Does Protein Powder Expire? Shelf Life and Big Bag Value
What a best before date on protein powder means, how long whey lasts, the signs it has gone off, and how long a 5kg bag takes to finish, with bag-size prices per 25g of protein checked in September 2026.
Protein powder does not go off in the way milk does. It carries a best before date, not a use by date, and the UK government says food past its best before is usually safe to eat but may not be as good. A 2016 study in the Journal of Dairy Science found whey protein lasted at least 18 months at room temperature, so shelf life is rarely the reason to avoid a big bag. Price is. On bulk.com, Bulk Pure Whey Protein was cheapest per gram in the 4kg bag at 97p per 25g of protein, while the 5kg bag cost £1.06, checked 17 September 2026. Buy the 4kg bag and keep it sealed and dry.
Protein powder is a dry food with very little water in it, so it keeps far longer than most things in your kitchen. This guide covers what the date on the bag means, how long whey lasts, how to spot a bag that has gone off, and the question that matters more for your wallet: whether a 5kg bag is really cheaper per gram. Prices come from bulk.com, checked on 17 September 2026, and the nutrition figures from Bulk's own label.
Keep protein powder sealed and dry, and check the best before date printed on the bag or tub.
How long does protein powder last unopened?
Whey protein lasts at least 18 months at normal room temperature. That figure comes from a 2016 study in the Journal of Dairy Science by Tunick and colleagues at the US Department of Agriculture, which stored whey protein concentrate for up to 18 months. At lower temperatures it still passed after 18 months. At 35°C, which is hotter than a UK kitchen, its shelf life fell to 9 months. For your own bag, the best before date printed on it is the figure to go by.
How long does protein powder last once opened?
Opening the bag does not start a separate clock. The best before date still applies, as long as you reseal the bag after each scoop and keep it somewhere cool and dry. Bulk's storage advice for Bulk Pure Whey Protein is exactly that: store it in a cool dry place and reseal the pouch after use. Keep it out of the fridge, where condensation can get in, and away from the kettle or a sunny windowsill. Use a dry scoop every time, because a wet scoop is the quickest way to put moisture into the bag.
Is it safe to use protein powder after the best before date?
Usually, yes. Protein powder carries a best before date, not a use by date. GOV.UK guidance says that after the best before date, food is usually safe to eat but may not be of the same quality. Use by dates are the ones about safety, and protein powder does not carry one. What you lose over time is quality: the 2016 study found the amino acid lysine fell in stored whey, so a very old bag may deliver slightly less usable protein than the label says, and it may taste flat.
How can you tell if protein powder has gone off?
Check it with your eyes and nose before you scoop. Throw it away if you see any of these:
A sour, rancid or cardboard smell. Fresh powder smells of its flavouring and not much else.
A colour change. Whey that is breaking down turns yellow, which the 2016 study recorded as storage went on.
Hard lumps that will not break up. Clumping means moisture has got in.
Any mould or damp patches. Bin the whole bag, not just the top layer.
A few soft lumps that crumble between your fingers are normal and are not a sign it has gone off.
Does whey go off faster than vegan or casein protein?
We have not found published storage studies comparing them, so the honest answer is to go by each bag's best before date. The 2016 study covered whey protein concentrate only. Whey, casein, clear whey and plant proteins are all sold as dry powders, and the same things spoil all of them: moisture, heat and air. If you want to compare the types on price rather than shelf life, our guides to whey isolate vs concentrate, casein and clear whey cover each one.
How long does it take to finish a 5kg bag?
About five months at one scoop a day. A 5kg bag of Bulk Pure Whey Protein holds 156 scoops at the 32g serving on its label, and each scoop gives 23g of protein. At two scoops a day the same bag lasts about 11 weeks. Either way you finish it long before the 18 months that whey kept in the 2016 study.
Bag size
32g scoops
One scoop a day
Two scoops a day
1kg
31
About 1 month
About 2 weeks
2kg
62
About 2 months
About 1 month
4kg
125
About 4 months
About 9 weeks
5kg
156
About 5 months
About 11 weeks
Based on the 32g serving on the Bulk Pure Whey Protein label. Other brands use scoops of 25g to 35g, so check yours. For how many scoops you actually need, see how much protein powder per day.
Is a 5kg bag cheaper per gram than a 1kg bag?
Yes, but the biggest bag is not always the cheapest one. On bulk.com, Bulk Pure Whey Protein cost £1.16 per 25g of protein in the 1kg bag and £1.06 in the 5kg bag, checked 17 September 2026. The 4kg bag was cheaper than both at 97p per 25g of protein, which makes it 17% cheaper per gram than the 1kg bag. The 5kg bag cost 9% more per gram than the 4kg bag.
Bulk Pure Whey Protein
Price
Cost per 25g of protein
1kg
£32.99
£1.16
2kg
£56.99
£1.00
4kg
£109.99
97p
5kg
£149.99
£1.06
One-off prices on bulk.com with no code applied, checked 17 September 2026. Every flavour was the same price in each size. Bulk Pure Whey Protein contains 71g of protein per 100g. Bulk runs frequent sales, which change these prices, so check the live figure before you buy.
Myprotein shows the same pattern on a smaller scale. Myprotein Impact Whey cost £32.99 for 1kg and £148.99 for 5kg on myprotein.com, checked 19 and 22 September 2026, so the 5kg bag was 10% cheaper per gram of powder. The 5kg bag was only on sale in Chocolate Mint when we checked, so the saving comes with less choice of flavour. Our Myprotein Impact Whey vs Bulk Pure Whey review compares the two powders head to head.
If the lowest price per gram matters more than protein per scoop, a cheaper whey in a mid-sized bag can beat both. Bulk Basic Whey Protein cost 61p per 25g of protein in the 2.5kg bag at £35.99 on bulk.com, checked 22 September 2026. It contains 59g of protein per 100g, less than Bulk Pure Whey Protein, so each scoop gives you less protein, but a 2.5kg bag is easier to finish.
Usually, yes, if it has been kept sealed and dry. Protein powder carries a best before date, and GOV.UK guidance says food past its best before date is usually safe to eat but may not be of the same quality. Do not use it if it smells sour or rancid, has changed colour, or has hard lumps that will not break up, which are signs moisture has got in.
How long does whey protein last once it's opened?
There is no separate opened date on most bags: the best before date still applies as long as you reseal the bag and keep it cool and dry. Bulk's own storage advice for Bulk Pure Whey Protein is to store it in a cool dry place and reseal the pouch after use. Damp and heat are what shorten its life.
Does protein powder go bad if it gets damp?
Damp is the main thing that spoils it. Powder that takes on moisture clumps, cakes and can grow mould, and at that point it should be thrown away. In a 2016 Journal of Dairy Science study, whey stored in hot, humid conditions took on water, caked and turned yellow, and samples kept at 35°C were pulled from the study by 12 months.
Is it worth buying 5kg of protein?
Only if the price per gram is lower, which is not always the case. Bulk Pure Whey Protein cost £1.06 per 25g of protein in the 5kg bag on bulk.com, against 97p in the 4kg bag, checked 17 September 2026. At one 32g scoop a day, a 5kg bag lasts about five months, well inside the 18 months or more that whey kept in a 2016 storage study.
Does clear whey go off quicker than normal whey?
We have not found storage data for clear whey specifically. It is a dry whey isolate powder, so the same rules apply: follow the best before date, keep it sealed and dry, and bin it if it smells off or clumps. Our clear whey guide compares the brands on price.
