Few home chefs get beyond the use of gelatin and corn starch or flour when it comes to thickening juices, syrups, sauces or gravies. Commercially, whether in food production factories or in high end restaurant kitchens, a class of thickeners known as hydrocolloids is used. These come in powder forms either white or off white in color.
This class of thickeners is used to create what I call special effects. Think about that high clarity beautiful fruit based sauce a chef ladles onto your dessert plate. Or think about serving sauces or syrups at home that retain their clarity instead of becoming murky from using a product like corn starch.
Besides appearance, this class of thickeners create behaviors that differ from each other ... some set when heated while others set when cooled. Some can be reheated to redo a process of liquefying followed by gelling or thickening. When grilling meat, a sauce that thickens when heated is surely superior to one that becomes thin and runs off the meat. Conversely, a syrup is better for pancakes or waffles or ice cream if it is relatively thick at room temperature instead of being thin and runny.
Technically gelatin is a hydrocolloid, the only one that is a protein, so the reader should not feel intimidated by the word hydrocolloid. All the term means is a natural product that merges physically (vs. chemically) with water to form molecular groups that thicken whatever food is being made.
For example, agar agar is extracted from red seaweed. Locust bean gum comes from a seed. Gum arabic comes from tree sap. Pectin comes from fruit. Xanthan gum is the result of microbe produced fermentation. All are complex sugars, and this list is not complete. For my purpose, it is enough that you know these products are natural.
Xanthan gum is a typical product found in salad dressings as it helps merge chemically and physically different products like oil and vinegar so that they tend to stay together when used on your salad, rather than separate from each other.
I have a supply of agar agar that I purchased through BulkFoods.com. I've also added xanthan gum to my pantry and that product is available also at BulkFoods.com and other places. The idea is that a simple Internet search via search engines like Google will provide you with web site links to a variety of sources happy to sell you what you want, inexpensively. For example, agar agar and xanthan gum can be purchased for roughly $12 per pound and a pound is one heck of a lot of product compared to how much you will use in any given recipe. Likely a pound of either product will last you at least five years.
With a bit of searching you will find good recipes that use hydrocolloids. But try to learn a bit about each first so you can recognize errors that are unfortunately too typical with many Internet recipes. Alas, I've found that too many home cook suppliers of recipes to popular web sites use, shall I say, poetic license in describing their results from the stated ingredients and procedures. In any event, agar agar is stirred into cold water a bit at a time to avoid clumping and left to hydrate for five minutes. Then the mixture is heated barely to boiling and stirred to cause it to mix well and then it is introduced into the product being thickened/gelled, with stirring, and it sets upon cooling to around 95 degrees F. Xanthan gum thickens at both room temperature and higher temperatures so it is quite versatile and all it takes to hydrate it is a blender.
Hydrocolloids are typically used in certain amount ranges based on the total weight of the product being thickened. Thus, for a syrup made from fruit juice and pulp that will also contain sugar the total weight of the juice and pulp and sugar will be used to calculate how much of a given hydrocolloid to use. But note that I am talking about the final weight of the syrup, so if evaporation during boiling is significant then the final weight will be less than that of the original ingredients.
Agar agar is used at the approximate rate of 0.5% to 0.8% by total product weight. Using more or less of it will produce either more or less thickening/gelling, and the choice is yours to make. Don't be afraid to experiment, for that is how we learn best.
Xanthan gum is used at the approximate rate of 0.25% to 0.75% by total product weight, depending on the degree of thickness desired. In my reading about Xanthan gum I was informed that using too much can produce ugly, slimy results.
Hydrocolloids need to go from powder form to mix with water such that they become hydrated. This can be accomplished with relative ease by mixing vigorously with cold water via a blender or by hand. It depends on which product is being hydrated. You also need to learn that hydration will occur best in pure water, not in a syrup that already contains sugar. Hydration will sometimes be impeded if the product is too acidic, so the answer is to hydrate the thickening agent prior to introducing it into an acidic product.
Sometimes thickened product can be processed in a blender to create a creamy consistency that will be maintained at serving time. The point, as usual, is to do some research on the physical behaviors of a hydrocolloid and to find or create recipes where the presence of the hydrocolloid and how it is introduced into the product works best.
In short, this discussion of hydrocolloids barely scratches the surface of the subject, yet you already know roughly how much to use of the two discussed products and how and when to use them to get the desired effect in your final presentation.
You might give yourself a homework assignment to learn which hydrocolloids are best for gelling as opposed to thickening. Yes, the Internet is a great source for this kind of information. Use it.