What factors affect the shelf life of foodstuffs?

If we buy a bag of wheat for example, a shelf life of 1-2 years is usually indicated. However, if we don’t just simply put the bag of wheat into some random corner but rather observe some basic factors, we can for instance in the case of durum wheat also achieve a shelf life of 10 or 20 years, even reports of wheat which has been stored for 30 years and is still edible can be found.

Which factors determine the shelf live of wheat or other types of corn, of legumes such as beans, of rice or also of durum wheat pasta?

First, we should ask ourselves what exactly makes food spoil, makes it inedible or unappetising.

Either we are simply dealing with pests which completely interpret our food supply as the land of plenty or it is micro-organisms such as special mould cultures and bacteria which make food inedible or poisonous for us, if nothing else by means of their decomposition products. In addition to the aerobic, which means the oxygen breathing micro-organisms, there are a handful of anaerobic, e.g. CO2 breathing micro-organisms which however in our case, do not play a particularly important role.

These tiny animals and micro-organisms need – as we humans do – a special environment in order to live. So, we preserve our food by not “inviting” them in the first place and by depriving them of their basic survival needs from the outset – in the case of insects, especially the oxygen which they breath; in the case of aerobic micro-organisms also the oxygen as well as the specially required warm, moist environment and in the case of some anaerobic micro-organisms also the carbon dioxide.

The creeping oxidation of food in the presence of oxygen already leads to alterations in taste. So here, the removal of O2 also helps.

The solution is obvious:

as dry as possible,

at the lowest possible temperatures,

in an oxygen- (and if applicable CO2-) free environment,

protected against sunlight

and in a resistant air- and watertight container!

Indeed, I can balance out the factors among one another however; no factor can be truly regarded independent of the others. Can I only ensure a degree of moisture amounting to 12% for wheat instead of 10% but in exchange store at a constant temperature of 10°C instead of 20°C, this way the lower temperature will be able to somewhat compensate for the higher degree of moisture. On the other hand, if I store in an oxygen-rich environment or if my wheat has a degree of moisture amounting to 14%, this in each case significantly limits the shelf life.

It is not surprising that the stated ideal storage conditions correspond to those which ensure the longest possible germination capacity of seed even though the duration of the germination capacity reaches limitations considerably faster than the basic edibility.

Let’s take a closer look at the individual factors.

The degree of moisture

The degree of moisture of staple food such as wheat or beans has an enormous effect on their shelf life. Respective guideline values for the ratio degree of moisture: shelf life are mainly based on research results from James F. Harrington (1916-2002), the so-called "Harrington rules of thumb".

According to one of these rules the shelf life (in cases of consistent germination capacity) of seed is cut in half with each increase of the degree of moisture by 1 percent (Harrington, 1972). This rule is regarded as applicable for a degree of moisture from 5% to 13% and can of course be applied the other way around; if you reduce the degree of moisture by 1%, the shelf life is then doubled. As of a 13% degree of moisture, mould cultures and the increased self-heating due to respiration provide for a disproportionately high decline of the germination capacity.

For us, it is not primarily about the germination capacity but rather about the edibility. Nevertheless, we can draw on these values as a basis. The wheat which is to be stored should not have more than 13% moisture (in Germany however, wheat with a degree of moisture of under 14% is still regarded as "suitable to store" but this value applies to the storage in special silos and not to long-term storage). Apparently, at an 11-12% degree of moisture you can already achieve quite a long shelf life over many years taking all other factors into account, even more if storage takes place at low temperatures (maximum 12°C). 10% or less are ideal for long-term storage, even for the duration of 10-30 years.

Measuring moisture
The degree of moisture of wheat can be conducted with the help of moisture meters. These are available in specialist shops and on the Internet from various retailers and cost between EUR 50 and several hundred EUR. The more expensive, the more precise the measuring results. A moisture meter which I purchased for EUR 50 has e.g. an accuracy of +/- 0.6% for values under 14% and measures the degree of moisture of wheat, rye, maize and rice in a measuring range from 8%-20%.

Reducing the degree of moisture
A 25kg bag of organic wheat which I purchased had a degree of moisture amounting to approx. 12-13% shortly after the purchase; after I had opened and stored it for several days in a heated, very dry room, the degree of moisture decreased to 11.5%, after approx. 2-3 weeks under 10%. To speed up the dehydrating process you can, if necessary, also spread the wheat out on the floor.

Very reliable, even if it is somewhat time-consuming, is moisture reduction by means of dehydrating agents. They absorb the moisture in their environment. If e.g. given into an airtight pail together with corn, you dehumidify the corn. For this however, a relatively large quantity is required. Example: We want to reduce the contents of moisture of 20 kg of corn with a degree of moisture of 15% to 10%. When we multiply the weight (20kg) with the degree of moisture (15%) we get the total proportion of water: 20kg x 0.15 = 3kg of water (= 3 litres of water). From that, we now want to dehumidify one third (from 15% to 10%), that is one litre of water. Very good, dry dehydrating agents can draw up to 40% of their dead weight in water, so you require at least 2.5 kg of the dehydrating agent (in Europe the calculation is in fact done in dehydration units (German = TME). 2.5 kg normally comply with approx. 80 TME) in order to extract one litre of water. The dehydrating agent (e.g. silica gel) within a dustproof bag would be placed into an airtight container and after approximately 1 to 2 weeks the used up dehydrating agent would be removed and the moisture reduced corn (after a check measurement) would be sealed and stored taking the other factors into account.

By the way, many dehydrating agents are reusable by heating them in the oven, letting them dry and then they are reactivated.

In the case of very large quantities of corn it is recommended to entrust an agricultural provider with the dehydration process. They have very large drying installations at their disposal especially for this purpose.

The temperature

As a basic principle the following applies: the lower the storage temperature the longer the shelf life. According to another Harrington rule of thumb, the durability of seed is doubled at temperature reductions of respectively 5.6°C in the temperature range of 0°C to 50°C. This rule requires a consistent degree of moisture and is only a general rule of thumb. If the degree of moisture amounts to less than 14%, then of course the durability does not suffer from the minus degrees, as frost damage cannot result.

If for example durum wheat is stored at approx. 20°C (i.e. room temperature) with a degree of moisture amounting to approx. 10% under the elimination of oxygen and taking the other factors into account, then a shelf life of 8-12 years can surely be assumed. If the storage temperature is reduced from 20°C to a consistent 10°C, this will extend the shelf life by many years.

The following summary can be found on many American information sites for crisis provision. Admittedly, I cannot cite any evidence for these values and cannot specifically assume any liability for this information however, in my opinion it procures a rough idea of the role temperature plays for the maximisation of shelf life. As we can see, the actual shelf life is of course already subject to other factors and according to that can be longer but also significantly shorter. The chart values are based on the assumption that corn under the elimination of oxygen, with a degree of moisture under 10% is stored in an air- and watertight container in a dark environment. Experience has apparently shown that a shelf life of at least 10 years could be achieved at a consistent storage temperature of approx. 21°C. Now, if the Harrington rule of thumb is applied which originally does not refer to the general shelf life of seed but rather to its durability at maintained germination capacity, the following values are obtained:

TemperatureYears

4,2°C

40

9,8°C

30

15,4°C

20

21°C

10

26,6°C

5

32,2°C

2,5

37,8°C

1,25

These values cannot be so absurd. After all, there are numerous reports on corn which has been stored for way over 10 years which was classified by means of laboratory analysis as “absolutely edible”.

If we, for the moment, disregard artificial cooling by means of refrigerators and freezers, we normally achieve the lowest temperatures when storing in the earth, so for example in the basement. In buildings, the coolest area is the north side.

Incidentally, it must be observed that not only the temperature itself but also the extent and the frequency of variations in temperature influence the shelf life. Storage at consistent low temperatures maximises the shelf life.

The oxygen-free environment

A couple of years ago, a rather time-consuming method was used in crisis provision in order to extract oxygen from containers filled with staple food: The oxygen was simply replaced with nitrogen by conducting nitrogen into the bottom of the container. In this way, the oxygen was pressed out of the container bit by bit due to its heavier molecular weight. Another method was the application of dry ice (firm carbon dioxide) which – if placed under the corn in the container- sublimated to carbon dioxide and therefore pressed the oxygen upwards out of the container bit by bit.

Finally, the oxygen absorbers paved their way into the food supply or rather preservation of food. These packages which are food safe on the outside are mainly filled with iron powder which oxidises in the presence of oxygen, so it binds it by reacting with it. So oxygen absorbers do not replace the oxygen in the air but rather bind it. In a closed container this creates a partial vacuum contrary to the above mentioned filling with nitrogen. The percent oxygen in air of approx. 21% is – provided that sufficient absorbers are available- absolutely withdrawn.

Indeed the successful application of oxygen absorbers is also subject to several important factors (for further information please see How to use oxygen absorbers) however; they offer inexperienced private citizens a truly simple, uncomplicated and effective solution. Your can find our product range of oxygen absorbers here .

Of course the use of a sturdy or also flexible container (see factor 5) must also be observed. After all, the emerging partial vacuum can pull the walls of an unstable or not completely filled pail / container inwards and therefore give in. That can, in some cases, lead to a tear in the wall of the pail or to a deformation which in return affects the air and water impermeability of the container on the lid.

Protection against sunlight

On the one hand, sunlight produces heat, so this would counteract the aspired low temperature and on the other hand, the ultraviolet light can influence the shelf life of food. A dark environment is therefore recommended for the long-term storage of corn or similar staple food.

An air- and water(steam) -tight container

If I have withdrawn moisture from the corn or rather have provided for a low degree of moisture and additionally withdrawn oxygen by means of oxygen absorbers, then these conditions are of course to be maintained in terms of long-term storage. So it is imperative that the container for storage is air- and water(steam) -tight.

Of course I can fill my corn into cans or pails or barrels. This option is widely used – also when oxygen absorbers are applied- for long-term storage of corn and other staple food. In the case of filling of cans however, a respective sealing machine is required. In the case of pails or barrels I have to ensure that I am dealing with food safe HDPE (Heavy Duty Polyethylene) containers with rubber sealing covers. Unlike in the USA, the offer of such HDPE containers with rubber seals (gasket lids) is relatively limited in Europe or rather relatively expensive (20 litre containers e.g. 20-30 Euros). In my opinion, the very high quality products of the Dutch company CURTEC (http://www.curtec.com) are best suitable, in particular the wide range of barrels (wide-necked barrels, extra wide-necked barrels, UV-resistant barrels), all food safe, water- and airtight in various sizes. Reference sources can be obtained upon enquiry.

For direct filling into cans or barrels, even HDPE (thick-walled) barrels / pails there is however still the risk of the walls giving in because of the emerging partial vacuum of the oxygen removal. This has already happened to me several times. Furthermore, this option is relatively expensive, as we have seen.


Aluminium laminated bag – the ideal container? According to this, the filling into “flexible” containers seems to be ideal, which are still air- and watertight, a type of flexible can, which can be easily sealed.
Normal plastic bags or sacks are not suitable for this because they do not have a sufficient air and steam barrier. However, I can ensure this barrier by means of an aluminium laminate in the bag. I am talking about so-called aluminium compound bags which we offer in our shop . It is a matter of plastic bags which normally have a food safe polyethylene coating on the inside, an aluminium film in the middle (as air and steam barrier) and on the outside a flexible, stable polyester coating.

The advantage of these bags is that they already offer a very high air, light and steam barrier as of a bag thickness of approximately 110 µm (micron / micrometer) and on top of that they can flexibly adapt to the partial vacuum which results from using oxygen absorbers. There is a reason, why exactly these bags are used as packaging for the long-term storage of freeze-dried meals, emergency rations or also other food. Compared to 20-30 Euros for a food safe, air- and watertight 20 litre barrel, I also only pay approximately 3 Euros for a bag with a holding capacity of more than 30 litres or approximately 4 Euros for bag with a holding capacity of more than 25kg of wheat.

Basically, the bags are very capable of bearing, but of course susceptible to pointy objects. Therefore, especially in the case of larger filling quantities, you should store the bags in respective resistant containers. The filled bags themselves are also very difficult to stack. However now, it doesn’t have to be absolutely water- and airtight pails or barrels anymore. Instead, I can use a normal pail for example (price approx. 3 Euros for a 20 litre pail incl. a lid).

As the case may be, further costs can accrue if you, instead of using an iron, select a much more convenient and safer solution for sealing the bags: professional bag sealers such as manual welding guns e.g. (price 200 Euros).

You can find more information about the application of aluminium compound bags here under "How to use Aluminium-laminated bags". Our product range of aluminium compound bags can be found here.