MYLAR BAGS (20)
The word “Mylar” is a trademark of the DuPont corporation for a special
type of polyester film. Typically made in thin sheets, it has a high tensile
strength and is used in a wide variety of industrial settings.
In food storage, particularly for the long term, it is commonly found as
a laminate with Mylar as the top layer, a very thin aluminum foil in the
middle and one or more other types of plastic films on the bottom acting
as sealant plies. This laminate combination possesses a high resistance to
the passage of oxygen, carbon dioxide, nitrogen, other gasses, water vapor,
and light which is what makes it valuable for our purposes.
Unfortunately, it has a poor puncture resistance so must be used as an interior
liner for more puncture resistant containers rather than as a stand-alone package.
Food grade aluminized Mylar complies with US FDA requirements and is
safe to be in contact with all food types except alcoholic.
For food use, Mylar is most commonly available as pre-made bags of various sizes.
Flat sheets or rolls of the material might also be found from which bags could be
fashioned as well. When Mylar bags are used by the storage food industry
they are generally for products sealed in plastic buckets.
The reason for doing this is the High Density PolyEthylene (HDPE) from which
the pails are made is somewhat porous to gasses.
This means that small molecules, such as oxygen (O2), can slowly pass
through the plastic and come into contact with the food inside.
The problem is further compounded if oxygen absorbers are used,
as the result of their absorbing action is to lower the air pressure
inside the container unless it has first been carefully flushed with an
inert gas such as nitrogen. How fast this migration activity will occur
is a function of the specific plastic formulation, its wall thickness and the
air pressure inside the container. In order to gain the maximum possible
shelf life a second gas barrier, the Mylar bag, is used inside the pail.
Whether the use of these bags is necessary for your home packaged
storage foods depends on how oxygen sensitive the food item is and how
long you want it to stay at its best. If the container is made of a gas
impervious material such as metal or glass then a second gas barrier
inside is not needed. If it is HDPE or a plastic with similar properties
and you want to get the longest possible storage life (say 10+ yrs for grain)
then Mylar is a good idea. If you’re going to use the grain in four to five years
or less then it is not needed. Provided the oxygen has been purged from the
container in the first place, either with a proper flushing technique, or by absorption,
there will not have been sufficient O2 infiltration to seriously impact the food.
Particularly oxygen sensitive foods such as dry milk powders that are to be
kept in plastic containers for more than two years would benefit from the use
of Mylar. Naturally, storage temperature and moisture content is going to play a major role as well.
HOW DO I USE MYLAR BAGS?
Sealing food in Mylar bags is a straight-forward affair, but it may take a bit
of practice to get it right, so purchase one or two more bags than you think
you’ll need in case you don’t immediately get the hang of it.