Nitrogen protein quality

A number of procedures used to determine protein quality involve bioassays. Bioassays require feeding live animals protein ingredients for a specified period of time, and then estimating the nutritive value of the protein. Two such assays are the rat-based protein efficiency ratio (PER) bioassay and the human nitrogen balance assay (Dimes et al., 1994). Animal feeding experiments require chemical analyses of both the dietary inputs and then the metabolic output of the animal (e.g., body composition analysis, fecal sample analysis, collection, and assay for urine) from which the efficiency of protein metabolism can be predicted as well as how the protein supports animal growth and cell maintenance. 

Animal assays. When assessing the nature of a protein quality reduction, the conventional methods of protein quality measurement have certain limitations. For example, a reduced TD in an NPU assay is not necessarily the result of a reduced protein digestion. Obviously the same result will be obtained if the increase of fecal nitrogen is caused by an enhanced excretion of endogenous protein or if there is a fixation of metabolic nitrogen by the colonic micro-flora. 

Laboratory tests such as urease activity, protein dispersibility index (PDI), nitrogen solubility index (NSI), thiamine, and water absorption have been found valuable in monitoring daily production for protein quality. But biological chick and/or rat assays are the only reliable means currently available for predetermining the nutritional value of whole soybean protein they must be conducted periodically to verify results of chemical tests (31). If whole soybeans are to be used in a mixture containing 20% or more soybean meal, 5% or more urea, and 20% or more molasses, or an equivalent mixture, and exposed to hot, humid storage conditions, it is advisable that the urease activity of the whole soybeans not exceed 0.12 increase in pH (31). Extruded or roasted soybeans properly treated for cattle to increase bypass protein should have urease values of less than 0.05 pH rise. A urease rise of 0.05-0.20 is an indication of proper treatment for swine and poultry. 

The study in monkeys involved lactalbumin or soybean protein-based diets. Nitrogen balance was attained with the feeding of relatively low levels of lactalbumin (0.15 g N/day) but relatively high levels of soy protein (0.30 g N/day). Another point is that lactalbumin supported a positive N balance at lower concentrations than did soy protein. These findings indicate that lactalbumin is a higher-quality protein than soy protein. The concept of protein quality is explored next. Lactalbumin is a higher-quality protein than soy protein, as the soy protein is somewhat deficient in methionine. For this reason, animal diets based on soybean protein are usually supplemented with methionine. 

A negative nitrogen balance represents a state of protein deficiency, in which the body is breaking down tissues faster than they are being replaced. The ingestion of insufhcient amounts of protein, or food with poor protein quality, cau result in serious medical conditions in which an individual s overall health is compromised. The immune system is severely affected the amount of blood plasma decreases, leading to medical conditions such as anemia or edema aud the body becomes vulnerable to infectious diseases and other serious conditions. Protein malnutrition in infants is called kwashiorkor, and it poses a major health problem in developing countries, such as Africa, Central and South America, and certain parts of Asia. An infant with kwashiorkor suffers from poor muscle and tissue development, loss of appetite, mottled skin, patchy hair, diarrhea, edema, and, eventually, death (similar symptoms are preseut in adults with protein deficiency). Treatment or prevention of this condition lies in adequate consumption of protein-rich foods [106]. 

The nutritive value of a protein depends on its capacity to provide nitrogen and amino acids in adequate amounts to meet the requirements of an organism. Thus, in theory the most logical approach for evaluating protein quality is to compare amino acid content (taking bioavailability into account) of a food with human amino acid requirements. A number of comparisons have been made using reference patterns such as those derived from egg and milk protein. The first major change in procedure was substitution of a provisional pattern of amino acid requirements for the egg protein standard. 

About 950 g/kg of the nitrogen in oilseed meals is present as true protein, which has an apparent digestibility of 0.75-0.90 and is of good quality. AVhen biological value is used as the criterion for judging protein quality, that of the oilseed proteins is considerably higher than that of the cereals (Table 23.3). 

Slope ratio method— This is a modification of the nitrogen balance index wherein various levels of protein are fed adlibitum to growing animals. Then growth rate and protein intake are plotted on a graph, and the slope of this line is taken as the measure of protein quality. 

A procedure employed to determine protein quality. It is the proportion of the nitrogen in the food that is retained by the tissues of the body. In other words, it is the amount of nitrogen contained in the food consumed, minus the amount lost in the urine and feces, divided by the amount of nitrogen in the food. 

Wayler, A., Queiroz, E, Scrimshaw, N.S., Steinke, F.H., Rand, W.M., and Young, V.R. (1983). Nitrogen balance studies in young men to assess the protein quality of an isolated soy protein in relation to meat proteins. J Nutr 113 2485-2491. 

Fermentation is known to improve protein quality due to higher nitrogen digestibility and the de novo production of some amino acids such as lysine and tryptophan. In addition, these processes lower the presence of antinutritional factors (Chavan and Kadam 1989a). 

---