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I've been making chocolate truffles recently, and I'm pretty pleased with the result, with one exception.

I make a ganache (different mixtures of cream, butter, chocolate and flavourings) and form into balls. I chill them, and dip in tempered chocolate, and letting is set at room temperature. (I'm tempering the chocolate in a sous-vide water bath at ~33°C, according to Jeff Potter's 'Cooking for Geeks')

The chocolate has great snap, and a good shine, but the chocolate shrinks while it sets, which often causes the coat to crack or a small puncture appears through which the ganache seeps out.

I understand the physics behind it (chocolate shrinks as it sets), but I'm wondering what to do about it. Surely I can't be the first person to have this problem.

  • Are you sure it's the chocolate coating shrinking, or could it be the ganache balls expanding? You did say you chilled them before dipping. – GdD Mar 21 '17 at 12:11
  • According to 'On Food and Cooking '(2004) p711: "It turns out that tempered chocolate shrinks by about 2% in each dimension as it solidifies, because the fat molecules in the stable crystals are more densely packed than they were in liquid form." He goes on to say that it can cause the thin coating on a candy or truffle to crack, especially if the filling is cold and expands slightly when coated with the warm chocolate." So maybe there's no accepter work-around, or I'm confident that he would have mentioned it. – Popup Mar 21 '17 at 13:20
  • I would suggest bringing your ganache balls up to room temperature as a possibility, less temperature change means less expansion, might be worth a try. You could also try double dipping the balls. – GdD Mar 21 '17 at 15:15
  • I think the thermal expansion of the filling is close to negligible here. The thermal expansion of water is less than 1 * 10⁻⁴ K⁻¹. thus it would need to heat up more than 200°C in order to equal the 2% expansion given by McGee for the reduction in volume caused by the chocolate setting. If they came from a cold fridge into a warm room we may calculate with a temperature difference of 20°C, so the thermal expansion of the filling will be less than 1/10 of the contraction caused by the phase-change of the coating. – Popup Mar 21 '17 at 15:32
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    I don't know how to prevent it, but you shouldn't be working with cold ganache, independently of the expansion/shrinking problem. cooking.stackexchange.com/a/21142/4638. – rumtscho Mar 21 '17 at 17:10
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Maybe use slightly less filling?

You're forming the ganache balls freehand, you mentioned, so it might be possible to form a little pocket or gap somewhere in the filling as you roll it (maybe with a toothpick), so that when the chocolate shrinks there's space for the extra filling to go instead of bursting the shell. I think it would work better if there was a way for the air to get out - a pinprick through the chocolate to the air pocket, which can be covered over after the fact by a dab of chocolate or decoration. But if the difference in volume is small, it may work without that.

Alternately, you could control the effect by piercing the truffle through the chocolate coating, say, on what will be the bottom as it sits. A little filling will probably leak out when it cools, but that can be scraped off, the hole will be small and controllable, and again - covered up with just a dab of melted chocolate or other decoration, if you want. Much less chance of cracking or other visible damage.

Finally, you might try dipping in two layers. If there's a gap, however small, for the filling to equalize out of your first shell won't crack or puncture - and a larger gap will have a smoother curve. A second thin shell, overlapping and smoothing over the first gap, should render it nearly invisible. You could dip about half at a time, or three quarters for a bit more overlap, or almost the whole truffle twice over for more chocolate per truffle, as you prefer.

  • What typically happens is that the coat punctures at its thinnest point and the ganache seeps out. While an air pocket would probably help, I can't see how to keep it from filling with chocolate during the coating. I don't see how double dipping could be an alternative, though. When the outer layer sets it will be on top of a solid chocolate ball, which can not change shape like the ganache could. It seems to me that this would inevitably cause the outermost layer to crack. But I haven't tried it myself. – Popup Mar 23 '17 at 16:33
  • @Popup - for the air pocket, you need to have it inside the ganache ball - picture scooping out a chunk, taking half that chunk and flattening and smoothing it over the dug in hole - the space isn't visible, chocolate can't get in. For double dipping, the point is the gap. The chocolate shell shrinks, and the filling will slightly bulge out where the chocolate didn't cover - that is why a bigger gap will leave a smoother curve. The second dipping will cover the gap that the first coat didn't, but since the ganache has already equalized, the second coat shouldn't be under pressure. – Megha Mar 23 '17 at 16:45
  • Thanks, @Megha for the added info, but I'm fairly confident that the problem isn't thermal expansion, but rather that the chocolate shrinks as it sets (2% in each direction, according to McGee). I think one solution is to let the truffles set slower, so that it can flow to accommodate the volume change. – Popup Mar 23 '17 at 16:59
  • @Popup - As for your clarification, I see little practical difference between expanding filling and shrinking shell, the problem I see is the mismatch and the resulting pressure - but that's just my opinion. As for your idea, it might help, and I hope it works. It's probably worth trying, anyway :) – Megha Mar 23 '17 at 17:09

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