Inside Our Food

Fermentation of Yoghurt and the Chemistry Behind it

Fermentation is a natural process that can be used in a number of ways for a wide variety of food products. As a fermented food, yoghurt results from the bacterial transformation of milk. Let's discover more about the fermentation of yoghurt and the microbiology that turns milk into yoghurt.

Microorganisms in fermented foods

Fermented foods are those in which microorganisms have transformed relatively complex substances into simpler ones.1 This simple process can change the food characteristics completely, turning grape juice into wine or milk into yoghurt.

The microorganisms in charge of this transformation are called “ferments”, and they are generally bacteria or yeasts. Fermentation is the natural process that specific microorganisms use in order to obtain energy for growth and development.

The science behind yoghurt fermentation

In nature, the growth of one bacterial strain usually prevents others from growing since they compete for the same nutrients. But that’s not what happens when yoghurt is made. Instead, the two bacteria used in yoghurt production, Lactobacillus delbruekii sp. bulgaricus and Streptococcus thermophilus, help each other grow until they reach a stable balance.2 Together, they transform the lactose naturally present in milk into lactic acid, creating yoghurt. 

How yoghurt bacteria positively interact

S. thermophilus grows better than L. bulgaricus in a neutral, high-oxygen environment like milk, so it starts growing first. It uses oxygen and produces new compounds, creating conditions that allow L. bulgaricus to kick off its metabolism and start to grow.2

Now, it’s the turn of L. bulgaricus to take the lead by breaking some of the proteins in milk down into amino acids. This makes it easier for S. thermophilus to collect the nutrients it needs to keep on growing.2

Getting the right consistency

As they grow, both bacterial strains consume the lactose naturally present in milk and transform it into lactic acid. The more lactose they convert into lactic acid, the more acidic the milk becomes.2 Once the milk sufficiently becomes acidic, caseins (proteins found in milk) begin to clump together, which changes the consistency of the milk to form a thicker substance: yoghurt. These bacterial strains in yoghurt actually prevent other bacterial growth, which would typically spoil milk. This is why fermentation is a way of conserving food.

Stopping the fermentation process: lower the temperature

Once milk has turned into yoghurt and we have the desired flavour and texture, we need to stop the yoghurt fermentation process. The way to do this is to cool it down, as lower temperatures slow the growth of the two bacteria driving the process. Keeping the yoghurt in lower temperatures helps retain the yoghurt’s texture and flavour by preventing it from becoming more acidic.3

Created by Paulina Cerna Fraga.

Related articles

Most viewed

Inside Our Food

Titanium Dioxide in Food | Is It Safe?

Kelly Oakes

You might not have heard of titanium dioxide, but you’ve probably eaten it – it’s…

The Future

How Health Claims Are Regulated

Bridget Benelam

Have you ever worried that health claims that you see on food labels are exaggerated or simply made…

Inside Our Food

E-numbers | Could We Live Without Them?

Claudia Parms

There’s a lot of mystery around E-numbers. You might have found yourself wondering, what exactly…

Inside Our Food

Animal Testing in Food Research | Ask the Expert

Madhura Rao, Dr Alie de Boer

It might come as a surprise, but some foods or ingredients are tested on animals. Moving away from…

Inside Our Food

What Are Rice Noodles and How Are They Made?

Samanta Oon

Rice is one of the most important grains in Asian cuisine. It is so important that in several Asian…

Inside Our Food

How to Get the Most Goodness From Your Garlic

Lottie Bingham

Garlic has been used as medicine for centuries, and the latest research reveals that it is for good…

Earth First

Where Does Jackfruit Come From and How Is It Grown?

Madhura Rao

A large, spiky, green-coloured fruit called ‘jackfruit’ has been making appearances at…

Inside Our Food

How is Sugar Made?

Madhura Rao

What is sugar? If you like chemistry, you might say ‘an organic chemical’. If you enjoy…

Earth First

What Does Jackfruit Taste Like & How Do You Eat It?

Madhura Rao

Jackfruit can be enjoyed as a dessert, in a curry, mixed with barbecue sauce, and in so many other…

Earth First

Nutritional Yeast: How It’s Made

Samanta Oon

Nutritional yeast are golden powdery flakes that add a whiff of nutty, cheesy umami when sprinkled…

Earth First

Sourdough Starter: How it Works

Sedeer el Showk

Baking sourdough bread has become an increasingly popular pastime and source of comfort for many…

Inside Our Food

What’s the Difference Between Probiotics and Prebiotics?

Lydia Melville

Our bodies contain just as many microbial cells as human cells, if not more - and most of these…

Keep updated with the latest news about your food with our newsletter

Follow Us