the shelf life is an important factor to health. bacteria are everywhere, and foods left unused too long will often get large amounts of bacterial living in them. it may be dangerous to eat them and lead to food poisoning. the shelf life itself cannot always be trusted to tell how safe it is to eat a certain item of food. for example, pasteurized milk can remain fresh for five days after its sell-by date if it is refrigerated properly. in contrast, if milk already has harmful bacteria, the use-by dates do not matter.