Converting a Fridge into a Curing Chamber - Part 2 - Controlling Humidity

At the end of Converting a Fridge into a Curing Chamber - Part 1 - Controlling Temperature we'd just acquired a hygrometer to check the humidity in our fridge now that we've got it running a 12°C...

...The chances are that the humidity's not at the level we want, but before we start to do something about it, let's just look at what relative humidity is.

When we refer to humidity in percentage terms, what we are referring to is the percentage of moisture in the air, relative to the maximum that the air can hold, at that temperature. Warm air can hold a lot more water than cold air before it becomes saturated, so air at 20°C with 100% relative humidity (RH) will have a lot more moisture in it than air at 12°C with the same 100% relative humidity. Let's make that 'doubly clear': if we cool air, the relative humidity will increase, even though the amount of water in it won't change. This means that the relative humidity of any air we introduce into the fridge will increase as it cools.

Hourly weather statisticsPlaces with different ambient humidity will require different solutions to the problem; the dry of the desert is very different to the wet of the rain-forest.

Don't think that things will be easy because the UK has a temperate climate. Take yesterday as an example: at 7am the relative humidity was 100%, but by 6pm it was only 38% (click the image on the right to see further details). For this reason, control of humidity by the introduction of fresh air using a fan, is unlikely to work here, even though it does in climates with constant low humidity.

So what do we do to control the relative humidity? Well, I suggest that you do absolutely nothing! Instead, go and make some chorizo, or any other salami type product that's fairly thin. Don't spend a lot of money doing it though; it may become a sacrificial sausage later! "Why?", I hear you ask. Well, experience has taught me that drying chambers with salami in them behave very differently than empty ones. Using your fridge to make some salami will give a truer indication of how the humidity will behave.

So you make your chorizo, ferment it as per the recipe, then put it into the fridge at 12°C. Now see what the humidity is using the hygrometer that you bought. Hopefully, it'll start around 85% and gradually reduce over the next couple of weeks to around 72% as the weight of the sausage drops. I say, "Hopefully", but if your luck's anything like mine, then there's no chance of this happening!

It's more likely that one of 3 things will happen:

  • the relative humidity will be too low,
  • it'll be too high,
  • or, it could go up and down like a yo-yo: too low, too high, but never just right!
The first could result in 'case hardening', whereby the outside of the sausage dries too quickly and prevents moisture escaping from the middle - characterised by a dark ring just under the surface of the sausage; the second, in unwanted moulds, and the third, well, take it from me, you don't want that!

Low Relative Humidity

The first of these problems is the easiest to tackle: all we need to do is introduce moisture into the fridge. This could be something as simple as a damp towel (changed regularly to keep things safe), a bowl of water, a bowl of damp sand, or humidity beads/solutions used in cigar humidors. A bowl of damp salt, in theory, will release and absorb water as needed to maintain 70% - 75% RH. An alternative, and one chosen by many, myself included, is to place a humidifier in the fridge. An Ultrasonic one is the type to go for. Get one with a manual on-off switch rather than a digital one. If it doesn't have a hygrostat (humidity controller) built in, it will require an external one to control it. A digital one is best. I speak from experience: the analogue one that I used, shown on my post about Safely Drying Meat and Sausage, was difficult to set accurately and very slow to react.

Digital hygrostat, digital humidity controllerThe one I now use is very similar to my digital thermostat with the exception that it will only control a humidifier OR a dehumidifier at any one time.

Similar controllers can be found by searching for 'humidity controller' on Ebay or Google. A similar product can also be found at www.suntekstore.co.uk, as can temperature controllers. Other options may include this combined unit. These are the cheapest options I can find at present - if you find others please let me know.

In choosing a humidifier, it's a case of balancing size against how often you'll need to fill it. Units also vary considerably in price - so shop around. A larger unit will run for longer without filling, but it'll take up more space in the chamber. It's a case of striking a balance for your own circumstances.

Drying chamber - some ideas are just plain daftHigh Relative Humidity

I think I've tried everything in my battle against high humidity.

It's been a real pain! I've had tubs of salt, silica gel, fans, and even a dehumidifier in the chamber at one stage - there's a photo on the right to prove it! Salt worked reasonably well, as long as I changed it every couple of days (I alternated two trays of salt). The idea of a fan which introduces air from outside the fridge appeals, until you realise that air (say) at 70%RH at 20°C when outside the fridge, will be 100% humidity when cooled. The dehumidifier took up half the fridge and was far to 'robust' an answer to the problem: all it did was cause case hardening.

It was a member of the sausagemaking.org forum who supplied me with a method that works. It relies on the fact that when the fridge motor/compressor is running, the RH drops due to moisture in the air condensing on the 'cooling plate' in the fridge. Although somewhat bizarre, the way to make this happen is to have a heat source in the fridge that operates when the humidity is too high. Heating the fridge causes the fridge motor to run, and the humidity falls. It works! I now have a 1ft tubular heater mounted in my fridge; I got it for less than £20 online, from Amazon. It's connected to a humidity controller, like the one above. The controllers can be set to operate in a 'humidify' or 'dehumidify' mode.

Fluctuating Humidity - too high and too low

If you're very lucky your fridge/chamber humidity will be either too low or too high: preferably the former. But, in the real world, there are likely to be times when it's too low, and others, when it's too high. I've yet to find an answer to this, other than to have both of the solutions above.

The control box that I use has a temperature controller (thermostat) and two humidity controllers (hygrostats): one for low relative humidity, and one for high. Overkill? Maybe. But it covers most eventualities and makes life easier.

In the next part, Converting a Fridge into a Curing Chamber - Part 3 - The Electrics, we'll look at wiring it all up.


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There are four comments

protopigeon

Thank you for this blog, and this post in particular.

I’m currently in the “alpha” stage of building my own curing chamber (I’m in Leicestershire too, so I have the same weather as you :))

So far I have a 6ft tall larder fridge, an ultrasonic humidifier, and a reptile vivarium controller (temp / hum / timer) to control it all with.

I’ve been monitoring the parameters in the fridge this week, and although the temperature’s OK (11C-15C), the humidity yo-yos dramatically (I’m guessing that once the humidifier’s been on, then when the fridge comes on it hits the dewpoint), so I’m looking at how I can make it more stable.

I don’t really want to have to drill holes in this fridge if I can help it, and I’m no electrician (hence the all in one vivarium controller – no wiring required), so I think I’ll buy another humidity controller, and then try the heater method like you suggest in this post.

Thanks again for a great blog.

protopigeon, - 26-05-’13 09:57
Stephen Hodgdon

Hi there, great blog, I’ve really enjoyed reading everything. I’ve just finished the first run of salami and things went pretty well in my converted fridge. I got a little bit of case hardening and I’m curious to get your input. I live in Montana, and we usually have very dry conditions here, so I was using a computer fan to draw air into the chamber a few times a day when humidity got too high. However during some rain storms this spring its been very humid, thus negating the drying effect of the fan. So, I did what you did and put a small heater inside.

My issue is that when the fridge kicks on it blows cold air via the fan, into the chamber. I’m concerned this is contributing to the case hardening problem. I’m curious if you’ve seen this problem as well. It definitely works to decrease the RH, but I’m concerned that kicking the fridge on every hour or two will lead to this being a recurring problem. Any input would be greatly appreciated. Thanks so much…

Stephen Hodgdon, - 28-03-’15 19:20
Phil

Hi Stephen,
I had a similar problem when I used a converted wine fridge that used a fan-cooling system. I never did solve the problem fully and eventually decided to changed to a fridge without a fan. Using a heater will not help in this situation.

The best I came up with was to use trays of salt to reduce the humidity. Swap them as they become less effective; you can dry the salt and reuse it.

Sorry not to be of more help.

Phil, - 29-03-’15 14:37
Stephen Hodgdon

Thanks for the input Phil, I’ll keep plugging away.

Stephen Hodgdon, - 30-03-’15 05:19

I'm somewhat incapacitated at present so replies may take some time. Please post urgent enquiries at the www.sausagemaking.org forum.

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