Module 3. Processing of cream

Lesson 7

7.1 Introduction

The important factor in standardisation of cream is the fat content. Standardization means adjusting the fat content in cream to a desired level so as to meet legal requirements of the cream products and to have ease in churning in butter manufacture and minimize fat losses in buttermilk. In market cream, if the fat content is higher than the specified requirement, then financial returns will suffer. If the fat content is low, then the cream may not meet regulatory requirements and also lack desired functional properties, such as viscosity or whipping. Too low a fat content will result in a high buttermilk volume; this will not give returns as much as the skim-milk. Standardisation is also an important facet of market milk production, and much of the technology is applicable to cream and milk.

7.2 Purpose of Standardization

Standardization of cream refers to the adjustment, i.e. raising or lowering, of the fat of cream to a desired value, so as to conform to the legal or other requirements of the process.

1. To prepare consumer cream products with fat content to meet legal/quality standards

2. To avoid economic losses accompanying excess fat content in preparaed product

3. To minimize fat losses in buttermilk

7.3 Methods of Fat Estimation in Cream

To achieve accurate standardisation, the fat content should ideally be controlled as it leaves the separator. For small-scale operations, however, standardising in bulk is the norm. The separator is adjusted to give a slightly higher fat content than required and a suitable diluent such as skim-milk is added to obtain the required fat content. Such dilutions can be calculated conveniently by using Pearson's square. A very important consideration in this operation is the measurement of fat content.

Normally a reasonably quick measurement method such as Babcock or Gerber methods is required. Accurate methods such as Werner-Schmidt or Rose-Gottlieb method are time consuming. The Babcock and Gerber methods, which rely on volumetric measurement of extracted fat, are quicker, but are a little less accurate. Most dairy factories, however, possess Milko-testers, Milko-scans or instruments that will give rapid measurements of good accuracy provided they are set in the correct mode with a properly prescribed diluent in the cream. Once the fat content of the cream has been determined, it is necessary to add the correct proportion of diluent (e.g. skim-milk or whole milk) to get the required fat content.

Cream storage tanks may be fitted with load cells, but are normally fitted with volumetric measures of contents, and as such the density of the cream should be taken into account as higher fat contents give lower densities.

7.4 Calculations for Standardization of Cream

Example 1

5,300 litre quantity of cream with a fat content of 42·3 per cent is available. Cream with a fat content of 40 per cent is required. How much skim-milk should be added assuming that skim-milk contains 0 per cent fat.

Quantity of fat in cream = 5300 x 0.423 x p kg = 2241.9 p kg

where p is the density of the cream.

The total amount of cream that would have this quantity of fat as 40 per cent of its content would be

2241.9p x 100/40= 5605p kg = 5605p/p' l

where p' is the new density of the cream.

305 (= 5605-5300) litres of Skim milk shall be added to 5300 litres of cream to obtain 5605 litres of standardized cream.

Alternatively, the Pearson square method can be used with more convenience as shown below: (to be calculated )

Example 2

If in the above example whole milk with 4·2 per cent fat content were to be used as a diluent, the calculations will be as shown below:

The essentials of the method are shown in Fig.7.1. The fat content of the cream used is set in the top left-hand corner of a square (42.3 per cent) with the fat content of the diluent at the bottom left-hand corner (4.2 per cent). The required fat content is then placed at the intersection of the square's diagonals (40 per cent) and the propor­tions of the cream and the whole milk are simply obtained by subtraction along the diagonals.

Cream required = 40 - 4.2 = 35·8 parts Whole milk required = 42.3-40 = 2·3 parts

Thus the amount of whole milk added to 5300 I of cream should be

5300 × 2.3/35.81 = 3411

i.e. the cream should be made up to 56411 with whole milk.

Fig. 7.1 Pearson's square as a means of calculating proportions of components for standardising.

These are only approximations, as strictly speaking the calculation should be based on weight rather than volume. However, if the change in fat content between the original and the standardised cream is not too great, then the approximation of equal density can be made. The relative inaccuracy of the tank volume measurements and any air entrained in the cream will also give a false volume measurement. Because of these inaccuracies, final analytical checking of fat content is advisable.

In high volume factories, it is more convenient to produce cream of a required fat content directly from the separator. This can be done purely by adjusting the flow of skim-milk and cream via back pressures for a fixed flow of incoming milk. Provided the fat content of the milk is known, the back pressures on the cream and skim-milk lines can be adjusted with special valves to provide the required proportional flow of skim-milk and cream. The important factors in such an operation are that standard conditions prevail during separation and that flow regulators are accurately calibrated. Flow measuring devices that assist these operations are available.

More sophisticated operations require an accurate measurement of milk fat content as the cream leaves the separator. A convenient method of ascertaining the fat content of cream is to measure its density, which is a function of the fat content. For example, in the temperature range 40-80°C density (kg/m3) can be computed using an equation

D =1038.2 – 0.17T – 0.003T2 - Ǿ (133.7 - 475.5/T)


T= Temperature in °C
Ǿ = Fractional fat Content


Density of cream = Fat % x Density of Fat + Skim milk % x Density of skim milk/ 100

As cream density is largely influenced by fat content and the use of an in-line densitometer can provide a constant monitor on fat content. For standardization, a constant density is required, and in this regard the unsophisticated principle of maintaining a constant volume at a constant weight can be used. A U –shaped tube of liquid is counterbalanced by weights, and any movement of the tube through losing balance can provide a signal to a controller to restore the balance. A diagram of such an instrument is given fig 7.2 and 7.3. It also contains density measurement as a means of direct standardization.

The densitometer is sensitive to vibration, and air incorporation in the cream will greatly affect its accuracy. Other mass flow meters that relate density and total flow of liquid are available. Less sophisticated systems that incorporate a Milkotest are available and are less sensitive to environmental changes, although not as accurate as a densitometer. There have been very significant advances in control systems, which incorporate microprocessors to receive signals from measuring devices and transmit them to operate controls. Figure 7.3 illustrates a typical arrangement for producing standardized cream. Figure 7.3 shows an extended system for producing standardized milk and cream. The major perturbations for such systems are changes of milk feed silos, which may change milk composition as well as feed pressure to the separator, and de-sludgings of the separator. The efficiency of a system depends very much on the rapidity of response to such changes and the re-establishment of the required conditions through design and tuning of the feedback control loops.


Fig. 7.2 System for automatic production of standardised cream
(ALFAST, courtesy of Alfa-Level)


Fig. 7.3 System for automatic production of standardised cream and milk
(ALFAST, courtesy of Alfa-Level)

Last modified: Saturday, 3 November 2012, 6:33 AM