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Here you can read about: Yogurt

learn about making homemade yogurts in the sections below.


What is Yogurt?

Making yogurts is an old tradition believed to originate from Central Asia and that is why you will sometimes hear it spoken of as “the Bulgarian’s drink”, even today. It was not until the early part of the 20th century the tradition won fame across the world as Russian biologist Ilyich Mekhnikov praised yogurt as the means to a long healthy life.

Yogurts and other fermented dairy products is a result of lactic acid bacteria fermenting the milk by converting lactose (milk sugars) in the milk into lactic acid and hence, acidifying the milk. As these beneficial bacteria feed on lactose, lactic acid levels raise coherently and the drop in pH make it increasingly hard for the proteins to stay in solution. As a result, the proteins begin to cluster together, form bonds and tangle up in masses which is what appears as a thickening of the milk.

Storebought milk has almost always been pasteurized (heat processed) with the purpose of killing off any harmful bacteria but the process kills off beneficial lactic acid bacteria too, and therefore they must be re-introduced to the milk to be able to make yogurt from it. Applying a culture with lactic acid bacteria will promote correct fermentation and enhance the development of palatable flavors.

The class of lactic acid bacteria that are used to make yogurts from are thermophilic, meaning they are heat-loving species that thrives optimally betweeen 30°C and 45°C. The species that are most commonly used are Lactobacillus bulgaricus, Streptococcus thermophillus, Lactobacillus lactis and Lactobacillus helveticus.

The difference between making yogurt and soured milk is only the mix of lactic acid bacteria we use to ferment the milk. Lactic acid bacteria for soured milk must be mesophilic, which simmply refers to a class of bacteria that prefer lower temperatures and thrive best around room temperature. Soured milk can therefore be made simply by leaving the milk to ferment somewhere in the house with a steady room temperature. Yogurt can be made in the exact same way if you can find a place in you house that provides a constant 42°C which is the optimum fermentation temperature for yogurts.

If your house does not provide the optimal temperature to make yogurts anywhere, you can use an oven or an electrical yogurt maker to ensures a constant temperature thoughout the fermentation process. Or simply, do it the old fashoin way by heating the milk to 42°C, add a starter culture and wrap the pot in a warm thick blanket. Place the whole thing somewhere where it will will keep warm for up to 12 hours - a thermobox is often a good choice.

Soured milk

What is Soured milk?

When milk is left at room temperature, natural occurring bacteria begin to develop and grow rapidly in a process of converting milk sugars (lactose) into lactic acid. Buildup of acid is what causes the well-known sourness in milk after a little while.

- The bacteria behind this process are lactic acid bacteria –

When lactic acid builds, acidity increases and pH levels drop making it increasingly hard for the proteins to stay in solution. This will cause the milk to coagulate and thicken and the outcome is what we know as soured milk.

Soured milk is usually made from milk only – if you use cream instead of milk, the outcome will be a crème fraiche (soured cream).

Fermentation happens the same whether you use whole milk, skimmed or semi skimmed milk – all kinds ferment equally well, as they contain equal amounts of lactose and proteins. The only thing that differentiate them from one another is the fat content and since fats does not play any active role in the fermentation process - choose a milk with the fat content you prefer.

Lactic acid bacteria are everywhere - in the air, on our skin, in our gut, on fruits and in vegetables – we cannot avoid them.

If we leave milk uncovered, lactic acid bacteria from the air will soon make their way in to the milk and begin to develop, converting lactose into lactic acid.

There is a vast variety of lactic acid bacteria species; some contributes with palatable flavors we like, others, not so much. They live alongside other species of bacteria and fungi that occur almost everywhere – all competing for their share of whatever milk you just left out for them to colonize. Such random and wild fermentation could potentially be making it less of a palatable experience.

Even the milking process induce bacteria to the milk and this is the reason why milk is pasteurized, to avoid growth of any undesirable bacteria. Since milk pasteurization also kills off naturally occurring good lactic acid bacteria, we must re-culture the milk with these bacteria prior to fermentation. Culturing milk with particular species of bacteria ensures correct fermentation and palatable flavors. When we add lactic acid bacteria cultures, the bacteria species in them will rapidly outnumber any undesirable bacteria that may have found their way into the milk. Rapid rising levels of acidity from these bacteria will naturally inhibit the growth of competing bacteria not liking acidic conditions. This is the reason why soured milk can often be stored for quite some time and keep fresh.

A batch of freshly made soured milk with the right combination lactic acid bacteria species can be perfectly all right to use as a culture for the next batch of soured milk. Simply take out a small portion, mix it in with the milk for the next batch - using approximately ½ cup of freshly made soured milk to 1 liter of milk. Leave the milk at room temperature for about 24 hours and another 12 hours in the refrigerator. Then it will be ready to enjoy! This process can be used repeatedly, over and over again.

However, be aware that bacteria from the surrounding environment may also find its way to your preparation somewhere along the process. By repeating the process many times, you may risk other types of lactic acid bacteria takes the upper hand and influence the flavoring in an undesirable way. A balanced combination of lactic acid bacteria is often crucial for the flavor and for us like it. If the balance between them is significantly distorted, flavors can be too and you must start over by making a new batch of soured milk from scratch, using a balanced culture.

Buttermilk contains the same types of lactic acid bacteria as soured milk, therefore small amounts of that can be used to culture a batch of soured milk.

Alternatively, you can use freeze-dried starter cultures for making soured dairy products. These have been readily made with correct balance of bacteria that will bring out nice palatable flavors in your products. Take a pinch of freeze-dried culture, dissolve it in a small amount of water or milk, stir well and add that in to your large portion of milk. Then follow the procedure as described above (leave for 24 hours at room temperature, covered, and then another 12 hours in the refrigerator).

Lactic acid bacteria in soured milk are mesophilic, a group of bacteria that thrives around room at temperatures between 15°C and 30°C. The most commonly used species are; Lactocossus lactis, Lactocossus diacetylactis and Lactocossus cremoris. We stock such a mix in our web shop.

Soured cream

What is Soured cream / Creme fraiche?

Soured cream is simply fermented cream.

In the very same way milk can be fermented to soured milk or yogurt, cream can be fermented to soured cream.

Soured cream can be prepared with various fat contents simply by diluting cream with milk before you start the fermentation process.

Fat content in cream varies from country to country. He following applies to Denmark:

  • Using full fat cream with no adding of milk, will provide you with a soured cream with a fat content like what was in the cream e.g. 38% fat.

  • Using 1 part of milk to 1 part of cream provides you with a soured cream with app. 20% fat.

  • Using 3 parts of milk to 1 part of cream provides you with a soured cream with app. 10% fat.

For more detailed information on milk fermentation, please read the information on Soured milk and Yogurt.Øverst på formularen


Cultures for Soured Milk, Yogurts, Kefirs and Cheeses

The unique flavor in fermented produce and cheese is determined by a complex inter play between different enzymes, bacteria, fungus and milk content of sugars (lactose), proteins and fats.

As enzymes, bacteria and fungus break down the milk ingredients, substances such as lactic acid, fatty acids, amino acids, aldehyde and ketones appear as a byproduct of this breakdown. This is what determines the complexity of flavors in cheeses.  

Below you will find a list of cultures commonly used for Soured Milk, Yogurts, Kefirs and Cheese.

Lactic acid bacteria

Leaving milk at room temperature will cause its naturally occurring content of lactic acid bacteria to begin break down milk sugars (lactose) into lactate, and this is what causes a soured flavor. In this process of breaking down milk sugars, acidity drop, and the milk proteins (Caseins) can no longer hold apart and will join together, forming that characteristic thickening of the milk. This is what we know as soured milk.

These lactic acid bacteria make their way in to the milk during the milk process, living on the skins of the cows. Even with the strictest of precautions, this contamination cannot be prevented.

Lactobacillus bulgaricus

There are many different types of lactic acid bacteria, each contributing with their own unique flavor as they break down and digest lactose. The two most important groups of bacteria to know are the mesophilic and the thermophiles. The mesophilic bacteria thrive at temperature levels between 15°C - 30°C and is significant to soured milk and buttermilk. The thermophilic bacteria thrive at higher temperature between 30°C - 45°C and is significant to especially yogurts.

Other lactic acid bacteria will break down lactic acid to Ketones and aldehydes, substances that play an important part in the complex flavoring of cheeses.

Oenicoccus oeni

When milk is pasteurized, the majority of lactic acid bacteria die off, keeping the milk fresh for longer. For this reason, cultures must always be applied when using pasteurized milk for making soured milk, yogurts and cheese.

The reason why pasteurized milk goes sour anyway when left out for a few days, is due to the small amount of lactic acid bacteria that was not killed off during pasteurization, besides exposure various other bacteria from the surrounding environment and air. When left exposed to air other bacteria will soon make their way in to the milk and speed up the break down the lactose to lactic acid. Lactic acid bacteria thrive in anaerobic conditions, so even if kept in air tight container, milk will still turn sour however it does take longer when there is no exposure to bacteria from the surrounding environment.

A handful of bacterial species even have the ability to transform oxygen into hydrogen peroxide and ethanol (alcohol). A mechanism that is quite useful to us, when preserving and fermenting vegetables with lactic acid bacteria, as this reduction of oxygen will provide an aerobic atmosphere that will limit the growth of bad bacteria. The acidity created by lactic acid bacteria will also limit the growth of bad bacteria.

Cheese mould

Different types of yeast cultures are responsible for the development of the characteristic flavor, known to moulded cheeses such as brie or camembert (white mould) or Danablu, Roquefort, Gorgonzola and Stilton (blue mould). As the yeast feed of the milk, it produces various substances belonging to the penicillin group. These are fungi, not surprisingly, often referred to as penicillin fungi.

Penicilium candidum

Eye Formation Culture

The round holes famous to cheeses such as Emmental, develops due to bacteria, producing high amounts of carbon dioxide (CO2) in the process of digesting nutrients in the milk. They are naturally occurring everywhere.

Proprionibacteria shermanii

Yeast Cultures

Yeast cultures are mainly used for making Kefirs but may also added to cheeses, by spraying them on the outside of the cheese, causing unique flavoring to build. Like Red mould cultures, yeasts cultures can also contribute to the development of a stronger cheese crust with greater resilience to bad bacteria.

Saccharomyces cerevisae

Red mould cultures

Red mould cultures can be sprayed on to the outside of cheeses in order to bring out unique flavors. Like yeast cultures, they contribute to the development of a stronger cheese crust with greater resilience to bad bacteria.

Brevibacterium linens

List of cultures for making Soured Milk, Yogurts, Kefir and Cheese

Lactic acid bacteria growing at low temperature (Mesophile)







Lactic acid bacteria growing at high temperature (Termophile)




bulgaricus / delbrueckii













Flavour developing lactic acid bacteria


mesenteroides subsp. cremoris


mesenteroides subsp. dextranicum





Eye formation culture


freudenreichii subsp. shermanii

White moulds


candidum (Brie/Camembert)


candidum (Camembert)

Blue moulds




glaucum (Gorgonzola)

Yeast culture















Red mould culture



Yogurt chemistry

Fermented milk - Chemistry

Leaving milk or cream at room temperature will cause naturally occurring bacteria within, to grow and develop as they digest and convert milk sugars (lactose) into lactic acid. Hence, lactic acid builds up and cause acidification as pH levels drops.

- These bacteria are known as lactic acid bacteria –

When lactic acid bacteria thrive, and increase in numbers, acidification follows which make it increasingly difficult for the proteins to stay in solution and cause them to tangle up and thicken the milk. The result is what we know to be soured milk.

The same process transforms cream into soured cream. Yogurt develops in the exact same way, however differs by the type of bacterial species that drives the process and produce lactic acid.

A more technical explanation on what happens when we create soured milk, soured cream and yogurt, follows below:

The majority of the proteins found in milk are round shaped formations known as micelles.

Due to their structure, the surface has negative polarity that pushes every little micelle away from other micelles. Continuously pushing apart, ensuring the longest possible distance between, these proteins are distributed evenly throughout the milk.

Milk also contains round shaped formations with fat. These fat globules has neutral polarity, meaning they will not be drawn towards anything, nor pushed away. As fat has a density lower than water, the fat globules will slowly float to the top. This is what forms the creamy layer on the surface of the milk.

Lactic acid on the contrary has positive polarity and when it develops during fermentation, it balances out some of the negative polarity between the protein micelles.

As levels of acidity rises, positive polarity rises and micelles are able to move closer towards each other. The longer milk is left to ferment the more acidic, hence micelles come closer.

When the protein micelles come up close, bonds sticking them together develops and prevent them from moving less freely.

This explains the characteristic thickening of milk during fermentation.

Fermented milk is made either by allowing naturally occurring bacteria in the milk to develop and produce lactic acid or by adding an acid to the milk, such as citric acid. To accomplish palatable flavors, specific lactic acid bacteria cultures are often added as various bacteria species do such more than produce lactic acid. They produce a variety of substances that contributes with significant and aromatic flavorings to the end products.

What differentiate soured milk and yogurts from one another is the species of bacteria responsible for the production of lactic acid. In soured milk these bacteria are mesophilic, meaning they thrive ideally around room temperature, approximately 25°C, whilst lactic acid bacteria in yogurts are thermophilic, meaning these species love heat and thrive at their optimum around 42°C.

Soured cream is made with mesophilic bacteria like soured milk or thermophilic bacteria like yogurt and fat contents in soured cream are not influenced by the presence of bacteria. It always reflects fat contents prior to fermentation and depends on how much milk was added to the cream.

Mesophilic cultures are typically a mix of the following species:


Lactococcus lactis

Lactococcus cremoris

Lactococcus diacetylactis

Leuconostoc mesenteroides


Thermophilic cultures are typically a mix of the following species:

Lactobacillus bulgaricus

(Normal yogurt)

Streptococcus thermophillus

Bifidobacterium lactis

(Probiotic yogurt)

Lactobacillus acidophilus

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