Milk to Plastic

By:Becca Copple



Physical status of milk

About 87% of milk is water, in which the other constituents are distributed in various forms. We distinguish among several kinds of distribution according to the type and size of particle present in the liquid.

Kind of solution

Particle diameter (nm)

Ionic solution


Molecular solution


Colloid (fine dispersion)


Coarse dispersio

(suspension or emulsion)


In milk we find examples of emulsions, colloids, molecular and ionic solutions.

pH and acidity

An acid is a substance which dissociates to produce hydrogen ions in solution. A base (alkaline) is a substance which produces hydroxyl ions in solution. It can equally be stated that an acid is a substance which donates a proton and a base is a substance which accepts a proton.

The symbol pH is used to denote acidity; it is inversely related to hydrogen ion concentration.

Neutrality is pH 7

Acidity is less than pH 7

Alkalinity is more than pH 7

Fresh milk has a pH of 6.7 and is therefore slightly acidic.

When an acid is mixed with a base, neutralisation takes place; similarly a base will be neutralised by an acid.

Milk fat

If milk is left to stand, a layer of cream forms on the surface. The cream differs considerably in appearance from the lower layer of skim milk.

Under the microscope cream can be seen to consist of a large number of spheres of varying sizes floating in the milk. Each sphere is surrounded by a thin skin—the fat globule membrane—which acts as the emulsifying agent for the fat suspended in milk (Figure 3).The membrane protects the fat from enzymes and prevents the globules coalescing into butter grains. The fat is present as an oil-in-water emulsion: this emulsion can be broken by mechanical action such

Fats are partly solid at room temperature. The term oil is reserved for fats that are completely liquid at room temperature. Fats and oils are soluble in non-polar solvents, e.g. ether.

About 98% of milk fat is a mixture of triacyl glycerides. There are also neutral lipids, fat-soluble vitamins and pigments (e.g. carotene, which gives butter its yellow colour), sterols and waxes. Fats supply the body with a concentrated source of energy: oxidation of fat in the body yields 9 calories/g. Milk fat acts as a solvent for the fat-soluble vitamins A, D, E and K and also supplies essential fatty acids (linoleic, linolenic and arachidonic).

A fatty-acid molecule comprises a hydrocarbon chain and a carboxyl group (-COOH). In saturated fatty acids the carbon atoms are linked in a chain by single bonds. In unsaturated fatty acids there is one double bond and in poly-unsaturated fatty acids there is more than one double bond. Examples of each type of fatty acid are shown in Figure 4.

Fatty acids vary in chain length from 4 carbon atoms, as in butyric acid (found only in butterfat), to 20 carbon atoms, as in arachidonic acid. Nearly all the fatty acids in milk contain an even number of carbon atoms.

Fatty acids can also vary in degree of unsaturation, e.g. C18:0 stearic (saturated), C18:1 oleic (one double bond), C18:2 linoleic (two double bonds), C18:3 linolenic (three double bonds).

The most important fatty acids found in milk triglycerides are shown in Table 2. Fatty acids are esterified with glycerol as follows:

Glycerol + fatty acids → triglyceride (fat) + water

The melting point and hardness of the fatty acid is affected by:

the length of the carbon chain, and

the degree of unsaturation.

As chain length increases, melting point increases. As the degree of unsaturation increases, the melting point decreases.

Fats composed of short-chain, unsaturated fatty acids have low melting points and are liquid at room temperature, i.e. oils. Fats high in long-chain saturated fatty acids have high melting points and are solid at room temperature. Butterfat is a mixture of fatty acids with different melting points, and therefore does not have a distinct melting point. Since butterfat melts gradually over the temperature range of 0–40°C, some of the fat is liquid and some solid at temperatures between 16 and 25°C. The ratio of solid to liquid fat at the time of churning influences the rate of churning and the yield and quality of butter.

Fats readily absorb flavours. For example, butter made in a smoked gourd has a smokey flavour.

Fats in foods are subject to two types of deterioration that affect the flavour of food products.

Hydrolytic rancidity: In hydrolytic rancidity, fatty acids are broken off from the glycerol molecule by lipase enzymes produced by milk bacteria. The resulting free fatty acids are volatile and contribute significantly to the flavour of the product.

Oxidative rancidity: Oxidative rancidity occurs when fatty acids are oxidised. In milk products it causes tallowy flavours. Oxidative rancidity of dry butterfat causes off-flavours in recombined milk.

Milk proteins

Proteins are an extremely important class of naturally occurring compounds that are essential to all life processes. They perform a variety of functions in living organisms ranging from providing structure to reproduction. Milk proteins represent one of the greatest contributions of milk to human nutrition. Proteins are polymers of amino acids. Only 20 different amino acids occur, regularly in proteins. They have the general structure:

R represents the organic radical. Each amino acid has a different radical and this affects the properties of the acid. The content and sequence of amino acids in a protein therefore affect its properties. Some proteins contain substances other than amino acids, e.g. lipoproteins contain fat and protein. Such proteins are called conjugated proteins:

Phosphoproteins: Phosphate is linked chemically to these proteins—examples include casein in milk and phosphoproteins in egg yolk.

Lipoproteins: These combinations of lipid and protein are excellent emulsifying agents. Lipoproteins are found in milk and egg yolk.

Chromoproteins: These are proteins with a coloured prosthetic group and include haemoglobin and myoglobin.


Casein was first separated from milk in 1830, by adding acid to milk, thus establishing its existence as a distinct protein. In 1895 the whey proteins were separated into globulin and albumin fractions.

It was subsequently shown that casein is made up of a number of fractions and is therefore heterogeneous. The whey proteins are also made up of a number of distinct proteins as shown in the scheme in Figure 5.

Casein is easily separated from milk, either by acid precipitation or by adding rennin. In cheese-making most of the casein is recovered with the milk fat. Casein can also be recovered from skim milk as a separate product.

Casein is dispersed in milk in the form of micelles. The micelles are stabilised by the Κ-casein. Caseins are hydrophobic but Κ-casein contains a hydrophilic portion known as the glycomacropeptide and it is this that stabilises the micelles. The structure of the micelles is not fully understood.

When the pH of milk is changed, the acidic or basic groups of the proteins will be neutralised. At the pH at which the positive charge on a protein equals exactly the negative charge, the net total charge of the protein is zero. This pH is called the isoelectric point of the protein (pH 4.6 for casein). If an acid is added to milk, or if acid-producing bacteria are allowed to grow in milk, the pH falls. As the pH falls the charge on casein falls and it precipitates. Hence milk curdles as it sours, or the casein precipitates more completely at low pH.

Whey proteins

After the fat and casein have been removed from milk, one is left with whey, which contains the soluble milk salts, milk sugar and the remainder of the milk proteins. Like the proteins in eggs, whey proteins can be coagulated by heat. When coagulated, they can be recovered with caseins in the manufacture of acid-type cheeses. The whey proteins are made up of a number of distinct proteins, the most important of which are b-lactoglobulin and lactoglobulin. b-lactoglobulin accounts for about 50% of the whey proteins, and has a high content of essential amino acids. It forms a complex with Κ-casein when milk is heated to more than 75°C, and this complex affects the functional properties of milk. Denaturation of b-lactoglobulin causes the cooked flavour of heated milk.

Other milk proteins

In addition to the major protein fractions outlined, milk contains a number of enzymes. The main enzymes present are lipases, which cause rancidity, particularly in homogenised milk, and phosphatase enzymes, which catalyse the hydrolysis of organic phosphates. Measuring the inactivation of alkaline phosphatase is a method of testing the effectiveness of pasteurisation of milk.

Peroxidase enzymes, which catalyse the breakdown of hydrogen peroxide to water and oxygen, are also present. Lactoperoxidase can be activated and use is made of this for milk preservation.

Milk also contains protease enzymes, which catalyse the hydrolysis of proteins, and lactalbumin, bovine serum albumin, the immune globulins and lactoferrin, which protect the young calf against infection.

Milk carbohydrates

Lactose is the major carbohydrate fraction in milk. It is made up of two sugars, glucose and galactose (Figure 6). The average lactose content of milk varies between 4.7 and 4.9%, though milk from individual cows may vary more. Mastitis reduces

Lactose is a source of energy for the young calf, and provides 4 calories/g of lactose metabolised. It is less soluble in water than sucrose and is also less sweet. It can be broken down to glucose and galactose by bacteria that have the enzyme b-galactosidase. The glucose and galactose can then be fermented to lactic acid. This occurs when milk goes sour. Under controlled conditions they can also be fermented to other acids to give a desired flavour, such as propionic acid fermentation in Swiss-cheese manufacture.

Lactose is present in milk in molecular solution. In cheese-making lactose remains in the whey fraction. It has been recovered from whey for use in the pharmaceutical industry, where its low solubility in water makes it suitable for coating tablets. It is used to fortify baby-food formula. Lactose can be sprayed on silage to increase the rate of acid development in silage fermentation. It can be converted into ethanol using certain strains of yeast, and the yeast biomass recovered and used as animal feed. However, these processes are expensive and a large throughput is necessary for them to be profitable. For smallholders, whey is best used as a food without any further processing.

Heating milk to above 100oC causes lactose to combine irreversibly with the milk proteins. This reduces the nutritional value of the milk and also turns it brown.

Because lactose is not as soluble in water as sucrose, adding sucrose to milk forces lactose out of solution and it crystallises. This causes sandiness in such products as ice cream. Special processing is required to crystallise lactose when manufacturing products such as instant skim milk powders.

Some people are unable to metabolise lactose and suffer from an allergy as a result. Pre-treatment of milk with lactase enzyme breaks down the lactose and helps overcome this difficulty.

In addition to lactose, milk contains traces of glucose and galactose. Carbohydrates are also present in association with protein. Κ-casein, which stabilises the casein system, is a carbohydrate-containing protein.

Minor milk constituents

In addition to the major constituents discussed above, milk also contains a number of organic and inorganic compounds in small or trace amounts, some of which affect both the processing and nutritional properties of milk.

Milk vitamins

Milk contains the fat-soluble vitamins A, D, E and K in association with the fat fraction and water-soluble vitamins B complex and C in association with the water phase. Vitamins are unstable and processing can therefore reduce the effective vitamin content of milk.

Plastic? In milk? Well, sort of. You made a substance called CASEIN. It's from the latin word meaning "cheese." CasEin occurs when the protien in the milk meets the acid in the vinegar. The casein in milk does not mix with the acid and so it forms blobs. True plastics, called poymers, are a little different.

How It Works:

Plastics are all similar, containing molecules that are repeated over and over again into a chain called polymers. Milk contains molecules of a protein called casein. When milk is added to an acid, such as vinegar, the pH of the milk changes. The pH change causes the casein molecules to unfold and reorganize into long chains, curdling the milk. The curds can then be kneaded and molded as casein plastic.

Fun Facts:

  • Casein plastic is commonly used to make fountain pens!

  • Casein plastic is extremely environmentally friendly, because it will decompose over time, unlike plastics made from petroleum products.

  • The National Plastic’s CenterMuseum, located in Leominster, MA, hosts exhibits about the history, manufacturing, and recycling of plastics!

  • Companies are beginning to research and develop ways to manufacture plastic from renewable resources, such as plants. Chemists at Pacific Northwest National Laboratory have been studying how to use glucose as fuels, plastics, and other petroleum products.

  • Casein plastic was first manufactured in London in 1900. It was used to make jewelry and buttons. Today, there are thousands of Casein jewelry collectors around

Normally these protein molecules repel each other, allowing them to float about without clumping, but when the pH of their solution changes, they can suddenly attract one another and form clumps. This is exactly what happens when milk curdles. As the pH drops and becomes more acidic, the protein (casein) molecules attract one another and become "curdles" floating in a solution of translucent whey. This clumping reaction happens more swiftly at warmer temperatures than it does at cold temperatures.



If I use milk and add a certain amount to vinegar and microwave it then it will turn into curds.


  • Mugs or other heat-resistant cups (large enough to hold more than 8 oz. of liquid)

  • Masking tape

  • Pen or permanent marker

  • Teaspoon measuring spoon

  • White vinegar (at least 8 oz.)

  • Milk (at least 12 cups)

  • Microwavable liquid measuring cup)should be large enough to hold 4 cups of milk )

  • Microwave

  • Cooking or candy thermometer

  • Spoons (4)

  • Cotton cloth

  • Rubber bands (4)

  • Clear plastic or glass drinking cups (4),( each large enough to hold 8 oz. of liquid

  • Kitchen scale, should be accurate to 1 gram

  • Wax paper (in 12 identical pieces); each piece should be smaller than the weighing surface of the kitchen scale

  • Paper towels

  • Lab notebook


1)Using the masking tape and pen, label the five mugs: 1, 2, 4, 8 and10.(make sure all mugs are same size)

2)Use the measuring spoon to add 1 teaspoon (tsp.) of white vinegar to mug number 1. 2 tsp, to the mug number 2. 4 tsp. to the mug numbered 4 and 8 tsp. to the mug number 8 and 10 tsp to the cup labeled 10

3)in a large measuring bowl heat up 4 cups of milk in the microwave. There is no exact amount of time because all microwaves are different. Just start with 4 minutes at 50%(so you don't scorch the milk) and go from there.

4) if needed have an adult check the milk with a thermometer to make sure the milk is at least 49°C or 120°F , if when you vheck the milk and it is not that hot yet put it back in the microwave for another minute or so (on 50%). If you get it warmer than 49°C it is fine.

5) in a note book recorded how long it took you to get the milk to 49°C ,also record the final tempture(when you repeat these steps try and get about the same temperatures over again)

6) Carefully pour 1 cup of hot milk in to each of the five mugs with vinegar in them. (You may need to ask an adult to pour the hot milk for you.)

7) record what you see happening in each mug, write them down in a data table.In at least one of the mugs you should see that the milk has separated into white clumps (called curds).

8)Make sure to pour the milk in to all five of the mugs at the same time so that the milk is the same temperature across all four vinegar amounts.recored which amounts of vinegar have formed of curds.

9)Write down any other observations

10) make a table in your lab notebook to write down your data (by the end of this lab you should have 3 tables)

11)Mix each mug of hot milk and vinegar slowly with a spoon for a few seconds. This helps makes sure the vinegar reacts as much as possible to the milk.

12)Meanwhile, take one of the cotton-cloth squares and attach it with a rubber band to the top of one of the clear cups so that it completely covers the cup's opening. This will make a sieve.

( Make sure some of the cloth hangs down a little bit from the inside of the cup, this way you have room to pour your liquid.)

13)Repeat this step with the other four clear cups.

14)Label the clear cups 1, 2, 4,8 and 10 with the tape and pen.

15)Let the milk and vinegar mixture cool down then carefully pour the mixture from mug #1 into the clear cup with the #1 and cotton cloth sieve on it.

16) the curds , if any, will collect in the sieve the leftover liquid will just be in the might want to do this step over a sink just in case you spill.

17)Record in your table in your note book what the clear liquid in your cup looks like.write this info down for all cups.

18)on cup #1 remove the rubber band sieve carefully over a sink. Scrape the curds off the cloth,pick up the curds, if there are any, squeeze all the extra water out of them. Next knee the curds together, into a ball. It will look a little bit like bread dough.

20)Weigh the ball of casein plastic on a kitchen scale (use grams) using a piece of wax paper to keep the scale clean. Record the weight in your table.

21)before you weigh the curds make sure your scale is set to 0, this will make sure your weight is more accurate. Put a piece of wax paper on top of the scale each time before you use it get a new piece before each time u measure.

22)The amount of casein plastic each recipe makes is called the yield for that recipe. The more plastic, as measured by weight in this case, the greater the yield.

23)Repeat steps 7-10 for the other three mugs of milk and vinegar.

24)If you want to make your casein plastic into something, you can color, shape, or mold it now (within an hour of making the plastic dough) and then leave it to dry on some paper towels for at least 48 hours.

Statically analysis:

As the teaspoon of vinegar of increased, the amount and weight of the curds also increased.


Claim: Microwaving milk and adding vinegar creates a chemical reaction that causes a curd to form, later hardening into casein plastic.

Evidence: According to our data table every amount of vinegar added to the milk had a reaction occur. The more vinegar you added the more curds would form from the reaction, meaning cup #10 with 10 teaspoons had the most and cup #1 with 1 teaspoon had the least amount of curds from the chemical reaction.

Reasoning: Every time you add vinegar to warm milk you will get a reaction that creates a curd. I know this because when you add the vinegar to the warm milk the pH of the milk drops making it more acidic. The protein casien moleclues attract to each other and that's what becomes the curds. Those curds when hardening become the casien plastic. Creating this mixture of milk and vinegar you will get a curd due to the changes in the pH causing the chemical reaction to take place.


When you add vinegar to microwaved milk it creates a reaction and makes curds which harden it to casein plastic.


In my case I wish I would of had a scale that weighs smaller objects, the one I had was in ounces so I had to use smaller numbers. If I would of had a scale maybe in grams you could see more of a difference between each increment of vinegar, making the lab easier to read.


My experiment is also a bit of a history fact fact, this casien plastic was use back in the 1800-1900's for jewelry, fountain pens etc. you can still use this plastic but most people prefer not to, but with my results you can know how much vinegar you will need. It's a fun family friendly easy to do little experiment, to just have fun with.


I think one way of improving this experiment to make the increments of vi gear go up by ore like do 1,3,6,9 and 12 this way you see more of a difference with the weight of the casien at the end.
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