Ask your students if they have ever seen anyone start to cry in the kitchen while preparing a meal. It could be because the person doesn’t want to make dinner, but it’s more likely something that the cook can’t control. Or can it be controlled?
Many recipes start with onions and garlic as a base. The person making the meal will chop these ingredients before beginning to prepare a recipe. When you see someone begin to tear up when chopping an onion, there’s actually a scientific reason behind it.
Onions are considered to be root vegetables as the entire bulb is grown in the ground. The onion grows to maturity according to the scheduled amount of daylight. Different varieties are grown across the country. When planted, the onion has green stalks that grow above the ground, and the bulb is below soil level. The onion plant that is allowed to grow for the longest amount of time will produce the biggest bulb.
Explain to your students there are different types of onions as far as shape, and size, and color. Onions are part of the “Genus Allium” family. They can be grown on farms and also in the home garden. Wondering who grew the world’s largest onion? Share this fun fact with your students: http://www.dailymail.co.uk/news/article-2203295/Worlds-largest-onion-weighing-18lb-smashes-records-Now-thats-eye-watering.html
Now, ask your students what vegetables can be grown in a garden. Make a list of the answers. Go over the list and ask what makes an onion different from the other vegetables? Explain that you can chop, with a knife, all the vegetables you grow in a garden or buy at a market. But, when you slice into the onion you will start to produce tears as you work with it.
This is where the science comes into your lesson. Tell your students that when onions are busy growing they are drawing in sulfur from the soil. Ask your students to come up with possible reasons why the process of cutting the onion would cause someone’s eyes to make tears. Give your students some onions to touch and observe.
Share with your students that when you start to slice an onion you are cutting and separating the cell walls of the onion. Until that moment, some enzymes in the onion have not mixed with each other, or the surrounding air. When you slice an onion the enzymes and amino acid sulfoxide chemicals are now free to mix together and form a gas. This is what produces the potent smell of an onion. The gas created from this mixture slowly starts to waft up and reach the eyes of the person cutting the onion. The resulting gas mixes with the water in your eyes to create a stinging sensation and tears.
The tears formed when slicing an onion are a kind of reactive tear. The lachrymal glands, located near the tear ducts in the eye, produce tears to flush out any irritant like dust or smoke. In this case, the irritant is the gas created from the enzymatic reaction of slicing an onion. The tears are the body’s natural defense to flush out the irritant once the gas reaches the eyes.
Is there a way to cut an onion that doesn’t result in tears? Explain to your students they are going to try three different ways of cutting an onion and make observations. First, the onion will be cut normally. Second, goggles will be worn as the onion is cut, and the third demonstration will involve cutting an onion that has been chilled in a freezer for 15 minutes. Have your students make predictions and create a list of possible outcomes. Ask for volunteers to be a part of the three demonstrations. (Make it clear that some participants will feel the effect of the irritant and a stinging sensation in their eyes.)
In conclusion, have a discussion with your students about the results. Guide the discussion to explain how chilling the onion reduces the chemical reaction that takes place when the onion is cut. The reaction of the enzymes is not as strong when they are cold.
Scientists are also working on a variety of onion with lower levels of the enzymes that produce the tears when the onion is cut. Tell your students they can share what they learned about slicing a chilled onion, to avoid tears in the kitchen, with the cook in their home. The cook will be happy to get such a tip!
You will need:
3 large white or yellow onions
1 knife for cutting
(You can do the cutting and have your students gathered around you in each group for the same results.)
Several pairs of goggles
A cooler filled with ice or frozen freezer packs