A new method for determining protein solubility index (PSI) based on extraction with 5 mM alkali hydroxide and its correlation with trypsin inhibitor activity in soybean products

Authors: Liu, Keshun

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2022
Publication Date: 10/7/2022
Citation: Liu, K. 2022. A new method for determining protein solubility index (PSI) based on extraction with 5 mM alkali hydroxide and its correlation with trypsin inhibitor activity in soybean products. Journal of the American Oil Chemists' Society. 99(10):855–871. https://doi.org/10.1002/aocs.12643.
DOI: https://doi.org/10.1002/aocs.12643

Interpretive Summary: There has been an increasing use of plant proteins as food, feed, and industrial materials in recent years. Accurate assessment of protein quality in protein products is essential. Broadly speaking, protein quality affects both the nutritional values and functional properties of a protein product. Both determine how the product can be used for a particular application. One major factor affecting protein quality is the degree of protein denaturation, which relates directly to the degree of heating. Unfortunately, it is difficult to precisely measure structural changes of protein molecules resulting from heat denaturation, while assessing protein quality in vivo is costly and time-consuming. Therefore, several laboratory procedures have been used to indirectly measure the degree of protein denaturation (or heating), including nitrogen solubility index (NSI), protein dispersibility index (PDI), urease activity (UA), and protein solubility in 0.2% KOH (PS-KOH). However, since these methods were developed some 80 years ago, they either require specific equipment that is expensive, outdated or unavailable and/or are limited to certain products or applications. To address these issues, researchers at USDA-ARS, Aberdeen, Idaho, developed a new protein quality index, hereby referred to as the protein solubility index (PSI), after investigating many relevant factors that centered on two major stages: protein extraction and nitrogen analysis. The new method features 5 mM alkali hydroxide extraction with magnetic stirring at room temperature, N analysis of dried residues, and small sample size. There is a strong or very strong correlation between PSI and trypsin inhibitor activity, a major antinutritional factor in soybean and other legume protein products. Hopefully, PSI can serve as a unified index to replace most, if not all, of the indirect indices currently used for protein quality measurement. With easy performance and good repeatability, this would eliminate confusion of multiple tests and terminologies used currently by relevant industries and scientific communities.

Technical Abstract: Protein quality affects the nutritional values and functional properties of protein products. For better utilization, it is important to assess protein quality accurately and cost-effectively. For decades, several indirect indices have been relied on, including nitrogen solubility index, protein dispersibility index, urease activity, and protein solubility in 0.2% potassium hydroxide. However, each existing index has limitations or practical difficulties for measurement, while different procedures and terminologies often cause confusion. For rectifying this situation, a new protein quality index, the protein solubility index (PSI), was developed. This required investigation of many factors, including alkali hydroxide type and concentration, solid to solvent ratio, extraction time, magnetic stirring speed, magnetic stirrer type, and centrifugation force and time. For nitrogen analysis, a combustion method was used, where effects of using extracts or residues, sample moisture, sample mass, and the sequence of drying and weighing residues were also investigated. The new PSI method employs 5 mM alkali hydroxide extraction, magnetic stirring, nitrogen analysis of dried residues and small sample size. It allows simultaneous running of multiple samples. Furthermore, the correlations between trypsin inhibitor activity (TIA) and PSI were either strong or very strong (r = 0.747**-0.974**), depending on sample sets. The 5 mM extraction procedure produced a stronger correlation between protein extractability and TIA than the extraction procedure for the TIA assay (10 mM NaOH, 3 hr). PSI has a potential to replace most, if not all, of the indirect indices for protein quality of various products, providing a unified index for relevant industries.