The term thermal processing applies to a range of heat treatments used for food processing. In general, the point of thermal processing is to kill pathogens and inactivate enzymes that cause negative changes to the food during storage. The most common type of thermal processing is the kind that happens in the kitchen at mealtime. Even the most domestically challenged among us have heated something in the microwave and have therefore "thermally processed" something.
Pasteurization constitutes one of the milder forms of thermal processing. Ultra-high temperature and sterilization methods kill all microorganisms in the food, while milder heat treatments like thermization and pasteurization only kill some of them. Why not use a higher temperature if it will kill more pathogens? The answer is that higher temperatures change the characteristics of the food. Since milk is what most people think of in relation to pasteurization, we'll use the pasteurization of milk throughout the rest of this article to show how pasteurization works.
At higher temperatures, as with UHT, several things happen to milk that make it less desirable to consumers:
- The proteins in milk are altered, changing how the milk acts when used to make other foods like cheese.
- Protective enzymes in milk are inactivated, making it more susceptible to spoilage.
- The Maillard reaction, a chemical reaction between proteins and sugars, occurs at higher heats and causes browning, discoloring the milk.
- The milk may taste "cooked."
If you look at the sidebar, you'll notice that each method of thermal processing requires a certain length of time. For example, HTST pasteurization takes 15 seconds. This minimum time requirement is based on the thermal death kinetics of the bacteria. No, that's not the name of a death metal band; it's a way to describe the conditions needed to kill bacteria. The D-value is the amount of time it takes to kill 90 percent of one type of bacteria at a particular temperature. The higher the temperature is, the lower the D-value, and vice versa [source: Lewis].
The pasteurization of milk kills off the most heat-sensitive pathogens but retains the qualities of milk that consumers expect: creamy texture, fresh flavor and milky-white color.