Shelf Life Issues, and Temperature in Particular
Nothing lasts forever – least of all, alas, ourselves. But, if we can ignore that most relevant of all issues (at least for now), let’s instead look at how long any food item will last, and how can we give it maximum longevity.
In a very simplified form, the life of any food item varies depending on a number of different factors. Some types of food are more sensitive to some storage issues, others are less sensitive, and of course, some types of food start off with only short shelf lives and little chance of extending them, whereas other food items are inherently long-lived and can be extended considerably further.
It seems that the current ‘state of the art’ for extended shelf life products is expressed in the commonly found freeze-dried foods that offer the promise of a 25 year shelf life. Is this realistic? Ask us in twenty-something years!
Not to get ahead of the article, but even a tin of freeze-dried product with a 25 year shelf life claim can have a longer or shorter life depending on a number of facts as to how it is stored. Of course, the tin of product has been designed to control many of those factors just by nature of it being a sealed tin, but others still apply.
So let’s consider the major factors as they relate to any and all food products and the shelf life you can expect from them :
- Bio-activity (ie fresh fruit or meat going ‘rotten’, seed germinating, etc)
- Water/moisture – (Both humidity and liquid water can trigger or accelerate bio-activity)
- Oxygen (and sometimes other gases too)
- Light (although some things may be activated also by the lack of light)
- External contamination/pests/etc
- Temperature (usually the cooler the better)
The one reliable constant factor that can be said in all cases is that the warmer the temperature an item is stored at, the more rapidly it will spoil, whereas the cooler the temperature, the longer it will last.
Having said that, there are practical limits to how cold you should chill items down to. Sometimes it is not a good thing to freeze items – the freezing of the moisture inside the item may break up the item’s cells, making it go mushy when unfrozen. But in most cases, keeping an item down around 40° or thereabouts will appreciably extend the life of the item in question.
How to Tell if Food is Still Good or Bad?
The words ‘good’ and ‘bad’ need quote marks around them in the heading, because they are subjective measures rather than absolute measures. And they can have several different meanings.
One measure of bad food is when an item has become unhealthy to eat – when it has acquired a significant level of harmful bacteria or toxins such as to make a person eating the food item unwell. Even this is not an absolute measure, because some people have stronger stomachs than others.
Another measure is when a food item no longer looks attractive or smells appealing. These of course are subjective measures too, and it is fair to say that what looks unappealing to a person in middle class suburban comfort today might look extremely appealing to a starving person post TEOTWAWKI tomorrow.
Our sense of ‘bad’ food is a fairly reliable predictor of safe/unsafe food. If food looks and smells bad, and if you proceed to taste it and it tastes bad too, it probably is bad.
Yet another measure is when the nutritional value of the food item has reduced down to something close to zero. This is impossible to detect by looking/smelling/tasting; but you’ll find out empirically. If after eating a food item for several days, your weight goes down and health deteriorates, probably the food has lost most of its nutritional value.
MREs as an Interesting Example of Time vs Temperature
Just exactly how much extra storage life can you expect by keeping your stores cooler than you otherwise would? There’s no exact answer to that question, but a couple of elaborate tests of MREs give us some clues.
Testing of the MRE formulation in the 1980s (which included free-dried food items) by the Army’s Natick laboratory were conducted, using a panel of ‘average’ people, and having them do subjective taste testing, with the results then averaged to try and create some consistent measures.
These taste tests showed that the shelf life of MREs ranged from as short as one month if the MREs were stored at 120° to as long as 130 months if the MREs were stored at 60°.
The report said that longer shelf life would be possible at temperatures below 60°, but the test did not have the time to study this. It also said that food which was rejected by the panelists as unappealing was still nutritious and healthy, even though it no longer looked, smelled or tasted appealing.
Here’s a table showing the results.
(from previous row)
(from previous row)
The key learning point here is not so much the increase in shelf life between 120° and 110°, but more relevantly, the increase in shelf life between 70° (the temperature are houses are at, most of the time) and 60°.
The implication of this is that we can increase the shelf life of many products that we store by a sizeable amount – 30%, which in many cases will mean another year or more of shelf life – by doing nothing more complicated than keeping them in the coolest part of the house. That closet in your basement, rather than upstairs in the sunny pantry.
After the reformulation of MREs, a new set of tests was run. The results aren’t directly mappable to the other set of results, and show the new MREs have much shorter shelf lives. Here’s the best guess we can make on the new results to show them in similar format to the old ones
(from previous row)
(from previous row)
These results are interesting because they add another data point – the additional extension in shelf life at 50° rather than at 60°.
The earlier set of results showed a 30% shelf life extension by going from 70° down to 60°; this newer set shows a lower 20% extension, with a further 25% extension if dropping the temperature by another 10°. While not quite as drastic as the earlier set of results, 20% is still better than nothing, and probably for no more effort than moving your stores from one part of your house to another, and in the possible event that this cool area averages closer to 50° than 60°, you’re getting as much as 45% extra life compared to leaving them in the warmer part of your house.
Here’s a good page with some interesting pictures of applesauce and cheese spread (click them to see more) that has been stored at different temperatures and times.
Why the Variation in Shelf Life Extensions?
You’ll see there is no constant variation in shelf life extensions per ten degrees of temperature change – either in terms of months or percentages. This is slightly puzzling because most chemical reactions speed up at a constant rate related to temperature increases, and so we would normally expect to see steady percentage changes.
There are two factors at issue here for why some ten degree steps have larger or smaller impacts on shelf life than others.
The first is sampling and testing errors. There is no scientific exact way of rating food as good or bad based on appearance and taste, so the personal preferences of the samplers will add an element of randomness to the numbers. We’d suggest all numbers be viewed as plus or minus 10% of reality, which enables us to say, for example in the second set of results ‘10% plus a 10% sampling error is within the same zone as 25% less a 10% sampling error’.
The second is that different processes are triggered at different temperature points. Some processes might be dormant at all temperatures below a certain number, but ‘wake up’ and start impacting on shelf life above a certain point.
What Does the Shelf Life Statement on a Food Item Mean?
So there you are, looking at your can of baked beans, and studying the ‘Best By’ date printed on it. How was that determined, and what happens the day after this date?
The USDA has a page that explains some of the distinctions between ‘Use By’, ‘Sell By’, and ‘Best By’ dates. Some of what they say is vague and meaningless, and the key take-away point is that these dates for non-perishable food items (ie not raw meat, fruit, etc) are not very exact or scientific.
We suspect – but absolutely don’t know for sure – that some food manufacturers find themselves steering a compromise path between setting dates that are too ridiculously short (which might discourage people from buying their products) and dates which are extended well into the future (which would reduce the amount of food people either junk or eat in a rush due to it being about to expire, and which also could create liability if the food is not well stored).
The significance of the date you see is not actually the date itself, but how far into the future it is. If we say that, to be on the safe side, manufacturers assume a storage temperature of 80°, and you actually store the item at 50°, then this could be enough to give you an extra 65% of storage life. So if the date shows you have two years remaining when you buy the item, maybe that means you really have, in your cooler store, an equivalent of just over three years.
Plus also remember that these dates are when the food first starts to become less than prime/perfect. There’s an unknown amount of extra time into the future before it becomes appreciably less than appealing.
Pure Seed and Grain Storage – More Temperature Dependent
Here is an excellent page of information about storing grains and seeds, including rice and wheat. It cites the USDA in claiming that a 10.1°F change in temperature will halve or double the shelf life of the product being stored (depending on if the temperature goes up or down).
The two main points from this page are to keep items cool and oxygen free, and that whole wheat keeps a lot longer than flour, and white rice much longer than brown – you probably already knew these things, and now you’ll understand why, too.
These rates of change are much greater than observed with the MREs. We guess this may be because the MREs have been treated to give them some longevity so the usual ‘laws’ of biological activity and their dependence on temperature have been modified.
Special Case : Medicines
Very good news here. The expiry dates on medicines are ridiculously conservative. With the exception of insulin, liquid suspension antibiotics and nitroglycerin, most medications can be considered to remain active and potent for ten years beyond their expiration dates.
Again, the same as other items, the cooler you keep them, the longer they’ll last.
Useful Tip : Check Shelf Life in the Store Before Buying
You probably know, when buying milk or other products with a short shelf life, it pays to check the expiry date before selecting the product in the store. Sometimes you’ll note a carton several back from the one in front might have a week or more of extra shelf life – ie, the milk cartons in front offer one week, and the ones behind them offer two weeks. If you’re not sure how long the carton of milk will last you, it for sure makes sense to take one from the back of the line.
Surprisingly, this is true of other items too. We were in a local supermarket today and observed cans of tomatoes on sale. Tomatoes – an acidic product – typically have a shorter shelf life. Some of the cans were showing an expiry of March 2013 (ie 10 months from now) but some of them were showing an expiry of March 2014 (ie 22 months from now – more than twice as long).
And if we were to store them in a cool place, getting perhaps a 50% extension in shelf life as a result, that would get us an extra 18 months of storage time on the longer dated product.
This was an astonishing difference in shelf life statements. So if you’re buying even canned goods that you won’t be immediately opening and eating, be sure to check the shelf life statements on the cans, and choose your cans accordingly.
Shelf life statements on food items you purchase are not exact magical dates whereby the food is perfect up until midnight on that date, and then useless/dangerous from that point forwards. If anything, these dates represent the shortest amount of life you can expect from food items, not the longest.
Shelf life of all items is massively influenced by temperature. A change of 10° may as much as double (or halve) the shelf life of food items; there will be less impact on pre-processed items, more impact on unprocessed items.
Always keep everything as cool as possible, but most products should be kept slightly above freezing point.