Fermentation of Yogurt and the Chemistry Behind it

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

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 yogurt.

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

The science behind yogurt 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 yogurt is made. Instead, the two bacteria used in yogurt 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 yogurt. 

How yogurt 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 up oxygen and produces new compounds, which creates 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: yogurt. These bacterial strains in yogurt actually prevent other bacterial growth, which would typically spoil milk. This is why fermentation is a way of conservation.

Stopping the fermentation process: lower the temperature

Once milk has turned into yogurt and we got the desired flavour and texture, we need to stop the yogurt 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 yogurt in lower temperatures helps retain the yogurt’s texture and flavour by preventing it from becoming more acidic.³

Created by Paulina Cerna Fraga.

Have you ever tried yogurt fermentation or fermenting something else at home? Let us know in the comments below!