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How Yogurt Is Made | Ask The Expert

Ever wondered how yogurt is made? Here at FoodUnfolded, we wanted to know not only how yogurt is made in factories, but also whether we could make it at home. Carolina Moyano, biochemist working in the research and innovation department of one of yogurt’s biggest producers, was the person to ask.

What goes into making yogurt? What does the process look like?

First, you need to source good quality milk. Cows need to be healthy. If they have an infection in the udder, for example, the microorganisms infecting the cow’s udder could end up in the milk. But healthy cows are still not enough. The milk needs to be sourced in clean conditions – in farms, dirt could end up in the milk while it’s being taken from the cow. However, to be really sure that milk is clean of microorganisms, you have to pasteurise it before starting your process. This has to be done even if you are very tidy and source milk in very good and clean conditions in the farm, because there will always be some microorganisms in the mix. It’s completely normal. So when the milk reaches the factory, it gets pasteurised. That’s how you get your best raw material for yogurt. 

Once you have good, pasteurised milk, you add specific microbial organisms (bacteria cultures) to it. In order to create the environment that the microorganisms need to grow at their best, you need to keep your mix at a specific temperature depending on different kinds of bacteria you want to use. The bacteria will then feed off the sugar naturally present in milk – lactose – to get the necessary energy they need to grow. In this process of getting their energy out of sugar, bacteria will make the milk more acidic, turning it into what we call “lactic acid”. The more bacteria grow in your yogurt, the more acidic it will be, because the bacteria will have consumed more sugar in it. Also, milk has proteins called caseins. Caseins normally have a specific arrangement when milk’s pH is neutral. However, the more acidic the pH of the milk becomes, the more caseins aggregate together. That’s what creates the texture in yogurt, and that’s the reason why yogurt is thicker than milk. 

Is acidity in yogurt just dependent on how many bacteria have grown in it, or are there other factors at play?

This is a very interesting question. There are a lot of factors at play that result in a more acidic yogurt. First of all, different bacteria have different capacities to create acidic environments – some of them are more tolerant to acidic environments than others, so they keep growing even when the yogurt is already quite acidic. Other strains are not as tolerant, and after a certain acidity level has been reached, they will stop growing – so they won’t make yogurt more acidic than that. 

Another factor is the length of the fermentation process. In industrial production, you want to create a product which always has more or less the same acidity. In a factory, you can choose to stop the fermentation process by cooling down the mix. You bring the temperature down so that the bacteria are left at ease to continue their metabolism, but they also won’t reproduce at high rates anymore. That’s, of course, harder to do at home. 

A third factor could be the amount of sugar added to the product. The more sugary a product is, the less you will perceive the acidity in your mouth. 

Is the sugar added at the beginning or at the end, after the cooling process? 

The sugar is added at the beginning. After the milk is pasteurised, they split the milk into two ingredients - skimmed milk and cream. The milk is split because you’ll need different amounts of each ingredient, depending on the type of yogurt, for example high-fat or low-fat yogurts. When you create the base for your yogurt, you add the proportions of ingredients that are necessary for the yogurt type – for example the amount of cream will be higher in full-fat yogurts, and it will be lower in low-fat yogurts. After you’ve created your base by mixing skimmed milk and cream, you also add sugar. 

Couldn’t bacteria feed off that sugar though?

You shouldn’t really be worried about whether microorganisms are going to use that added sugar instead of the sugar naturally present in milk, because microorganisms have their preferences. They are quite lazy, we could say, and will always do what is more efficient for their development. So they will only really use the lactose in milk. 

Ok so now we have our base mix and we need to add the bacteria. How do you choose your bacteria cultures?

I’ll start by explaining the past, but I promise there’s a reason. Just a century ago, scientists were starting to find out that thicker milks had bacteria fermenting in them. That meant finally putting a name on what many cultures around the world had been doing for a long time – there is evidence of fermentation since 6000 B.C.E. Back in the day, people would use animal stomachs as bags to preserve milk. They had discovered that something happened when they used stomachs – milk would get a bit more acidic, but it would remain drinkable for longer. But of course, they had no notion of bacteria, rotting, fermentation and so on. Ilya Metchnikoff, a Russian zoologist working at the Pasteur Institute in the 20th Century (best known for his pioneering research in immunology), was one of the first to recognise that these bacteria cultures could help us in some way. He had made some observations in populations from North Eastern Europe, a region that has a long tradition of fermented milk, and had deduced that it brought health benefits. He could be considered the father of what we today know as “probiotics”. 

I talked briefly about the history of yogurt to show the contrast it has to our research today. Today, we know a lot about bacteria cultures. Producers have suppliers who can offer different types of “symbiosis” – a mix of strains – depending on the end result the producer would like to reach. Suppliers are now able to provide the best strains to reach certain levels of acidity and specific kinds of texture. Everything is very much more standardised. 

However, to call your product “yogurt” you can’t put whatever bacteria strain you want. You have to use at least the Lactobacillus bulgaricus and Streptococcus thermophilus. Under most local regulations, you need to have these two strains together to be able to call your product “yogurt”. 

When was yogurt defined as having these two strains?

The definition comes from the Codex Alimentarius, an international standard for different foods. It has a chapter on fermented milk, and most countries accept this definition. 

Editor’s note: The Codex Alimentarius was introduced in 1961 by the FAO. The Codex Alimentarius’ goal is to protect the health of consumers and ensure fair practices in the international food trade. The proposed standard for fermented milk products was introduced in the Codex Alimentarius in 2000.

Do these bacteria strains in yogurt have probiotic effects on our guts?

Actually, these two strains don’t reach our guts alive. They are killed by the acidity in our stomachs, so they don’t really interact with our microbiota. But it’s true that sometimes you can have a yogurt to which you add additional strains that have different properties and can interact with the microbiota – whether that translates into a health benefit is something that can be discussed. 

How is bacteria added to the mix? 

After pasteurising your base mix (cream and skimmed milk) you have to let it cool down before you add the bacteria strains, otherwise you’ll kill them. The ideal scenario is not to add your bacteria until you have reached the optimal temperature for the reproduction of your strains in the mix. Each strain has an optimal temperature for growth: for the Lactobacillus and Streptococcus strains, their temperature range is between 28 and 36°C. 

Once you add the bacteria to the mix, you let them do their thing. The bacteria have the nutrients they need – the lactose in milk – so they will grow and make the base increasingly more acidic. And then you have two options. If you’re at home, you’ll probably just have to let them continue, and at a certain point the microorganisms won’t be able to grow further because the yogurt has reached an acidity that is too high for them to be able to continue growing. But in industry production, you might not want to reach that point – it might be too acidic for consumers. So after 5-10 hours, you cool the mix down. When you cool it down, the microorganisms might continue reproducing just a little bit, but at a very, very slow pace. 

How do these industrial processes take place? 

It’s a very interesting question. I was so surprised when I first saw the factory. It’s a huge space. I think that a batch can be around 10 tonnes. So everything happens at a very big scale. Usually you have two huge tanks outside the factory. These are for the skimmed milk and the cream, your main raw materials. The two big tanks are usually connected to the inside of the factory through pipes. And these pipes generally directly reach the place in which you’ll do your mix – another big tank. 

But you can’t really see what happens inside the tanks and pipes. You can’t see anything because usually tanks and pipes are double-coated to be able to regulate their temperature. The fun part, though, is when you see the product being injected in the yogurt pots. There are robots with different arms. The filling system has five little tubes, and when they have finished filling the pot, a drop falls from each little tube, creating a little flower. It’s really cute. 

How about texture – are different kinds of yogurts made differently?

Drinkable and stirred yogurts are made in the big tank I was describing earlier. The mixing process happens all the time, and that’s why you have this stirred texture, because the yogurt has been stirred throughout the whole process. But for set yogurt, the one that looks like custard and takes exactly the shape of the pack, you actually put it in the packs straight away without steering it. And the packs go to a hot chamber which keeps the temperature that the bacteria need for fermentation. 

What role do people play in the yogurt factory? 

You always have people in production who are in charge of the line and following the process, because you know, anything can happen. Then you have the people from quality, who will generally be checking that the conditions are ok, that you have reached the pH that you need and so on. And then there are the people who handle the filling pots, once they are done.

It seems that there’s a lot of food safety involved, especially when you’re dealing with bacteria and microorganisms. Would you say it’s preferable not to make yogurt at home?

I’m not against making your own yogurt at home. I have! My grandma’s sister would always make it, and I would make it with her. She had a little machine. She would usually put milk and powder milk, plus a small spoon of a yogurt she liked. 

But when you make it at home you will have much more variability. In a factory everything is quite controlled and standardised, and you have the guarantee that you will obtain the same product again and again.

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