Food Physics: Dessert Edition

By Cassie Martin

It’s that time of year again, readers! Every week from September through December, hundreds of people line the halls of Harvard University eagerly waiting to get their hands on one of the hottest tickets in town and a chance to sample some truly delectable creations. If you love food and are even mildly interested in science, then Harvard’s free lecture series, Science + Cooking: From Haute Cuisine to the Science of Soft Matter, is the place to be.

Last night’s lecture featured Bill Yosses, the White House pastry chef and frequent contributor to the series. He kicked-off the night with a Youtube video of two Tesla coils playing House of the Rising Sun. At first I thought it was a techno cover until I saw the bolts of electricity flash across the screen. “Oh, this is gonna be good,” I thought.

Yosses presented the concept of elasticity and how it informs the texture and flavor of desserts to make for an incredible dining experience. The first half of the lecture felt like a high school physics class. We watched cool, old-timey gadgets build up and discharge static electricity; we watched Yosses bend the glowing green ray inside of a cathode ray tube with a magnetic field; we brushed up on surface tension; and we picked up some cool science history facts along the way. 

Did you know that Benjamin Franklin used one of the gadgets–called a Leyden jar–in his infamous kite experiment? Neither did I. Franklin’s portrait is also the only portrait in the White House not of a president or first lady.

White House Pasty Chef Bill Yosses teaches the crowd about electricity and magnetism. He uses these concepts to create intricate desserts. Photo courtesy of Cassie Martin.
White House Pasty Chef Bill Yosses teaches the crowd about electricity and magnetism. He uses these concepts to create intricate desserts. A Leyden jar can also be seen in the foreground. Photo courtesy of Cassie Martin.

After a quick overview of the science, the demonstrations began. Yosses’ demos covered some building blocks of desserts including foam, gel, and sugar–here are the coolest ones:

Peaches are one of my favorite fruits, so when he brought out peach puree to make foam I was excited, hoping I would get to taste it. Foam is a congregation of bubbles held together by surface tension and electrical charge that builds up between between the molecules. Immersion blenders are generally used to beat air into liquids, which releases more flavor molecules, according to Yosses. At one point, he pulled out a metal container brimming with fog. Liquid nitrogen is used by chefs to manipulate surface tension to get the right consistency (without adding unhealthy ingredients such as butter or oil) and preserve the flavor of food. It has a much lower boiling point than water, so it requires less energy to disrupt bonds which means that fewer flavor molecules are lost to heat. 

For one of the gel demos, which demonstrated bond formation and the cross-linking of molecules, he dipped a spoonful of hibiscus sauce into a mixture of water and a gelling agent (I’m unsure of it’s name). The gelling agent bound to the sauce, forming a skin around the outside. “It’s like an egg yolk, still liquid in the middle,” he said. After a few minutes, he scooped out a purple ball and added it to the plate of desserts.

Yosses ended the evening sculpting, but it looked more like glass-blowing. Under a red heat lamp sat a blob of sugar stuck between liquid and solid phases. Chefs commonly refer this as glass–the sugar is malleable, has a shiny quality, and becomes very brittle once it cools. Initial heating to 320 degrees Fahrenheit disrupts the crystalline structure resulting in a soft consistency, but the structure reforms upon cooling. Yosses carefully stretched the ball of sugar, wrapping small strands around a plastic stick. When he removed the wrapped strands of sugar from under the heat lamp, he was left with a hardened sugar coil which was promptly added to the dessert plate.

A plate of sugar and gelatin based desserts made by manipulating their molecular structure. Photo courtesy of Cassie Martin.
A plate of sugar and gelatin based desserts made by manipulating their molecular structure. Photo courtesy of Cassie Martin.

The Take Away:

Molecular gastronomy is giving chefs new tools and re-purposing old ones to make food better–not just by improving how a dessert looks or tastes, but it’s healthfulness too. Manipulating food at the molecular level makes it possible to achieve the same textures and flavors without adding notoriously unhealthy ingredients such as butter, oil, and fat. Yosses hopes his legacy will be known for “including desserts as part of a healthy diet, restoring food as a pleasurable experience, and preserving flavor,” he said.

The Critique:

The lecture was fun and informative. I learned something new and I sampled cocoa beans and fruit gel–can’t get much better than that. The one criticism I have is that there were a few times where I was unsure of how the science applied to his cooking techniques. I wish he would have explained the science alongside his demos instead of separating the two, so I could get a better understanding of what I was watching.

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