Evaporated Skim or Whole Milk
After the raw milk is clarified and standardized, it is given a pre-heating treatment of 93-100° C for 10 to 25 min or 115-128° C for 1 to 6 min.. There are several benefits to this treatment:
- increases the concentrated milk stability during sterilization; decreases the chance of coagulation taking place during storage
- decreases the initial microbial load
- modifies the viscosity of the final product
- milk enters the evaporator already hot
Milk is then concentrated at low temperatures by vacuum evaporation. This process is based on the physical law that the boiling point of a liquid is lowered when the liquid is exposed to a pressure below atmospheric pressure. In this case, the boiling point is lowered to approximately 40-45° C. This results in little to no cooked flavour. The milk is concentrated to 30-40% total solids.
The evaporated milk is then homogenized to improve the milkfat emulsion stability. There are other benefits particular to this type of product:
- increased white colour
- increased viscosity
- decreased coagulation ability
A second standardization is done at this time to ensure the proper salt balance is present. The ability of milk to withstand intensive heat treatment depends to a great degree on its salt balance.
The product at this point is quite perishable. The fat is easily oxidized and the microbial load, although decreased, is still a threat. The evaporated milk at this stage is often shipped by the tanker for use in other products.
In order to extend the shelf life, evaporated milk can be packaged in cans and then sterilized in an autoclave. Continuous flow sterilization followed by packaging under aseptic conditions is also done. While the sterilization process produces a light brown colouration, the product can be successfully stored for up to a year.