Re: The chemical properties of aluminum foil & plastics wrap?

Actually its the physical properties of foil and plastic wraps that will 
have the impact you describe.  Basically the difference is due to the 
barrier properties of the material.

Aluminum foil gives a complete barrier to the transfer of gases and 
vapors, assuming the the product is well wrapped without holes or cracks.  
Plastics, on the other hand, allow some quanitity of gases to pass through 
the material.  The amount will depend on the type of material and its 
weight.

If you wrap a lemon in aluminum foil you create an environment that is 
much different from that in plastic.  The enzyme systems would continue to 
function, however if they are dependent on oxygen the amount available 
would decrease and the rate of reaction would slow down significantly or 
stop.  Bacterial growth would see the same changes.  Aerobic (oxygen 
loving) bacteria could grow for a while, as the oxygen is depleted they 
stop growing and anaerobic (oxygen hating) or facultative anaerobes 
(tolerate some oxygen) would take over.  They grow very differently and 
metabolize very differently than anaerobes.  The appearance might be the 
same but the smell would vary enormously due to the way the food is 
metabolized (eaten) by the bacteria.

Plastic wrap would behave much differently.  If this is something you did 
at home, you probably used a Saran type wrap.  Saran wrap is very good at 
holding in moisture, but poor in holding oxygen.  This means that the wrap 
will continually be allowing oxygen inside.  Aerobes will continue to 
grow, anaerobes and facultative anaerobes won't be able to grow because 
there is just to much oxygen available.  Again, the smell will be 
different because of the differences in the way the food is metabolized 
and the by-products that are created by this "bacterial digestion".