Kitchen Science – Sink or Float – Bobbing for Apples (and other vegetables and fruits)

It is almost Halloween and this game is often played by children. So the question is do all apples float? That led to more questions – what about pumpkins, lime, banana, pomegranates or coconuts? Why do they float or sink? Inspired by the thought, I decided to check it out on my own. I started with an apple, pomegranate, and coconut. I then started taking vegetables out of my fridge and started playing with what I have. There were some surprises.

So here are my results:

Fruits and vegetables that float: apple, banana,  tomato, madras cucumber (squash family), coconut, eggplant, capsicum and believe it or not, watermelon.

Fruits and vegetables that sink: pomegranate, lime (surprises), potato

Why do coconuts and watermelons float? Think about it!

Why do some sink and some float?

This is because of the density of the fruits/ vegetables (in this case).

Density is defined as mass per unit volume or the measure of how heavy a substance is for its bulk or volume. But what does density have to do with floating or sinking? We all have heard about Archimedes, the guy who ran out of his bathtub into the street, butt naked, shouting  “Eureka”. But do we know why he shouted Eureka? Well, he discovered the principle, ‘The Archimedes Principle’ that governs whether an object floats or sinks in a fluid. He is also credited for calculating the value of pi.

Explanation of Archimedes Principle: Archimedes Principle states that when an object is immersed in a fluid, the buoyant force is equal to the weight of the fluid displaced by the object. This is what we learned in school. But what does it mean?

Let us go back to our bobbing of apples. If our bowl is filled completely with water and then an apple is added to it, some water spills out (just like the water that spilled out of the bathtub when Archimedes got into the tub). If you weight the water that spills out, it would equal the upward force (buoyant force) on the object. From that buoyant force, the density of the apple can be determined. or we can also say that the apple is kept afloat (buoyed up) by a force equal to the weight of the water it displaces.

So, if the object’s (vegetables and fruits in this case) density is less than that of water, it will float and if its density is greater than that of water, it will sink.

Let us look at examples in the kitchen while cooking.

  1. When ravioli, and gnocchi are cooked, they first sink when you put them in the boiling water. This is because they are more dense than water. But as their starch granules expand in the hot water, their density decreases. When their density becomes less than the density of water, they float and we know that they are cooked.
  2. When we fry vadas (Indian savory fried snack) or doughnut holes in hot oil, they are less dense than oil and floats. But as the bottom surface cooks, it loses water and becomes more dense. The bottom is now more dense than the top and it turns and cooks on the other side.

Sources:

Wolke, R. L. (2005). What Einstein Told His Cook 2. New York: W.W. Norton and Company.

https://www.livescience.com/58839-archimedes-principle.html

 

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chaiwithpreethi

Daughter, Wife, Mother, Teacher, Student! Love to cook and bake, eat and travel. Continue to learn!

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