Can You Slice Onions the Day Before?

One of the most important tasks in a professional kitchen is Mis En Place, which is basically preparing everything that is needed before cooking.

This is extremely important for several reasons: all ingredients are sliced, diced, and measured, time-consuming special prep like blending fruits or toasting nuts are out of the way, the actual cooking process is made easier, and so many more.

However, there is some fresh produce that must be readied only a few minutes before firing up the stovetop to maintain its freshness or to prevent certain chemical reactions which may change the flavor of the dish or make it go bad faster.

Some say that the aromatic vegetable onion is one of these.
So what’s the deal with slicing these beforehand?

Slicing onions the day before is alright as long as these are stored in an air-tight container and then kept in the fridge. The flavor will change because it will be more pungent but this won’t go bad that fast and be toxic, as some people might attest to.

Then again, it is hard contradicting age-old beliefs at times.

And believe it or not, this misconception has been around for centuries.

Separating the Rumors From the Truth

The notion that eating a piece of onion which have been sliced a day ago can cause food poisoning arises out of the belief that this edible bulb draws in the air around it, including the negative elements like disease-causing microorganisms and viruses.

Yes, some will swear that this vegetable is a good remedy for the common cold.

Up to this day, you will still hear people putting cut-up bulbs inside the socks and then wearing them overnight while others just leave a few inside a room of a sick individual and keep it there while that person is sleeping.

The former stems from an old Chinese reflexology practice, which considers the feet to be connected to various internal organs.

The latter originates from the 1500s when the Bubonic Plague afflicted swaths of Europe.

The National Onion Association of the time urged people to place cut-up pieces of this pungent bulb around the house to eradicate the miasma or noxious air.

Understanding the Science

Scientific studies have not found a direct correlation between onions and their capability to cleanse the air or treat certain diseases.

But as there is no study to refute the ancient belief, some doctors also say there is nothing wrong with these practices.

It won’t cure you but it won’t make things worse either.

Also, this vegetable – like many others – is good for you. It is a rich source of Vitamin C which strengthens the immune system and is packed with flavonoids which are said to lower the risk of cancer and other inflammatory diseases.

It’s Not Poisonous…

But it won’t taste good either.

With all that, the belief that a day-old pre-sliced onion can kill you does not have any scientific basis.

However, cutting these in advance does make it more pungent because when an onion is sliced, the cells burst and release alliinases.

This enzyme then interacts with the sulfur-packed amino acids that form thiosulfinates.

You would notice the strong sulfurous smell which may circulate inside the fridge or room.

The longer it sits uncovered, the stronger that smell will be.

Worse, the sharp taste just might overpower the other flavors in your dish.

That’s bad, particularly if you just want this aromatic to merely mingle with the other seasonings and ingredients and not be the focal point of the dish.

The Right Way to Prep

If you need to Mis En Place several hours to a day before you cook, here are some helpful tips about our featured ingredient:

• Store in an airtight container

If you’re having a piece, wrapping each half in a plastic film then putting it in the fridge.

If you’re dicing, slicing, or julienning, place everything in a container and then keep it in the fridge.

Better yet, store everything in separate air-tight containers.

You don’t want this to mingle too long with other ingredients because the ensuing chemical reactions might ruin it all the next day.

• Rinse with water before using

If you aren’t fond of the strong taste or smell, rinse the sliced pieces in cold water right before you’re going to use it.

This effectively takes that pungency.

• Base your Mis En Place decision with the dish you’re making

If it’s a stew, go ahead and do this earlier.

If you’re going to use this as a soup base or eat this fresh in a burger or a salad, it would be best to do your chopping board chores right before you cook.

• Peel today, slice tomorrow

The best way to go is to peel this vegetable, keep this in the fridge overnight in an airtight container and do your slicing and dicing the next day.

Aside from ensuring that your food is fresh, you won’t have to worry about too much onion in your dish.

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My name is Andy Wang, and I'm a retired chef. I used to work at the City Vineyard restaurant in NYC. I also had a culinary degree from the Institute of Culinary Education in New York. And this blog is where I share my love for knives and cooking with people like you.