What causes milk to decay when stayed overnight and what can prevent it?

1. Bacteria Growth: Milk naturally contains bacteria, which, when left at room temperature, rapidly multiply. These bacteria convert the lactose (milk sugar) into lactic acid, causing milk to sour and spoil.

2. Temperature: The warmer the temperature, the faster bacteria multiply. Leaving milk at room temperature for several hours or overnight significantly increases bacterial growth and spoilage.

Preventing Milk Decay:

1. Refrigeration: The most effective way to prevent milk decay is to promptly refrigerate it after opening or purchase. Keep milk at a consistent temperature below 40°F (4°C) to slow bacterial growth.

2. Air Exposure: Limiting air exposure can help reduce bacterial contamination. Always keep milk containers tightly closed to prevent air from entering and promoting bacterial growth.

3. Cleanliness: Cleanliness is crucial to prevent external bacteria from contaminating milk. Wash your hands thoroughly before handling milk containers, and make sure the refrigerator is clean to prevent cross-contamination.

4. Use-By Date: Milk typically has a "use-by" date indicated on the packaging. While this date doesn't guarantee spoilage, it provides a reference point for optimal freshness and quality.

5. Boiling Milk: Boiling milk before consumption can kill harmful bacteria and extend its shelf life. However, it may alter the taste and nutritional composition.

6. Pasteurization: Most commercially available milk undergoes pasteurization, a heat treatment that kills bacteria while preserving milk quality.

7. UHT (Ultra-High Temperature) Processing: UHT processing involves heating milk at an even higher temperature than pasteurization, making it shelf-stable at room temperature for extended periods.

By following these preventive measures, you can significantly extend the shelf life of milk and ensure its safety and quality for consumption.