The solubility of powder in water is one of the key characteristics in the quality of food powders. This property directly impacts the ease of use and nutritional value of the product. The process of powder dissolution typically consists of three main stages: wetting, dispersion, and dissolution, each of which is measured by specific indicators. These indicators include the Wetting Index (WI), Dispersion Index (DI), and Solubility Index (SI).
Wetting Index (WI) measures the powder's ability to absorb water and its penetration into the surface of the powder particles.
Dispersion Index (DI) indicates the powder’s ability to separate into individual particles when dispersed in water.
Solubility Index (SI) shows the overall ability of the powder to dissolve in water.
To assess the solubility of powders, Shock and colleagues (2012) defined the "Instantaneous" criterion (IC), which applies to powders that simultaneously exhibit fast wetting (WI < 20 seconds), high dispersion (DI > 95%), and optimal solubility (SI > 99%).
Impact of Fat on Solubility
The solubility of milk powder is influenced by the composition and structure of the powder, particularly the fat and protein composition. Full cream milk powder, due to the hydrophobic nature of the free fat particles, requires more time to wet and disperses less efficiently compared to non-fat milk powder. This characteristic makes it harder to dissolve in water. To improve these properties, natural surfactants like lecithin or humectants such as amorphous carbohydrates are commonly used to facilitate the dissolution process.
Impact of Protein on Solubility
Protein powders, such as Whey Protein Concentrate (WPC) and Whey Protein Isolate (WPI), typically have difficulties dissolving in liquids due to their high protein content. These powders, especially those with a high protein concentration, have a strong tendency to absorb water and immediately wet the surface upon contact with water. This leads to the formation of a gelatinous layer at the water-powder interface, preventing water from penetrating the protein particles and making uniform dissolution difficult. Rather than dispersing quickly, these powders tend to form clumps when stirred in liquids.
