Expert: Carol Schlitt - 11/13/2007

Question
I make the simple powdered sugar/milk icing for my cut-out cookies.  I don't refrigerate the cookies.  So what makes the milk to not sour since it is left out for several days?  And for that matter does sour milk make you sick?  My mother always used sour milk to make chocolate cake.  Which leads me to another question: what kinds of cakes (and cookies?) should be refrigerated?

Answer
Hi June,

The reason simple sugar/milk icings don't sour is because the ration of sugar to milk is such that it does not provide enough liquid to support the growth of organisms that cause milk to sour.  Soured milk is an indication that bacteria has grown to a level to change the pH. While milk may still be safe, we recommend not consuming milk that has naturally soured (without the addition of an acid such as lemon juice or vinegar) because it could be masking food borne bacteria that do not give off visible/flavor signals. It is possible to sour your own milk with the addition of acid and this is perfectly safe.

Cakes and cookies that contain unbaked or cooked dairy products should be refrigerated.  Frostings made with sour cream and cream cheese such as that used on carrot cake should be refrigerated for safety.

Hope this answers your questions.  If not, please let me know.

Carol