Curd

Milk coagulates when calcium chloride is added.

Difficulty:
Danger:
Duration:
15 minutes
Experiment's video preview

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Remove protective gloves before lighting the candles.
  • Keep a bowl of water nearby when working with fire.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

May I use low-fat milk in this experiment?

Yes, it should work. Low-fat milk does contain casein micelles that can “stick together” to produce cottage cheese!

Step-by-step instructions

Milk consists of water with tiny drops of fat and protein clots that are both suspended in it. As long as the protein clots are small enough, they can float freely. However, they can be coagulated, for example, by adding calcium chloride CaCl2.

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The reaction of calcium chloride and milk proteins speeds up when heated.

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The resulting clots are so large that they cannot pass through the filter paper. Cloudy milk whey passes through the filter, and what is left in the filter is called curd.

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

What is milk from a chemical perspective?

Milk is a complex biological mixture of proteins, fats, sugars, vitamins, and minerals dissolved in water. Its main protein component - casein - consists of a hydrophilic (water-loving) “head” and a hydrophobic (water-fearing) “tail”.

The molecules of proteins and fats are not really water-soluble. They are, however, evenly dispersed throughout milk, which is why milk looks uniform. The molecules of casein protein in milk form small ball-like structures called micelles. The hydrophobic “tails” of casein molecules are located in the centers of the micelles, while the hydrophilic “heads’ are located outside, in water.

Learn more

Each micelle consists of thousands of protein molecules. A micelle is millions of times larger than a water molecule. Together, casein micelles and milk fats give milk its opacity and white color.

What happens when we add calcium chloride to milk?

When an excess of calcium chloride is added to milk, the micelles of casein protein stick to each other. The surfaces of the micelles and the drops of fat have a slight negative charge and are attracted to calcium ions Ca2+, which are abundant in the solution. As a result, the calcium ions bind the micelles and drops of fat, and the milk curdles. This phenomenon is used in the industrial production of cheese.

Learn more

Milk and dairy products are good natural sources of calcium. Calcium ions are present in the milk even before we add calcium chloride solution CaCl2. Calcium ions Ca2+ additionally bind casein molecules in the micelles with each other.

What is left after filtration?

We have separated the curd we made from the liquid using filter paper. Now let’s examine the filtered liquid more closely. This transparent, yellowish, rather sour substance is whey. It is used as a food additive as well as a source of easily digested proteins and necessary microelements.

What is milk curdling?

The curdling of milk is a complicated process involving the formation of insoluble protein clots. The primary function of cow milk is to feed calves -- and in nature, milk curdles in a calf’s stomach, where casein precipitates due to the acidic medium. After this, casein clots are digested with the help of stomach enzymes (so-called “rennet”) and the enzymes in the fresh milk.

Learn more

Even since ancient times, milk had been a part of our diet. After a while, the ability of milk to curdle was discovered. Over time, the same bacteria which produce lactic acid (C3H6O3) propagate in milk, and as a result, the milk becomes acidic and curdles.

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