How is unsweetened plant-based yogurt possible? For plant-based (soya, almond, cashew, etc.) unsweetened yogurt, how does the yogurt bacteria grow and develop without sugar?

From online recipes for soya yogurt, one tablespoon of sugar per 1 liter of unsweetened soy milk is needed to promote bacterial fermentation. As these plant-based milk alternatives themselves do not contain sugar (lactose/ milk sugar), for which is the basic food for yogurt bacteria (eg. Lactobacillus delbrueckii subsp. bulgaricus/ Streptococcus salivarius subsp. thermophilus/ bifidobacteria).

Also, there is not much sourness in some brands of soya yogurt (eg. Sojade etc). I wonder instead of sugar, what is added to cover the lactic acid that was produced by the yogurt bacteria during fermentation?

Would like to try making unsweetened soya yogurt at home. Appreciate any insights :)
Actually, the yogurt cultures, usually some strain of the group N Streptococcus and Leuconostoc species, mesophilic cultures, converts the lactose into lactic acid, which gives the diary product it's tart taste.
Furthermore, bacterial enzymes transform the milk carbohydrates into oligosaccharides, some of which have prebiotic properties.
Different LAB (Lactic Acid Bacteria) produce different fermentation products, although they have in common that they are alive in the product and can interact with microbiota during intestinal transit and the cells of the intestinal wall.
Fermentation of soy milk with lactic acid bacteria offers a means of
preserving soy milk and the possibility of modifying the characteristic
flavor and texture to make it more acceptable to Western taste.
It is possible to make soy milk yogurt-like products with acceptable texture
and clean acid flavor. The choice of fermenting organisms is limited to
those that can ferment the sugars typical of soy milk. i.e. stachyose.
raffinose or sucrose. unless sugars fermented by the desired cultures
are added to the soy milk.
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