Yoghurt Making in the Classroom

Home-made Yoghurt

Making Yoghurt in the Classroom

This activity combines a study of the history and science of yoghurt making with a practical activity that can be performed in most classroom settings.

Yoghurt making is an effective method of introducing scientific concepts to students. Aimed towards middle secondary students, the information below can be used to create a worksheet or Powerpoint presentation that can be introduced before the practical activity.

The History of Yoghurt

The exact origins of yoghurt are relatively hazy, but it is thought to have originated accidentally over 4500 years ago when milk was left to become sour. It was definitely used as a food in the Middle East and Turkey from the second century AD. Yoghurt was introduced into the Western world as a health food by a scientist named Mechnikov, who suspected that it was responsible for the longevity of the Bulgarian people.

Yoghurt and Biotechnology

Biotechnology is defined as the use of living things to make products beneficial to mankind. As such, yoghurt making and other fermentation processes can be regarded as among the most important and early examples of this science.

Fermentation involves the use of bacteria and other microbes to manufacture alcohol, lactic acid or other products useful to man. Yoghurt is the result of specialised lactic acid bacteria (Lactobacillus bulgaricus and Streptococcus thermophilus) converting the lactose in milk to lactic acid and energy. Because this process does not require oxygen it is known as anaerobic respiration.

Lactobacillus bulgaricus and Streptococcus thermophilus are both ‘homofermenters’, which means that lactic acid is their only fermentation product. Other species of bacteria, however, are ‘heterofermenters’, producing carbon dioxide and ethanol in addition to lactic acid. These additional products can add flavour to various fermented foods.

The word equation for the production of lactic acid by homofermenters is:

Glucose/lactose → lactic acid

The word equation for the production of lactic acid by heterofermenters is:

Glucose/lactose → lactic acid + ethanol + carbon dioxide

The Characteristics of Yoghurt

The lactic acid produced in the production of yoghurt acts to curdle milk and enhance its flavour. It also performs a natural sterilising role by helping to reduce the growth of bacteria that cause food poisoning. The lactic acid bacteria are also "gut friendly," which means they foster the growth of helpful intestinal microbes and also counteract toxins.

Ironically, many commercial yoghurts are pasteurised, which destroys the live bacteria present in the yoghurt and their beneficial properties. As a consequence, the best yoghurts to buy are the organic and natural ones.

Other Uses for Lactic Acid Fermentation

Lactic acid fermentation is also used to produce foods such as pickles, cheeses, sauerkraut, sourdough breads, kefir, kimchi (a Korean dish of pickled vegetables) , cassava and nham (Thai fermented fresh pork). Again, the lactic acid creates a unique flavour and also serves a preservative function.

Making Natural Yoghurt: Materials and Ingredients

Note that the materials and ingredients listed are based on the requirements of one group of 4-6 students

•                Portable hotplate or portable microwave oven

•                Thermometer

•                1 teaspoon natural yoghurt to use as a starter culture

•                Small saucepan, 500ml beaker (or a plastic bowl if using the microwave oven)

•                250 ml milk

•                1 plastic cup

•                Incubator or Thermos flask

Method for Making Natural Yoghurt

1.             Pour the milk into the saucepan, beaker or plastic bowl.

2.             Heat the milk to 90°C and maintain it roughly at this temperature for 10 minutes.

3.             Now allow the milk to cool. When it has reached 40°C, mix in 1 teaspoon of natural yoghurt.

4.             Pour the mixture into a plastic cup and incubate it for at least 12 hours at around 30-40°C. If a school incubator is not available a Thermos flask or other insulated container could be used.

5.             The mixture should now have a thick consistency and the characteristic smell and taste of yoghurt (see figure below).

Follow Up Work for Students

Students can investigate why the milk needed to be heated to a certain temperature then cooled to 40°C. Note that this is essentially because high temperatures kill off any harmful bacteria and also restructure the milk proteins so that when they set they won’t form lumpy curds. Cooling is required so that the lactic acid bacteria don’t get killed when they are added to the mixture.

References

Eat Yoghurt, "History of Yoghurt," eatyoghurt.com, 2010. Accessed 8/4/2010

Tempeh info, "Lactic Acid Fermentation," tempeh.info, 2010. Accessed 9/4/2010

Wordpress, "Yoghurt Making," wordpress.com, 2008. Accessed 9/4/2010