Standards Development

Scope

This document specifies a method to determine soluble proteins in 5 mM (0.02% NaOH) hydroxide solution in meals of oilseeds, pulses, and cereals.

Purpose

Recently, a new method was developed to measure protein solubility index (PSI) in protein meals of oilseeds, pulses and cereals. It features extracting soluble proteins with a very low hydroxide solution (5 mM, equivalent to 0.02% NaOH or 0.028% KOH), measuring nitrogen content of the dried residue by Dumas’ method or the extract by Kjeldahl, and calculating percentage of nitrogen mass in the extract over that in the original sample. An interlaboratory test was conducted thereafter to evaluate the performance of the new method. It involved 16 laboratories from 10 countries to measure PSI of 12 selected samples of oilseeds, pulses, and cereals. Based on the published results of the study, the new PSI method was approved as AOCS Method Ba 15-2023. The purpose of this new work is to specify the method and propose it as an International Standard. Indeed, Proteins are important components of almost all food and feed products. In describing protein quality, protein solubility is the primary parameter that correlates well with the degree of heating. Decades ago, several simple methods were developed to measure protein solubility of oilseed meals (particularly soymeal), including nitrogen solubility index (NSI), protein dispersibility index (PDI), and protein solubility in 0.2% KOH (PS-KOH). These methods have been standardized as official ones: NSI as American Oil Chemists Society (AOCS) Method Ba 11-65, PDI as AOCS Method 10b-09, and PS-KOH as ISO 14244:2014. They are widely used as indirect indicators of nutritional quality and/or functional properties of not only oilseed meals but also other plant protein products (such as those of cereals and pulses). Unfortunately, decades later, these methods become outdated and have some practical issues. First, PDI and NSI methods require specific blenders or mixers that are unavailable or uncontrollable, while PS-KOH targets heated or overheated protein products. Second, they all use Kjeldahl method to measure soluble nitrogen (proteins) in extracts. Yet, the Dumas (combustion) method is gaining popularity because it is simpler and faster, uses lower amounts of samples, and generates no chemical waste. Simply replacing Kjeldahl with the Dumas method to measure soluble nitrogen in extracts can lead to variable results. For overcoming the above issues, the new PSI method stated above was developed. In addition to solving the above issues, compared to ISO Method 14244:2014, AOCS Method Ba 15- 2023 gives a larger range for a set of samples, uses smaller sample size, and offers simultaneous running of multiple samples. It also gives a higher positive correlation with trypsin inhibitor activity in soybean products. Collaborative trial has been done previously.

No complementary tests are planned for this ISO project.