I have adapted a recipe from The British Larder. If you have not been on her website, you really must. She is a truly inspired chef and is opening her very own restaurant in Suffolk, England soon.
Jean Pierre Wybauw, I know that it isn't all about temperature. I would love to find a recipe, or perhaps adapt this recipe, so I can use the refractometer to measure the doneness of the sugar syrup.
I would prefer to err on the side of softness rather than hardness when it comes to a delicate sweet like this. I will definitely be making this again! I used my rhubarb simple syrup and it was perfect.
When I was studying home economics at university, I learned that if a gelatin is allowed to set at a higher temperature (say room temperature) rather than a lower temperature (such as the refrigerator), the resultant jelly would be firmer and maintain that firmness better at room temperature. Hence, the suggestion not to let this set in the refrigerator. See Cooks Notes below.
Rhubarb Turkish Delight adapted from The British Larder
2 cups of rhubarb simple syrup
1 envelope of gelatin powder
2 tablespoons of cornstarch
1/4 + 1/4 cup of water
2 tablespoons of cornstarch and 4 tablespoons of icing sugar
Line the bottom and sides of a standard loaf pan with plastic wrap. Sift the cornstarch with the icing sugar and set aside.
Soften the gelatin powder in 1/4 cup of warm water. Let it bloom until fully hydrated. At the same time, dissolve the cornstarch in 1/4 cup of cold water. Mix to make a slurry.
Put the rhubarb simple syrup in a non-reactive pot. Add the softened gelatin and the cornstarch mixture. Stir to blend completely.
Heat over medium high heat for about 15 - 20 minutes, approximately 112C.
Remove from the heat and cool slightly. Then pour into the prepared pan. Let sit 12 hours at room temperature. Do not refrigerate.
Do not store in the refrigerator. Store in a tightly closed container in a cold room. This will keep up to 3 months.
Temperature. With a definite concentration and time for setting, solidification occurs only with a definite temperature. For a high solidifying temperature a high concentration of gelatin is required. The slower a gelatin solution is cooled, the higher the temperature at which it solidifies. A gelatin solution may be cooled down, by packing ice around its container, to a temperature below that at which setting would occur if a longer time is allowed. On the other hand, gelatin solutions may be mixed and left standing for 2 or 3 hours at room temperature. If they are then put in ice or the refrigerator they set quite rapidly and as if gel nuclei might have started to form while standing at room temperature. At low temperatures all gelatins become firmer. At high temperatures they are liquid, for no gel occurs at temperatures above 35°C. with any concentration of gelatin. With the same concentration one gel may set at 10°C, another at 12°, still another at 14° to 16°C, and others require lower temperatures for setting. The gelatins that set at higher temperatures show jellies of greater firmness than those that set at low temperatures. The ones requiring lower temperatures than 10°C. to set do not serve very well, for they also soften at a low temperature.