an insider’s guide to use by dates

When I finished university and started my first job as a food scientist, I was fascinated by the process of determining the shelf life of products.

There were all manner of fancy simulated tests, but as I quickly realised, the only real way to know how a product is going to change over time is to keep a few packets and taste them as time progresses. Simple but time consuming – especially if your product is expected to last for a year or more.

what’s the difference between best before and use-by?

Best before tends to be used on products that have a longer shelf life like cereal or chocolate biscuits. The intention is to give you a guide but in most cases, if the product hasn’t been exposed to extreme heat and is unopened, it will still be edible. There are subtle changes such as loss in flavour that occur over time, so it may not taste as good as a packet fresh off the factory floor, but not enough difference to warrant wasting good food.

Use-by, is usually associated with shorter shelf life, more high risk products like milk and eggs. In this case I tend to obey the use-by date. Although I have been known to risk milk in my tea after it has expired and sometimes it’s fine (say if the carton has only just been opened) and of course then there are the times where it’s in date but makes a curdled mess of your tea (usually because someone has left the milk out of the fridge!). Which reminds me – trust your instincts. Just because something is in date doesn’t mean it’s 100% safe – if it smells funny, ditch it, regardless of the date.

how do best before dates get chosen?

I’m not familiar with the process for shorter life products like meat and milk. But I assume there is extensive testing both of changes to flavour, appearance and microbial populations to determine the use-by time period.

For things with longer shelf lives, like cereal and chocolate biscuits, I’ve been actively involved. Basically a whole heap of the product is placed in room temperature (and sometimes other temperatures as well) storage. Samples are taken on a regular basis and evaluated for appearance, texture and flavour until the desired shelf life time has passed, or the product becomes unacceptable for sale. The time period is then chosen based on how the product has performed.

Generally there is a safety margin built in of a few months. So if the product started to show signs of flavour loss or other undesirable attributes at say, 8 months, then the shelf life would be set at say, 6 months.

when should you throw food out?

Trust your instincts.

In most cases food will start to show visible signs of spoilage before any terrible disease causing bacteria get going. So if there is mould or fungi, or if it smells funny it’s probably a good idea to chuck it.

But if it seems fine, it probably is.

If you’re feeding people who may have sensitive immune systems like pregnant ladies, children or anyone who’s been ill, it’s best to play it safe.

how do I maximise the shelf life of my food?

I could write a whole book about this. But there are 2 key things to think about for maximising the shelf life of your ingredients: temperature and exposure to air.

As a rule of thumb, the cooler things are, the longer they last. So it’s a good idea to make sure your fridge and freezer are working properly. And if possible, have your dry food stored in the coolest part of the kitchen.

Of course not all ingredients are best refrigerated, especially some veg. For more on this see the tips on storing your fresh ingredients

Exposure to air plays a few different roles.

In dry goods like cereal, once you open the packet and air gets in the cereal is likely to pickup moisture from the air.

Oxidation is another result of air exposure, just think of the difference between a whole apple and one that’s been cut in half.

For moister products like yoghurt or sauces, exposure to the air means exposure to bacteria and fungi which can lead to spoilage of your food. This is why an unopened jar of tomato paste can sit on the shelf for years but only lasts a few weeks (if that) in the fridge once you’ve opened it.

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