Final answer:
Milk goes bad despite pasteurization because the process reduces but does not eliminate all bacteria. Additionally, enzymes in the milk can break down proteins and fats over time. Even refrigeration only slows down, but does not stop, these spoilage processes.
Step-by-step explanation:
Why Milk Goes Bad Despite Pasteurization
Pasteurization is a process developed by Louis Pasteur where milk is heated to a certain temperature for a specified amount of time to kill pathogens that can spoil milk and cause disease. This process significantly reduces the amount of bacteria in milk but does not make it sterile. Since bacteria can still be present in the milk in small amounts or can be introduced after opening, eventually milk still goes bad. Two common methods of pasteurization are high-temperature short-time (HTST) and ultra-high-temperature (UHT) pasteurization.
When pasteurized milk is stored in the refrigerator, the cold temperature slows down molecular movement and reactions. However, over time, the energy from molecular collisions can allow bacteria to multiply or cause chemical changes, leading to milk spoilage. The bacteria can produce lactic acid, which gives the milk a sour taste and unpleasant smell. In contrast, when milk is stored at room temperature, the higher energy state of the molecules allows these reactions to happen much faster, causing the milk to spoil more quickly. UHT pasteurization allows milk to be stored for longer periods without refrigeration due to the higher temperatures used, which further reduces the amount of bacteria and enzymes that can cause spoilage.
Even with pasteurization, milk still contains some bacteria that can proliferate over time or enzymes that can cause breakdown of the milk proteins and fats. Therefore, pasteurized milk will eventually spoil due to microbial growth or enzymatic activity if not consumed within a certain period after pasteurization.