The Microbiology of Yogurt/Yoghurt

This is the first in 2-part mini-series on making yoghurt at home. For SEO reasons I’ll spell yoghurt in the incorrect American style – “yogurt” – from now on. But for the record, “yoghurt” is the correct spelling. Yogurt may be one of humanities oldest fermented milk-based foods, and has been produced for at least 5,000 years. Yogurt-making in these times served two important purposes. Milk needs to be heated to near-boiling, or even boiled for a period of time, prior to making yoghurt. The boiling not only helps the milk set, but also kills the pathogens commonly found in raw milk. The boiled milk is then fermented, producing an acidic product that preserves the milk. Thus, by turning milk into yogurt, our ancestors were able to convert milk into a safe and storable product. There was an additional advantage as well – although our ancestors may not have been aware of it. Yogurt fermentation produces folate, a critical B vitamin. Folate deficiency can result in blood clotting issues, birth defects, and a range of other health concerns.

goats in a field.
A source of quality milk is essential for a good yogurt fermentation

Today the availability of pasteurized and filtered milk eliminates the risk of infection, and folate-enriched foods are common (as is year-round availability of leafy greens), so these benefits are no longer a major attraction of yogurt. Rather, it is a popular and delicious food. It is also one of the main sources of probioitics in the modern diet. It is also a great example of a synergistic fermentation, where two organisms cooperate to grow and ferment a food.

The Major Players

As I mentioned above, yogurt fermentation is synergistic, with two different bacterial species working together to ferment the milk. So who are they, and what do they do?

Lactobacillus delbrueckii, and most often a specific sub-species (Lactobacillus delbrueckii subsp. bulgaricus), is the main player in yogurt fermentation. In fact, some yogurts are fermented solely with this strain. L. delbrueckii thrives at typical yogurt fermentation temperature (42-46C/107-115F), and in a few hours can consume much of the lactose in the yogurt, converting it to lactic acid. L. delbrueckii is also responsible for producing many of the compounds which give yogurt its unique flavours and aromas.

Streptococcus thermophilus is the other member of the team. On the grand scale of yogurt fermentation, S. thermophilus is the minor player. But it is also critical, as without this microbe most yogurt fermentations will not complete. S. thermophilus produces some of the acidity and flavour compounds present in yogurt, but its main role is to help L. delbrueckii. Interestingly, early yogurt likely lacked S. thermophilus, as genetic evidence indicates that it has evolved over the past 3,000 years to its modern form – possibly evolving from a previously pathogenic species!

So how do these two bacteria synergize? It all comes down to nutrition. L. delbrueckii is able to break down the proteins found in milk much more efficiently than is S. thermophilus, producing free amino acids and short peptides. S. thermophilus needs these to support its own growth, converting them into new proteins and a range of other compounds needed for its growth. In return, S. thermophilus produces large amounts of folic acid and ornithine – compounds that L. delbrueckii cannot produce in large amounts, but which are critical for its ability to make DNA. Thus, by fermenting together, these two species are able to grow in milk far better than either could alone.

BFF’s indeed.

What Happens During a Yogurt Fermentation?

On the surface yogurt fermentation is very simple. Bacteria in the yogurt ferment the lactose in the milk and form lactic acid. The lactic acid decreases the pH of the milk, making it acidic, thereby providing the flavourful “tang” and preservation of the milk. Of course, a lot more goes on during this fermentation:

pathway illustrating the yogurt fermentation pathway
An overly complex diagram of the major products produced during fermentation. Figure prepared in BioRender.

Lactic acid fermentation: Both L. delbrueckii and S. thermophilus use the same pathway to break down lactose. Lactose is imported into the cell, where an enzyme (β-galactosidase) breaks lactose down into its two constituent sugars – glucose and galactose. These are then broken down via the homofermentative (homolactic) pathway into lactic acid, thus providing the cells with energy.

Proteolysis: One of the critical roles L. delbrueckii plays in the cooperative fermentation of yogurt is the production of a protease that is secreted into the milk. This protease breaks down milk proteins into their constituent amino acids. This serves two important roles. 1) It provides these amino acids to S. thermophilus, which has trouble breaking down proteins on its own. 2) This helps to coagulate the milk, making for a thicker yogurt. This same process is why sour beers – and quick sours in particular – have issues with head retention, as the same protease will break down head-retention proteins in wort.

Folate Production: Yogurt tends to have 2 to 4 times the folate of the starting milk, and can be a good source of this important vitamin. Folate is made by S. thermophilus (and in some cases, strains of L. delbrueckii will produce it as well), and is used by these bacteria for synthesizing DNA.

EPS Production: Exopolysaccharide (EPS) is made by both L. delbrueckii and S. thermophilus. This is the same pathway which can lead to “sickness” or “ropiness” in mixed-fermentation beers, although in the case of beer, Pediococcus is usually the culprit. During EPS synthesis, Lactobacillus and Streptococcus convert lactose into unusual sugars that other bacteria have difficulty eating, and then link these sugars together into long chains. This allows them to “store” these sugars for later consumption, but from our perspective, this EPS helps to create the thick and creamy texture of the yogurt.

Flavour Production: Yogurt contains hundreds of chemicals that contribute to its flavour and aroma, many of which are produced during fermentation. The primary flavours are chemicals well-known to brewers: acetaldehyde (green apple), diacetyl (buttery), butyric acid (cheese/pungent; rancid butter when in excess), and ethyl acetate (solvent/fruity). While these are off-flavours in beer, they are important flavours in yogurt, and provide much of its unique character. Lactobacillus will produce some of these in beer wort (especially acetaldehyde and diacetyl, as these are made from sugars), which is why it is important to give yeast enough time to clear up these compounds when making sour and mixed-fermentation beer.

Taking Advantage of Yogurt Fermentation in Beer

milk ready to be turned into yogurt
Milk isn’t the only thing that yogurt cultures can ferment!

This section almost doesn’t need to be written, as the use as yogurt as a starter culture for quick/kettle soured beers is common. While not an optimal species for souring wort, the L. delbrueckii strains used in yogurt are up to the job, and can be used successfully to make great beer. An even better option is to find a brand of Greek or Balkan yogurt that uses Lactobacillus acidophilus instead of (or alongside) L. delbrueckii. L. acidophilus appears to be better at utilizing the sugars found in wort than is L. delbrueckii, providing faster acidification and often a lower finishing pH. My own experiments have found that S. thermophilus does not fare well in wort, appearing to die off in just a few days, so there is no concerns of S. thermophilus contributing unwanted flavours during wort souring.

A concern (or interest) that is occasionally raised when using yogurt as a starter culture for beer production is the presence of probiotic bacteria in the yogurt. Some are concerned that the Bifidobacterium probiotic added to many yogurts may produce unwanted off-flavours, or alternatively, may provide a probiotic benefit to the beer. I’ve covered this topic before in some detail (posts 1, 2), but the short version is that Bifidobacterium won’t ruin your wort, nor will you get a probiotic beer.

All the above said, you don’t want to simply toss some yogurt into your beer, as the fats in the yogurt can ruin your beers head retention and create a haze. Likewise, the bacteria in yogurt may not be in the best shape as they’ve likely been sitting in yogurt, at refrigeration temperature, for several weeks. To use yogurt as a culture for souring beer:

  1. Prepare an ~1 L wort starter with a gravity of ~1.020. A starter containing only DME is sufficient.
  2. Warm the starter to between 37-43C (98-110F) and add a tablespoon of yogurt. Stir in the yogurt, then leave the starter (unstirred) at this temperature for a day.
  3. If you have a pH meter or pH strips, check the starter to make sure it is below pH 4.0.
  4. Pitch into wort at a temperature between 37-43C (98-110F), and hold for 12-48 hours until the desired acidity is reached.
  5. Either cool your wort and pitch yeast, or boil/pasteurize the wort before pitching yeast, and continue as per usual.

Note that yogurt cultures are unlikely to be hop resistant and will probably die if you so much as wave a hop cone above the boil kettle. So this approach should only be used for unhopped worts. The soured wort can be boiled and hopped afterwards, if desired.

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