Hint: How to Break Down an Onion

Recorded October 30, 2013. Revised November 25, 2019. Revised May 29, 2021  

This discussion of how to turn an onion into slices and reasonably-uniform dice was adopted from a science of cooking course I teach to older adults at the local university. My hope was to show how understanding onion anatomy helpss to produce the best shape for the dish you are preparing, efficiently, safely, and with a minimum of tears.

An onion is formed of concentric layers made of tightly-packed tubules that transport water from the roots up to the leaves and carry nutrients made in the leaves down to the bulb for storage. These vessels run parallel to each other from root to leaf tip. This means that when you slice in the 'polar' direction, that is, parallel to polar axis from root to stem, you will cause the least cellular damage. Use this cut when you want onion pieces to keep their shape during cooking or perhaps in a salad. On the other hand, the equatorial cut slices through all the vessels, and so causes more cellular injury. Use this cut when you want the onions to break down, such as for caramelized onions.

To form dice, cut the onion in half through the poles. I usually rinse and dry the halves to reduce eye irritation. Retain a portion of the root when peeling. Then slice in the polar direction along the radii following the shallow grooves in the surface, stopping just short of the root. Then, to form dice of about equal thickness, cut across the leaves in the equatorial plane. No need to use the slow, dangerous horizontal cut that is classic.

Injured cells release an irritant chemical, syn-propanethial-S-oxide. Use a sharp knife to produce in fewer tears. Also, cold onions release less irritant because chemistry is slower when it's cold.