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 dairy 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. 

Bacteria in your food - it sounds bad, right? In fact, many bacteria bring us health benefits. These "good bacteria" are known as probiotics and include the two types of bacteria found in yoghurt that I just mentioned. But not all yoghurts contain probiotics. For the best chance of a yoghurt bursting with friendly bacteria, look out for "active cultures" or "live cultures" on the label and avoid heat treated yoghurts - high temperatures can kill off good bacteria.

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 down some of the proteins in dairy milk into amino acids. This makes it easier for S. thermophilus to collect the nutrients it needs to keep on growing.²

The carefully blended bacteria that will be added to milk to make yoghurt is known as a "starter."

Getting the right consistency

The yoghurt-making bacteria need to be kept at the right temperature to thrive and do their work. The period when yoghurt is kept warm to allow probiotics to proliferate is called "incubation". 

As they grow over time, both bacterial strains consume the lactose naturally present in milk and transform it into lactic acid - giving that classic tangy flavour. 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 finished thickening into yoghurt and we have the desired creamy 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.