The Can It Academy Series: 3 of 12

food preservation

Class #3 of the Can It Academy Food Preservation and Cottage Arts Certificate Program: Dehydration, Freezing, and Vacuum-Packing

Week three of the food preservation class was a huge science lesson on water activity levels in food and what the safety levels are regarding bacteria growth, etc… Chef Miller tied it all together while teaching us proper dehydration methods and equipment, as well as proper freezer storage, flash-freezing at home, and vacuum-sealing for freshness.

Why is it important to know a food’s water activity? To predict the growth of bacteria and mold, of course.  We can leave most of this science up to the commercial producers of food to figure out, but having this knowledge helps us home cooks/food preservers understand the science behind what we are doing and why it’s important to do it right.

food preservation

Many commercial producers use this handy-dandy super expensive meter to figure out water levels. Pure water, by the way, reads a 1.0 level; salt water a 0.76.  Bacteria  thrives in 0.9 – 0.6, so foods under 0.6 are deemed safe from bacterial growth.  These are usually dry goods or items like honey. Here’s a bit more info from UC Davis.

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On dehydration: Dehydrating food came before cooked food.  It is thought that our ancestors started cooking as a means to speed up the dehydration process. Chef Miller went over the different types of dehydration techniques – from just leaving food out in the sun, to solar dehydrators, to the oven and electric dehydrators, as well.

To dehydrate in the sun, you will need 80 degree weather and 60% humidity or less.  And although we could certainly have taken advantage of that this past summer in Los Angeles, it’s a bit tougher to do elsewhere or in the cooler months. You’ll also need to bring your food items in at night because of the increased humidity.  A solar dehydrator helps solve some of these issues. It’s useful at 60% humidity and the dew point is usually not a factor. There are a lot of great tutorials online on how to build a solar dehydrator. Below is a cheap electric dehydrator and a hanging sun dehydrator, handy to have for keeping creatures out and The Big One.


The oven, set to below 140 degrees, is another way to dehydrate; as well as electric dehydrators (like the one in the photo above left and below left). The one below left can also be used to make yogurt and bread proofing. Handy! We used this model for a few class demonstrations…

electric dehydrator

We did a quick fruit puree with applesauce as the binder and made fruit leather in about two hours (pouring it on the silicone pad, bottom left, and adding coconut to the one on the bottom right.)  The photo top right shows cut up pieces of salsa. And Chef Miller also did a marinara sauce to show us the great dehydration applications for sauces, too.

fruit leather

Chef Miller made a batch of dehydrated sauerkraut, then ground and shifted it to a powder for a recipe he’s working on.  Dehydrators are not just for drying fruit – the possibilities are endless!

Dehydrated kale with a splash of balsamic for flavor (done in about an hour):

dehydrated kale

dehydrated kale

dehydrated kale

dried sauerkraut

Dehydrated sauerkraut  

This is dehydrated and ground onion and garlic – and the fresh taste of homemade powders was night and day from store bought!  Below are minced, dried onions along with flash-frozen berries.

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Freezing: Chef Miller went over in detail the best practices for freezing.  He did his own make-shift flash freezing: first by (carefully) crushing dry ice and placing it on a cookie sheet, then placing the cookie sheet in a cooler with another cookie sheet on top with fresh berries.  Within 20 minutes, we were sampling frozen fruit that tasted as fresh as it did previously with a cold, denser consistency as it was still frozen, of course. That was a great snack and I need to remember this technique for warm summer days.  Frozen fruit in photo above right, and prepping it, below:

freezing fruit

Great take-away tips for freezing:

  • Use bags (or containers) designed specifically for freezing.  (I have to admit, I always thought the bags in the store that said ‘freezer’ were just a marketing ploy to get more money out of me!  Apparently they really are made just for freezer usage! Duh!)
  • Bag items, remove all air, and store flat until frozen – then you can stack them.
  • The most important part of freezing is a marker!  Write down what it is, the amount, and the date you froze it.
  • Don’t fill containers more than half way.
  • Oxygen = Freezer Burn! Remove the air by either using a straw to suck air out or a vacuum-sealing device like this one, below left…

vacuum sealer

We spent most of this class taking notes, as there was so much information. Hopefully this will help give you some great ideas on additional ways to preserve the fruits and vegetables in your home.  There really is no reason anything should go to waste. There are times when I buy too much, grow too much, or have food that might go bad if I’m going out of town. So why not slice it up evenly and either dehydrate it or freeze it?

Make sure to come back for class #4 next week. You can find previous classes here:

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