Converting a Fridge into a Curing Chamber - Part 1 - Controlling Temperature

lomo dryingAlthough it's a common topic of discussion on the sausage making forum, I've never got around to writing about converting my fridge into an air-drying chamber. Before I start, I accept no responsibility for what you do with the information here; it's up to you to ensure that what you do is safe and complies with any relevant regulations/legislation. If in doubt, please seek the advice of a professionally qualified person.

This information relates to converting a fridge for use in the UK. For details of converting a fridge in the US, please see this article on the Cured Meats website.

Firstly, let's look at the conditions we need; there are 3 main phases during the process of making air-dried products, curing, fermentation and drying. Ideal conditions for these are:

  • Curing - normal fridge temperatures are fine, ideally at the higher end around 5° - 8°C.
  • Fermenting - used when making sausage and occasionally with dried meats. The product is held at temperatures around 24°C with a very high relative humidity, around 90% - 95%, for a period that can vary from 12 hours to several days, to enable bacteria to make the sausage more acidic, which makes it safe to eat. The exact temperatures and times depend on the specific bacteria added, so follow the manufacturers guidelines or the recipe carefully.
  • Drying - a period of weeks, or months, during which we want the product to dry slowly and evenly which will add to its safety. The conditions for doing this are ideally between 10° - 15°C with a relative humidity between 70% and 85%. Our aim is to keep the humidity of the chamber just slightly below that of the product, whilst it dries. Regular changes of air are also beneficial. My own experience, along with that of fellow home sausage-makers, suggests that there are less problems when the drying is takes place at the lower end of this temperature range. Many favour a temperature of 12°C, or thereabouts, as do I.

Most people have little problem creating the conditions for the first two phases, but often have problems with the third; modern houses tend not to have places with these conditions. A cellar or pantry is often ideal, or can be adapted easily. If you can beg or borrow the use of one, then do so: the larger the area, the easier it seems to be to control. However, for the rest of us, the only economic option is to adapt a fridge or freezer to create the conditions required.

Firstly, you'll need a fridge to convert - frost free is the type to go for as these tend to have very low humidity; it's easier to increase humidity than decrease it! Once acquired, set it to its minimum setting; by that I mean the setting that will make it run at its highest temperature. Check the temperature inside the fridge with a thermometer: a simple room thermometer, or a fridge thermometer, from the local supermarket for a couple of pounds is fine. If the temperature's around 11 - 12°C or above, you're laughing. The chances are it won't be! Assuming it's not, you'll need to take over control of the fridge's thermostat by using an external thermostat/temperature controller. A digital thermostat is best. The first chamber I made used an analogue one and it wasn't really accurate enough.

Temperature controllers can be purchased quite cheaply from Amazon or Ebay by searching for 'digital temperature controller'. A similar product can also be found at www.conrad-uk.com and www.suntekstore.co.uk. Mine's a STC-1000 like this:

Digital Temperature controller

This type can control cooling AND heating. It's worth checking, and getting one that can do both, as some types can only control cooling OR heating. Either type's OK, but the one that does both can be useful for other things as well, and in my case, is used as part of the system that controls the humidity of my fridge. People who are more proficient at DIY than I am, may wish to mount their chosen controller into the door of the fridge; I chose not to, as by having it separate, it can also be used for other things. The simplest way to use it is to mount it in a project box which can be bought for a few £'s from Ebay.

Digital temperature controller

I had the box made up for me professionally, it's not actually the one I use to control my fridge as that has hygrostat (humidity controller) as well. However, this one shows what's required to control just the fridge temperature more clearly than my full control box. The thermostat has separate outlets for cooling and heating which each have their own sockets. The black wire on the left is the temperature sensor and the white on the right, the power supply from the mains. The control box pictured contains relays that switch the power, controlled by the thermostat. You may chose to rely on the 10 amp relays in the controller but assuming that you choose to use a relay, the one I used in my full set up was a Finder DPNO relay from RS-online (Stock No. 245-2431). Details of the wiring, using a relay, are given later.

To use this you plug the thermostat into the mains and the fridge into the socket on the project box that is controlled by the 'cooling' side of the thermostat. You then set the thermostat according to the manufacturer's instructions. It allows you to set your required temperature and also the number of degrees above this that it should allow the temperature to go before turning the fridge back on. You can also set a time delay to ensure that the fridge motor isn't 'cycled' on and off too quickly. All you then need to do is set the fridge to its coldest setting. The controller will now take over instead of the fridge's own thermostat. A setting of 12°C with a difference of 1° and a delay of the maximum 10 minutes works fine for my fridge. Others may vary; it's a case of trial and error until the temperature stays within the parameters required.

Now, it's at this stage that I suggest you buy a hygrometer if you've not already got one. We need to see what the humidity of the fridge is when running at about 12°C. Just Google 'hygrometer' and you'll find loads of places that sell them, or there's always Ebay and Amazon - they're available from a couple of pound upwards...

...we'll look at what we do when we've got it in Converting a Fridge into a Curing Chamber - Part 2 - Controlling Humidity.


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There are four comments

Rich Deveau-Maxwell

Hi there, I am looking to create a curing chamber along the above lines using a under the counter bar chiller. Will the wiring setup etc differ to that required for a domestic fridge?

Rich Deveau-Maxwell, - 17-01-’15 22:01
Phil

There should be no trouble with the wiring as the fridge just plugs in to the controller.There may be issues fitting the humidifier in if it’s a small unit, Some units of that type have quite a large fan as part of their workings; it can be a problem. I had this with my conversion of a wine fridge.

Phil, (URL) - 17-01-’15 23:10
steve

Great article, most I’ve come across are all US based. Can you explain why you chose to use an additional relay rather than rely on the ones in the temperature controller.
I’m not that great on electricals and would prefer to keep things as simple as possible but don’t want to have to retrofit.

Also is there any problem in just using a hygrometer placed in the fridge and manually check a couple of times a day – or does the environment change quite frequently and spoil the meat

Many thanks

Steve

steve, - 27-05-’15 11:47
Phil

Hi Steve, sorry for the delay replying – I’m laid up at present.

The simple answer is that I did it because my electronics expert told me to. It’s something to do with the ability of some things to peak at far higher amps than their normal running amps. It also ensures that the system meets a requirement that the gaps between contacts have to be more than 3mm (I think) i.e. He knew the specification of the independent relay but could not be sure of the ‘actual’ specification of the one in the controller.

That said, I know that many people use them successfully relying on the built in relay.

What it does mean is that, if I want to, I can use the box for other projects where the item being controlled is greater than 10 amps.

Don’t do anything about humidity control until you’re sure you need it. Monitor the chamber with product in it using an hygrometer before deciding what to do – as per Part II. Using hygrostats etc should be seen as a last resort.

Hope this helps.

Phil, (URL) - 01-06-’15 21:42

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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