Versaperm Vapour Permeability measurement

Why do cornflakes go soggy?
Food and packaging – Testing for the omnipresent effects of water vapour
By Chris Roberts, Director Versaperm

 

Well, the simple answer to “Why do cornflakes go soggy” is probably “Milk!” - but in truth, it’s more complex than that. Much more complex.

It is important that every food product reaches the consumer in perfect condition, but there are two separate factors that affect it. The product itself and the packaging.

Some products are moist and can not be allowed to lose moisture, some are dry and should not absorb it, sometimes, (in a pie for example), both types are present and moisture must not even be allowed to migrate, for example, from the filling to the crust. Added to this it can be difficult to maintain the quality of frozen foods, prevent freezer burn and produce packaging that actually helps preserve the excellence and characteristics of the product. Chemical reactions, biological decay and changes to the physical structure of the ingredients are all too widespread.

The single most common cause of all of this is water. But not in it’s liquid state. Water as a liquid behaves well and is widely understood, many materials are totally impervious to it and it is easy to protect products from its effects. The problem is water vapour – it passes through materials that are totally impervious liquid water, it’s omnipresent, it’s devious and it can even find the most microscopic channels on its way to spoiling our food.

One intuitive answer would be freezing – everyone knows that freezing keeps the ice solid and stable? Wrong! It is actually water vapour evaporating from frozen products that causes “Freezer Burn”, turning meat and other products brown and giving them greyish white spots and blooms. Water Vapour is far more insidious than water, it can escape from, or attack frozen products, products sealed in containers or protected by coated papers, it can even travel through the sides of a plastic container or a foil sachet. And, worst of all, the products that you intuitively “feel” will protect food best are often the wrong choice as water vapour can behave in a very counter-intuitive way.

Techniques developed for testing water vapour can be extremely useful – in fact they can even be useful if the product is not sensitive to water, as the vapour acts as a tracer and indicates the route other gases would follow. Modern equipment can measure the moisture flow via primary and secondary containers, sachets, wrappings, bags, click-fit caps, threaded closures, pouches, bottles, tubs, solid seals and all manner of other packaging. It allows the testing of the seal and the packaging under “real-world” conditions.

Similarly, various parts of the product themselves can be tested – pastry, linings, bases etc.

Traditionally, tests were based on a “gravimetric” technique, which involved measuring the change in weight of special substances as they absorb water from the sample. The biggest single drawback in this was the time it took – measurements would take days, and sometimes weeks. Today, a simple WVTR (Water Vapour Transmission) meter can be used to produce results that are accurate to around one part in a million - sometimes in as little as half an hour.

Testing

The continuous parts of packaging, such as the fabric of a bag, the film over a frozen meal or the material in a sachet, are the parts that are most vulnerable to the diffusion of water.

To test the permeability of flat materials such as cornflake lining bags, coated card or paper, films, or rigid foodstuffs (such as pastry, linings or pizza bases), specific temperature and humidity conditions are applied to one side of the material and a dry gas (normally nitrogen) is passed over the other side. The amount of water vapour that has passed through the sample can then be measured - and reliable and accurate results can be obtained simply by reading them off a LCD panel.

There are two ways to test the permeability of “closed containers” such as frozen packed meat, pre-prepared meals or products in tubs, jars or tins. In the first case you simply place the product in a special chamber. With tubs, jars, tins or other containers, the most reliable method is to fill them with water and seal normally. If possible, this should be done on production scale equipment, as preparing them in the laboratory can give different results from a production run. The container is then placed inside the chamber, through which dry gas is channelled. In this way any water vapour that passes through the container walls can be detected easily, and a reading can be obtained as soon as the diffusion rate is steady. Accuracy is typically in the parts per million range, or even, in some cases, parts per 100 million.

A different method involves placing the container in a special humidity and temperature controlled chamber and then passing the dry gas through the container itself. While this offers a measure of the water vapour entering the container, as opposed to leaving it, careful sealing is required. In most cases, the vapour permeation rates in one direction match those in the other, and the more reliable former method is used.

Any part of a package can be tested for permeability – either by using a specially manufactured jig to hold just that component in the testing environment, or by sealing off the other parts of the container using a non-permeable material.

Common materials include treated papers and polymeric films, especially laminated polymeric films. While most polymers offer very good resistance to liquid water, with the exception of a few such as EVOH, PVOH and cellulose, there is little correlation between resistance to liquid water and water vapour - so a material that is good in one case might have little effect on the other. Some of the best polymeric barriers to water vapour include PVDC (polyvinylidenechloride) and PCTFE (polychlorotrifluoroethylene). The best films are laminates that include a component of aluminium, either as a discrete layer or as a result of a metalisation process.

Many food manufacturing and packing companies are beginning to use these techniques when they design their packaging, as they provides invaluable information on the protection of the products. For instance, as is often the case in the real world, measuring the permeability of the finished packaging is far more reliable than simply calculating it from theoretical data on the materials. With increasing innovation in packaging and the need for novelty in marketing, the norms and limits of packaging materials will continue to be stretched, further enhancing the need for ad hoc testing.

So the moral of the story is, if you don’t like soggy cornflakes, you have to test! Simple “intuition” can be misleading - and having to call back products, especially after the razzmatazz of a new launch or a new exciting packaging may well be giving your competitors too much food for thought!