Jamie Oliver's Food Revolution

[by Naveen]

As I struggle through the latest round of problem sets, it's easy to lose perspective and become entrapped in a web of partial differential equations. However, Jamie Oliver's Food Revolution project has been on my mind a lot lately and helps me keep in mind what really matters. At the latest TED conference earlier this year, he proposed his wish to "to create a strong, sustainable movement to educate every child about food, inspire families to cook again and empower people everywhere to fight obesity.”

I feel totally disconnected from the public school system right now, but his message resonates me for several reasons:
  • I'm helping develop the "Science of Cooking" class to be taught at Harvard this fall. The class itself is only open to Harvard undergraduates, but many other people have expressed interest in the course. I think that food is a great way to introduce all types of people (including elementary school students) to topics ranging from physics and chemistry to neuroscience and ecology.
  • I'm also helping organize a TEDx event in Cambridge, inspired by Jamie Oliver's TED wish. I've already discovered many fascinating new ways of thinking about the issues he raised by reaching out to potential speakers.
  • I recently finished reading Mindless Eating by Brian Wansink, Born Round by Frank Bruni, and Switch by Chip and Dan Heath, which explore the intersections of food, psychology, and behavior change.
  • One of my role models for giving effective presentations, Garr Reynolds, is similarly inspired by the Food Revolution project (see here or here, for instance) and his written several times about the contrasts between fast food in America and traditional Japanese cuisine.
I'd love to hear more about your thoughts of Jamie Oliver's project, but now I need to return to the realm of reaction-diffusion equations.


Cheese rind: eating my research

[by Naveen]

For my microbiology class final paper, I am planning to write about the microbiology of cheese rinds. Until recently, I didn't appreciate that nearly all cheese rinds (with the except of wax-coated cheeses), are thick layers of microbial communities, known as biofilms. For the past year, I was studying biofilms in a rather different context. I was examining a single strain of bacteria, known as Bacillus subtilis, to try to understand the mechanics of its growth. The cheese rinds are far more complicated, with successive waves of colonization by various microbes over the course of several months:
  1. Lactic acid bacteria are first to the scene and convert lactose to lactic acid.
  2. Yeast cells eat the lactic acid, which de-acidifies the cheese.
  3. New waves of bacteria can now colonize the curd. If the cheese is brined, then these are predominantly salt-tolerant bacteria.
  4. Fungi can also colonize the cheese later in the aging process.
Each of these microbes has its own mix of peptases and lipases, which can break down the proteins and fats, respectively, in the curd to produce flavorful compounds and change the texture of the cheese. One of the first people to thoroughly study cheese from a microbiological perspective was Sister Noella Marcellino, also known as the Cheese Nun, who got a Fulbright grant to travel around France and study the micro-ecology of all types of artisan cheeses. Below is a figure from one of her papers (N Marcellino and D R Benson, Applied and Environmental Microbiology, Nov. 1992, p. 3448-3454), which shows some of the microbial diversity living on the surface of a piece of St. Nectaire cheese:

At the top (a) are fungal spores and collapsed hyphae (chains of cells). Below that (b) is a layer of yeast and bacterial colonies. More fungal hyphae can be seen growing further inwards (c). The boundary with the curd (d) can be seen near the bottom of the figure. The total thickness of the rind is about 1.5 mm.

Categorizing this microbial diversity is just the first step. In my paper for the class, I am proposing a study to figure out how these microbes are interacting. One well-known method is through quorum-sensing, in which bacteria send out small signal molecules into their environment and listen for the concentration of these same molecules. If there are a lot of the same species around, the concentration of these molecules is high, so the bacteria know they are not alone and start behaving in new ways, such as forming a biofilm.

Cheese is a great system for studying microbial ecology, since it's more interesting a single species on an agar plate, but still far simpler than the overwhelming diversity found in nature. It's another potential topic for my future gastro-science PhD.


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.