design process (mass and energy balance)
PROCESS OF MAKING MILK POWDERED
SEPARATION/STANDARDIZATION
The conventional process for the production of milk powders starts with taking the raw milk received at the dairy factory and pasteurizing and separating it into skim milk and cream using a centrifugal cream separator. If whole milk powder (WMP) is to be manufactured, a portion of the cream is added back to the skim milk to produce a milk with a standardized fat content
PREHEATING
The next step in the process is “preheating” during which the standardised milk is heated to temperatures between 75 and 120C and held for a specified time from a few seconds up to several minutes. Preheating causes a controlled denaturation of the whey proteins in the milk and it destroys bacteria, inactivates enzymes, generates natural antioxidants and imparts heat stability. Preheating may be either indirect (via heat exchangers), or direct (via steam injection or infusion into the product), or a mixture of the two.
EVAPORATION
In the evaporator the preheated milk is concentrated in stages or “effects” from around 9.0% total solids content for skim milk and 13% for whole milk, up to 45-52% total solids. This is achieved by boiling the milk under a vacuum at temperatures below 72C in a falling film on the inside of vertical tubes, and removing the water as vapour.
This vapour, which may be mechanically or thermally compressed, is then used to heat the milk in the next effect of the evaporator which may be operated at a lower pressure and temperature than the preceding effect.
SPRAY DRYING
Spray drying involves atomising the milk concentrate from the evaporator into fine droplets. This is done inside a large drying chamber in a flow of hot air (up to 200C) using either a spinning disk atomiser or a series of high pressure nozzles. The milk droplets are cooled by evaporation and they never reach the temperature of the air.
PACKAGING AND STORAGE
Milk powders are immensely more stable than fresh milk but protection from moisture, oxygen, light and heat is needed in order to maintain their quality and shelf life
WMPs are often packed under nitrogen gas to protect the product from oxidation and to maintain their flavour and extend their keeping quality. Packaging is chosen to provide a barrier to moisture, oxygen and light. Bags generally consist of several layers to provide strength and the necessary barrier properties.
Shipments of milk powder should never suffer prolonged exposure to direct sunshine especially in tropical countries