Dairy and Food Engineering

Lesson 4. Chemical properties of milk and milk products-I

4.1 DENSITY

Density is defined as the mass of substance divided by the volume occupied. Its unit in SI is the kilogram per cubic meter (kg m^-3). At 5°C water has a density of 1.00 g/ml or 10³ kg m^-3.

The addition of solids, e.g. minerals, sugar, protein will increase the density, whereas oil and fat will decrease the density. The density of fluid is usually measured with a hydrometer. The density is temperature dependent, so temperature should always be recorded. The density of milk usually falls within the range of 1028 - 1035 kg m^-3 depending on the composition. It is generally measured with a special hydrometer known as a lactometer and the result can be used to estimate total solids.

The densities of the solid constituents are regarded as fat (930), water (1000), and milk solids-not-fat (l035 kg m^-3). Fat contents range between 1 and 10 per cent and the total solids determination is based on the following equation.

T = 0.25 ρ + 1.2.2 F + 0.72…………………………………………………... (1)

where T=total solids [w/w], ρ = 1000 (density in g/ml). F=fat percentage.

Fat is determined separately usually by the Gerber method.

Kessler (1981) presents relationship for the density of whole milk and cream over the temperature range 0 - 150°C.

Whole milk; ρ = 1033.7 - 0.2308 θ - 2.46 x 10^-3θ² ………………………….(2)

Cream (20 % fat) ; ρ = 1013.8 - 0.3179 θ - 1.95 x 10^-3θ² ……………………(3)

where θ = temperature, ̊C.

 

Density is useful for monitoring changes occurring during processing. e.g. evaporation, or for checking whether extraneous water has entered the product. When dealing with solids, it is necessary to differentiate between solid density and bulk density, particularly with particulate matter and powder. The solid or particle density is mass of solid/volume of solid, and it will take into account the presence of air within the solid. For particulate matter, it can be determined either by flotation using liquids of known specific gravity or by using a density bottle.

The density of milk varies between 1.027 and 1.035 g/cm³ as a function of the type and quantity of the dispersed particles (emulsified, colloidal or soluble). With increasing fat content, the density decreases; density increases with an increasing protein, lactose and mineral content. Density is influenced by temperature, average values for densities at 20°C and 1.013 bar (standard atmospheric pressure) in g/cm³ are:

Whole milk                        1.027-1.032                                    Cream                  1.0035-1

Whey                                1.025-1.027                                    Skimmed milk       1.033-1.036

Buttermilk (heat serum)      1.0214

 Density is technologically important when calculating the raw milk quantity, when determining adulterations, for automatic fat standardization, for the determination of water in buttermilk and when manufacturing concentrates for longlife products.

A rapid method for the determination of density in milk uses the lactodensitometer.

The solid density is important in separation processes. e.g. centrifugation of cheese fines, cyclone operation and the pneumatic or hydraulic transport of powders and particulate matter.

Bulk density is an important property, particularly for the transportation and storage of bulk particulate material e.g. fruit, grain, powders. It is the mass of material divided by the total volume occupied.

In most cases, it is important to have a high bulk density. The bulk density of milk powder is affected by processing condition. The bulk density increases as the total solids increases. The particle density increases as the total solids increase indicating that less air is incorporated into the particles at higher total solids. Injection of air or nitrogen into the product immediately before atomisation may reduce the bulk density and agglomeration achieved by a re-wetting process substantially decreases the bulk density.

The method of atomisation will also affect the bulk density. Early designs of atomiser wheel produced powder 0.45 - 0.55 g/ml, whereas later designs typified by the vaned wheel produced bulk densities of ·0.55- 0.65 g/ml; later designs have used steam to occlude air from the fluid. Jet nozzles can produce powders with bulk densities as high as 0.83 g/ml.

The porosity is the fraction of the total volume which is occupied by air between particles. It does not take into account air within the particles.

Porosity = (ρ_s – ρ_b) / ρ_s

where ρ_s = solid density and ρ_b = bulk-density

The density of whole milk powder ranges from 43 to 51 per cent.

4.2 Boiling and Freezing Point

The boiling point for milk is 100.2 ̊C, slightly higher than for pure water. The boiling point of both cow and buffalo milk ranges from 100.2 ̊C to 101̊C with an average of 100.5 ̊C. The boiling point ranges slightly with the percentage of solids present in milk.

 The freezing point of cow milk and buffalo milk ranges from – 0.535 ̊C to – 0.59 ̊C with an average of – 0.553 ̊C depending on the lactose, proteins and mineral content.

The average freezing point of raw milk is at -0.526 ̊C; of pasteurized milk in the range -0.517 to -0.521 ̊C. This is influenced (see also the boiling point) by the dissolved lactose and the similarly dissolved ions of the milk salt and a few other compounds with a relatively low molecular mass.

This relationship permits to detect adulteration with water or with additives e.g. salts, disinfectants or neutralizing agents.

The freezing point of raw milk has a lower value of -0.515 ̊C, values of < -0.5 ̊C indicate adulteration with water. Values of > -0.62 ̊C indicate adulteration with salts.

The freezing point can be modified by gassing/ degassing of milk, lactose splitting or pH modification.

The presence of dissolved substances elevates the boiling point of a solution.

Dissolved substances lower the freezing point of a solution, since milk is a solution containing salts and sugars its freezing point is lower than that of water.

The fortification of milk with milk powder or lactose lowers the freezing point.