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(Yes, I know the question sounds super-trivial... but bear with me here)

Most chocolates (especially milk-chocolate) tend to melt into this sticky (albeit delicious) mess at temperatures slightly above room temperature. Why does this happen?

From my experience (as a high-school student), most organic solids (at room temperature) tend to either sublime or decompose at moderate temperatures; by "moderate", I mean a temperature range between 25 degrees Celsius (typical room temp) and 50 degrees Celsius (a very hot day in Arizona).

Now why do I find this particular property (ease of melting) of chocolate intriguing all of a sudden? Because chocolate is an organic cocktail of all sorts of compounds such as sugars, lipids, proteins, alkaloids, blah blah blah; yet in the moderate temperature range, it doesn't sublime nor does it decompose (chill the molten chocolate, and you'll still get chocolate)... it first softens and then melts, unlike most other organic substances in that temperature range.

So, what makes chocolate so special? And why does milk chocolate (tend to) melt faster than dark chocolate (higher-cocoa content).

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    $\begingroup$ Chocolate manufacturers need to formulate their products differently for different markets. For example, Cadbury's is popular in the UK and Australia but it is slightly different in Australia to suit the typically higher temperatures. $\endgroup$ – badjohn Jun 3 '17 at 8:44
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    $\begingroup$ Not if you cover it with a hard shell, someone should market such a chocolate product ;-) . $\endgroup$ – Oscar Lanzi Jun 3 '17 at 11:35
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    $\begingroup$ @Oscar Smarties ^ ;) $\endgroup$ – paracetamol Jun 3 '17 at 13:39
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    $\begingroup$ I would also note that a large number of organic solids do not significantly sublime or decompose in the 25°C - 50°C range under a standard pressure. For example: polypropylene, glucose, cellulose, nylon, urea, and many many more. It’s simply not true that most organic solids perform in the manner you say. $\endgroup$ – user46088 Jun 5 '17 at 2:56
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    $\begingroup$ So it melts in your mouth and not in your hands $\endgroup$ – Keltari Jun 5 '17 at 9:44
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tl;dr The main structural component of chocolate is cocoa butter, which is a blend of fatty acids (primarily oleic, palmitic, and stearic acids). Cocoa butter has multiple crystal structures, and manufacturers target a specific form which melts at around 33 °C. The fact that chocolate is about 90% sugar and cocoa butter is why chocolate is (chemically) insensitive to temperature changes.

While chocolate itself is a giant mess of hundreds of different compounds, the primary structural material of chocolate is fat. Wikipedia lists a typical composition (by mass) of about 60% sugar, 30% fats, and 10% proteins/other. So it's not terribly surprising that chocolate doesn't sublime or decompose. You probably wouldn't expect a ball of butter + sugar to suddenly decompose at 50 °C, and for the most part, chocolate is chemically pretty similar.

Now, what's interesting about cocoa butter is that it can form multiple crystal structures, and it's actually a bit tricky to get the right crystal structure out. You can read about the process in this RSC article if you'd like. The one that most chocolatiers are after is polymorph V, which is glossy and melts just below body temperature.

However, a different polymorph, polymorph, VI is the most stable form of cocoa butter, and it has a melting point that's a bit higher. It also tends to have a faint white crust on it (known as "fat bloom"). As a result, it's not as pleasant to look at, not as pleasant to eat (since it takes some chewing on to get it to melt), and generally undesirable.

On the right, a form V polymorph. On the left, the form VI polymorph. All chocolate eventually spontaneously converts to the form on the left.

Since form VI is the most stable, all chocolate eventually goes "bad" and becomes not as delicious as the maker originally intended. If you've ever bitten into an old chocolate bar and thought it was a bit waxy and unmelty, you've experienced this interconversion. I've been told by someone who did research on crystal structure kinetics that chocolate manufacturers spend lots and lots of money on research in hopes of being able to slow down or stop this process.

Now, I'd like to take stab at addressing two points in the question:

chill the molten chocolate, and you'll still get chocolate

Yes, but is it the exact same chocolate as you got before? What if you take the molten chocolate and put it in the freezer, or, heaven forbid, drop it in liquid nitrogen? Does the resulting chocolate still melt at the same temperature? My suspicion (based on this chart) is that you'll have generated some of the lower-melting cocoa forms, and thus the chocolate will get soft and melt at a much lower temperature than previously.

And why does milk chocolate (tend to) melt faster than dark chocolate (higher-cocoa content)?

I don't know. If I were to hazard a guess, I would say it's because milk chocolate tends to contain more sugar than dark chocolate, and so you're seeing some form of freezing-point depression. However, if it turns out that, at similar concentrations of sugar, milk chocolate still melts more quickly, I'd have no idea.

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  • $\begingroup$ Milk chocolate could melt faster b/c there is more sugar which means less amount of fatty material that needs to phase transition (less latent heat needed). $\endgroup$ – Kevin Kostlan Jun 3 '17 at 20:50
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    $\begingroup$ Simply chilling melted chocolate will not recover the as-manufactured texture. Tempering manipulates the mix of types of cocoa butter crystals to get a desirable texture. $\endgroup$ – Patricia Shanahan Jun 3 '17 at 23:16
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    $\begingroup$ Milk chocolate is made by adding milk fat, so it has more fat total. Dark chocolate has more coca solids (remember the "90% cocoa" marketing?), so it has less fat total. A "99% cocoa" chocolate is pretty much pressed cocoa powder so it hardly melts at all. $\endgroup$ – Agent_L Jun 5 '17 at 9:24
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Chipbuster's answer is excellent. Let me add in an important point: chocolate is not a homogenous solution, but an emulsion where all the sugar, protein etc is emulsified in the fat content. As in the other post it is said cocoa butter is polymorphic and generally crystallize around room temperature. When it is melt, you have a regular emulsion (like creams or butter), and when it is solidified it is still the same emulsion, but with the solid fat.

It also means adding other fats can shift the melting point: milk chocolate contains lot of milk powder, therefore the fat-phase is only partially cocabutter, diluted with milk fat (same as butter or cream), which makes lower melting point and also the crystallinity of the fat is somewhat compromised.

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I think you are seriously overestimating chocolate's stability of consistence because these days all but the most expensive kinds of chocolate are stuffed with emulsifiers generally keeping the consistence together. Before soy (and milk) was used in that function, you had to prepare chocolate in a Conche to get an enjoyable consistency.

Also if you are working with "organic" couverture without additional emulsifiers, getting your chocolates to dry without having cocoa butter bloom out all over them within hours and turn them into a shoddy mottled whitish-grey is a real nightmare.

Been there, done that, had to do lots of overtime.

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    $\begingroup$ Pretty sure conches are still used, at least for decent-quality chocolate, even if it contains emuslifiers – it's not just for mixing, also for first grinding the still-solid particles down to unnoticable size. $\endgroup$ – leftaroundabout Jun 4 '17 at 20:59

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