MadSci Network: Other

Re: Which type of bread molds the fastest?

Date: Fri Apr 28 17:43:25 2000
Posted By: Phyllis Stumbo, Staff, Nutrition, University of Iowa
Area of science: Other
ID: 953505285.Ot

How quickly a bread will mold depends on several things, one being the 
"water activity" of the bread.  An earlier Mad Scientist message gave the 
following information about bread molding:

"Scientists measure "water activity", that is water that is not bound into 
the product, to help determine "shelf life", or how long a product can 
remain on the shelf and be edible.  Pure water has an activity of 1.0 and 0 
is bone dry.  Bacteria generally stop growing at a water activity less than 
.91, but molds grow on drier foods, until the water activity drops below 
.81.  Bread has a water activity of about .95.  However, another factor 
about bread that determines how quickly it spoils is its pH (or acidity).  
Bread has a pH of about 5.3 - 5.8 which means it is slightly acid.  This 
also helps keep it safe to eat on the shelf."

One way to estimate how quickly bread will mold would be to look at its 
water content.  For example the percent of water in several breads is:  
Navaho fry bread 26%, banana bread 29%, toasted white bread 30%, 
plain white bread 36%, Oat bran bread 44% and canned Boston brown bread 

But this is not the whole story, because other ingredients in the 
bread may affect the "water activity".  Water activity is measured by 
putting a bread sample in a tight container and measuring the amount of 
water that vaporizes from the product.  If you want to learn more about how 
food producers determine the water activity of their bread visit the web 
sites of manufacturers who make the instruments bread makers use to test 
their product.  One place to study this question is at the following URL:

The bottom line is that the dryer the bread the less likely it is to mold.  
Toasting bread slows subsequent mold growth, as does refrigerating bread.  
And much commercial bread contains a chemical to slow mold growth.  
The best policy is to buy only as much bread as you can eat within a few 
days or store bread in the refrigerator or freezer until needed.


Phyllis Stumbo, University of Iowa, Iowa City, IA

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