Why does sour cream have a ‘use by’ date? Can it go off?

Yummy! via FlickrUnwittingly pouring gone off milk over the breakfast cereal is one of the worst ways to start a day. Milk that has gone bad tastes sour and smells revolting. There is no salvaging it, and there is no salvaging the corn flakes (please don’t try washing them, it doesn’t work – I tried).

Saying that soured cream has a use-by date, therefore sounds ridiculous – how can something that has already ‘gone off’ possibly ‘go off’ any more? Well it can, because there is a big difference between a milk product ‘souring’ and a milk product spoiling.

Sour cream, like crème fraiche, is a fermented milk product made in a similar way to yoghurt. It is made by mixing cream with certain types of bacteria, called ‘lactic bacteria’. These bacteria feed off the sugars in the cream (lactose) producing lactic acid as a biproduct. It is the acid (lactic acid) that gives sour cream its tangy taste and changes the structure of the proteins in the cream (casein), causing it to thicken.

Spoiling is different to souring and happens when a variety of bacteria (and possibly molds) grow in the milk – breaking down the fats, protein and sugars in the milk, changing its taste completely and producing some very unpalatable biproducts. And even though your shop-bought sour cream has been pasteurised to eliminate harmful bacteria, it is not completely sterile – very low levels of bacteria will be in there. After about two weeks from milking, these bacteria will have multiplied enough to spoil the sour cream. So ignore the use-by date at your peril.

The best way to have a dairy product that won’t go off easily is to buy it UHT processed – that is, heated to a very high temperature making it sterile of essentially all bacteria. The downside, as you probably know, is that UHT milk products tend to taste a bit naff.

That said, even UHT milk on your cereal is better than crying over soured milk.

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Image source: James LeVeque, Flickr

Article by Stuart Farrimond

June 30, 2014

Doctor Stu is editor of Guru Magazine. He originally trained as a medical doctor before deciding to branch out into lecturing, writing, editing and science communication. He drinks far too much coffee, eats lots of ice cream and has a bizarre love of keeping fit.
You can check out Doctor Stu’s blog at realdoctorstu.com or his poncy personal website stuartfarrimond.com. Here's his .

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