Inclusion of gas in casein and leaf protein precipitate particles and its effect on particle density

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Abstract
Floating precipitates often encountered in laboratory and industrial protein precipitate handling operations include isoelectric casein precipitates, casein precipitates formed by salting‐out with ammonium sulphate, and heat precipitated leaf (lucerne) protein. These were investigated by measuring water‐free densities and the variation in particle moisture content with temperature, and for each type of protein precipitate the density of degassed particles was greater than the density of the suspending medium, though the density difference was always small. Floating protein precipitates were therefore caused by the inclusion of gas in precipitate particles. Gas bubbles are easily incorporated into protein solutions. Furthermore for many types of protein precipitate, the physicochemical changes used to cause protein precipitation also drive dissolved gas from solution. Calculations showed that the amount of gas evolved is sufficient to cause flotation of all the precipitate formed.