Analyses of Protein Quality

Determining protein quality analysis is important in food science, particularly for developing foods with targeted nutritional value, and in animal feeding and husbandry. Protein is the key component in the diet of any farmed species, particularly in aquaculture and the pet food industry, making an accurate assessment of protein utilization critically important. Protein quality analysis provides an estimate of the content and bioavailability of indispensable or dietary essential amino acids. 

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. 

USING PROTEIN EFFICIENCY RATIO (PER) TO DETERMINE PROTEIN QUALITY 

PER is a method to metabolize or determine the quality of protein in foods. Quality is measured by the amount of usable protein and the growth resulting from it through an animal assay. Formerly, this method was used as the standard method for all protein quality analysis. However, there is some question as to whether or not it is a valid measurement. This is because PER does not account for the differences in amino acid requirements between humans and rats (Seligson and Mackey, 1984), nor does PER account for the protein needed for cell maintenance. Therefore, PER results often overestimate the requirements for some amino acids and underestimate others. Specifically, PER tends to underestimate the protein quality of lysine-deficient proteins such as wheat gluten (Hackler, 1977). 

However, since the PER is an in vivo test, protein digestibility and amino acid bioavailability are encompassed to some extent within the assay. Despite these advantages, it is difficult to determine the individual contribution of digestibility and bioavailability of individual amino acids, or of individual proteins in a complex mixture, on overall protein quality. There are indications that the assay can be shortened from 4 weeks to 2 weeks with little loss in accuracy (Hackler, 1977). 

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Amino acid analysis remains an indispensable tool in a variety of biological research and development fields, e.g. the biochemical study of proteins, quality control in biotechnology and nutrition, and in clinical analyses. The classic chromatographic technologies will be for the foreseeable future the major quantitative tools for amino acid analyses. The techniques are deceptively difficult and there remains a need to standardize techniques for good quantitation. 

In recent years, a number of workers have published amino acid analyses of the sweet potato (38, 43, 35, 22, 18). The overall picture is that the sweet potato amino acid pattern is of good nutritional quality but that the variability of individual amino acids both within the same cultivar and across cultivars is very high. For example, Walter et al. (44) reported that with the exception of aromatic amino acids, every essential amino acid has a score of less than 100 in one or more cultivars. The amino acid score is defined as the g of amino acid in 100 g of test protein divided by the number of g of that amino acid in the FAO/WHO reference pattern times 100. Bradbury et al. (22) showed that, for the same cultivar, environmental effects on the amino acid patterns is significant. For three cultivars, they found a mean percent standard deviation for all amino acids of 24.2,... 

Note that turbidity measurement plays an important role in many types of routine chemical analyses (e.g. nephelometric determination of water quality, the evaluation of the concentration of barium and sulphate, or potassium determination with sodium tetraphenylbo-rate in water analysis, and determination of seric proteins). Also, the characterization of turbidity measurements is very important when evaluating some sources of uncertainty in gravimetry used in valid chemical metrology [5],... 

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