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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 24 September, 1998 Next Week's Question
Turning milk into homemade moo glue
Question
I have a recipe to make homemade glue. The recipe asks you to heat skim milk and vinegar until the milk curdles. Then it asks to remove the liquid from the mixture and add sodium bicarbonate to the curdles. What is sodium bicarbonate and why is it an important ingredient to make the glue?

Question
To answer this question, let's first ask another question: "What exactly is the 'homemade glue' that you make using this recipe"? The sticky material that you are making is a chemical called casein. It has been used as an adhesive for a long time. In the late 1800's, it was first used as a wood glue in Switzerland, and is even used today to glue the seams of some grocery sacks. Casein is also used to fortify bread, and is often an ingredient of non-dairy coffee cream. You can eat it, and it is actually good for you!

In your experiment, you are separating casein from milk. Casein is found naturally in cow's milk: however, the amount of casein in a typical glass of milk would only fill a couple of thimbles! To get this sticky stuff away from the rest of the milk, which isn't very sticky, you add vinegar. Vinegar contains an acid, and it causes the casein to precipitate (in other words, it turns the casein into a solid). Now, the casein can be easily separated from the rest of the milk, which is a liquid. The problem now is that casein isn't very sticky after being reacted with acid. Plus, these curds are kind of like cottage cheese - you wouldn't want to stick paper together with this, would you? (yuck!). To make it into a glue, you must add a base. A base is the chemical opposite of an acid. In your experiment, the base is sodium bicarbonate (commonly known as 'baking soda'). When you add it to the curds, it reacts to make the casein soluble again (no longer chunky!), and it is now sticky like a glue.