Figure 1 shows the multiple end product of milk, in the form of milk. We have whole milk, homogenized fresh milk, pasteurized milk, UHT milk and milk powder. Ever wonder why we have so many processes for just obtaining our milk? Food processing is an area where we focus on the consumers and their demands. In other words, we as food technologist exist because of you.
The demand for fresh milk are there because there are people who demand for freshness and getting optimal nutrients from the fresh milk. Some prefer whole milk, which is a direct product of the cow. Whole milk contains large fat globules that affect the palatability. Imagine drinking a chunk of fat. That is why most fresh milk is homogenized. Homogenization breaks the large fat globules into small milk particles, distributing the milk fat evenly across the milk. Well, then it really is "homogenized fresh milk" and not just fresh milk. If you are a fresh milk drinker, are you aware of the homogenization process?
A question that you might ask: Why I can leave the UHT milk or milk powder at room temperature whereas pasteurized milk needs to stay refrigerated. The answer lies with the degree of processing. Pasteurization only reduces the quantity of pathogenic bacteria and is unable to kill all microorganisms. Pathogenic bacteria typically grow and metabolize faster at room temperature, slowing down at a low refrigeration temperature. That is why we need to couple pasteurized milk with refrigeration at 4°C.
If pasteurized milk is kept at temperature above 4°C, then the souring of milk occurs. Bacteria digest lactose, a sugar found in the milk into lactic acids. Lactic acid causes the sour taste in the milk. It is an indicator of the milk are highly contaminated with pathogenic bacteria. No wonder, my mum refrained me from drinking sour milk.
Ultra-High Temperature processing, as its name indicated, uses high temperature (exceeding 135 °C) for 1-2 seconds. The use of high temperature not only capable to deactivate all microorganisms but also destroys some of the milk nutrients. That is you will see the fluid milk to turn into mild yellow. It is due to Maillard (browning) reaction that occurs during the processing.
Milk powder is a result of drying process. In this case, high temperature for few hours. How does milk powder turn yellow when fluid milk is off-white? Similarly, it is due to browning effects that occurs at high temperature. High temperature speeds the browning process and having it for a long duration of drying time make the yellow intensity stronger. Why is drying still works for milk even if it browns the milk? Drying is a process of free-water removal from the food products. The removal of free water inhibits microorganism growth, making it an excellent hurdle for milk product's safety. Thus, milk powder is highly shelf-stable and need no refrigeration.
Well, how do you as a consumer chooses which milk to get? Let me know and share some ideas.