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Could you maybe enlighten me a bit about the function of the lid on the melanger? My only guess would be that it keeps heat in. And maybe keep dust and other unwanted particles out. But during refining/conching we expect water to evaporate, volatile acids to be driven off, and the chocolate mixture to be aerated. Intuitively, all this seems to be limited if the lid is on. Your advice in the dark chocolate video is to keep the lid off the first three hours to let the water evaporate, and then put it on for the remainder of the time. But aren't acids still being driven off after those initial three hours, which might be limited by the lid on?
Why should you use the lid?
That really sums it up.
It is for dust and other unwanted cats* from getting into your chocolate. Depending upon how full your melanger is, occasionally you can get a little spatter and the lid will contain that also.
You are correct, you are losing water and other volatile compounds (mostly acids) and the water is the reason I suggest you leave it off for a few hours. If you don’t you can get water droplets forming on the underside of the lid and then dripping back into chocolate, which counter intuitively can cause the chocolate to thicken (it is because it is going in as a drop, so it is concentrated).
The lid isn’t limiting the acids or other volatile compounds from escaping though, and may be speeding it (more in a moment). It would slow it down if it were sealed but it isn’t. There is plenty of space around the lid for air to get in and compounds to get out. The only reason moisture is a concern is that relative to everything else there is just so much of it and it condenses on the lid. All the other compounds are so diffuse that they don’t aggregate to form droplets. They stay as a vapor and easily escape.
I don’t see it as intuitive at all that the cover (it isn’t really a lid since it sets above the melanger) would inhibit the gases from escaping. If you cook a pot roast in the oven and cover it with a snug lid, you still smell it so something must be escaping.
It really comes down to the fact that gases exert a pressure so even with a semi-sealed lid they would find their way out. It may be not as fast with an open lid, but the difference between a floating cover and no cover is insignificant. It does not limit it in the least.
As a last thought, I have observed something that on the surface is counter intuitive. A lid, under certain circumstances, may actually increase how fast volatiles are released. How?
With the lid on you can trap heat generated by the friction of grinding. That additional heat can make the temperature rise and the chocolate will off-gas faster. I see this every winter with people writing in that the recipe they have been using all summer suddenly tastes different. It is usually because the room is a different temperature so the chocolate is a different temperature. So keep that in mind as we go into the cooler season.
This is also one variable that needs to be tracked as your batch size increases. Larger batches will generate more heat. With the lid off it tends to equalize and you don’t see the temperature go up as much. With the lid on, you can see a temperature increase. I’ve seen a 20 F jump from 2 lbs to 8 lbs in certain batches with the lid on.
That is one of the reasons I tend to not keep a lid on my chocolate, but I make mine in a closed room so dust, and cats, are a rarity. It’s your call. If you decide to keep the lid on don’t fret about it slowing down the off gassing.
And here, have the picture of a cat again because it is fun.
*no cats were harmed in the writing of the article.