What is the difference between solubility and suspension stability?

ery little actually. But there are some differences. It is possible to have a protein powder that is highly soluble but displays poor suspension stability. Conversely, it is also possible to encounter a protein powder with good suspension stability and relatively low solubility. Suspension stability, as discussed above, is really a measure of how well the powder stays suspended in water after it is dispersed into ambient temperature water. All other things being equal, this is really a matter of how impervious the outer layer of the powder particle is to water penetration. A water-impervious outer layer will make a powder display poor suspension stability (because it is taking the water too long to penetrate into the inner layers of the particle and make the particle truly soluble). Solubility of a protein is usually measured by dispersing a protein powder in ambient temperature water, followed by heating the protein dispersion to at least50°C (and usually 70°C) and holding the protein dispersion at that temperature for a specified period of time to allow for maximum hydration/solubilizing of the powder (the dispersion is then cooled to room temperature and centrifuged to determine the percentage of soluble versus insoluble material present). The elevated temperature hold allows water to penetrate even the most impervious of particle outer layers, thereby allowing the water to penetrate the particles maximally. Water penetrated particles will have reached their maximum solubility level. So you see…a powder that might display lower suspension stability (due to a more impervious outer layer) could actually display high solubility (once the water has thoroughly penetrated into the particles).