Just like a sourdough starter, yogurt is a self supplying food. Once you’ve got yogurt, all you need is extra milk and some patience. With those you continue making new yogurt from your yogurt! Just as with a sourdough starter, the bacteria will do all the work for you. They ferment the milk and transform it into a thick, creamy, yogurt.
We’ll dive deeper into the exact steps required to make yogurt, both in a factory, and at home to help you understand exactly how yogurt is made.
What is yogurt?
Yogurt is acidified milk, it is milk that has turned sour through lactic acid fermentation, but in a very controlled manner, through adding bacteria. The only necessary ingredients for making yogurt are milk and those bacteria, the bacteria do all the work for you. That said, there are a lot of different styles of yogurt, through variations in the processing steps, type of milk or type of bacteria.
In a lot of countries (including the US and the EU) the term yogurt is protected, meaning that if a manufacturer wants to call their process a yogurt it has to meet a set of criteria. These criteria involves aspect such as protein content, acidity (pH-value) and the presence of live bacteria.
Which bacteria are used to make yogurt?
In order to make yogurt, bacteria should be chosen that can transform lactose (the sugar in milk) into lactic acid. This process is called lactic acid fermentation and is done by lactic acid bacteria. The two most commonly used species for making yogurt are cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These two micro bacteria help one another, which is why they are commonly used together, by producing molecules that the other bacteria needs for proliferation. They are synergistic.
Depending on the country, you will also find yogurts made with Streptococcus thermophilus and any Lactobacillus species. If other species are used it doesn’t tend to become a yogurt, but a fermented milk or other ‘fermented milk product’.
Every yogurt has their own distinct set of bacteria which are used for the production process, these are called cultures. For manufacturers it is very important to ensure their cultures stay alive (and can thus do their job) and are made of the right micro organisms.
At home this isn’t easy, but as long as you have a yogurt with those cultures inside it, you can use that yogurt as a source of micro organisms! It’s a bit like making and maintaining a sourdough starter, but here the yogurt itself is what maintains the bacteria and keeps them alive.
What happens when making yogurt?
Once the bacteria have been added to the milk they start producing lactic acid, due to lactic acid fermentation. The steady production of lactic acid will slowly bring down the pH-value of the milk, making it more sour. This results in the proteins, the whey and caseins naturally present in milk, to form a gel structure. As opposed to cheese making, the casein micelles don’t fully fall apart but form a tight network with the whey proteins. This network can hold on to the moisture in the milk.
Apart from the acidification and thickening of the milk flavours are developed along the way as well. This is again due to the micro organisms!
In order for these processes to occur the way we want them to, you have to prepare your milk, getting it ready for yogurt production.
Preparing milk for yogurt production
Most yogurt production factories will prepare the milk, before it can be transformed in yogurt. The preparation is done to improve the quality of the yogurt (better consistency) but also to improve the safety of the yogurt. You will notice that the steps are quite similar as those used with production of dairy ice cream.
In a factory this starts by standardizing the milk. During this process the fat (cream) is separated from the rest of the milk and then added back in a controlled manner. This is done since seasonal and other variations will result in differences in composition of the milk. Without standardization the fat content would change between batches, making it harder to control further processing. It would also make semi-skimmed milk for instance, impossible to make.
In some cases additional ingredients are also added to the milk at this point, for example, additional dried milk or milk proteins.
After this standardization the milk is pasteurized. Pasteurization kills most micro organisms in milk. When the milk will be used for making yogurt, it is pasteurized more intensely in the factory than regular milk would. The reason for this is that pasteurization also breaks down, denatures, whey proteins. Whey proteins are one of the two types of proteins in milk (the other being caseins). Denaturation of proteins results in a loss of their 3D structure. As a result they can thicken a liquid more easily through interactions between whey proteins themselves, but also through interactions with the casein proteins.
The last step before actual yogurt production begins is homogenization. This process breaks down the fat droplets in milk and makes them all the same size (it’s used during ice cream production as well). If manufacturers wouldn’t do this, the larger fat droplets will have a tendency to float to the top of the liquid. Fat has a lower density than water and the larger particle size makes it easier for the two phases to split (similar mechanism as sedimentation).
By making all fat droplets the same size, the final yogurt will be more stable and consistent, all important for factory run processes.
Preparing milk for making yogurt at home
You can also make yogurt at home (see the end of this post for a recipe). In most cases you, you will start with store bought milk. This milk has already been standardized (see the fat content on the label?), pasteurized and homogenized. Nevertheless, most recipes will call for pasteurizing your milk again. The main reason for pasteurizing the milk again is that, as we mentioned above, the factory pasteurization process for milk is less intense than that for milk meant for yogurt. In other words, there are still proteins to be broken down to help with the curd formation.
Also, pasteurization doesn’t kill all micro organisms. As a result, your store bought milk will have some micro organisms in it again. However, you want to make it as easy as possible for your yogurt micro organisms to thrive. The less competition, the better, hence the extra pasteurization.
Making yogurt – Adding the bacteria
Once the milk is ready to go, it is time to actually make the yogurt. This is done by first bringing the temperature of the milk to about 40°C. This is the temperature your lactic acid bacteria like to live in and it stimulates their growth. Once the milk is the desired temperature (to hot and the bacteria will die, too cold and the process will take too long), the bacteria themselves are added. In a factory the bacteria are added as a culture, but at home, you can add bacteria by simply adding some yogurt you still have. The yogurt will still contain live bacteria, which can continue to grow in the new yogurt.
The milk + bacteria is held at the fermentation temperature for several hours. During this time the bacteria produce lactic acid through lactic acid fermentation as well as other components such as flavours. For some yogurts the fermentation takes place in the final package, whereas in others it takes place in a tank.
It depends on the factory and style of yogurt how long this process will take exactly. Manufacturers can measure the pH-level to determine whether their fermentation process has been completed. Once the fermentation process has been completed the yogurt should be cooled down to ensure the bacteria completely stop the fermentation and to prevent spoilage of the yogurt. Thanks to the low pH value though (around 4,4), yogurt won’t spoil as easily as milk, essentially it’s a way to preserve milk.
Strained yogurt for a thicker yogurt
Yogurt made from milk itself, without any added powders, contains a lot of moisture. As a result, these yogurts aren’t as thick. Therefore, manufacturers may strain these yogurts are fermentation. During straining the excess moisture will drain away, resulting in a thicker yogurt. It does make yogurt more expensive since you need more milk to make the same amount of yogurt.
Making yogurt at home
To summarize this whole process, it is great to actually make yogurt at home and see how your yogurt turns out. You can experiment with different flavours, styles, etc. When you make yogurt it is very helpful if you can properly control the temperature during fermentation, to keep it at that approx. 40°C. There are a lot of different ways to do this, but in this recipe we use an Instantpot that can you set at yogurt making temperature automatically.
A note on InstantPot sealing rings
For several months we used the InstantPot for both yogurt and more spicy dishes and noticed that the yogurt would always faintly taste like what we made in it before. No matter how good we would clean the inner pan, you would taste it. After smelling the white sealing ring of our lid we knew what the problem was: the ring strongly smelled of savoury food. for other savoury food with plenty of spices this wasn’t an issue, the flavour of the new food would be a lot stronger than that what was in the ring. However, for yogurt it is.
Therefore, buy a second InstantPot sealing ring. The InstantPot comes with one standard white/translucent sealing ring in the inside of your InstantPot lid. It is super easy to remove. We bought a set of a red and blue ring and use the blue one solely for yogurt making and the like. The red one will serve as the replacement of the white one once it is worn out.
- 1 liter of milk (we used 2% fat, more fat will give a creamier, thicker yogurt)*
- generous spoonful of yogurt that contain live bacteria (if you've made your yogurt at home before, it will contain live bacteria, for store bought, check the label)
- Take the InstantPot and pour the milk in the inner pan. Close the InstantPot. You will first have to pasteurize your milk again. Even if you bought pasteurized milk this step is useful, it will denature some of the proteins which will help thicken the yogurt.
- Click the button 'yogurt' until it shows the option 'more' and says 'boil'.
- Let the InstantPot do its thing, it will now pasteurize the milk.** Once the program is finished (it will start beeping), take the bowl out of the InstantPot and leave to cool down to 40-45C. You can speed this up by placing it in an ice bath or your fridge.
- Place the pot back in the InstantPot (make sure it is clean on the outside!). Close the lid again and now select the 'normal' program within the yogurt menu.
- A timer will pop up, select the time you'd prefer. We use 11-13h, the longer you ferment, the more tangy the yogurt. We've also tried 10h which gave a perfectly fine yogurt, we just prefer a slightly thicker and more sour yogurt.
- Leave it one until the program is finished.
- Take the bowl from the InstantPot, your milk should have thickened considerably, become sour and look and taste like yogurt. It is likely that you will see some water floating in between pieces of yogurt. This is normal, just mix the yogurt through and you will see that it will all thicken.
- Pour the yogurt in a suitable bowl with a lid and leave to cool in the fridge. It will thicken further in the fridge.
- Once it's been cooled you can choose to strain it to thicken in any further by using a cheese cloth. We find that our yogurt is thick enough for our purposes so don't do this.
* 1 liter will work in most sizes Instantpot, you can easily multiply this recipe. We tend to make 2-3 liter of yogurt at a time. If you use more milk, make sure to increase the amount of yogurt accordingly.
** If you do not have an Instantpot you can do this step by heating your milk in a pan on the stove until it has reached at least 80C (if you're using raw milk, use your regular pasteurization method to ensure the milk is free of harmful bacteria). You will need to stir once in a while to prevent it from sticking to the bottom. Then leave to cool down.
In Western cultures you will mostly find yogurt during breakfast & dessert, with some fruits and oats for instance. Yogurt though is found in a lot of different cuisines and is also great with savoury food. In general it works well with a lot of Indian curries (e.g. butter chicken) for instance. Also, yogurt, thanks to its acidity, is great for marinating meat, softening it along the way. Last but not least, yogurt is used in various flatbreads, such as pita, to create a softer texture.
FAO, Codex Alimentarius on Milk and milk products, 2011, 2nd edition, link
University of Guelph, The dairy science & technology eBook, maintained by Professor H. Douglas Goff, link
Krämer, J. Lebensmittelmikrobiologie, 2007, 5th edition
Poltronieri, P., Microbiology in Dairy Processing: Challenges and Opportunities, 2017, chapters: 15.3.5, link
Foods under the microscope, Yogurt, link ; this website has great microscope photos of a lot of foods, including yogurt