Dairy products like yoghurt have a notoriously short shelf life once opened, especially when purchased in large tubs.

Buying in larger quantities is often more cost-effective but comes with the pressure to use up yoghurt before it spoils, which often manifests as a watery, lumpy consistency and a sour smell.

Yoghurt products with a high protein content tend to separate less, according to Robert Roberts, head of department of food science at Pennsyyvania State University.

He told Martha Stewart that Greek yoghurt, which has been strained to remove most of the whey, has a higher protein content, which is why you'll find less liquid in a container of Greek yoghurt than regular yoghurt that hasn't been strained.

As for other yoghurts, whey residue commonly appears once you've opened the product, as it has been jostled around from the store to your home to the refrigerator and so on, said Robert.

It is easy to prevent, however, as Martha Stewart suggests. Sharing her recipe for homemade yoghurt, she said: "When stored properly in an airtight container, your homemade yoghurt should last about two weeks in the refrigerator."

Her preferred container is a sterilised glass jar with a tight-fitting lid. Those who buy yoghurt in large containers with a paper lid can transfer the product into an airtight container for longevity.

This should slow down the process that causes whey separation and watery residue to sit on top of the yoghurt.

While you cannot entirely avoid the liquid from forming, you can return the yoghurt to its original state by stirring the whey back in or pouring it off the top.

Even if you choose to remove it, the liquid has several uses. It can be added to smoothies, dressings, and soups for extra vitamins and nutrients.

As for thickening watery yoghurt, whether homemade or shop-bought, straining it can help.

Martha Stewart said: "Simply place the yoghurt in a strainer lined with a few layers of cheesecloth or paper towels and rest the strainer over a bowl. Let it sit in the refrigerator for 24 to 48 hours, then discard the liquid. You'll be left with a thicker yoghurt, more similar to Greek yoghurt."

Return the yoghurt to the fridge, avoiding the door shelving as this has the most fluctuating temperatures.

Contrary to popular belief, Martha recommends avoiding the freezer for long-term storage. She claims this is because the texture can change once defrosted.