The solubility index was developed by the American Dairy Products Institute. It measures the amount of product sediment after the application of low centrifugal forces under specified conditions—a high value indicates that the product is less soluble. Following are maximum values for various types of NDM.

This method may also be used to measure dry whey, dry buttermilk and dry whole milk powder. Other methods for determining solubility of dairy proteins include the Nitrogen Solubility Index (NSI) and the Protein Dispersibility Index (PDI). NSI has been accepted as an international standard by the International Dairy Federation.