There are different ways of fermenting milk to produce yogurt, with variations arising from the types of bacteria used. Two common live cultures stem from the organisms Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus.
By adding these organisms to milk, the bacteria break down the milk sugar lactose (which is molecularly a disaccharide) into two different forms of sugar: glucose, and galactose (these are monosaccharides.) The bacteria then proceed to take in the glucose and secrete the galactose. This creates a sour-tasting yogurt. To meet consumer tastes, sucrose is normally added.
Through selective culturing to create mutant strains, the scientists have reversed the process leading to the bacteria taking into the galactose and secreting the glucose. This was accomplished by successive rounds of culturing, so that the organisms no longer consume glucose.
The culturing process was enhanced by using a special medium that contained a substance called 2-deoxyglucose, which is similar to glucose but toxic to cells. The various rounds of culturing created bacterial mutant strains incapable of taking in or transporting glucose.
In studies using the modified bacteria, a yogurt was produced that had the same level of sweetness but far less sugar. This is because the yogurt was rich in glucose. When presented to a taste panel it was found that the glucose rich yogurt did not need very much sucrose added to it.
The reason this happens is because glucose can be as sweet as sucrose, but a lower quantity is needed in order to produce the same sweet sensation.
The same process has also succeeded in reducing the lactose content, making the yogurt suitable for those with lactose intolerance. It is now hoped the process can be commericalized.
The findings are discussed in an article published in the journal Applied and Environmental Microbiology. The paper is titled “Enhancing the sweetness of yoghurt through metabolic remodeling of carbohydrate metabolism in Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus.”
