MadSci Network: Biochemistry

Re: Plastic Wrap in the freezer

Date: Thu Apr 27 14:54:10 2000
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Biochemistry
ID: 954854372.Bc

The ice or moisture inside the wrap is neither good nor bad; it is a 
Ordinarily, bread contains some free moisture or water that is not bound 
to the starch and protein in the bread.  The free moisture can vaporize 
and condense within the bread but, some of the vapor will escape and the 
bread becomes harder and "stale". 

If you cover bread in a moisture resistant wrap, then the free moisture is 
trapped inside the wrap.  The water vapor bounces off the wrap and back 
into the bread.  To see how much moisture is hitting the wrapper, put an 
ice cube on the wrapper.  The condensed water you see is how much water 
vapor was hitting that spot.  In other words, as the water vapor atoms hit 
the cold spot, the ice absorbs their energy, and the vapor turns back to 
liquid.  Thus, the ice crystals you see on the inside of a wrapper 
originated from free moisture inside the bread that condensed on the 
cooling wrapper. 

The fresher the bread, the more moisture within it.  But some breads have 
more free moisture that others.

Take a stick of fresh bread that has a hard crust, such as French bread 
and divide it.  Put half in a plastic bag, leave the other half open or in 
a paper bag.  Check the sticks after 8-24 hours.  You'll see the half in 
the plastic bag has become softer than before; this is because the free 
moisture has equilibrated between the soft interior and the formerly dry 
crust. The half left open will have become harder (unless it's a rainy or 
very humid day) because some of the free moisture has escaped.  Dry bread 
is hard bread; unbuttered toast is another example of hard dry bread.  

Keeping the moisture in is not necessarily a good thing.  If you kept the 
two sticks of bread for several days, you would notice that the open bread 
would get harder and crustier. The bread in the plastic bag would remain 
soft but grow mold.  Wrapping the bread and putting it in a freezer keeps 
the moisture in and prevents mold growth. 

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