What are curds and whey?
Explore the chemical reaction that is in involved in making curds
and whey.
Setting: Indoors
Time: < 30 minutes
Concepts: acids; chemical reactions
Skills: Comparing & Contrasting, observing
Subject(s):
Chemistry
Life Sciences
Ages:
9-11
12-14
Materials:
• 2 glasses or clear plastic cups
• Homogenized or 2% white milk (not skim or 1%)
• White vinegar – 5 mL (or 1 teaspoon)
• Strong paper towels, coffee filter or cheesecloth
• Lemon juice (optional for follow-up activity)
• Tray or sink
• Measuring spoon
Safety First!
Vinegar, lemon juice and baking soda will irritate sensitive skin and tissue. Do not
rub your eyes while handling these items. Wash your hands immediately after you
complete this activity.
What to do:
1. Pour milk into a glass so that it is ¼ full. Allow it to come to room temperature (this
should take about 15 minutes).
2. Carefully look at the milk. Describe what you see.
3. Add 5 mL (1 teaspoon) of vinegar to the milk.
4. Wait about 5 minutes.
5. Swirl the milk around and look at the sides of the glass. What do you see? How has the
milk changed?
6. Use the paper towels, coffee filter or cheesecloth as a strainer to separate the curds
(solid part) and whey (liquid part) in the tray or sink.
7. Squeeze most of the liquid out of the curds.
What’s happening?
Milk is a mixture and contains water, sugar (lactose), protein, fats and minerals. The vinegar
helps to curdle the milk. Curdling happens when the solids in the milk (the proteins and fats)
clump together and form a new substance, which are called curds. The curdling is the result of a
chemical reaction between the vinegar (acid) and the fats and proteins. You can separate the
curds out of the milk by pouring the clumpy mess through a filter like a strong paper towel or
piece of cheesecloth. The liquid that passes through the paper towel or cheesecloth is called
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What are curds and whey?
whey. These are the same “curds and whey” that appear in the nursery rhyme “Little Miss
Muffet.” Curdling can also happen if heat or salt are added to milk.
Why does it matter?
Curdling is what happens when you keep milk after its expiration date has passed or if it has
been left unrefrigerated too long. This is what it means when people say the milk has "gone
bad” - it will taste very different from milk that is still fresh. This “natural curdling” or spoiling is
also caused by chemical reactions that create acids in the milk as it gets old or warm. Those
acids cause the proteins and fats to clump together, however the milk actually spoils because of
the growth of bacteria that also occurs during this time.
Curdled milk is still safe to cook with. You can also buy curds in the grocery store. In fact,
cheese is actually made from curdled milk! Cottage cheese is a great example.
Investigate further!
•
•
Vinegar is a weak acid and so is lemon juice. What happens when you add lemon juice
instead of vinegar?
Learn more here: Cheese-making Process – You Tube (Video – 4:48 min.)
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