Why You Can’t Defrost a Salad…Yet

Vicki Cobb
The Master Chef of Kids' Hands-on Science

---

At left in the back is a fresh spinach leaf that has been treated with trehalose, frozen and defrosted.. The droopy one has been frozen and defrosted without trehalose. It's what is giving us hope that we may soon be able to defrost a salad.

     When it comes to preserving a fresh taste in food to be eaten at some later time, nothing beats freezing it.  That was the discovery made by Clarence Birdseye in 1924. He had been working in northern Canada and noticed that fish caught by the native Canadian Inuits froze almost instantly in the frigid winter air.  It was just as delicious when cooked and eaten months later as it was on the day it was fresh.  Birdseye figured that if food was frozen quickly at very cold temperatures, large ice crystals couldn’t form to damage the food and make it mushy.  His flash-freezing process made him very rich.  

     The problem isn’t so much the freezing of food as what happens when it’s defrosted. See for yourself.  Stick a stalk of celery in your freezer. The next day defrost it.  Want to eat it?  Compare it to a fresh unfrozen stalk.  The perky structure of fresh celery is destroyed by ice.  Water has the very unusual property of expanding and taking up more space when it changes into ice than when in a liquid state.  That’s why ice cubes float and frozen unopened soda cans bulge.    Expanding ice crystals destroy the cell walls of plants.  Quickly freezing fresh food keeps the ice crystals smaller than slower freezing, but they are still large enough to destroy the cell walls of delicate vegetables like spinach or lettuce. But if you defrost frozen spinach from the supermarket it is beyond limp.  So a salad you can defrost and serve as if it were fresh has seemed like an impossible dream.  

​     Federico Gomez, a Swedish scientist, is working to change this.  Like Birdseye he took a close look at nature, specifically at plants that stay alive in very cold climates.  He discovered that they contain a sugar called trehalose (tree-HAL-ose) that works like a natural antifreeze.  Could he find a way to get trehalose into spinach leaves?  If so, would the trehalose protect the structure of the spinach and keep it crisp after defrosting?  This picture shows the results.  The leaf on the left was treated with trehalose.  The one on the right was untreated.  He froze and defrosted both.  The treated leaf is as crisp as if it had never been frozen!  

​     Just because there is success in a lab doesn’t mean a defrosted salad will show up on your dinner plate any time soon.  But these results are enough to keep the research going. 

     Move over Clarence Birdseye! ​

---

This is a nice fresh, crisp piece of celery...

...and this is the same piece of celery after being frozen and then defrosted. Yuk!

---

Cobb has revised her classic book, Science Experiments You Can Eat.  While doing her research, she came across this work of Frederico Gomez.  She bought trehalose on line and soaked some slices of parsnip and zucchini in a trehalose solution, hoping that the sugar would be absorbed by the plant cells.  But when she froze them and defrosted them, it didn't work.  Dr. Gomez got the sugar inside the plant cells by removing some water from between the cells in a vacuum chamber, soaking the leaves in a trehalose solution (which  moved the sugar into the spaces outside the cells)  and then exposing the leaves with a mild electric shock to get the sugar through the cell walls.  Vicki didn't have the equipment to do all this but she tried anyway. The book was published in 2016.

Vicki Cobb is a member of iNK's Authors on Call and is available for classroom programs through Field Trip Zoom,  a terrific technology that requires only a computer, wifi, and a webcam.  Click here to find out more.

---

MLA 8 Citation
Cobb, Vicki. "Why You Can't Defrost a Salad...Yet." Nonfiction Minute, iNK Think
     Tank, 11 01 2018, www.nonfictionminute.org/the-nonfiction-minute/
     Why-You-Can't-Defrost-a-Salad-Yet.