Why refrigerated chocolate tastes better

Posted on February 18, 2013 by Dan Jenkinson

The difference of opinion when it comes to whether chocolate tastes better out of the fridge or not has caused many a discussion between me and my peers, and has almost torn friendships apart (not really); but after one of my lecture courses from last semester, I can finally definitively say that chocolate DOES in fact taste better from the fridge.

First, a little background. This is all to do with a phenomenon known as polymorphism, which is the ability for a solid to exist in more than one crystal structure. Each crystal structure is called a polymorph, and each polymorph has its own set of distinct properties.

The main ingredient in chocolate, cocoa butter, has six polymorphs which can be distinguished between each other by measuring their melting points:

Polymorph I, 16-18 degrees
Polymorph II, 22-24 degrees
Polymorph III, 24-26 degrees
Polymorph IV, 26-28 degrees
Polymorph V, 32-34 degrees
Polymorph VI, 34-36 degrees

Polymorphs I – IV are not suitable for making chocolate since they are too sticky and unstable at room temperature. Polymorph VI is the most stable, but tastes bland and is too brittle. Polymorph V is the ideal form for eating (but of course, sod’s law dictates that it’s the hardest to manufacture).

This is great, I hear you saying, but why does this mean I should keep my chocolate in the fridge? Well… at room temperature, the fatty molecules in polymorph V have enough energy to slowly (days/weeks scale) convert to polymorph VI. This transformation in in the crystal structure is facilitated by the vibrational energy stored in the molecules which allow the molecules to wriggle about and realign with each other. This can be stopped by keeping your chocolate in a cool, dark place (i.e., the fridge!!) to make sure the molecules don’t have enough vibrational energy to convert to polymorph VI.

You then might ask how you can tell this has happened? The change in crystal structure is usually accompanied by something called ‘fat bloom,’ which is where the chocolate begins to look dusty, and pale spots appear on the surface as shown in the attached image. We’ve all been there (you’re incredibly lucky if you haven’t). It’s off putting, but still safe to eat. It happens because of partial melting in the solid which cases the fats within it to rise to the surface. It’s this strange occurrence that leads me to believe that keeping my chocolate in the fridge is in fact the correct way to keep it, and also why all the chocolate I bought on my exchange year in Australia just didn’t taste as good as the stuff at home in the UK due to their hotter climate!

For the original blog, follow this link:
http://danthechemist.wordpress.com/2013/02/12/why-refrigerated-chocolate-tastes-better/

The Chemistry of Chocolate? Eggcellent.*

Posted on April 10, 2011 by Holly Rogers

Part I

With Easter coming up, and so many people giving up their sweet tooth for Lent, it seems like chocolate is on everyone’s mind. So what better topic for my first blog post for Science Brainwaves than the science of chocolate…

The first record of chocolate dates back to around 1500 BC, when the Aztecs and Mayans began drinking a cold, bitter mixture of cocoa and water. They named this cacaoatl, which literally translates to “foamy water”. Delicious. Three thousand years later, chocolate reached the European mainland, and from then on, there was no stopping it. By 1831, John Cadbury was selling drinking chocolate, and in 1876, Nestle released the first bar of milk chocolate onto the market.

Despite the huge variety of chocolate on offer in the shops, the basic recipe is very simple – cocoa butter, sugar, cocoa solids and milk solids. The difference in taste comes mainly down to different amounts of each ingredient, although dark chocolate has no milk solids (which is why it tastes less “creamy”) and white chocolate has no cocoa solids, which explains the pale colour.

The way cocoa beans are processed are the key to how the end result will taste. After being picked, they’re fermented for about a week, dried for a fortnight and then transported to a factory. Fermenting the beans adds around 30 new chemicals to the mix, some useful and some not, so at the factory, the beans undergo a process known as the Maillard reaction. It has at least 9 steps, and converts the amino acids in the cocoa beans (which don’t taste of anything) to aldehydes (which generally taste great). Depending on how hot the reaction is, how acidic you make it and how long you run it for, you can get over 1000 different tastes from the Maillard reaction, including coffee, caramel and roasting meat.

Next, cocoa solids (made from grinding the beans) are conched. This means warming them up, grinding them with sugar and blowing air over the top. The heat removes any volatile (easily evaporated) chemicals from the beans, and the air whisks them away. The main thing removed from the beans is acetic acid, which is more commonly known as household vinegar.

Conching the cocoa solids decides what quality the chocolate is. The smaller they get, the better the chocolate – anything greater than 0.03mm (0.003cm, or three hundred thousandths of a metre) wide and the chocolate will feel gritty in your mouth. European chocolatiers, like the Belgians and Swiss, prefer very, very fine particles, which makes it melt more slowly in your mouth, and explains why European chocolate is more expensive.

A useful tip is to never store chocolate anywhere too warm or too cold. If you leave it in a warm room, you get a “fat bloom” – the white powder you sometimes find when you keep an Easter egg for too long, or forget about the Dairy Milk in your handbag. The increased heat means the fat rises to the top where you can see it. You can get rid of the bloom by gently warming it, then letting it cool down slowly, but it’s tricky to get it right without ending up with a puddle of chocolate. If you keep your chocolate in the fridge, you’ll get a “sugar bloom”. Any water vapour inside the fridge or the packet will collect on the surface of the chocolate as it cools down, dissolving any sugar it can reach. This eventually crystallizes on the top, giving you a gritty layer of sugar on your chocolate. There’s no way to undo this, so store your KitKat at room temperature for the best taste and texture.

* Well done if you made it past the pun! The alternative was “The Easter Bun(ny)sen.” In part II, I’ll be looking at why chocolate makes you feel good, and how it can help you stay awake. Title suggestions gratefully received!

Chocolate cravings: ‘it weren’t my fault guv, my orbitofrontal cortex dun it’

Posted on April 7, 2010 by admin

Given my own over-indulgence this Easter, I needed little excuse to hunt around in the neuroscience literature for some possibly-beyond-consious-control neural mechanism that means my unabashed chocolate scoffing is entirely not my fault (of course, that raises the whole issue of consciousness and the neural basis of free will, but that’s another story for another blog post…).

It’s an issue that isn’t entirely out of my remit, so this is also a great opportunity to intoduce a few key ideas related to my own research that will probably form the bulk of some future blogposts. I spend most of my time studying a group of structures bang in the middle of the brain known as the ‘basal ganglia’ (not, as my sister calls them, the basal danglies). They’re pretty old, evolutionarily speaking, and are heavily implicated in lots of different functions.

A couple of important functions here are believed to be learning associations between stimuli and reward (think Pavlov’s dog learning to associate a bell with food), and in representing predictions of the ‘reward value’ of an event or stimulus. A quick caveat here: in the neurosciences, ‘reward’ is a term that can be bandied about a bit carelessly without being properly defined. This can lead to a lot of talking across purposes and confusing poor PhD students in meetings (true story). So, just to be clear, by reward I don’t just mean something that is intrinsically pleasurable, though this is included in the term. I regard reward as something that tends to ‘reinforce’ the behaviour that brought it about; it encourages us to do again whatever caused the reward**. So, ‘reward value’ may be thought of as the degree to which an event or stimulus is either pleasurable or reinforcing.

It has been demonstrated widely that (expected) reward value may be encoded in a region of the basal ganglia known as the ventral striatum (though trying to figure out whether it’s pronounced stree-ah-tum, stree-ay-tum or stry-ay-tum has robbed me of a disproportionate amount of good procrastination time). This part of the brain is also implicated in influencing the actions we take based on motivational information. Projecting to this area is a region known as orbitofrontal cortex. This is a ‘new’ part of the brain in evolutionary terms, and sits just above the eyeballs. It too appears to represent information relating to reward value, and has also been implicated in high level functions like suppressing instinctive responses and urges. People who have damage to this region often show behavioural dysfunction such as impulsitivity and compulsiveness.***

So – where does chocolate come into all this? I can only speak for myself of course, but I certainly find chocolate rewarding. Spectacularly rewarding. So rewarding that I’ve had to stop bringing spare change to work since the arrival of ‘Claudia’, our beautiful new departmental vending machine, for fear of ending up the size of a house before the year is out (it’s the galaxy caramels that really do a lot of damage). To study the – rewarding, amongst other – effects of the sight and taste of chocolate on neural activity, Edmund Rolls and Ciara McCabe of Oxford University**** performed an fMRI study examining neural responses to chocolate in chocolate cravers and non-cravers. The results were pretty interesting.

First of all, cravers showed more brain activity than non-cravers at the sight and the taste of chocolate in the orbitofrontal cortex (which I have mentioned above). Even more interesting, sight and taste combined produced an effect even greater than the sum of the effects of sight and taste alone, and this was also yet more pronounced in cravers. Cravers also showed a high correlation between brain activity here and how pleasant they said they found chocolate. This shows that the higher the ‘pleasantness’ rating, the stronger the activity was. Again, this was the case more for cravers than non cravers.

What is really interesting though, is that brain regions involved directly in the representation of taste, most specifically the anterior insula, did not show greater activation in cravers. Neither was activity here correlated with pleasantness ratings. Also, while ventral striatum showed greater activation in cravers at the sight of chocolate, there was no difference here between cravers and non cravers for the taste of chocolate.

This might all seem like an awful lot of interactions, but let’s look at it simply: brain regions involved in representing reward value, and those involved in suppressing instinctive behaviour, were generally more active in cravers, particularly when anticipating – rather than consuming – chocolate. Regions involved in taste showed no difference. Cravers also seemed more aware of their own future responses to receiving chocolate. The upshot is that it probably isn’t the actual sensory experience of eating chocolate that influences cravings, but that the anticipation of a pleasurable experience is greater for cravers, and the subsequent reward is represented to a greater degree. It may be that cravers have a stronger learned association between the notion of chocolate (including the sight of it) and the expectation of reward – this would explain the greater drive for cravers to eat chocolate.

So next time you find yourself craving a dairy milk, don’t be too hard on yourself. Clearly, you’re just a brilliant learner, and that association between chocolate and reward is incredibly strongly represented in your highly efficient brain. At least, that’s what I like to tell myself…

* Disclaimer bumpf: I nabbed this picture from wikipedia. I don’t own it… I think I’m allowed to use it.

**Those in the know will be surprised that I haven’t mentioned the ‘phasic dopamine’ signal here. It is certainly relevant and may well act as a ‘do-it-again’ signal. The reason I haven’t gotten into it here is because the exact nature of the signal is still hotly debated and to attempt to outline it in a paragraph or two would be to traverse a minefield/labyrinth/dreary conference hall of empassioned academics. However, watch this space, and it may well form the bulk of an extended future blog post.

***One famous case is that of Phineas Gage whose frontal lobes were severely damaged when a tamping iron impaled his skull.

****The super interested can find the original paper here