Fermented milk products: cultures and fermentation
Lactic acid bacteria behind yogurt, skyr, cultured buttermilk and traditional buttermilk
Fermented milk products are milk products where milk is fermented with lactic acid bacteria that break down lactose (milk sugar) and convert it to lactic acid. This gives the characteristic sour taste and thicker consistency that characterise products such as yoghurt, sour milk, buttermilk and skyr. For you as a dairy technician, culture selection is one of the most important professional decisions in the production of these goods.
§Mesophilic and thermophilic cultures
Lactic acid bacteria are broadly divided into two groups based on the temperature at which they thrive best. Mesophilic bacteria convert milk into products such as sour milk, cultured buttermilk, and traditional buttermilk at lower temperatures, while thermophilic bacteria are behind yogurt, which is fermented at a higher temperature. The choice of culture is therefore not just a matter of taste — it is a biological prerequisite for what product you end up with.
| Product | Crop type | Characteristic |
|---|---|---|
| Yoghurt | Thermophilic bacteria | Fermented at higher temperature, thick and sour |
| Sour milk / sour cream / ylette | Mesophilic bacteria | Fermented at lower temperature, milder acidity |
| Buttermilk | Mesophilic bacteria | By-product from soured cream/butter churning |
| Skyr | Thermophile/mesophile culture blend | Si-process gives high protein and low fat content |
§How the contract affects what the apprentice can be asked to do
The actual process happens by leading milk or cream to a fermentation tank where a lactic acid culture is added. Depending on the product the mixture then stands for a set number of hours at a specific temperature until the desired acidity and consistency is reached. The taste depends especially on two things: which lactic acid bacteria is used and how fat the milk or cream is.
- 01Choose culture based on product requirement — thermophilic for yoghurt, mesophilic for sour milk types
- 02Keep temperature stable throughout fermentation
- 03Monitor pH continuously to hit the right acidity level
- 04Stop fermentation at the right time — too long gives too high acidity
§Skyr — a product made with straining technique
Skyr is a soured milk product, originally from Iceland but also produced at Danish dairies, which is distinguished by being strained so a large part of the whey is removed after fermentation. This gives a product with high protein content and low fat content compared to regular yogurt, and the process requires extra control of yield and consistency.
§More than taste: durability and aroma
Lactic acid bacteria contribute not only to taste and consistency — they also extend product shelf life and create aroma. This makes the fermentation step a central quality factor in the entire production, and one reason why dairies work closely with culture suppliers to find the right bacterial strains for each product.