How Does Plain Milk Turn Into Yogurt When Nothing Sour Is Added?

What Do Yogurt Makers Actually Add to the Milk?

Only a culture of bacteria. The United States Food and Drug Administration (FDA) writes a legal definition of yogurt, and it describes the process in one sentence: milk ingredients are cultured with bacteria that make lactic acid, namely Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus [1]. The milk is pasteurized (heated to kill unwanted germs) before the culture goes in, so the bacteria you add are the ones in charge [1]. Other countries have their own rules, but these two bacteria are the classic pair.

A starter, then, is not a sour ingredient or a chemical that sets the milk. It is a small group of living bacteria that start fermenting the milk. What exactly do they do there?

Where Does the Sourness in Yogurt Come From?

It comes from the bacteria. These are lactic acid bacteria, and the name says what they do: they produce lactic acid from lactose, which gives yogurt its tangy taste [2]. Milk already contains lactose, so the bacteria find their food ready in the container. Making a sour substance out of food is called fermentation.

This is the main difference from the lemon-juice trick. With lemon juice, you pour in acid from outside, all at once. In yogurt, the acid is made inside the milk, bit by bit, as the bacteria multiply. The researchers who measure this often track the fermentation by measuring how far the milk’s pH falls, an easy signal of how well it is going [2].

Why Does a Little Acid Make Milk Set?

Milk is not a clear solution. Its main proteins are the caseins, which are packed into tiny clusters called casein micelles that stay spread through the liquid [4]. Fresh milk stays smooth because each cluster has negatively charged strands on its outside, and clusters with the same charge push each other away.

Acid supplies protons (hydrogen ions), which are positively charged. According to a computer-simulation study, the extra protons cancel out (“screen”) the negative charge on those strands, so the clusters no longer push each other apart; the same paper puts the point where casein is nearly uncharged, called the isoelectric point, at about pH 4.6 [4]. Once the clusters stop repelling each other, they can stick together, and the sticking clusters form a three-dimensional network that traps the watery part of the milk. That network is the gel you spoon out. The same paper notes that this kind of acid-driven clumping is the main way yogurt is made [4].

The number 4.6 shows up in the law too. The FDA says yogurt must have a pH of 4.6 or lower, measured within 24 hours after the container is filled [1]. (pH is a scale of acidity: the lower the number, the more acidic the liquid.) I would not read that rule as saying exactly what happens to the casein at every moment, since it is a food standard, but it fits the picture: a finished yogurt is acidic enough for casein to clump.

Why Do Yogurt Makers Use Two Kinds of Bacteria?

One kind could make lactic acid on its own, so why a pair? Because each helps the other grow, and together they get the job done faster. Researchers call this protocooperation. The abstract of a doctoral thesis from Wageningen University reports that S. thermophilus supplies formic acid, folic acid and carbon dioxide, while L. bulgaricus breaks milk protein into peptides, which are then split further into amino acids [3]. Peptides and amino acids are the building blocks of protein, and bacteria need them to grow. A 2022 study from the University of Stuttgart says the same thing from the other side: the strong protein-cutting activity of L. bulgaricus makes peptides and amino acids available to S. thermophilus [2].

A simple way to picture it is two cooks sharing a kitchen, where one chops the ingredients the other needs and the second passes back a spice the first cannot make. That is only a picture; the real exchange involves many substances, and the Stuttgart team found that amino acid release in a mixed culture was not simply the sum of what each species released alone [2]. They also found that some amino acids are made and then used up again, which shows how tangled the trade is [2].

The same abstract reports that in mixed culture the bacteria also made more exopolysaccharides, sugar-based chains that raise the thickness of yogurt, and that a model estimated about 50% more bacterial growth per unit of lactose than in single cultures [3]. Both are results from one research project, not a universal rule for every yogurt.

What Does This Note Leave Unanswered?

Quite a lot, and that is a good place to start your own digging. I did not find a source I could read that gives the exact share of lactose that remains in finished yogurt, the temperatures and times that home recipes use, or how much the pairing varies between bacterial strains, so I have not given numbers for them. They are good questions to take to the original papers.

How Can You Look for Yourself?

• Read the label on a tub of plain yogurt. Does it list the names of bacteria, and do they match the two in this note? Some products are heat-treated after culturing, which the FDA definition allows [1], so a label may or may not mention live cultures.
• With an adult, put a little cold milk and a little yogurt from the same fridge in two small cups. Compare how each looks, how it moves when you tilt the cup, and how it smells. Only taste food that has been kept cold and is still within its use-by date.
• Skim the abstract of the Stuttgart paper [2] and find where it says amino acid release in mixed culture was not the sum of the single cultures. Why would that surprise the authors?

Sources

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