Chocolate won't melt

Question

I'm trying to melt some Belcolate Vietnam 45 milk chocolate, pictured below

in order to make it easy I decided to just put some pieces in a small pot and put it in the oven at a low temperature (this chocolate melts at 40-45 degrees C) for hours, thinking the heat (50 C) would propagate / melt it without me having to stir or do anything.

However not only is it not melting, it is sort of "drying".
Trying this again but this time I put the small pot on a temperature-controlled hot plate, making sure the heat isn't too high. After some hours I noticed the chocolate pieces have sort of "cracked" and transparent liquid was slightly oozing from them but they were not melting, only having become slightly glossy at the surface. So I decided to stir them and, again, they were REALLY dry and cracking at the bottom, to the point I'd call them "cooked", where they were touching the bottom of the pot.

I do not understand what is physically happening. I know chocolate usually has a low thermal capacity, meaning it absorbs heat quickly and melts, usually without transferring that heat to the rest of the chocolate but I don't think this is what I am observing and plus, I thought that if I were to leave it for hours at the same, say 50 C, temperature it would melt but eventually transfer the heat to the rest and thus the whole thing would be melted liquidy.

If it matters - the chocolate is a bit old, I wasn't able to store it properly, so it has been sitting at some summer (~20-30 C) temperatures and it has got "white dust" on top of it, you know the one that appears when chocolate is old, as far as I know it's the cocoa butter going to the surface.

Can anyone explain what and why I am seeing?
At this point I only want to melt the chocolate, nothing else.

Here are the ingredients of the chocolate:
Sugar, cocoa butter, whole milk powder, cocoa mass, emulsifier, soy lecithin (E322), flavouring: natural vanilla

Answer 1

Chocolate is a peculiar substance. It consists of cocoa butter (a fat), cocoa solids, sugar and possibly some other components like what you listed. To get its smooth texture it goes through quite a process. Part of that is called conching and consists of grinding the heated mass for an extended period of time, which produces very fine sugar and cocoa solid particles. We're talking micrometer level size here. Without this process chocolate would remain grainy and brittle. Then there's tempering which consists of heating the chocolate to a certain temperature and then cooling it to another temperature, all with the purpose of controlling which types of crystals the cocoa butter forms while cooling. This eventually yields the solid mass we know as chocolate with a uniform texture, that is solid thanks to a specific crystalline structure.

As you might imagine, which so much going into the formation of solid chocolate, it doesn't take much to mess it back up. An example is getting some water into melted chocolate or while it's melting. The water allows sugar to dissolve into it, breaking up what should be an emulsion and causing the chocolate to "seize". It turns into a lumpy stiff mess instead of the smooth viscous liquid you want. This doesn't take much water, as the sugar getting out of the emulsion will form a "seed" for the crystallization of more sugar. This is similar to how caramel containing butter and/or cream can start splitting if there's sugar crystals sticking to the side of the pot. They act as a base for further crystallization, taking sugar out of the mix and causing the fat to pool.

Another thing that could mess up the melting process is heat control. Like rumtscho said in their answer, an oven is probably not precise enough for the melting process. Maybe some professional convection ovens with humidity control might do the trick. A hot plate is probably too hot as well, and possibly not precise enough either, despite the temperature control. In either case, you mention a timeline of hours. That is waaaay to long. Even with good temperature control the chocolate would start drying out, or uneven temperature gradients would cause some parts to seize.

The classical and reliable way of melting chocolate is to use a double boiler. Bring a pot of water to the boil, then reduce the heat until it's simmering. Place a suitable container over it. This could be a metal or tempered glass bowl. Something that fits neatly over the pot and can withstand the heat. However, its bottom should ideally not touch the water. The idea is that the container will be heated up by the steam from the water, which provides a gentle and controlled heating. Put the chocolate into this container. Let it gently heat up for a little while until you notice the chocolate in direct contact with the container starts to soften up, then gently stir to distribute the heat and let the solid chunks dissolve into the already molten chocolate. From when the water is boiling the whole process would take less than 10 minutes, depending on how much chocolate you're melting. Even with this setup, if you leave it on a bit too long, you might notice the chocolate at the bottom starts to become harder, like it's solidifying again. This is some early splitting of the cocoa butter and solids. Usually you can fix that by scraping along the bottom of the container and folding this more solid mass back into the molten chocolate. One of those flexible silicon spatulas is ideal for this, but make sure it's heat resistant.

Careful handling the container. It can become quite hot and when lifting it from the pot, steam can escape which can rapidly cause burns. Protect your hands with a towel or oven mitts.

Answer 2

The explanation is that you have overheated your chocolate, and it has seized.

The whole oven idea isn't all that practical. An oven isn't a sous vide setup. It doesn't hold a constant temperature, it just cycles a heating element on and off. The temperature varies a lot around a constant value, both in time and space, and with some luck, that constant value isn't too far from 50 C, but even that isn't a given.

So, when you turned on your oven, the heating element's radiation hit the dark chocolate full blast, and its surface probably shot up to a high temperature fairly quickly. With time, the whole block must have overheated. In any case, the symptoms you describe, "cracked" and "oozing transparent liquid" are very normal for overheated, distempered chocolate. It didn't dry out, it just separated into cocoa butter and cooca solids, losing its tempered crystalline structure.

To sum it up, the oven isn't suitable for this kind of work. Use a bain marie, and make sure you are 1) monitoring it closely all the time, and 2) stirring it well.

Answer 3

The ‘white dust’ is likely ‘sugar bloom’: https://www.southernliving.com/chocolate-bloom-7693364

Supposedly it’s still fine to be used in baking recipes, but I’ve had issues with getting it to melt and spread. It always seemed to me to be better to use in something like chocolate chip / chunk cookies where you don’t need to get it to a liquid state like what you were doing.

Water may help. Although a little bit of water will cause it to seize, in an episode of Good Eats, Alton Brown mentioned that adding enough water will prevent it from seizing (episode Art of Darkness, but unfortunately the website Good Eats Fan Page is no longer with us to grab a transcript)