What is Rahns Foam churning theory?

This theory was put forward by Rahn (1928). According to Rahn, cream (and also milk) contains a foam producing substance which gets solidified gradually when cream (or milk) is agitated. 

During churning first foam is produced. The fat globules then, due to surface tension, tend to concentrate on the foam bubble and thus are bought into such close contact that clumping of fat globules take place. Subsequently the foam producing substance assumes a solid character and the foam collapse. The fat globules then coalesce and butter is formed. 

According to Rahn’s theory, fat in cream at churning time is completely crystallized and the pass in to butter with their membrane intact and thus butter is a compact mass of fat globules in which butter milk, water and air are distributed as small globules. Rahn’s theory was based on his findings that air was necessary for normal churning of butter. Application of normal amount of mechanical agitation, in the absence of air did not result in churning of cream. The effect of overloading of churn resulting in increased churning time supported this theory (in case of overloading the churn, there was no sufficient space in the churn for the formation of required amount of foam hence more time). 

This theory was, however, subsequently criticized because of the fact that foam formation i.e. presence of air, is not required in some of the continuous butter making processes developed subsequently.