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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?

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    I am sure thickening occurred well before 98 C, but I would guess your recipe's proportions are such that the thickening effect is too subtle to "coat" a spoon. What are your proportions? (Note, as long as you heat your custard base to at least 72 C before chilling it, you have denatured most of the yolk proteins and Pasteurized the base, so you can go ahead and make the gelato from it even if it does not "coat" a spoon.)
    – Bruce Goldstein
    Dec 30, 2011 at 13:40

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No, it doesn't have to thicken, at least not in the way custards like creme caramel thicken. "Coat the back of the spoon" is just a misnomer, a term known to experienced cooks which confuses everybody who learns to cook on their own.

For "Coat the back of the spoon", read the comment riotburn left at the question What is the correct consistency of a cream soup?. It says: "The best way to describe 'coat the back of a spoon' is where if you dipped the spoon into the soup, take it out and holder over the pot, you'll find thin film (<1mm thick)."

For the ice cream, heat to between 80 and 85°C. I've explained it somewhere else, but in short, this gives you enough thickening without overcooking. If it is really gelato and not French style, it should be thickened with starch anyway. Also, watch videos on the Internet for how to make ice cream, they show the correct consistency. I think Allrecipes had a good one, but their search is so strange, I can't find it and don't know if they have it or not.

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    rumtscho: Many gelato recipes (like other cooked-custard recipes) call for starch, but not all. I have a friend from Tuscany whose family recipe does not call for it, and a quick Google search turns up as many that don't as do (though I get it's hard to say which link really gives an "authentic" gelato). It depends what effect you want. Also, while I often heat my custards to 80°C, I'm not sure why you set 80°C as a minimum: the various yolk proteins have denaturing points between 64.5°C and 70°C. Can you point me to your longer explanation?
    – Bruce Goldstein
    Dec 30, 2011 at 14:42
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    @BruceGoldstein In Italy, "gelato" is the native word for "ice cream". When I speak English, I use it as a loanword to mean a certain category for ice cream, which contains starch, after the classification proposed by Lebowitz. I am aware that in an Italian café, you can get ice cream with or without starch, and it will always be called gelato. As for the temperature, there are many kinds of proteins in egg, and they all denature at different temperatures. You are correct that the first start at 65°C, but I have found that custards are not thick enough if stopped at that temperature.
    – rumtscho ♦
    Dec 30, 2011 at 15:45
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    @BruceGoldstein (2 of 2): On the other hand, temperatures above 85°C denature enough proteins to get a separation of lumpy thickened parts and whey in the custard. So, you want to go close to 85°C, but not above. A 5°C interval is quite convenient to work with in the average kitchen, and produces consistently good results. You could stop at lower temperatures, but you won't have your custard as thick as it should be.
    – rumtscho ♦
    Dec 30, 2011 at 15:47
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    Gelato served in proper Gelatorias never includes cornstarch - generally it is only added in Sicilian recipes. Gelato with cornstarch has a very different flavor and texture and is not recommended.
    – RunLoop
    Dec 29, 2019 at 4:17
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I studied gelato making in Bologna. We never used starch to thicken gelato. The mixture shouldn't thicken like custard. Heating is only required to pasteurise the mixture. For a mixture with egg in it should be heated to 65C or 149F. Use a digital thermometer.

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