Making Yoghurt | Health | Biology
I’m sure you know that yoghurt comes from milk But did you know that bacteria are also a key ingredient?
Yes – bacteria! But don’t worry, these are good bacteria and are often called probiotics.
Your gut has thousands of types of bacteria happily living inside, whose job it is to aid digestion. Yoghurt is thought to be a good vehicle to bring more of these good bacteria in.
So in this video, we’re going to learn a bit more about how yoghurt is made.
First off we need milk
Before we start the next steps, all of our equipment needs to be sterilised to kill off any unwanted bacteria and other microorganisms.
Now we’re ready to start.
The milk is heated to somewhere between 85 and 95 degrees celsius for between 15 and 30 minutes. This is a process called pasturisation, and is also done to kill off any bacteria and other microorganisms that are naturally found in the milk.
The milk is then homogenised. This breaks down the fat droplets in milk to make them smaller so that they stay suspended in the yoghurt, rather than sinking and making it all lumpy.
So we’ve sterilised our equipment, then pasturised and homogenised the milk.
Now the mixture is cooled to 40 to 45 degree celsius. Now the mixture is cooled to 40 to 45 degrees Celsius and we add our special yoghurt-making bacteria. The mixture needs to be cooled first so that the bacteria aren’t killed by the high temperature.
You might be wondering why on earth are we adding bacteria when we’ve gone to all the effort so far of killing all the bacteria??!!
We’re adding lactic acid bacteria, which are also used in cheese manufacturing. The two species used to make yoghurt are lactobacilli and streptococci. But don’t worry, you don’t need to learn these names, just remember that lactic acid bacteria are added.
These lactic acid bacteria are known as the starter culture.
The bacteria ferment the milk and change it into yoghurt. Lactose is the main sugar in milk, and during fermentation the bacteria turn the lactose into lactic acid.
The bacteria also start to digest the milk proteins.
Fermentation takes many hours and during this time, the mixture is kept at the optimum temperature of 40 to 45 degrees celsius so that the bacteria can work fastest.
As fermentation produces lactic acid, a type of acid, the pH of the milk drops to about pH 4.4. This is why natural yoghurt has a sharp, tangy taste.
As the pH levels drop, the milk solidifies to become raw yoghurt. Once the pH drops too low, the lactic-acid bacteria that make the yoghurt also stop working so the yoghurt doesn’t become too solid.
The pH drop also stops any bad bacteria growing.
The solidification happens because the proteins begin to coagulate
Coagulation is when something which is liquid turns into a more solid state.
So we’ve sterilised our equipment, then pasturised and homogenised the milk. Added lactic acid bacteria so fermentation happens, and then coagulation.
The mixture is now stirred and cooled to 5 degrees celsius. And we’ve made natural, unflavoured yoghurt!
At this stage, we can add flavourings and fruit. Now it’s ready to be packaged and sold. Delicious!
Can you match the words to the definitions of the different stages of the yoghurt making process? Pause the video and give it a go.
Now what about completing the missing words? Again, pause the video and work them out.
So there we have the process of yoghurt making, from milk to a tasty strawberry flavoured yoghurt