How does unopened milk spoil even while refrigerated?

While refrigeration can slow down the spoiling process, it cannot completely stop it. Here are a few factors that can cause unopened milk to spoil even when refrigerated:

1. Storage Temperature: The ideal temperature for refrigerating milk is between 35°F and 40°F (2°C and 4°C). Slight fluctuations in temperature within this range are acceptable, but if the milk is exposed to higher temperatures for extended periods, the growth of spoilage-causing bacteria will accelerate.

2. Light Exposure: Milk is sensitive to light, especially sunlight. Exposure to light can damage the milk's riboflavin content (vitamin B2), reducing its nutritional value and contributing to off-flavors and spoilage. This is why milk is typically packaged in opaque containers or stored in a dark area of the refrigerator.

3. Microorganisms: Even before the milk is packaged, it can potentially contain tiny amounts of bacteria that may survive the pasteurization process or enter the milk due to imperfect sealing of the package. Under refrigerated conditions, the growth of these bacteria is usually slow, but they can eventually reach detectable levels and cause spoilage.

4. Packaging Integrity: Damage to the milk's packaging, such as tears, cracks, or punctures, can compromise its seal and allow the entry of spoilage-causing microorganisms. It's important to inspect the milk container for any signs of damage before purchasing or consuming it.

5. Milk Composition: The composition of milk can also influence its spoilage rate. Factors such as fat content, pH level, and the presence of dissolved oxygen can impact the growth of bacteria. For example, skim milk (with lower fat content) tends to spoil faster than whole milk.

By controlling these factors and following proper storage and handling practices, the shelf life of unopened milk can be extended, although it won't be completely immune to spoilage indefinitely.

Remember to always check the expiration date on the milk container and keep an eye out for any signs of spoilage, such as changes in odor, color, or texture, before consuming it.