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Skip to 0 minutes and 6 secondsAmazingly, from the early 1900s until about 1945, milk was commonly used to make various plastic ornaments, ranging from buttons, beads and other jewellery, to fountain pens, and comb and brush sets. The plastic, called milk plastic, or more usually casein plastic, was even used to make jewellery for Queen Mary of England! First milk was converted into curds, which were hardened on soaking in a solution of methanal in water. This activity explores making curds from cow's milk. Milk contains a family of proteins called casein - proteins, are comprised of long chains of amino acids that are linked together by amide bonds, called peptide bonds. Casein makes up around 80% of the proteins found in cow milk.

Skip to 0 minutes and 52 secondsThese proteins group together to form sphere-like structures called casein micelles, which form a suspension in milk. These micelles consist of casein molecules, together with ions including calcium, and they have negatively charged groups on the outer surface. On addition of ethanoic acid, or vinegar, the negatively charged groups in the casein micelles are protonated. This causes them to come apart and the proteins forming the micelle are no longer soluble and they form a precipitate of small white blobs - as the proteins clump together into a mass called curd. This process occurs naturally when milk sours. So, this activity involves adding vinegar to hot cow milk, and stirring it, to make curds.

Skip to 1 minute and 35 secondsCollect and dry the curds, and thoroughly knead it, like dough, to make a shape, or put it in a mould. You can colour it by mixing it with a food colouring, before letting it dry for at least 24 hours to form a hard object of your choice, which should be related to chemistry. Alternatively, when dry, you can it colour and decorate it using marker pens. You might like to try other common household acids, such as lemon juice - does this work as well as vinegar? What about adding the vinegar to hot milk at different temperatures? Use a thermometer to record the temperature and find out what is the best temperature for forming the maximum amount of curds.

Skip to 2 minutes and 12 secondsAlso, what about using other types of milk, such as skimmed milk? Take a picture of your chemistry-related object and post your results - we look forward to seeing your findings, from this 'hard' experiment.

Making a natural plastic

We are going to make a natural plastic using proteins. Casein is a mixture of proteins found in cow milk which can be extracted through the following method and then moulded into whatever shape you want.

The following ingredients will be required:
• Fresh cow’s milk
• 1-5% strength vinegar
• A variety of food colourings

The following items will be required
• A measuring cup
• A measuring spoon or tablespoon
• A selection of pans for heating
• A stirring spoon
• A strainer or sieve
• A roll of paper towels
• A selection of ice cube or cookie trays and moulds

Ensure that you read through all of the instructions below before beginning the experiment to ensure that you understand exactly what will happen at each step.

Step 1: Measuring and heating

Measure out approximately 1 cup (236 ml) of milk and pour into a saucepan. Heat until the milk becomes warm. However, do not heat excessively as the milk should not be left to boil.

measuring milk

heating milk

Step 2: Addition of Vinegar and food colouring

Measure out one tablespoon of vinegar. Add the vinegar to the warm milk and stir until the milk cools. At this point you can add some food colouring of your choice in order to add some colour to your protein moulds. Here we used green and red food colouring as well as doubling the volume of both milk and vinegar used, in order to process a larger batch of casein.

measuring vinegar

food colours

Step 3: Collection of precipitate

Once the milk has cooled some casein should have precipitated out of the solution. (The acid protonates the negatively charged casein micelles in milk and the neutral protein precipitates - even acidic drinks like Red Bull or Coca Cola can be used). Pour the milk through the strainer and collect any solid product. Dispose of any remaining milk down the sink.

The solid product is known as casein and contains a variety of proteins. Transfer the solid product to a piece of paper towel and fold the towel onto the casein in order to dry the product.

Continue transferring the casein to fresh paper towels until most of the water has been removed.

precipitate extraction

precipitate drying

Step 4: Moulding

Take the resulting casein and press the solid into any shape you want. Securing the casein into a mould or ice cube tray allows specific shapes to be made. A more detailed product can be achieved by slowly layering casein into the mould. Pushing down firmly in order to compact the solid each time.

Once placed into an appropriate mould the casein should be left to continue to dry. Warm areas between 30-40 °C, such as above radiators or in greenhouses, are ideal for storing the mould.

The final product should be casein cast into a shape based on the mould it was placed into. The solid is hard but sometimes brittle and so should be handled relatively carefully. Casein is non-toxic, however, we would not recommend consuming the results of the experiment.

Although the solid result may be ready after a few hours, leaving the casein in a mould for up to four days will result in the driest and strongest structure. Here we see moulds removed less than a day after being made. Although they are solid they are still “moist” and easily develop fractures as they have not fully dried.

After being left for over a week the moulds become very hard but also experience a slight shrinking in size. Be sure to account for this by using larger moulds if you want plastic objects of a specific size.

plastic

When you are done we would love to see your results! Upload a picture of your creation using our open Padlet or the Twitter hashtag #FLchemistry

Safety

  • Take care when warming milk: Warm milk can cause scalds; Take care when separating protein solid from warm milk and pour all warm milk down the sink once it is separated from the protein.

  • Avoid ingesting casein: It is strongly recommended that you do not consume any of the casein that you extract. Dispose of any casein in a food or general waste bin after use.

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This video is from the free online course:

Exploring Everyday Chemistry

University of York

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