Re: Testing for sour milk

Hi, Tim.  The expression "what a small world" is very true.  My daughter 
has been to Melbourne several times visiting the Hawkins family who moved 
there from New Brunswick, Canada a number of years ago.

But, on to your question......

At the start of your experiment, you'll find that milk has a pH of about 
6.6, already on the acid-side of neutral (pH 7.0).  Litmus paper will only 
indicate whether the material to be tested is acid or base so you are not 
likely to be able to differentiate the drop in pH with litmus paper.

However, there are several types of pH indicator paper that will enable you 
to determine the actual pH of the milk as it changes through your test 
period.  The pH indicator paper comes in strips that you dip in the milk (- 
just as you would with litmus paper -) and then compare the color of the 
strip to a chart which correlates that color to a specific pH reading 
(usually to one decimal place).

I'm sure that any commercial or school lab in your area would be willing to 
give you a number of the indicator strips to carry out your experiment.  
They are relatively inexpensive.  The strips cover different ranges of the 
pH scale so you will have to tell the lab that you would like to have 
strips that either cover a wide pH range (for example, from 3.0 to 9.0) or, 
narrower range strips that, together, cover the anticipated pH (for 
example, 3.4 to 4.8 AND 4.8 to 6.7).  Whe you are scrounging the indicator 
strips, be sure to ask for about three times more than you expect to need 
for the number of test readings you have planned.  As I said, they are not 
expensive and this will allow you to discard a few when you are getting 
used to interpreting the test strip versus the color chart.

There are usually extra color charts in every lab (- we don't like to 
throw out anything that may have a use another day-), but if the lab 
doesn't have a spare color chart for you, just take a close-up color photo 
of theirs.  The photo can later be used in your project display, too.   

Good luck with your experiment and the science fair, Tim.  I'm always 
excited when students see that there is science in food and everyday 
situations.

Carol Crouse
Food Technologist
The Food Chain Ltd.