Sweetener and flavoring for gelato are key to creating a tasty dessert, but they are also two of the things that can encourage ice crystals or freezing problems to happen. Use them, but use them wisely.
When it comes to sweeteners, whether you prefer sugar or a liquid sweetener, sweeten in moderation. Ideally, your gelato recipe should call for no more than about 25 to 35 percent sugar; use any more or any less and you'll risk making a gelato that's too hard or that may develop ice crystals as it freezes -- and don't forget that any fruits you add also contain sugars.
Also be careful when adding liqueurs and other spirits to your gelato mixture. Just like when adding sweeteners, a light hand is best when flavoring with alcohol -- not necessarily because the flavor will overwhelm the taste of your dessert, but because alcohol doesn't freeze at home-freezer temperatures; to avoid freezing problems, don't use more than 4 tablespoons per 1 quart of gelato base [source: Baker].