Zoom in on microorganisms


Streptococcus thermophilusZoomZoom
© Dennis Kunkel Microscopy, Inc.
Lactobacillus acidophilusZoomZoom
© Miloslav Kalab, Agriculture and Agri-Food Canada, Ottawa
Yogurt makingMovie ClipMovie Clip
© Production Cinémanima inc. and Armand-Frappier Museum

If It Wasn’t For Bacteria…We Wouldn’t Have Any Yogurt!

The transformation of milk into yogurt depends on the action of two bacteria: Lactobacillus bulgaricus and Streptococcus thermophilus. Both these bacteria consume lactose (milk sugar) and transform it into lactic acid, which causes the milk to coagulate and gives yogurt both its solid texture and its characteristic sour taste. The acidity also helps preserve yogurt, as it prevents certain harmful bacteria from multiplying.

When Lactobacillus bulgaricus and Streptococcus thermophilus are both active, it takes less time to coagulate the milk. One of these species releases substances that favour the growth of the other. Once a certain level of acidity is reached, bacterial growth slows down, and if the temperature is low enough, stops. Yogurt thus contains a certain number of live bacteria... in fact, about five hundred thousand per gram!

Yogurt Production, Step By Step

Preparation of the milk : pasteurisation, addition of sugar (if necessary), and topping up of the fat content.

Incubation : addition of fermenting agents (Lactobacillus bulgaricus and Streptococcus thermophilus) to the milk in incubation vessels. The bacteria are allowed to act for 14 to 16 hours, with the exact duration of incubation determined by the desired final acidity. The yogurt's fat and water content, as well as the presence of unwanted bacteria, are monitored throughout the incubation period.

Packaging : this step is highly automated in industrial facilities. Fruit jams are added to the yogurt, containers filled, aluminum foil sheets applied, covers put on, and best-before dates printed—all by a single machine operating at breakneck speed under the watchful eye of an operator.

Other bacteria that may also be present in yogurt: Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium bifidum and Bifidobacterium longum.