## TEACHING

## PHYSICS OF LIVING SYSTEMS, IN THE LAB AND IN THE KITCHEN

When we cook, whether it's a simple omelette or a fancy duck confit, we are inadvertently preoccupied with the physical properties of every bite (e.g. elasticity and viscosity), and making physical manipulations on animal and plant tissues (e.g. diffusion, heat transfer, fermentation). In this course we combine theory with experiments, and try to understand how physical laws shape the biological world around us, and ourselves, through demonstrations from the culinary world. The course aims to foster curiosity-driven research at the intersection of physics and biology, while also highlighting the importance of combining theory with simulations and experiments. Slides of my talks are available - have fun :)

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Buon Appetito!

## Elasticity

We took a bite of chocolate candy, noticing that what makes it delicious is not just the taste, but also the symphony of textures (crunchy, soft, brittle..). This is "mouthfeel", critical in the culinary world. In physics this is described by elasticity - the response of a solid material to mechanical perturbations. In class we discussed physical properties of elasticity, and what governs theses in animal and plant tissues, as well as polymer networks such as gluten. <slides>

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Master Baker Yuval Alhadeff, previously at R2M, now at Stybel flour mill, showed us the importanc of gluten elasticiy, comparing different types of dough.

## Elasticity Lab

In the lab we explored the dependence of elasticity of plant leaves on turgor pressure (water content). Students cut strips of lettuce leaves, at different drying stages, and measure their elasticity by measuring their deformation due to different weights. Plants with more water (higher turgor pressure) can stretch less (stiffer) - just like water balloons can stretch less compare to empty balloons.

## Viscosity

Liquids are also important for mouthfeel: soy sauce is salty, leaving a thin trace on food - however chocolate sauce tends to be thick, clinging to food and our palate. In physics this is viscosity, describing the resistance of fluid to flow. We discussed how fluids shape biological systems, with external flows shaping motility (swimming strategies differ at low/high Reynolds numbers), and internal flows shaping vasculature. <slides>

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Yaniv Gur Arye, Chief Chef at Strauss, thickened wine sauce using 4 different methods, including reduction, emulsions, starch (gelatinization), and modernist thickeners (polymers).

## Viscosity Lab

In this lab student learnt a method for measuring viscosity, measuring the velocity of a sphere falling in a fluid - set by the equilibrium between gravity and fluid resistance. They measured this for fluids with different viscosity, using the modernist thickener Xantham Gum, which is based on polymer networks. In this case the dependence on density does not hold...

## Surface Tension

Surface tension is the force that holds a drop of water together, and is a direct result of the inter-molecular forces of a liquid. While this sounds of mild importance - this phenomenon allows to create solids out of gas and liquids, such as whipped cream. Surfactants (e.g. egg yolks, soap) lower surface tension, and stabilize emulsions such as mayonnaise. Examples of biological systems governed by surface tension; pulmonary surfactants and cactus water collection. <slides>

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Chef Shalom Simcha Elbert, CEO of Tenne and head of R&D at OCD, demonstrated the magic of foams and emulsions.

## Surface Tension Lab

In this lab students measured surface tension of liquids in different ways. One method was through the size of drops released form a tube, assuming this is set by an equilibrium between surface tension forces and gravity. The second methods provided a visual demonstration of surface tension, moving a straw. Students measured force tension as a function of surfactant density (soap).

## Diffusion & Random Walks

This week we move from static properties to dynamics, discussing diffusion: both its macroscopic description (Fick's law) and microscopic (random walks). We also talked about the critical role of noise in biological systems - ranging from search processes to collective dynamics. <slides>

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Mixologist Ori Goffer demonstrated spherification - a a culinary process where drops of sodium alginate are placed in calcium. As the former diffuses into the latter, it creates a delicious gel sphere.

## Diffusion Random Walks Lab

In this lab the focus was on numerical simulations. Students simulated a random walk, fitting the ensemble to a Gaussian distribution, thus relating macroscopic to microscopic characteristics. They also simulated models of anomalous diffusion, identifying differences in properties.

## Heat Transfer

We talked about the definition of heat and its dependence on temperature, as well as its microscopic origins. We then moved to discussing dynamics of heat transfer, identifying that these are identical to diffusion! <slides>

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Chef Matan Abrahams, from Hudson Restaurant, demonstrated different heating protocols on steak, comparing sous-vide with pan-seared cooking.

## Heat Transfer Lab

In the lab students placed a block of tofu on a heating plate, and measured temperature profiles along the block over time. They fitted these to the solution of the Heat Equation, as well as an approximation of the mean squared displacement, extracting tofu's heat diffusivity. Like most food, it turns out to be close to the value for water - it's main ingredient.

## Population Dynamics

Last subject of the course. We discussed exponential growth and decay, including growth of tribbles (any Trekkies?), and the exponential decay of beer froth. We also discussed prey-predator models, and in particular described the population dynamics of yeast converting sugar into alcohol and CO2 - critical for beer. <slides>

Amir Neuman and brewmaster Alon Schwartz, from Schnitt microbrewery, showed us the magic behind their perfect beers.

Cheers!