Hi Koko,
Stevia may taste sweet like sugar but it does not function like it. The candylike texture of pralines is due to the sugar. Stevia juice is 30 times sweeter then sugar and the pure extract (Stevioside) is 250-400 times the sweetness of sugar.
This means Stevia has no bulking potential like sugar has. It is like saying the sweetness of one hard candy is found in a pinch size of steviaside.
The only other <sugar> that can act like sugar in cooking is xylitol (that I know of). If you make praline with this stuff, you will need many trials to get the texture right but you would NOT need stevia because xylitol is already as sweet as sugar with a similar bulking capacity.
As for truffles: I guess you could make a truffle using pure dark chocolate, cream and butter then add the pure stevia powder to sweeten it (I would not use the liquid extract because water and chocolate don't mix).
Don't forget that Stevia has a mild licorice-like taste so I do not know how it would clash with chocolate (just a thought)
In any event, what you propose requires many trials...
Good luck!
Luc
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