Protein Determination—Method Matters

TLDR

Protein content reported for foods varies with the analytical method, hindering direct comparison across studies. This study aimed to examine and compare different protein analytical methods. The authors assessed extraction‑dependent methods, measuring extraction yield across food matrices and evaluating nitrogen‑based conversions that can be affected by interfering substances. Most assays overestimated protein due to indirect nitrogen conversions and matrix interference, while amino‑acid analysis avoided these errors, so the authors recommend hydrolysis‑based amino‑acid analysis as the preferred method.

Abstract

The reported protein content of foods depends on the analytical method used for determination, making a direct comparison between studies difficult. The aim of this study was to examine and compare protein analytical methods. Some of these methods require extraction preceding analysis. The efficacy of protein extraction differs depending on food matrices and thus extraction yield was determined. Overall, most analytical methods overestimated the protein contents. The inaccuracies were linked to indirect measurements, i.e. nitrogen determination and subsequent conversion to protein, or interference from other chemical substances. Amino acid analysis is the only protein analysis method where interfering substances do not affect the results. Although there is potential for improvement in regards to the hydrolysis method, we recommend that this method should be the preferred for food protein determination.