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The gold-standard method to calculate protein content is to determine nitrogen levels, through Kjeldahl or Dumas, and subsequently convert this to protein using a conversion factor. However, this is slow, expensive and requires specialist equipment. This has led to the widespread use of colorimetric assays, principally the Bradford assay, as a rapid protein determination method. Limited research exists quantifying the accuracy of these methods for whey protein, using bovine serum albumin (BSA) as a quantifiable standard. To this end, standardised solutions were made of whey protein and BSA and protein content confirmed using Dumas. When tested with Bradford reagent, it was shown that standard solutions of whey protein were significantly less reactive than equivalent concentrations of BSA: thus, the use of BSA substantially underestimated the amount of protein present within a whey protein sample. This highlights that different proteins vary in their affinity for the reagent, indicating the need for researchers to create a sample-specific standard curve. Bradford data was shown to vary significantly from the reported protein concentration for a range of commercial whey proteins. Dumas was accurate only within a limited range, with higher margins of error being seen than commonly reported. The authors suggest against the use of BSA standard curves in the determination of protein within whey due to the varying protein-reagent affinities and recommend the use of any colorimetric assays with caution for heterogenous mixtures. It is hoped that highlighting these limitations will assist the development of more accurate protein determination methodologies, improving scientific quality.
Giles et al. (Tue,) studied this question.