I am trying to understand a little bit about science process between Fermentation (when sugar is turned into alcohol) and Priming (when fermentable sugar is turned into). I have done several batches of cider now and notice that every time fermentation transforms all sugar in juice into alcohol making it Very dry. Therefor i was thinking of stopping fermentation process early when there is still some 20% of natural sugars left in juice, but the question is: If I stop the fermentation and there is still 20% of sugar will the sugar left not just be turned into Co2 in priming process and thus make the drink more carbonated but still fully dry? I guess the bottom line is- is there a way to preserve natural sweetness and not turning it into alcohol in fermentation or Co2 in priming process?
Priming is really just another word for fermentation, sometimes called secondary fermentation.
If you stop fermentation when 20% of sugar remains, you normally do this by killing the yeast. This means it will not ferment further in the bottle.
On the other hand if you bottle it with 20% of the sugar left and some active yeast, you'll generally find fermentation continues. This typically leads to your bottles exploding! It is actually somewhat tricky to get a carbonated, but not fully fermented, product in basic homebrewing. You might try different yeasts but at least in my experience, this only slows things down. If sugar remains, it will eventually get fermented unless you pasteurise or ensure the temperature is kept low enough.
This is one reason that commercially, drinks are force-carbonated. They kill the yeast and stabilise the drink, then artificially carbonate it in the bottle using CO2.