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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.


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