Mass Transfer of Cooking-Chemical Engineering through Cooking


In this segment of Chemical Engineering in Cooking, we are going to explore the mass transfer principles behind events such as making Kool-Aid, and mixing pancake batter, to brewing coffee. As a child, almost everyone has made Kool-Aid, or at least watched their mother do it; however, not everyone has stopped to wonder why the water turns red and tastes sugary. The reason why is due to a concept called mass transfer.

Mass transfer is another important concept in the field of chemical engineering and science in general. Mass transfer is the movement of something that has mass. Since all matter has mass, then mass transfer just refers to the movement of all things. Mass has the tendency to move from an area of high concentration to an area of low concentration. What we mean by that is, it moves from an area where it is crowded to a more open space. This is just how people act in a crowd. To avoid feeling claustrophobic, many people will move from the center of a crowd to the outskirts to get more breathing room. Everything in nature does this, which is the basis behind mass transfer.

Mass transfer is often seen in many day-to-day situations. Have you ever sprayed perfume in one corner of the room and smelled it a few seconds later in another corner of the room? The perfume moved from the area where it was sprayed (high concentration) to an area where there was less perfume (low concentration). Mass transfer can also be seen when a glass of water is left on the table overnight. The next morning the glass is found empty. What happened to the water? Was it spilled? Probably not. The water probably turned into water vapor and spread into the room where there was a low concentration of water vapor. How many of you have ever poured bubble bath into a tub of running water? The bubbles don't just stay in the area where you poured them. Instead, they move throughout the bathtub evenly. This is just yet another example of mass transfer.

The types of mass transfer discussed above are examples of diffusion. The bath bubbles diffuse through the bathtub and the perfume diffuses through the air in the room. Diffusion can also be related to the example of making Kool-Aid.

Stirring the Kool-Aid to mix the water and sugar is another type of mass transfer called convection. Convection is the movement of mass due to forced fluid movement. Fluid, such as water, is forced to move when it is stirred. Convective mass transfer is a faster mass transfer than diffusion. Convective mass transfer is commonly applied to many cooking recipes. Whenever stirring is involved, you are applying convective mass transfer theories. Using a blender, beating an egg, adding food coloring to Christmas cookies, and kneading bread dough are all examples of mass transfer due to convection.

Why do many cookie recipes call for using a mixer on medium speed as opposed to mixing the dough with a spoon? See Answer.

Proceed to Diffusion
Proceed to making Kool-Aid
Proceed to mixing pancake batter
Proceed to making fruit salad
Proceed to Fun with Spaghetti

Proceed to Brewing Coffee

[Introduction | Kinetics | Heat Transfer | Mass Transfer | Bibliography]

This project was funded in part by the National Science Foundation and is advised by Dr. Masel and Dr. Blowers at the University of Arizona.

2007 Arizona Board of Regents for The University of Arizona