Final answer:
Milk is packaged in cardboard cartons primarily to protect vitamin D and other light-sensitive nutrients from degradation. The cartons also reduce heat exposure that can accelerate spoilage, and provide a barrier against freezer burn during storage.
Step-by-step explanation:
The reason that milk comes in cardboard cartons is to protect certain nutrients from being destroyed by exposure to light. One such nutrient is vitamin D, which is often added to milk. Vitamin D is sensitive to light and can degrade when exposed to sunlight or fluorescent lights. By storing milk in opaque cardboard cartons, exposure to light is minimized, helping to preserve the vitamin D content in the milk. These cartons also provide a barrier to heat, which can accelerate spoilage, as an increase in temperature can increase the rate of chemical reactions, leading to quicker spoilage. This is the same reason why refrigeration is important for preserving milk's freshness and nutritional quality.
Additionally, the packaging helps maintain the quality of the milk by preventing freezer burn when milk is stored in a freezer, provided it is wrapped tightly. The cardboard carton provides insulation and limits the exposure of the milk to air, which can cause freezer burn. This is why packaging plays a crucial role not only in protecting nutrients but also in extending the shelf life of perishable items like milk.