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
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.
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
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.
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
various parts of the product themselves can be tested – pastry, linings,
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.
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.
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.
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.
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
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.
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
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!