Summer has settled upon the northern hemisphere, which means that it’s time for sweet, sweet strawberries to be cheap and plentiful. But would you believe they taste even better in freeze-dried format? I wouldn’t have ever known until I happened to get on a health kick and was looking for new things to eat. I’m not sure I could have picked a more expensive snack, but that’s why we’re here — I wanted to start freeze-drying my own strawberries.
While I could have just dropped a couple grand and bought some kind of freeze-drying contraption, I just don’t have that kind of money. And besides, no good Hackaday article would have come out of that. So I started looking for alternative ways of getting the job done.
Dry Ice Is Nice
Early on in my web crawling on the topic, I came across this Valley Food Storage blog entry that seems to have just about all the information I could possibly want about the various methods of freeze-drying food. The one that caught my eye was the dry ice method, mostly because it’s only supposed to take 24 hours.
Here’s what you do, in a nutshell: wash, hull, and slice the strawberries, then put them in a resealable bag. Leave the bag open so the moisture can evaporate. Put these bags in the bottom of a large Styrofoam cooler, and lay the dry ice on top. Loosely affix the lid and wait 24 hours for the magic to happen.
I still had some questions. Does all the moisture simply evaporate? Or will there be a puddle at the bottom of the cooler that could threaten my tangy, crispy strawberries? One important question: should I break up the dry ice? My local grocer sells it in five-pound blocks, according to their site. The freeze-drying blog suggests doing a pound-for-pound match-up of fruit and dry ice, so I guess I’m freeze-drying five entire pounds of strawberries. Hopefully, this works out and I have tasty treats for a couple of weeks or months.
Preparation
In order to make this go as smoothly as possible, I bought both a strawberry huller and a combination fruit and egg slicer. Five pounds of strawberries is kind of a lot, eh? I’m thinking maybe I will break up the ice and try doing fewer strawberries in case it’s a complete failure.
I must have gotten rid of all our Styrofoam coolers, so I called the grocery store to make sure they have them. Unfortunately, my regular store doesn’t also have dry ice, but that’s okay — I kind of want to be ready with my cooler when I get the dry ice and not have to negotiate buying both while also handling the ice.
So my plan is to go out and get the cooler and the strawberries, then come back and wash the berries. Then I’ll go back out and get the dry ice and then hull and slice all the berries. In the meantime, I bought some food-safe desiccant packets that absorb moisture and change color. If this experiment works, I don’t want my crispy strawberries ruined by Midwestern humidity.
Actually Doing the Thing
So I went and bought the cooler and the strawberries. They were $2.99 for a 2 lb. box, so I bought two boxes, thinking that a little more poundage in dry ice than berries would be a good thing. I went back out to the other grocery store for the dry ice, and the person in the meat department told me they sell it in pellets now, in 3- and 6-lb. bags. So I asked for the latter. All that worrying about breaking it up for nothing!
Then it was go time. I got out my cutting board and resigned myself to hulling and slicing around 75 strawberries. But you know, it really didn’t take that long, especially once I got a rhythm going. I had no idea what the volume would be like, so I started throwing the slices into a gallon-sized bag. But then it seemed like too much mass, so I ended up with them spread across five quart-sized bags. I laid them in the bottom of the cooler in layers, and poured the dry ice pellets on top. Then I took the cooler down to the basement and made note of the time.
Since I ended up with six pounds of dry ice and only four pounds of strawberries, my intent is to check on things after 18 hours, even though it’s supposed to take 24. My concern is that the strawberries will get done drying out earlier than the 24-hour mark, and then start absorbing moisture from the air.
Fruits of Labor
I decided to check the strawberries a little early. There was no way the ice was going to last 24 hours, and I think it’s because I purposely put the lid on upside down to make it extra loose. The strawberries are almost frozen and are quite tasty, but they are nowhere near depleted of moisture. So I decided to get more ice and keep going with the experiment.
I went out and got another 6 lb. of pellets. This time, I layered everything, starting with ice in the bottom and ending with ice on top. This time, I put the lid on the right way, just loosely.
Totally Not Dry, But Tasty
Well, I checked them a few hours before the 24-hour mark, and the result looks much the same as the previous morning. Very cold berries that appear to have lost no moisture at all. They taste great, though, so I put them in the freezer to use in smoothies.
All in all, I would say that this was a good experiment. Considering I didn’t have anything I needed when I started out, I would say it was fairly cost-effective as well. Here’s how the pricing breaks down:
- 28-quart Styrofoam cooler: $4.99
- 4 lbs. of strawberries: $5.99
- 12 lbs. of dry ice at $1.99/lb.: $24
- a couple of resealable bags: $1
Total: $36, which is a little more than I paid for a big canister of freeze-dried strawberries on Amazon that lasted maybe a week. If this had worked, it would have been pretty cost-effective compared with buying them.
So, can you freeze-dry strawberries without a machine? Signs still point to yes, but I’m going to go ahead and blame the Midwestern humidity on this one. You can bet I’ll be trying this again in the winter, probably with fewer berries and smaller cooler. By the way, there was a small puddle underneath the cooler when it was all said and done.
Have you ever tried freeze-drying anything with dry ice? If so, how did it go? Do you have any tips? Let us know in the comments.
Main and thumbnail images via Unsplash
I think you mostly just froze them; to dry out they would need to be frozen for longer and probably kept in some sort of partial vacuum, because water sublimation rate from ice is bound to be slow. I wonder if clothes storage vacuum bags would work here.
Correct
Or an epoxy de-gassing setup. Some of those would be small enough to put in a cooler, though I imagine you wouldn’t want the dry ice under vacuum or it would sublimate more quickly. I suppose you could let them freeze with the dry ice first, then pull a vacuum while surrounding the chamber with more dry ice to keep it cold while the ice sublimates.
As well as vacuum you need condenser. Freeze dryers have minimum two components. Vacuum – quite a serious vacuum as other comments mentioned. Not just your Hoover. Plus very cold condenser to capture the water. That’s why they cost $10k+. Dry ice is cold enough to condense water at quite low vacuum.
>That’s why they cost $10k+.
$2300+ Google Harvest Right Small
This would be my first instinct. It might work with lower pressure in the bag. This will not work however due to CO2 filling the bag again. So there’s need for constantly sucking out the CO2 and moisture. Not worth the hazzle imo.
I bet you would have better luck if you used a conventional dehydrator first then put them into a vacuum bag with enough desiccant to absorb say 10% the dehydrated weight of the strawberries. I’m not sure if conventional dehydration step first would change the texture of the freeze dried product. And you may have a dehydrator and food sealer already.
Possibly even better results if you put them into bags with a flow net and vacuum port for fiberglass work to keep a constant vacuum on them. I think that vacuum is required for freeze drying to remove the sublimated moisture. But that will require a decent vacuum pump.
You’ve now got frozen strawberries. Stick them in a blender with cream and sugar, and you’ll make some lovely ice cream in minutes.
I’m fairly certain that you will not be able to freeze dry your food without a vacuum pump. The vacuum is needed to sublime the moisture from the food. Dry ice is essentially -78 degC. Notice the commercial machines all lay the food out similar to what you did and chill to ~-40 degF. You have the temperature. All you need is the vacuum. So dry ice on the bottom, food vacuum container connected to a vacuum pump, dry ice on top, vacuum for 24 hours should work. Sort of the reverse of a heated food dryer using a vacuum and cold.
-40 degF = -40 degC
Just saying…
I don’t know the equivalent in Banana degrees ;)
Which you definitely need to know if you are going to freeze dry banana daiquiris!
You can freeze it. You can dry it while frozen. But you can’t freeze-dry (“lyophilize”) without vacuum.
The point is to freeze it, then reduce the pressure below the triple point (<611 Pa) so liquid water can't exist, only solid or gas. In the absence of liquid, there's no surface tension, and the material retains its (frozen) shape.
Once frozen and depressurized, then *add* heat to sublimate the ice directly to vapor.
(Just found an envelope, lets use its backside) One mol of gas has the volume of 22,4 L at 0,1013 MPa, one mol water has 18 g and 1 L is about 1000 g. So the volume of the gas is about (22,4 L/ 18 g) * 1000 g/L = 1200 times the fluid. Since we want to go below the triple point and p*V=const. the gas expands to 101300 Pa/600 Pa times the volume; the vacuum pump would need to clear (101300/600)*1200=200 000 times the volume you want to dry. That’s why you absolutely want to condense as much as possible before the vacuum pump.
More precision may be added by someone with a slide rule, let’s see if the specific weight of ice (instead of water) and the amount of water in 1 kg strawberries changes the order of magnitude :)
I would say no, it might be half but still crazy amounts…
But that kind of explains it, I was sort of wondering where the freeze would figure in the actual freeze drying. I mean vacuum or low humidity are two means of drying, and low humidity can be achieved by freezing air and then heating it. But none of these require the actual subject to be cold!
But I do wonder how the actual guide followed was supposed to work? Frozen water is not more likely to evaporate, and ambient air circulating into the box would actually rise in relative humidity, due to being cooled down??
What about using a high-pressure-capable sealed container to let a bunch of dry ice melt into liquid CO2 and then displace the water before cracking it open to sublimate? Or does that give fizzy food?
Supercritical CO2 drying: you’ve described the process for making Strawberry Aerogel :-)
OK, seriously though, supercritical CO2 drying is how Silica Aerogel and a few other aerogels are made.
My daughter and I blew up a hard “Nalgene brand” (hence, not actually Nalgene material) water bottle in the refrigerator making Carbonated Fruit. Orange slices, grape halves, and a small piece of dry ice. Leave it overnight in the fridge and… normally you get Fizzy Fruit. This time we got a glass shelf blown apart and a mixture of glass and leftovers all over the inside. Naturally, I fully indemnified her and took the heat for this one…
The water carrying capacity of liquid or even supercritical CO2 is very low, youd spend far more money on equipment necessary to purge the water and recycle the CO2 then you would building or buying a freeze dryer.
https://amzn.eu/d/0dFYsdcP
Could you use something like this as your container? I thought the idea was that the fresh stuff is frozen, then allowed to warm back up (could even put that container in a warm water bath) in a vacuum such that the moisture sublimates straight off as a gas. How long it’s frozen for isn’t the issue, but freezing it quickly might help with texture of the final product.
Wow, you can buy dry ice in grocery stores in the US? In the Netherlands, It’s only sold in specialty stores and I only found them in bigger cities that have a fish harbour.
When I needed some for a achool project 23 years ago I had to make it myself with a CO² bottle with riser pipe, in a fairly big city.
Yup. And we sell out of dry ice in October because people like making spooky punch bowls that bubbling fog. Even relatively small towns in the US have maybe one or two grocery stores that carry dry ice. There’s a handful of companies that make regular delivers, and its profitable because dry ice is quite expensive when you buy it retail.
But healthcare isn’t available everywhere here, so please Americans, be careful when handling dry ice and use precautions such as gloves. And avoid giving guests dry ice in their cup, especially young children.
It depends where. In my part of New Jersey the only place I can buy it is a camping outfitter.
This is a very interesting article, but what about the preservation of nutrients, vitamins ?
I would be interested in dehydrated food and styrofoam if I were an astronaut, but as I am a European with my feet on the ground I like better to eat fresh, unprocessed and non-industrial foods. It is better for health and it consumes less energy.
So for strawberries, it will be in season, fresh and with whipped cream.
All the studies I’ve seen on freeze dried say nutritional value isn’t affected meaningfully, other than the reduction in water content which of course was largely the point…
And on the subject of consuming less energy in Europe that is probably true much of the time – between the grow houses in Spain and the fields further north the window that fresh fruit and veg can be found locally (ish anyway) is quite wide with stuff in season somewhere for much of the year.
However the transport costs are still substantial, and that doesn’t answer the question of what to do with the excess of a crop that won’t be eaten truly fresh – either you waste it and all the energy required to grow, harvest, transport and store it locally, or you find a market for it much further away, or you preserve it in some way for later in the year. So it could easily be much more energy efficient to create fruit leather or freeze dry, pickle etc from the local fresh produce so it keeps till you need it.
I freeze strawberries and raspberries in the freezer. Yeah, there is water… so it is how the product is laid out that makes it useful later, not just a block of berries frozen together. Dry Ice provides much faster and lower temperature freezing if that is what is required…. like alcohol cubes… the more they melt… the stronger the alcohol content of the drink… Will an alcohol content of 40% freeze? What about Kahlua cubes (16%) for iced coffee? If you like a loud bang you can feel… put a a cup of water into a large plastic soda bottle and a few pellets of dry ice, and watch from a short distance. Note, this is not an explosion… just an over-pressurized vessel failure. Lots of things and experiments you can do while in possession of dry ice.
Pop bottle + dry ice is a super common, and kinda dangerous, experiment. The main problem is that soda bottles are designed to hold pressure, and so only give way when they really fail, and they do so in a very shrapnelly way, if that’s a word. You’re probably alright if you’re behind a wall of some kind, though, but if it goes off before you’re clear, it can be ugly.
If instead, you stick some dry ice in a tupperware, or something like that fails more predictably at a lower pressure, it’s a lot safer. It can still make a decent fwoomp, and it demonstrates the expansion of solid into gas just about as well. Old film canisters are fun b/c they go off with just a tiny bit of dry ice, but you have to move fast because of the low volume.
Or make a cannon, and don’t pack it too tight.
Either way, the known, planned point of failure is actually a safety feature that keeps it from getting out of hand. IMO.
Nonsense.
The pieces of 2 liter bottle don’t have the mass to go far, much less far with speed. Not even the cap and neck.
Safe as houses. Good fun for the kids.
They’ll outgrow it once they get to ‘explosive pyromaniac’ psychosocial development stage…About 12 is normal. But don’t worry if you kid isn’t blowing stuff up by 14, worry at 16. You want them past that while still legal juveniles.
A buddy came into work one day with a large bandage on his arm. He told the story that his groceries had arrived on his doorstep and the frozen food was kept chilled by dry ice. So he had taken a soda bottle and filled it with tap water, put it in the kitchen sink, then dropped in a slice of the leftover dry ice and capped it. A short time later he thought “I should put a towel over it so it doesn’t spray water all over my kitchen.” As he reached over to place the towel, the bottle burst. The jagged shrapnel that used to be the neck of the bottle had slashed right through both the towel and his arm, which required several stitches.
Hardly “safe as houses.”
Sure it is.
Your friend is just a moron.
Darwin swings and misses.
Hint: There is no water needed in a dry ice bomb. It just speeds up the explosion.
If he had done it right, it would likely have blown the windows out of his kitchen though. Still dumb AF. Would have needed more dry ice.
I blame his parents. They almost certainly prevented him from doing what came naturally at 12.
“Glass doesn’t show up well on X-rays”
-Source: my orthopedic surgeon friend, after spending 18+ hours reconstructing someone’s hand who did this with a glass bottle.
Freeze drying is also used to dry water-damaged books.
Fun fact: The vacuum chamber used by Bendix in Ann Arbor to test moon-bound Apollo hardware in the 1960s, was used in 1980 to rescue books soaked by water used to fight a fire at a University of Michigan library. I’m pretty sure that (huge!) vacuum chamber is still in use by UM Aerospace.
U of M engineers drive trains. Bam
While you are playing with dry ice, there are other parallel experiments you can try, like ice cubes made out of your favorite alcoholic drink. You can try 40% alcohol by volume, and then try something like Kahlua at 16% and see how that works in your iced coffee.
Once you get a freeze drying machine you will never go back. I freeze dry everything. Stew? No problem. Eggs right after they came out of the cloaca? Heck yeah! Berries? Psh, that’s childs’ play. If you are serious about preserving food, you can’t go wrong with freeze drying. 10 years from now I will consume eggs from a beloved chicken that passed away years before I consumed them and I didn’t even have to store them in a freezer!
The Hen-o-matic cloacatron will advance mankind’s ability to master everything. An endless supply of eggs without feathers. Parallel continous production on a vast scale. 10 billion chickens at least on the planet right now.
Have some distilled water freeze-dried, very tasty, just don’t eat too much of it at one time (it is best used as Secret Ingredient).
And if you’re thirsty, you can just rehydrate it first!
You can do this in your freezer, just put the fruit in a paper bag, it takes about a year.
Perhaps having the berries in a sealed container hooked up to a vacuum pump would do the job. When working on air conditioning systems, the system needs to be completely devoid of moisture. A vaccum pump is used to cause any moisture to boil off, and get expelled through the pump.
Vacuum pumps for air conditioning service are easily available and inexpensive. Unfortunately they usually very loud.
I did some vacuum drying (no freeze) in college. It worked really well. I wonder if the freezing is really necessary. Especially if a cold trap were present.
I have attempted to build a DIY freezer dryer with parts from Harbor Fright, using a cold trap cooled with dry ice and rubbing alcohol (cool stuff!). Unfortunately the tubing plugged with water ice fairly quickly.
I have access to a Harvest Right, the super cold walls of the chamber is quite handy as it prevents most of the water from escaping though the oil in the vacuum pump.
The slow version of freeze drying is freezerburn, which occurs despite the lack of vacuum. Over time, the ice from unsealed food forms on the cold spots of the walls, or on the insides of airtight packaging. The dry ice, when it works, would be providing an extremely cold spot for ice to be captured, while also keeping the food frozen and producing some dry gas to surround the food rather than warm humid air.
If I was avoiding vacuum, I might try using a really low temperature ice melting salt so the ice doesn’t have to be scraped off the walls, and the freezer only has to stay cold enough to keep the food frozen instead of getting much colder to trap the ice. I’m not sure what the difference in sublimation rate is between dry ice temperatures and just barely freezing, when at atmospheric pressure.
Oh, and flash-frozen strawberries are available already sliced; that’d deal with the damage from freezing them yourself.
They definitely make small home freeze-dryers now, I saw ’em at the farm store a few months ago. They’re still not CHEAP, and I have no experience with them. Just sayin.
https://harvestright.com/product/home-freeze-dryer/ smallest is $2,300 with a not-super-awesome vacuum pump, but I’m sure works.
While freezing is part of the process (‘freeze dried’) the other part is _drying_ and freezing stuff really hard with dry ice won’t do the drying part. You need a decent vacuum for that.
But hey, it was an experiment. You learned something.
I used to maintain controllers for a wood kiln that used a vacuum to lower the boiling point of water enough to boil it out of the lumber at pretty low temperatures (well below a combustion danger). Still might cook the strawberries though
A few years ago I read about “carmelizing” wood, i.e. baking the wood around 400 degrees F until the heat thoroughly penetrated(sp). The heat made the cellulose undigestable for bacteria therefore less susceptible to decay.
Haven’t heard anything since then.
Leaving the strawberries on a Himalayan mountain top could probably freeze dry them without a machine.