What is the best way to preserve food? Why?

Answer 1

Almost all bacteria are killed by heat - hence there's heating methods of preservation (cans, etc). All bacteria need some moisture to grow - hence there's preservation by dessication. Aerobic bacteria need oxygen to grow - hence there's preservation by vacuum sealing. Other methods involve changing the chemistry of the environment to restrict bacterial growth, such as salting, acidification, etc. This list is not exhaustive.
My vote would go to dessication, although it's not appropriate for all foods. Bacteria are bloody smart little things (for things that don't think!) - when they detect the environment's getting harsh, they form spores which generally lie dormant despite whatever preservation method use - only to spring to life when you want to use the food. Probably the best way to try to kill the spores as well would be to heat the food to very high temperatures - but that would ruin the food itself! Final answer - different for different foods!

Answer 2

there are two types of bacteria
aerobic, which need oxygen to live
anaerobic, which don't.
the first are more common, which is why most food is sealed up to preserve it. tinned food is a good example.
the second is usually dealt with by freezing food. it severely restricts the metabolic activity of the bacteria.