Sparking Onions in the Microwave

by Wendy Wedum, Pondera County Extension

One of the fun things about being an Extension Agent and providing education are the many learning opportunities I get when people ask us questions.  It is fun because we often get all kinds of different questions.  The learning opportunity comes when I do not know the answer and need to do some research to learn the answer.

For example, last week a woman came into the office and asked, “Why do my onions spark in the microwave?”  She explained that her husband likes to have onions on his burgers and that she cuts them up and partially cooks them in the microwave first – this is where the sparking comes from.

I am familiar with peas, corn or beans bursting in the microwave from water that goes from a liquid to steam when the food is heated, but the issue with the onions causing sparks was a new one.

A search showed there are a couple possibilities why onions and other foods will spark in the microwave.  First, since all plants absorb water and nutrients from the soil, there is a strong chance that there are also small bits of dissolved minerals and metals in the veggies such as iron, potassium and copper.  According to the US Department of Agriculture, root vegetables are very good at storing these metallic nutrients.

Second, it could also be related to how the onion was cut up.  If the pieces are irregular or unevenly sized pieces, the smaller pieces are more likely cook more quickly and burn than larger pieces.  The other thing that happens are all the pointy cut edges or leafy edges of plants such as kale make opportunities for a spark to form. 

There are ways to reduce the amount of sparking.  Instead of cutting the vegetables into small pieces, leave them as large as possible and cut them after the cooking.  For the onion, cook whole slices.  This lowers the number of angular edges.  If cutting vegetables, then cut into uniform sizes – this prevents the pieces that are too small from cooking too fast.

Another method is to arrange the food evenly on the plate or in the bowl.  A small amount of liquid could be added or put a light covering over the food.  Then stir, turn or rotate the food halfway through the cooking time.  This helps even out the cooking and will reduce cold spots.

Link to Safely Microwaving Foods:
 https://www.fsis.usda.gov/shared/PDF/Microwave_Ovens_and_Food_Safety.pdf
Link to article on sparking foods: 

https://www.npr.org/sections/thesalt/2016/10/03/495975189/snap-crackle-kale-the-science-of-why-veggies-spark-in-the-microwave

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