Barrier properties are the packaging's ability to resistance the ingress of moisture, air, light and micro-organisms etc. These properties come from the inherent properties of the individual films that are used in the constructed laminate, and the measurement of these properties gives an indication of the amount of protection that is given.
Flexible films have large variations in their barrier properties in contrast to other singular materials such as cans and glass jars. The processor does not have to specify the degree of protection required when ordering cans or bottles because they are all a complete barrier. However because of the wide variations in film barrier properties, due to differences in types of films or even differences in the thickness of the same film, it is necessary for the food processor to understand the approximate amount and type of protection required for a given product or susceptible ingredient. For example, generally nuts require a good oxygen barrier. A PVdC coated PET or PP laminated to Polyethylene may do. Generally if there is a barrier requirement but the degree of barrier is unknown the simplest choice is a laminate with aluminium foil being a full barrier.
Alternatively, if there is only a limited range of films available it is more difficult for the processor to know whether they are suitable for the intended use. In these cases the producer should ask the film supplier whether the intended use will be suitable for the available film.
The barrier properties of films and other packaging materials are described by two main factors: the Water Vapour Transmission Rate (WVTR) and the Oxygen Transmission Rate (OTR). These are a measure of how much water vapour or oxygen is able to pass through a known area of packaging material in a given time (by convention this is usually the amount passing through one square metre of material in 24 hours) at a set ambient temperature and set humidity. The units of WVTR and OTR are therefore: g or ml/m2/24 h.
The higher the value of WVTR or OTR the more permeable the material is to moisture or air (or to put it another way, the lower the value the better the barrier to moisture or air). Because the permeability of most materials varies with the temperature and humidity of the surrounding air it is usual to measure WVTR and OTR under known air conditions (e.g. 25℃ and 65% relative humidity).
It is important for a food processor to know the conditions under which the food is likely to be stored and then get data on the barrier properties of the proposed packaging, which have been measured under similar conditions (available from the packaging supplier). This will enable the processor to assess the likely shelf-life of the food under these conditions