Bottles changing shape when stored in a refrigerator for a long time is typically due to a natural process known as thermal expansion and contraction. This phenomenon occurs because of the temperature difference between the contents of the bottle and the surrounding environment.
When you place a bottle with liquid inside the refrigerator, the liquid and the bottle itself are initially at room temperature. As the refrigerator cools the air inside, it also cools the contents of the bottle. During this cooling process, the liquid inside the bottle contracts and takes up less space.
Now, the bottle itself is made of a material (usually glass or plastic) that also contracts with the drop in temperature. The contraction of both the liquid and the bottle can lead to a reduction in pressure inside the bottle. This reduced pressure can cause the bottle's walls to be pulled inward slightly, changing its shape.
The degree of shape change can vary depending on factors such as the material and thickness of the bottle, the type of liquid inside, and the temperature difference between the inside of the fridge and the initial room temperature.
When you take the bottle out of the refrigerator and allow it to return to room temperature, the liquid and the bottle expand again. This expansion can cause the bottle to return to its original shape. However, repeated cycles of expansion and contraction over an extended period may lead to a gradual change in the bottle's shape over time.
It's worth noting that not all bottles may change shape noticeably, and the effect can be more pronounced in certain materials or designs. Also, if a bottle is hermetically sealed (completely airtight), the pressure changes might be more noticeable.
In summary, bottles change shape in the refrigerator due to the thermal expansion and contraction of both the liquid inside and the bottle material itself as they adjust to temperature changes. This is a natural and normal process, and it should not cause any significant issues with the bottle's functionality or safety.