PhD in Chocolate

[by Naveen]


a model of the solar system, constructed by master chocolatier Enric Rovira, one of the inspirations for this post

My latest obsession has been the science of chocolate. After several literature searches and interviews with people about the transformation from cocoa pods to chocolate bar, I'm starting to get a sense of feasible thesis topics, which would be both scientifically interesting and industrially relevant (as well as delicious). Two main frontiers in the chocolate world that can be explored from a soft matter physics perspective seem to be:

1. Design a simple, reliable "temper-meter." By melting and cooling molten chocolate in a specific way, chocolate makers can create a solid bar that is glossy on the surface and breaks cleanly. If this procedure isn't done right, the chocolate can be crumbly and develop an ugly whitish coating on the surface, called bloom. The main difference relates to how the cocoa fat molecules are stacked within the chocolate: well-tempered chocolate has all these molecules packed tightly together. Unfortunately, the only way to determine this conclusively is with x-ray diffraction, which isn't especially convenient for artisan chocolate makers. A more common method is to measure the temperature of the chocolate over time as it cools, which works okay, but apparently there is still considerable room for improvement.
2. Glossy coated almonds (or nibs, cocoa beans, etc.). In a technique known as panning, the nuts are placed in a large rotating vat and molten chocolate is slowly drizzled over them. As the nuts tumble around, a layer of chocolate gradually develops around each nut. Somewhat surprisingly, the resulting coating is smooth and fairly resistant to bloom, despite not undergoing the specific tempering process. However, it's still tricky to attain a glossy, rather than matte, surface on the product, so additional coatings are typically used in the industry.

Armed with my physics knowledge of mechanics, electricity and magnetism, acoustics, and (high-school level) chemistry, I think that I can discover some interesting things while I pursue these problems. If I'm incredibly lucky, I could help solve some major problems in the chocolate industry. At worst, I'll have fun playing around with chocolate.

what (free stuff) i got out of tedxcambridge

[by john]

as naveen alluded to, tedx was such a phenomenal event that it has so far eluded my attempts at summary or theme-threading. i learned so many things, from academic, communication, and even emotional standpoints, but i shouldn't write them all down, lest i use my budget of run-on sentences for the year.

instead, i'll look back via the very grad student lens of the freebies i got.

handshakes

i'm so glad i stumbled into a volunteer position that let me meet every speaker. mike and i were running the floor - mike the slides, me the stage - so we got to chat with and advise each speaker during their practice slots.

all this small talk and handshaking helped de-deify food gods like kenji alt and wylie dufresne, while also putting faces on some remarkable ideas and accomplishments, such as david waters' community servings.

files

the presentations all ran off of my computer, which means that i now have gigabytes of food porn (dufresne had a 14 minute slide deck) and great stock images like this from john gertsen:

see those wigs? those guys are colonizing, not thinking about what's in the bowl.

chocolate

francisco migoya supplied the audience with free chocolate-maple-brioche-bacon bars, delicious examples of his pursuit of non-traditional food pairings. my pimenton-cantelope combination last night seems downright pedestrian compared to that. which is good; it gives me license to explore.

straws

besides defending the thesis that the company you keep at a bar matters more than the drinks you drink there, john gertsen (and ted, of no. 9 park) presented an à la minute pousse-café. this strange collection of french words was made by raising two straws - one filled with bitters, one with cassis - up out of a glass of soda. the bitters floated, the cassis sank, et voila, layers!

the two barmen came well equipped with lots of straws, and i offhandedly mentioned i might steal some. (note: i have been lusting after such black straws for a while now - they're way more elegant for testing a drink than shaw's brand bendy straws.) when john gave the go-ahead, the cocktail geek in me sprang into action, stuffing these black straws into a gallon freezer bag.

well, john and i had been talking earlier that, despite his talk's message, it is alright to geek out every now and then. (phew!) so i just laughed after my kleptomania subsided, knowing that straws are cool, but the friends sipping through them will take precedence.

inside jokes

when you charge a chef's ipod on your computer, you find out what they name it:

awesome.

Chocolate: gateway into the sciences

[by Naveen]

My gastroscience research for the past month has focused on the physics of chocolate. It's taken me a while to write this post since I've been a bit overwhelmed by all the science involved in one of my favorite foods. Below is just a small subset of the questions one can investigate:

• Botany: where do cocoa plants grow?
• Microbiology: what's the best way to ferment the cocoa pods?
• Organic chemistry: what are the flavor molecules in cocoa particles?
• Physical chemistry: what is the structure of the cocoa fat?
• Rheology: what is the viscous/elastic behavior of chocolate at different temperatures?
• Physiology: how do we taste chocolate?
• Neuroscience: how does eating chocolate affect one's mood?

Since several books and numerous research articles have been written about the subject, in this post I'll just focus on the phase transitions in the cocoa fat. In elementary school I learned about phase transitions like ice melting or water boiling, but chocolate is way more complicated. One can't take a simplistic view when it comes to producing chocolate that has a smooth, glossy surface and that breaks cleanly (aka "tempering").

The fats in chocolate, like most foods, are triacylglyerides (TAGs), which have three fatty acids attached to a carbon backbone (#1 in the figure below). The fatty acids (labelled R1, R2, and R3), can be either saturated (straight) or unsaturated (with a kink), with the center fatty acid often being unsaturated (#2). The composition of fats changes depending on where the cocoa pods were harvested. Unlike the E-shape shown in the chemical formula, the fatty acids distribute themselves on opposite sides of the carbon backbone to form a chair or tuning fork shape (#3). These TAGs can stack in several different ways. In one of these forms, the chairs pack in a double length configuration (#4). A tighter packing is possible in a triple-length configuration (#5). This tighter packing has an observable effect, a chocolate bar can contract by 1 or 2% if it solidifies in this configuration.

The type of packing is determined by the temperature of the cocoa fat. The tighter packings require more energy to form, so their melting point is higher. In cocoa butter, there are six different types of packings (aka phases), labelled with either Greek letters or Roman numerals. The fifth form (Form V or Beta-1) is the goal when tempering the chocolate to achieve a nice, glossy appearance. In the tempering process, molten chocolate is cooled enough to allow the Form V crystals to form, but also to keep the temperature above the melting points of the undesired forms. Once enough Form V crystals have formed, the chocolate can be cooled all the way down to room temperature.

This is where things get complicated and I'm still working to sort out the details. The crystals can form over a range of temperatures and the crystal structures can transform among themselves in particular ways over times that can range from minutes to months. This is the cause of chocolate bloom, in which a dull, whitish coating can form on top of chocolate that is stored improperly. I've tried to show a rough idea of the types of crystals that form at different temperatures in the figure below, but I welcome the feedback from anyone with more authoritative knowledge on the subject.

This is just the tip of the iceberg when it comes to phase transitions in chocolate. The rate at which crystals nucleate, the effect of shearing the chocolate, the presence of other fats (like in milk chocolate), and numerous other variables can affect this process. There is certainly enough left to explore to fill an entire PhD dissertation.