I am making a vegetarian jelly using Carrageenan. I am hoping to make a batch to give out as samples to friends etc. (hoping to market it eventually). Does anyone know what concentration of citric acid to use as a preservative. Also, some commercial products include sodium citrate. Is that always needed?
You will have to use the amount specified in the recipe you are using. It varies with each fruit or vegetable. Further, making up your own recipe is not wise because of possible bacteria that could set in. You will want to check out recipes from sites like http://www.freshpreserving.com/tools/faqs.
I am assuming here that you mean jelly as in the preserve. It is not preserved with citric acid, it is preserved with sugar. I've heard as a rule of thumb that 1:1 jelly is shelf stable by itself, while anything between 1:1 and 1:2 can be canned using standard procedures and will be shelf stable until opened, keeping well in the fridge after that. Anything with more fruit than that has to be made from an empirically tested recipe without deviations from recipe ratios and process, possibly needing some amounts of acid (which are not sufficient by themselves to preserve!) and/or pressure canning. So, if your recipe has at least 33% sugar, the acid doesn't matter, if it has less, you cannot make it preservable by yourself and have to treat it like standard cooked food (keep in the fridge, and not too long).
If you meant that you are turning a gelatine-jelly recipe (typically called jello in the US) into a carrageenan-jelly recipe, that's not a preservable food. Any edible amount of acid will result in a food with a fridge life of some days, not shelf stable at all. It can be made shelf stable if you make gummy bears out of it, although I'm not sure carrageenan is suitable for that.
Citric acid is a natural preservative, 1tsp er quart of liquid to preserve the final product. As to allergies listed very little http://www.ehow.com/how_6315775_use-citric-acid-preservative.html