The simple answer to your question is the kind of starch they use. Most use cornstarch or tapioca starch, neither of which contain gluten, so most powdered sugar shouldn't contain gluten. If the type of starch isn't listed, don't buy that product.
For absolutely no gluten, it gets a bit more complicated.
A Google search for "Gluten Free Powdered Sugar" only yielded a few brands that make a "gluten-free" claim. Since most brands of powdered sugar don't contain any ingredients that would normally contain gluten, look for the line "This product is processed in a facility that also processes wheat". Choose a powdered sugar that lists the starch used as cornstarch, tapioca starch or other non-gluten starch and that doesn't have that line on the label. Of course I can only speak to US labeling, other countries may be different.
One brand, Domino Sugar, stood out to really stand by their claim of gluten free. Domino's Gluten Free Claim (C&H is Domino, BTW). I happen to have some C&H powdered sugar, so I looked at the label. There is no "Gluten-Free" claim on the label, but the ingredients listed are only sugar and corntarch, and there is no "this product is processed in a facility..." warning. Walmart brand also pops up on the "Gluten Free Powdered Sugar" search, they say "Naturally gluten-free food". The FDA requires that claim not be misleading, so if they do use equipment to process the sugar that also processes gluten-containing ingredients, the sugar cannot contain more that 20 ppm gluten.
There is one way to know for sure about any ingredient you use, or to even to check your final dish. Elisa Technologies, a very well respected name in medical laboratory testing, has put out home test strips sensitive to 10 ppm, that's half the concentration of gluten the FDA allows for a product to be certified "gluten-free". If a product tested contains less than 20 ppm gluten the US, Canada and the European Union allow that product to use "Gluten Free" on the label. FDA Announcement At over $10 a strip, you'd want to use them judiciously, but if I or someone I loved had serious health issues that did not allow gluten, I'd get these strips. Test Strips
It looks like you are trying to make sugar fondant. I often make a batch to use as seed crystals in my holiday fudge preparation.
Sugar Fondant
Sugar fondant is a crystalline sugar confection where the crystals are microscopic and suspended in a saturated solution of sugar. Its texture is very short, and the mouth-feel is creamy.
Creating sugar fondant is relatively easy. In brief, boil a syrup to softball stage and cool it undisturbed until around 50C followed by rapid agitation until the fondant is too difficult to work.
Now for some specifics.
Multiphase Solutions
One of the interesting features of boiling syrups is the temperature is intrinsically linked to the composition of the syrup. Unlike how boiling water transitions to steam at a constant temperature of 100C, syrups boil at a range of 110C to well over 200C. As water evaporates from the boiling syrup, the composition changes to contain a higher concentration of sugar and the boiling temperature rises.
Be warned. You are boiling a super-saturated solution. Any crystallized sugar introduced to the solution will not dissolve and it will seed crystallization during the cooling stages.
- Avoid stirring once the sugar has fully dissolved prior to boiling. Use a brush moistened with hot water to wipe away any crystallized sugar on the side of the pot during cooking.
- If you are adding any glucose to the syrup, add it after the syrup has come to boil to ensure that the other sugars have fully dissolved.
Candy Stages
The desired properties of a candy are principally derived from the candy stage to which you cook your syrup. The sugars remain mostly unchanged chemically (though disaccharides may break down into glucose and fructose) regardless of the candy stage or temperature.
When making sugar fondant, you are aiming for the softball stage which is 110C to 120C. You can take a dollop of hot syrup and drop it into cold water to check the stage in absence of a good thermometer or if altitude / humidity are affecting your candy. So long as you do not scorch the sugar, you can add water and lower the temperature to restore the syrup to the desired candy stage.
Crystallization
The most important part of a sugar fondant is the formation of microscopic crystals. The syrup must cool to 50C before agitation to create the desired crystal size and distribution.
One difficulty that I have found is that I cannot let the syrup cool within the bowl of my stand mixer - the syrup cools unevenly which typically induces crystal growth. This is especially difficult for fondant, as it must be worked for a significant amount of time to crystallize.
Depending on your desired purpose for the fondant, you may let the syrup cool to a lower temperature or agitate less to keep the texture longer or more pliable. If you agitate at a higher temperature, the syrup will form crystals that grow tremendously during agitation and make the fondant grainy or crunchy.
But Why Is It Crunchy?
Chances are, you have undissolved sugar in the syrup.
- Be sure all of the sugar dissolves before you add any glucose to your syrup.
- Don't stir once it begins to boil.
Your syrup might be at a higher candy stage. If you somehow managed to reach soft crack or hard crack, this might prevent the syrup from forming any crystals and resulting in an amorphous sugar glass.
Best Answer
Sugar does not cause sour food to be any less acidic. The difference is purely one of perception; we are wired by evolution to prefer sweet tastes and tend to perceive less of other tastes when a high sugar concentration is present.
Sugar does not ionize - it is not basic or acidic. Acidity itself is a chemical property, and only a base (such as baking soda or trisodium citrate) can actually neutralize it. Two other compounds - miraculin and curculin - actually alter our mouth chemistry and really do cause sour foods to taste sweet, without actually sweetening the food itself. But plain sugar does nothing at all - there is no chemical reaction happening. It's just masking other tastes, not neutralizing them.
The reason sugar doesn't mask bitterness as effectively as sourness is that sweet and sour have roughly similar taste thresholds, while most humans are extremely sensitive to bitter tastes. We can detect quinine (the reference solute for bitterness) in solution at 0.5 ppm, whereas sucrose isn't normally detectable at levels below 5000 ppm. The amount of sugar needed to mask a significant bitter taste is simply not practical.
If you want to neutralize a bitter taste, use salt instead.