Mylar is a clear film that block oxygen from passing through it. Often mylar is paired with foil for tensile strength ... which is why we associate the foil bags with mylar. So not all foil bags are mylar bags. Typically oxygen absorbers use powdered iron or iron filings and some form of salt.
The purpose of using mylar bags and oxygen absorbers is to remove and keep out oxygen. Just as iron becomes oxygenated and rusts, so does food... except it won't rust it will deteriorate. Oxygen supports aerobic bacteria and fungi. So by removing the oxygen you prevent these types of bacterias and fungi to grow on your food. One thing that MUST be remembered is you should NEVER store moist foods in these mylar bags. According to the LDS (The Church of Jesus Christ of the Latter Day Saints... aka the Mormons) "
Warning: Botulism poisoning may result if moist products are stored in packaging that reduces oxygen. When stored in airtight containers with oxygen absorbers, products must be dry (about 10% or less moisture content)." I don't follow the same beliefs as the LDS do but I have to say that I know they are great when it comes to prepping!
Botulism is a bacteria that produce one of the strongest toxins known to man. It takes just one single microgram to kill a person (Chuck Norris is the only exception... he sprinkles botulism on his cereal in the morning). A microgram is 1/1,000,000 (one millionth) of a gram.
The LDS also states "Products intended for longer-term storage must be dry (about 10% or less moisture content).
Products intended for longer-term storage must be dry (about 10% or less moisture content).
Warning: Botulism poisoning may result if moist products are stored in packaging that reduces oxygen.
Dry products that are
not suitable for longer-term storage due to moisture content, oils, or other concerns include:
longer-term storage due to moisture content, oils, or other concerns include:
Barley, pearled
Meat, dried (such as jerky)
Eggs, dried
Nuts
Flour, whole wheat
Rice, brown
Grains, milled (other than rolled oats)
Sugar, brown
Granola
Vegetables and fruits, dehydrated (unless dry enough, inside and out, to snap when bent)