MadSci Network: Chemistry

Re: Why does bread make cookies softer?

Date: Mon Aug 30 09:55:05 1999
Posted By: Daniel O'Brien, Grad student, Food Science, Rutgers University
Area of science: Chemistry
ID: 933087953.Ch


The reason that putting a piece of bread in with the cookies makes them 
soft again is interesting.  Good question.  First you have to understand 
that cookies get hard and stale because they lose moisture to the 
surrounding air.  Dry air passes over the cookies and essentially dries 
them out.  When your aunt puts the bread in the tupperware with the 
cookies, they exchange moisture through the air.  Bread contains a lot of 
water (fresh bread that is) compared to stale cookies.  When you put them 
together in tupperware, the bread loses water to the surrounding air and 
then the cookies attract the water and become soft again.  The key to this 
interaction however, and the reason the bread gets dry and hard, is because 
the cookies are more hygroscopic than the bread.  This means that they have 
a higher affinity for water because they contain lots of sugar.  Sugar, or 
sucrose in this case, has many hydroxyl groups in its molecules.  These 
groups love to bind with water, more so than the molecules in the bread.  
Therefore, the cookies remove moisture from the air, which throws off the 
equilibrium, and the air continues to remove more moisture from the bread 
to compensate.  Eventually you end up with soft cookies and dry bread.  
Hope this helped....

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