FAT RICH DAIRY PRODUCTS

Module 7. Butter-making process

Lesson 19
CHURNING OF CREAM

19.1 Introduction

Churning is the process of converting cream into butter through appropriate mechanical manipulations leading to the conversion of oil-in-water (O/W) emulsion of cream into water-in-oil (W/O) emulsion desired in butter. The emulsion change accompanied by removal of buttermilk and working of butter yields the desired structure and texture in the product.

19.2 Theories of Churning

The conversion of oil-in-water (O/W) emulsion of cream into water-in-oil (W/O) emulsion to form butter has been explained by various theories of churning. These are discussed below:

19.2.1 The phase reversal theory

This 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.

19.2.2 The Foam 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.

19.5 King’s Theory

King’s theory was proposed in 1930 and 1953 and it is regarded as the modern theory. According to this theory, what happens during churning is mid-way between the ‘Phase Reversal theory’ and Foam theory. The modern concept has been summarized by Mc Dowall as follows:

i) The fat in the cooled cream, at churning temperature, is present as clusters of fat globules. And within each globule it is present partly in solid and partly in liquid form.

ii) Agitation (churning) breaks up the clusters and causes foam formation. The globules become concentrated to some extent in the film around the air babble in the foam and thus are brought into close contact of each other.

iii) 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 phospholipid protein complex). The globules then adhere together to form larger and larger particles. Eventually these particles become visible as butter grains. The grains enclose some of the air from the foam. The fat still mainly remains in globular form.

iv) The working of the butter grains causes the globules to move over one another. Some of them, under the effect of friction and pressure cause some yields out a portion of the liquid fat, others are broken during working. Finally there is enough free liquid fat present to enclose the water droplets, air bubbles and intact fat globules.

19.6 Factors Influencing Churn ability of Cream

The factors which influence the churnability of cream can be classified into two groups as

i. The factors related to the initial character of the cream

ii. The conditions in the process of manufacturing

Factors related to the initial character of the cream includes chemical composition of the butter fat, size of fat globules, richness of cream and viscosity of cream while factors related to processing conditions are churning temperature, fullness of churn, speed of churn, design of churn etc. All these factors are discusses in the following sections.

19.6.1 Chemical composition of butter fat

Churnability of cream is greatly influenced by the proportion of soft fats (low melting point fat) and hard fats (high melting points). This proportion determines the degree of fat solidification in the cooled cream. If the proportion of soft fats is more than the churning period will be shortened, butter made will have less firmness and there will be more fat losses in butter milk. If the proportion of soft fats is low, it will prolong the churning period.

19.6.2 Richness of cream

The amount of fat in cream affects its churnability considerably. The richer the cream the sooner will be the completion of the churning provided the cream is not rich enough to be so thick as to cause the cream to adhere to the inside of the churn and thus escape agitation.

If rich cream is churned at a high temperature the butter will form in a remarkable short time, providing all other conditions are favourable. Thin cream churns much more slowly, and can be churned at high temperature than thick cream, without injuring the quality of butter when rich cream is churned at a high temperature and the butter forms in a short time (about 10 min), the butter will usually be greasy in body and will not contain a much of butter milk, which will be more or less difficult to remove on washing. When thick cream is churned, the butter does not break in the form of small round granules, as it does when thin cream is churned.

When thick cream is (36 to 38% fat) is churned at as high a temperature as is consistent with getting a good texture, the best result are obtained. This type of cream produces less butter milk and consequently less part loss in the butter milk and this will give increase over run and the breaking of the butter at the end of the churning will be such as to cause the granules to appear large and flaky, rather than small round granules. The more flaky granules of butter will retain more moisture than the small, harder granules under the same treatment.

When thick cream is churned and the temperature is moderately high, it is almost impossible to churn the butter into granules. This condition causes butter from thick cream to contain more moisture than butter from thin cream.

19.6.3 Viscosity of cream

The more viscous the cream, more time is required to complete the churning process. More viscosity diminishes the freedom of movement of the fat globules, lessens their opportunity of being brought together and retard coalescence, thereby increases churning time.

19.6.4 Size of fat globule

Cream containing large fat-globules (avg. diameter 4.6µ) churn more quickly than cream containing small globules. Cream containing small fat globules (avg. diameter 3.4 µ) churn with difficulty and require twice as much time to break as the large globule cream. The butter made from such cream has short grains and crumbly character.

19.6.5 Churning temperature

The temperature is one of the most influential factors in determining the churnability of cream. The higher the temperature of cream, the sooner the churning process will be completed. Too high a churning temperature is however not desirable. It causes the butter to contain soft lumps instead of in a flaky granular form. This is deleterious to the quality of the butter. It causes first a greasy texture of butter, and secondly, it causes the incorporation of too much butter milk in the butter. This butter milk contains lactose, curd, and water, which when present together in butter, are likely to sour and in other ways deteriorate the butter. Curd and lactose should be excluded from butter as much as possible, in order to eliminate food for bacteria which may be present. Too low temperature is also undesirable although it is better to have the temperature a little low rather than too high. Cream at low temperature becomes more viscous. On agitation in the churn such cream if it is very thick will adhere to the sides of the churn and rotate with it without agitating; consequently no churning will take place. Too low a temperature brings the butter in such a firm condition that it takes up salt with difficulty, and when this hard butter is being worked, a large portion of the water in the butter is expressed, and the overrun will be lessened to a great extent without increasing the commercial value of the butter.

The degree of hardness of the fat in cream is the governing factor in deciding the temperature during churning. The hardness of the fat depends upon:

(1) The season of the year.

(2) The individuality of the cow.

(3) The stage of lactation period.

(4) The kind of food fed for the cows.

All these factors influence the melting point of butter fat- The higher the melting point of butter fat, higher is the churning temperature and the lower the melting point of the fat; the lower is the churning temperature.

During spring, the cows yield milk containing a longer proportion of soft fats; consequently the churning temperature is always lower in the spring than in the winter. During the winter, when the cows are fed on dry food chiefly the harder fat increases in quantity, therefore, a higher churning temperature is necessary during that time.

The nature of food fed affects the melting point of butter to a considerable extent. Cotton seed will cause butter to become hard. When larger amount of cotton seed is fed, the butter assumes a crumby, hard condition.

It can be concluded that the churning temperature may vary between wide limits, but the average desirable churning temperature according to the season is

Winter --- 10-13°C; Summer--- 7-9°C

19.6.6 Amount of cream in churn

For maximum agitation to take place during churning, the cream must dash from side to side or from top to bottom. Optimum load for maximum agitation should be one third to one half full.

Overloading of the churn diminishes free space in the churn, diminishes concussions and leads to increase in churning time. The overloaded cream may be churned at higher temperature so that churning time is not prolonged but this is not recommended as higher temperature increases fat loss in butter milk and produces soft and leaky butter.

Under loading the churn is not economical for the manufacturer and at the same time the cream will adhere to the inner side of the churn and delay churning,

19.6.7 Nature of agitation

Proper agitation is necessary for churning cream into butter. The speed of the churn provides agitation to cream. So, the maximum speed of the churn is the speed that yields the maximum amount of agitation. It is dependent on the ratio of centrifugal force and gravity force. Centrifugal force should be less than gravitational force.

Calculating the speed of the churn

Where,

m= mass of the cream

N= speed of the churn

g= gravitational acceleration

R= distance from center of churn to the periphery

19.7 Churning Difficulties

The causes of churning difficulties are usually associated with the peculiar character of the cream and particularly where the source of cream is confined to a single herd. Usual causes of prolonged churning time and difficulty in formation of butter are excessive hardness of fat, small fat globules, use of thin cream for butter making and high protein content in cream.

i) Excessive hardness of fat: Winter cream usually contains more hard fats. Use of such cream for butter making prolongs churning time because it diminishes the ability of fat globules to coalesce during churning.

ii) Small fat globules: cream that contains small fat globules takes more time for churning as the ratio of membrane material to fat increases and thus provides increased protection to fat globules.

iii) High protein content: such cream delays butter formation because of increased viscosity that minimizes the force of concussion between the globules.

iv) Use of thin cream: Such cream also have increased protection due to higher membrane protein to fat ratio and also due to intervening serum that keeps globules apart during churning.