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

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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 h). 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.

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