The statement "so that bacterias would have something to eat" is incorrect on several levels- including grammatically.
Bacteria already have plenty to eat. There is a lot of sugar in milk. Cow's milk is 4-5% sugar. Additionally, giving the bacteria more to eat would allow them to create more acid and make the product more sour not less.
The bacteria used in yogurt making are lactobacilli and prefer munching on lactose anyway.
That said; Lactose tastes less sweet than the sucrose in table sugar. Adding sucrose will make the yogurt sweeter if that is your taste.
Making the yogurt thicker is done by
- accurately controlling the fermentation temperature,
- using higher fat milk,
- adding protein in the form of dry milk powder,
- or removing water after fermentation.
When I have added sugar to my yogurt it has not had a noticeable effect on the texture.
The biggest reason to heat milk to almost boiling before fermenting is that it improves the texture of the yogurt.
During fermentation the bacteria consume lactose and produce lactic acid which causes the milk proteins to denature and coagulate trapping most of the fat. The proteins involved are primarily the casein proteins.
When this happens, there is still quite a bit of protein left that isn't bound up in the new casein mesh. All of the albumin proteins are water soluble and will not add to the structure of the yogurt.
These albumin proteins denature when they are heated. For this reason recipes universally call for the milk to be heated to 190 and then cooled. The albumin is denatured and is able to tangle up with the casein during fermentation and add to the yogurt structure.
Skipping this step will make a very profound difference to the structure of your yogurt. Without it your yogurt will be thinner and much more fragile. When you scoop it there will be more whey and all that albumin will wash out in it.
According to Harold McGee (On Food and Cooking pp 48)
"These treatments improve the consistency of the yogurt by denaturing the whey protein lactoglobulin, whose otherwise unreactive molecules then participate by clustering on the surfaces of the casein particles. With the helpful interference of the lactoglobulins, the casein particles can only bond to each other at a few spots, and so gather not in clusters but in a fine matrix of chains that is much better at retaining liquid in its small interstices."
Best Answer
Recipes call for a certain amount of starter to maximize the chances that your starter bacteria will crowd out undesirable wild bacteria.
If you use too little starter you will increase the chances that some random bacteria will win the incubation war. Since you don't know what you will get this can be actually dangerous.
I would recommend making an initial batch with the ratio the starter recommends and then using that finished yogurt as a starter for a second, larger batch. Since you would then have the entire first batch to use as a starter you can scale up your recipe for larger batches.