Churning and its Theories

Churning refers to the process of agitation of cream at a suitable temperature until the fat globules adhere with each other forming larger and larger mass and leading to almost complete separation of fat and serum. During churning cream is slowly agitated with the paddles provided in the butter churn causing it to thicken and break into two parts, i.e., butter grains and buttermilk. At this stage, cold water at 10°C is added and agitation continued. Addition of water is necessary to help the cream to ‘break’ and hence it is known as break water. The volume of water should not exceed 25% of the total volume of cream. Churning continues until the butter granules are about the size of pea grains. In order to prepare good quality butter, the cream should possess good churnability. Good churnability refers to the ease of conversion of cream to butter, completeness of conversion and the time required for the formation of sufficiently firm butter grains. The body & texture of butter is also influenced by the duration of churning. Churnability of cream and body & texture of butter depends on the following factors:

a) Chemical composition of fat
b) Size of fat globules
c) Viscosity of cream
d) Temperature of cream at churning
e) Fat percentage of cream
f) Acidity of cream
g) Load of churn
h) Nature of agitation
i) Speed of churn
j) Pre-churning holding period

 

i Events during Churning

The sequence of events that occur during churning can be summarized as follows:

i) Churning is initiated with agitation of cream causing incorporation of air bubble into the cream. This, in turn, increases the volume of cream and air plasma interface.

ii) Surface- active forces like frictional forces cause partial disruption of fat globules membranes. Consequently, a part of the liquid fat leaves the globule and spreads over the surface of air bubble in the form of a thin layer.

iii) The fat film thus formed serves as a foam depressant causing the air bubble to burst. Also, the liquid fat serves as cementing material causing fat globules to join together.

iv) Eventually butter grains are formed which floats in the plasma, i.e., butter milk.

 

ii. Theories

There are three main theories of the churning of cream into butter. These are Fisher and Hooker’s Phase-Reversal Theory, Rahn’s Foam Theory, and King’s Modern Theory.

i) Fisher and Hooker’s Phase-Reversal Theory: According to this theory,churning is a process of phase reversal, i.e. changing an oil-in-water type emulsion (such as cream) to a water-in-oil type emulsion (such as butter).Agitation of cream in the churning process causes coalescence and clumping of fat globules until eventually the ratio of the surface area to the volume of fat units becomes so small that it can no longer contain all the buttermilk in stable form. The fat-in-water emulsion then suddenly breaks, yielding butter grains (consisting of an emulsion of water-in-butter-fat) and free buttermilk.But this theory fails to explain the results of microscopic studies, which reveal that a proportion of fat globules in butter are still intact in the worked butter.Hence butter is not a true water-in-fat emulsion whereas this theory assumes that butter is a water-in-oil type of emulsion.

ii) Rahn’s Foam Theory: According to this theory, the presence of foam/froth is essential for churning. It also postulates that there is a ‘foam-producing’ substance present in cream (and milk) and which gradually solidifies as the cream or milk is agitated. Foam is created during the churning period. The fat globules, due to surface tension effects, tend to concentrate and clump on the foam bubbles.The foam-producing substance assumes a solid character and causes the foam to collapse. The fat globules then coalesce and butter is formed.This theory also fails to explain continuous butter-making processes where in some cases foam formation (i.e. the presence of air) is not required. This theory assumes that the presence of foam is essential for churning.

iii) King’s Modern Theory. According to this theory, the true explanation of what takes place during churning appears to lie midway between the first two theories. The current concept of mechanism of the churning process may be summarized as follows:
 

• In cooled cream at churning temperature, the fat is present as clusters(clumps) of fat globules and within each globule it is present partly in solid and partly in liquid form.
•  Churning breaks up the clusters and causes foam/froth formation. The globules become concentrated to some extent in the film around the air bubbles in the foam and are thus brought into close contact with each other.
• The movement of the globules over one another in the foam film and the direct concussion between them causes a gradual wearing away of the emulsion-protecting surface layer of the phospholipids-protein complex. The globules then adhere together to form larger and larger particles. Eventually these particles become visible as butter grains (granules). As the granules form, they enclose some of the air from the foam. The fat in the granules is still mainly in globular form.
•  The working of butter grains causes the globules to move over one another.Under the effect of friction and pressure, some of them yield up a portion of the liquid fat. Others are broken up during working. Finally there is enough free liquid fat present to enclose all the water droplets, air bubbles and intact fat globules.