggested one reaction mechanism for NAM molecular aggregation in solution and gelation.