The preparation of recombined milk can be broken down into:
- Raw material handling
- Weighing and mixing
- Filtration, deareation, homogenisation and pasteurisation
If these steps are carried out correctly and the proper raw materials have been used, then further processing in the manufacture of each product usually follows the standard method of manufacture of the product from fresh milk.
Raw material handling
Water of high bacteriological and chemical quality, without any taste or flavour, is needed since these factors will influence the final product.
Milk powders must be of high microbiological quality. Milk powders are classified according to the heat treatment of the milk.
It is important to use milk powder ofa specific grade, especially in the manufacture of recombined cheese.
Different types of milk powder are used depending on the type of product to be manufactured.
Milk powders are typically received in 25 kg bags. In smaller plants, the powder is often manually unsacked directly into the mixing system, but in the larger plants the bags are emptied automatically. The powder is transferred pneumatically into large storage silos from which it is mechanically conveyed to the mixing operations.
Fats and oils
Unsalted butter can be used in the manufacture of recombined milk products but it must be kept under refrigerated storage. The most common sources of milk fat is anhydrous milk fat (AMF) which does not require such storage. It is usually necessary to heat the containers to melt the product prior to transferring it to the mixing operation. Once melted the AMF is transferred to a holding tank.
Dry additives such as sugar, stabilizers and emulsifiers may be handled in the same way as the milk powder.
Weighing and mixing
Batch weighing and mixing of individual ingredients is still the most common system in preparing milk products. This is because it is normally necessary to have a minimum mixing time of 10-15 minutes at the optimal temperature 40-50°C, depending on original powder-quality storage time and storage temperature, to ensure hydration of the milk powder before further treatment.
The method used for mixing the ingredients, and in particular the mixing of the water, milk powder, milkfat and eventually sugar, depends very much on the solids and viscosity of the finished product-mix.
Mixing systems for low viscous products
For small batch recombining operations, a simple propellor or turbine mixer, mounted on the jacketed mixing vat is adequate. The temperature of the mix is maintained by recirculating hot water or steam in the jacket.
Venturi jet mixer with recirculation loop
Larger batch operations use a recirculating loop around the mixing tank. Powder is added into the recirculation line, which also includes a small plate heat exchanger to adjust the temperature of the mix. The powder is usually introduced into the recirculation line after the pump, directly after a specially designed venturi unit which creates a vacuum within the pipe sucking the powder into the recirculating water. Any air drawn in at the same time is returned to the mixing tank and not directly into the suction side of the pump. The incorporation of air into the mix is kept to a minimum with the assistance of a valve to open or shut the exit from the powder feed hopper. The recirculation line is terminated below the liquid level in the mixing vat so that further air incorporation is reduced. An in-line static mixer can be installed further down the line and an agitator is also installed on the main mixing tank.
The fat may be added in the mixing tank, when the mixing temperature is 40-50°C or may be dosed continuously in the skimmed milk stream by a static mixer, just before homogenisation.
Mixing systems for high viscosity products
Water is filled into the mixing tank and circulated through a plate heat exchanger where it is heated to 60°C. Milk powder and water are mixed in a turbo-mixer recirculated over a mixing tank. The turbo-mixer ensures complete dissolution of the raw materials. After recombination of the milk powder, the sucrose is added in the same way. Oil or fat is dosed into the turbo-mixer by means of a positive pump.
Filtration, homogenization and pasteurisation
After mixing and hydration, the mix is ready for further processing.
From the balance tank the product is transferred to a filter arrangement to remove any undissolved milk particles.
In some plants de-aerators may be installed before homogenisation to be sure that there is no air in the product led to the homogeniser.
All combined milk products have to be homogenised. Recombined whole milk for use in the manufacture of cheese and recombined sweetened condensed milk require relatively low pressure homogenisation. The effect of homogenisation on the viscosity of recombined sweetened condensed milk is an important factor and can be used as a processing variable to help control the viscosity of the final product. Recombined whole milk for such applications as sterilized milks and yogurts, recombined ice cream and recombined evaporated milk should preferably be subject to two-stage homogenisation.
Pasteurising processes use APV plate heat exchangers. Pasteurisation is followed by further processing in the manufacture of each product.