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    Home » Why does milk curdle?

    Why does milk curdle?

    April 27, 2012 By Emma Vanstone 4 Comments

    Curdling is a process by which a liquid is transformed to a soft semisolid or solid mass. We can see this happen by a very simple experiement! You will need

    Orange or Lemon juice (citric acid)

    A glass

    A spoon

    Milk

    Sieve

    Method

    1. Half fill the glass with milk

    Curdling milk

    Describe the consistency of milk!

    What does the milk look like?

    What is it’s texture?

    Can you see bits in it?

    Milk is creamy white and is made up of lots of particles but together you cannot see them, milk looks smooth!

    2. Fill the rest of the glass with your orange juice

    Curdling milk, Science Sparks

    Add an acid – orange or lemon juice

    3. Mix Has anything changed?

    Results

    Milk is made up of lots of tiny particles which don’t separate easily. The acid makes the milk more acidic (lowers it pH) and this causes the particles to solidify and expand. They rise to the top as they don’t like to touch the acid (a bit like how oil and water don’t like each other).

    If you put a spoon in the top and lift it out you will now see all the particles on the spoon. You can compare this to what milk looks like dipped in without the orange juice added.

    Curdling milk, Science Sparks

    You can see the solid particles now on the spoon!

    Usually when we curdle something we have done something wrong in a cooking process, and is usually caused by acids and milk or eggs are being added incorrectly or a liquid being left over heat for a period of time and it means the food has gone bad or off. Curdling does have some benefits though and it is the method used to make cheese. Soft cheeses like ricotta and brie are examples of ‘curdled cheese’ and lactic acid is the acid used to complete this process.

    It is a good opportunity for some taste testing of cheese but do not drink the milk after curdling it otherwise you will get a funny tummy!

    Last Updated on February 4, 2014 by Emma Vanstone

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    Safety Notice

    Science Sparks ( Wild Sparks Enterprises Ltd ) are not liable for the actions of activity of any person who uses the information in this resource or in any of the suggested further resources. Science Sparks assume no liability with regard to injuries or damage to property that may occur as a result of using the information and carrying out the practical activities contained in this resource or in any of the suggested further resources.

    These activities are designed to be carried out by children working with a parent, guardian or other appropriate adult. The adult involved is fully responsible for ensuring that the activities are carried out safely.

    Reader Interactions

    Comments

    1. Jacob Winegrad

      February 17, 2015 at 1:20 am

      The curds do not actually form because they dislike acid. This occurs because the proteins initially all have negative charges on their outsides. This makes them dissolve well in water and also causes them to repel one another (because like charges repel each other). When acid it added it provides a positive charge to the proteins, which neutralizes their negative charges. This has two results:

      1) Because the proteins no longer have a negative charge on their surface they do not dissolve in water as well.
      2) Because the proteins now all have no charge (neutral) on their surface they can interact with each other and form larger particles.

      So they actually form because the proteins LOVE the acid and readily take it up. Whoa, science, you so crazy 🙂

      Reply
    2. RONICA EUGENE

      November 13, 2015 at 8:33 pm

      MILK AND ORANGE

      Reply

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