Master the Art of Blanching for Freeze-Drying

There is a lot of discussion around blanching online and whether it is desirable, useful, necessary, or superfluous.  Understanding the blanching process and the reasoning behind it is probably the best place to start because in truth, the answer is “All of the above”.

What is Blanching?

There are different methods for blanching depending on what your goals are and what you are treating.  Some are simply “dunking” or “scalding” in water that is at a rolling boil.  Others are more prolonged treatments (par-boiling), or specifically, to cause a physical change—such as deliberately compromising the tough outer skin to aid in freeze-drying (e.g. fresh grapes), and some techniques are to entirely remove the skins, as with tomatoes, in order to make a sauce.

An Example: Grapes

Typically, preparation for freeze-drying grapes included cutting each grape in half so that the interior flesh was exposed to the vacuum.  If the skin was left intact, you would get raisins, not freeze-dried grapes.  Cutting them in half at least left them looking like half-grapes instead of rendering particularly tough, chewy raisins.

That is not necessary, however, since if you blanch grapes, the flesh weakens, becomes more permeable to moisture, and allows them to freeze-dry while still retaining their appearance, size, and shape of being a grape.  If the skin splits a little bit, all the better!  Unlike rehydrating halved grapes, blanching lets you restore them, without washing away flavor in the water.

Why Do We Blanch?

Do it for “looks”

First of all, it is aesthetic.  If you’ve ever tossed some fresh broccoli in a freezer bag to use later, you’ll notice that it is very dark green and somewhat limp when you thaw it.  Why is it different from the commercial frozen broccoli that is so bright green and fresh looking?

Commercial manufacturers long ago learned that a quick blanch kept their vegetables looking bright and fresh, even after months in the freezer.  This is because blanching stops (slows actually, since it never really stops) the enzymatic processes, preserving the item’s current state.  That vivid “fresh” color translates to better sales.

Do it for “purpose”

Next, it’s functional.  Halting that enzymatic process also preserves the current flavor profile and texture.  It kills surface bacteria, reducing the chance of continued (albeit slow) deterioration.  And in some cases it is absolutely essential to reduce the toughness of vegetables such as fresh asparagus, broccoli, or cauliflower.

Do it for “ease and speed”

Getting the skins off of almonds, tomatoes, peaches, and their like is made simple with a dunk in the blanching pot!  Try blanching a tomato or ten the next time you want to make sauce.  Those skins will slide right off with just a little thumb pressure, and be no trouble at all!

Essential for Freeze Drying

Almost every fruit and veggie can benefit by blanching before freezing, and then freeze drying.  There are some exceptions of course, such as chopped onions, though some people swear by blanching onion rings (raw, obviously, without batter!). 

Anything that is delicate and naturally high in acid, like many fruits, can probably go without blanching, as well as peppers, rutabagas, leeks, chopped tomatoes, potatoes, winter squash, but there are plenty of lists online from folks with specific experience with an item that you’re trying for the first time.  If it can lose water easily, like chopped peppers, then blanching may be too harsh for it.

How Do You Blanch?

There is more to it than merely putting something in boiling water.  Duration varies widely depending on the item.  Artichoke hearts need time for the heat to reach all the way to the center, so they’ll have to stay in for seven minutes.  Green beans will only take about three minutes. Even then, you’re not done. 

First the blanching water has to be a large enough quantity to retain most of its heat when food is added.  People often recommend a gallon per pound.  First (for vegetables only) add a couple of large pinches of salt as the water starts to boil—in the neighborhood of two teaspoons or more in a large pot.  Once it is at a true rolling boil (defined as a boil that doesn’t stop when you stir it with a spoon), use a wire basket to add the batch of veggies.  The boil will slow or stop and you have to wait for the boil to resume, and then start timing.

However, once the time is up, you have to remove them right away and plunge them into ice water, (or running cold water) to stop the cooking immediately.  You’re not trying to cook them completely—you’re being a home-chemist and altering the processes at work inside the food itself.  You’re now a scientist!

In scientific terms, the enzymatic processes that you’re altering needs to stop quickly.  If you under-blanch something, those processes can be sped up.  If you don’t quench the blanch(shocking the item in cold water), the cooking process will continue and the food may get mushy and unappetizing. 

In order to stop this partial-cooking, you should quench your items for the same amount of time that you boiled them.  Those artichoke hearts will take the same seven minutes to cool, but snap beans will only take three.

There are charts available online (go to a reputable source) for everything you would like to blanch.  Grapes don’t need a timer as much since they have a neat characteristic.  They sink when you put them in boiling water, and when the blanching is complete, they float to the surface so you can scoop them up with a slotted spoon and get them into the icy water bath. This is because they expand, becoming less dense, stretching their skins (or even splitting it a little bit) making them ideal for freeze-drying.


It is important to remove all excess moisture from the blanched food, too.  Get those beans and things nicely toweled off before you put them on the freeze drying trays.  Extra water really slows the process.

Some of the best advice you can get is to pre-freeze these newly blanched items in a deep freezer (or your fridge freezer) to shorten the cycle time in your freeze dryer.  It’s not a question of operating costs, just time itself.  The faster it goes the more you can get done!

Stack those trays if you have stacking corners, or save some wine corks and cut out a notch in them so you can make your own.  Your fridge freezer is always going to be working, but an empty freezer works much harder than a full one because of thermal inertia.  When you open the door all the cold air falls out unless cold is trapped in the contents to make it almost immediate freezing again when the door is closed.  Keep it full!

The Takeaway

Blanching can make food prep easier, such as removing peach or tomato skins prior to further processing.  It certainly kills off surface bacteria guaranteeing better, longer storage whether frozen or freeze-dried.  It preserves color, vitamin content, appearance, texture, and just generally improves your food.

Best of all, for certain thick-skinned foods, like grapes and blueberries, it prevents them from turning into wrinkled, unappetizing, little raisin-like things.  By using heat to break down their tough skins, they can lose moisture properly and retain their shape and appearance.  Not incidentally, this also makes them easy to rehydrate to almost their original fresh condition.

This holds true for so many things, too.  Is your celery mushy?  Are you carrots limp?  All these little problems disappear when you take the extra step to blanch before you freeze or freeze dry.

Does the thought of taking on this task make you blanch?  Don’t fret; it’s easy, and your preserving-reputation will go through the roof because…you’re now a kitchen chemist—not just a cook!

Leave a comment

Please note, comments must be approved before they are published