Some history:

Fermented milks have been around for thousands of years and can be considered the "ancestor" of yoghurt. They are also easier to store than milk. Here are some examples : Ribot milk, a speciality enjoyed by the Gauls, Kefir, very popular in the Middle East and Koumys  in central Asia, made from mare, ass or camel milk.

The history of "yoghurt" goes back to the naturalist, Pliny the Elder, who noticed that the Barbarians were able to "thicken milk into a substance with a pleasant acidity". He describes yoghurt as a "divine essence" and as a remedy for several disorders.
In France, yoghurt made its real appearance in the reign of Francis I who was said to have been cured of an intestinal disorder by eating ewe’s milk yoghurt, prepared by a Turkish doctor.  

At the end of the nineteenth century, the 1908 Nobel prize-winner, Metchnikoff (1845-1916) studied the link attributing the longevity of Caucasian and Balkan mountain dwellers with their yoghurt intake. He in fact discovered the positive effects of yoghurts in the intestinal problems of newborn babies.

Yoghurt arrived on our tables in the twentieth century. In 1917, Isaac Carasso started to make yoghurt in Barcelona using industrial processes which were to act as the foundations of the development of yoghurt in Western Europe. In 1929, his son launched yoghurt in France.

What is yoghurt?

Yoghurt is the word used to describe a coagulated dairy product obtained by lactic fermentation through the action of 2 exclusive bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus, on milk (pasteurised, concentrated, partially skimmed enriched with dry extract).

Bacteria, in the end product, should be alive and present in large quantities:
"French regulation fixes the minimum quantity at 10 million bacteria per gram " (Decree n°88-1203, 30/12/98, art. 2 et al).   

The content of this definition was adopted in the International Norm of the "milk" CODEX Alimentarius commission. Furthermore, this commission, meeting on 2nd July 2003 in Rome for its 26th session, took a decision concerning an old debate on the definition of yoghurts and fermented milks: some countries, including France, would like to restrict this name to dairy products containing living flora, therefore non-sterilised. The health benefits of yoghurt and fermented milks are mainly linked to these living bacteria. The Codex Alimentarius has therefore adopted a norm specifying the technical characteristics essential  for yoghurt and fermented milk production and the labelling rules. It has been recognised that the identity of specific fermented milks such as yoghurt relies on the presence of living active ferments in the end product whose health effects are scientifically recognized.

The UBD (Use-By Date) on yoghurts is a maximum 30 days starting from the production date, as its characteristic bacteria should remain alive and in sufficient quantity.

How is it made?

Milk has a standard rate of fatty matter for the end product and may be enriched with dry dairy extract (milk powder) to improve its consistency. It is homogenized and treated at 90 °C for a few minutes (destruction of pathogenic germs) then cooled to reach the optimal fermentation temperature (around 45 °C). It is then inoculated with two specific bacteria: Lactobacillus bulgaricus and Streptococcus thermophilus . The first provides its acidity whereas the second develops its flavours.

From this point, different production processes come into play, depending on the texture required: 

"Set" yoghurt:

Once inoculated with the starter culture, it is packaged into cartons that are then steamed for three hours. The bacteria reproduce and attack lactose which is partially transformed into lactic acid, thus changing the protein structure and forming a "gel". Yoghurts are then placed in a ventilated cold chamber or cooling tunnel and stored at 2-4 °C.

"Stirred" yoghurt:

It is not fermented in its carton but in bulk, in vats. The curd is stirred then cooled before being poured into cartons, which are stored in a refrigerated environment.

"Drinking" yoghurt:

Once stirred, it is beaten in the vats before bottling.

Depending on the type of yoghurt, ingredients (pieces of fruit, honey, jam, flavouring, etc) are added before or after fermentation.

There is thus a wide range of products known as yoghurt on the market: "sweetened yoghurt", "flavoured yoghurt", "fruit yoghurt"...

What is fermented milk?

Whereas yoghurts are milk exclusively fermented with Lactobacillus bulgaricus and Streptococcus thermophilus, "fermented milk" concerns milk where other bacteria are used.
The most frequently used bacteria are currently Bifidobacterium longum, Bifidobacterium lactis and Lactobacillus acidophilus.
As in yoghurt, the lactic bacteria in fermented milk remain alive.

How are they made?

Their production processes are generally identical to those of yoghurt. Only the ferments and fermentation factors(temperature, length, quantity of starter culture) vary.

> source : www.syndifrais.fr