I have attempted to make coffee flavoured gelato. I have run into a problems specifically when the beaten egg yolk/sugar mixture is heated with the cream/milk mixture - until "it thickens enough to coat the back of a spoon".
I used two recipes - one with just plain cream and one with cream/milk. Both times - the final mixture, when heated, would not thicken enough to coat the back of a spoon. The mixture was heated once to 74° C and then again to 98° C.
I used chicken egg yolks the first time - they seemed too small. I used duck egg yolks the second time. Still no luck thickening.
I guess I would like to know when I should stop and take the mixture off the stove. If thickening doesn't happen after 20 minutes on medium high heat with a temperature of 98° C - then what?