Home practical: making yoghurt

In the third and final home practical experiment, you’ll turn milk into yoghurt using everyday items you can buy in a supermarket.

This practical is optional, you can just read the instructions if you’re unable to, or would prefer not to, carry out the practical yourself.

Lactic acid bacteria (LAB), predominantly Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus thermophilus are used in food production to turn milk (and non-dairy alternatives) into yoghurt on an industrial scale. LAB like to ferment sugars, particularly lactose, to produce energy. Unlike the yeast that you used in the Week 2 home practical, LAB produce lactic acid as the main by-product (not CO₂ or ethanol) of fermentation, which gives yoghurt its characteristic ‘tangy’ taste and texture.

The first step in yoghurt production is to heat the milk to around 80°C for a few minutes. This kills potential pathogens (pasteurisation) and also changes the structure of the milk proteins (it denatures them) so that they form a more stable gel-like structure. The milk is then cooled to between 40 – 45°C before the bacterial culture is added (If the bacteria are added when the milk is too hot they die). It is then kept at this temperature for 6 – 8 hours for the bacteria to grow and ferment the sugars into lactic acid. This causes the pH to drop, which helps prevent the growth of pathogens, but the LAB are able to tolerate the acidic conditions they create.

You can find a list of the items you need and instructions for carrying out the experiment in this PDF. It’s a good idea to read through all of these instructions at least once before starting the experiment. You may also like to post an image of your experiement on the home practical Padlet Wall (Note: Padlet.com is an external website).

Note: to provide access to a wider bank of images from Learners, this Padlet wall includes images and observations from previous runs of this course, which can be added to.

You may have read in the news that, due to the increasing popularity of Greek yoghurt, producers are finding it difficult to dispose of the watery by-product from this process, called acid whey. Such large volumes cannot be poured down the sewer because it is acidic and contains high levels of nutrients that would alter the natural balance in aquatic ecosystems, leading to algal blooms and oxygen depletion. Scientists are trying to find solutions to this problem – which you can read about here.

Discuss how successful your attempt to make yoghurt was. What did you use for the starter culture? What strains of LAB did it contain? How did your homemade yoghurt compare to shop-bought?