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vegetable blanching
Cooling fruits and vegetables in an ice bath immediately after blanching will stop the cooking process.

Blanching is important part of vegetable freezing process

Friday, July 9, 2021

Media Contact: Trisha Gedon | Communications Specialist | 405-744-3625 |

Asparagus, green beans, squash and okra are some favorites among vegetable growers. For many, those well-tended plants produce so much bounty, the family can’t eat it all before it spoils.

One way to preserve that summer taste is to freeze the extra fresh vegetables.

This isn’t just a matter of plucking the vegetables from the garden and tossing them in the freezer. Before freezing, many vegetables need an initial rapid-cooking process called blanching. Why is this important?

“Blanching is a term that literally means to make white, but it’s an important step in the preservation process. The goal is to ensure a higher quality finished product,” said David Hillock, Oklahoma State University Extension consumer horticulturist. “Blanching may seem like an unnecessary step, but it all comes down to the vegetable cells.” 

As living plants respire, they generate gases — mainly oxygen and carbon dioxide. These gases aren’t inhaled and exhaled in the manner humans breathe, but rather diffuse slowly into and out of a plant’s cells. Hillock said removing the remaining trapped gases helps to maintain quality by getting rid of excess oxygen, but the main benefit for frozen foods is to deactivate or “kill” the enzymes that abound in living plant tissues.

“These enzymes, or catalytic proteins, perform a host of useful functions in live plants. Some of these aren’t as useful to us as consumers. Some enzymes, for example, are responsible for the brown color that rapidly appears in cut peaches. Others can cause grassy or rancid flavors in corn and green beans,” he said. “If these enzymes aren’t deactivated by heating, they’ll continue to work very slowly in frozen foods. Therefore, most vegetables are blanched prior to freezing.”

Other benefits of blanching include cleaning the surface of dirt and organisms, brightening the color and helping to slow the loss of vitamins. It also softens or wilts vegetables and makes them easier to pack.

Barbara Brown, OSU Extension food specialist, identified the most recommended methods of blanching: water blanching and steam blanching.

“Start by washing, draining, sorting, trimming and cutting the vegetables. For water blanching, use about a gallon of boiling water per pound of prepared vegetables. Place the vegetables in the blanching basket and lower into water that is at a rolling boil. Blanching time starts as soon as the water returns to a full boil,” she said. “Remember, blanching time will vary depending on what food is being prepared for canning or freezing.”

Overblanching causes loss of flavor, color, vitamins and minerals. Underblanching stimulates the activity of enzymes and is worse than no blanching at all.

To steam blanch, use a pot with a tight-fitting lid and a basket that holds the food at least 3 inches above the boiling water. Begin timing as soon as the lid is on.

As soon as the blanching is complete, vegetables should be cooled quickly and thoroughly to stop the cooking process. Immediately plunge the basket of vegetables into a large quantity of cold water, 60 degrees Fahrenheit or below. Change the water frequently. Ice can be added to the water to keep it cold.

“The cooling process should take the same amount of time as the blanching process,” Brown said. "The National Center for Home Food Preservation provides an extensive list of blanching times for vegetables.”

There are a couple of options for freezing, too. The dry-pack method consists of packing the blanched vegetables into containers or freezer bags. Press out the air and seal tightly.

Another method is called tray-pack. Place the blanched vegetables on a shallow pan and put the pan in the freezer. As soon as the vegetables are frozen, put them into a freezer bag or container. Press out the air and seal tightly and return to them to the freezer.

OSU Extension also offers more food preservation information online.

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