What is phase reversal theory?

The phase reversal theory theory was proposed by Fischer and Hooker in 1927, the theory is therefore also referred as Fischer and Hooker’s theory. According to this theory churning is a process of phase reversal i.e. changing of oil in water emulsion (O/W) to water in oil emulsion (W/O). The stability of emulsion is related to the relative volumes of the two constituents present. When oil and water are mixed together, the resulting suspension may be a suspension of (o/w) or suspension of w/o. The type of emulsion obtained depends on the proportion of the two main constituents present, the order in which they are added and the type of emulsifier used. 

In churning cream, initially the ratio of surface area to volume (S/V) of the fat globules is large. When the churning proceeds, surface area decreases and with progressive churning, surface area keeps on decreasing. The reduced surface area can no longer hold all the butter milk so it breaks i.e. separates out. 

Agitation of cream during the churning process causes coalescence and clumping of fat globules until eventually the ratio of surface area to volume of the fat units becomes so small that the reduced surface area can no longer contain the butter milk in stable form. The O/W emulsion then suddenly breaks; giving butter grains consisting of an emulsion of W/O and free butter milk. The supportive evidence of this theory is established by the fact that in normal butter, water is not in continuous phase. It has been demonstrated that plastic cream, containing 80-82% fat, conducts electricity and it responds to the pH determination showing water is in continuous phase but butter is a very poor conductor of electricity and pH determination cannot be done on butter but only on serum separated from it.

Microscopic structural studies conducted by Rahn (1928) revealed that butter is not a true W/O emulsion. A proportion of globular fat are still intact in worked butter. He explained that since butter fat is cooled and largely crystallized before the start of churning, true W/O type emulsion is rarely possible.