Friday, January 3, 2014

Controlling Humidity in Your Cheese "Cave"

Steve (Bear) Murtaugh, Pittsburgh, Pennsylvania

A couple of months ago, I mentioned in an e-mail to Steve Murtaugh that I think controlling humidity is the biggest challenge for home cheese makers.  Steve has been retired from managing government labs since 2010, so he decided to tackle that problem.  He had just read an article about how salt retains humidity and he thought that might be useful.

Within a very short amount of time, Steve conducted experiments which demonstrate that salt does, indeed, retain humidity in a cheese "cave."  I believe this is very important information for all of us and I'm sure you will join me in thanking Steve for this contribution to our body of cheese making knowledge.  It is an outstanding achievement.

An easy "no frills" low-tech solution for temperature and humidity control while aging cheeses.

By Steve Murtaugh

Making a saturated salt solution in a ratio of 1 tablespoon of table salt and approximately one teaspoon of water, and mixing until a "slushy snow" consistency; yields a stable humidity controller when used in conjunction with an equal amount of water. 
One acts as a humidity source while one acts to regulate the humidity.  The salt solution takes an initial 12 hours to stabilize.  This solution, alone, in a closed sandwich bag along with a hygrometer may be used in calibrating the hygrometer. The solution will yield a stable 75% RH in such a small environment.

After 5 days of extensive testing, and 1 month of periodic "real world" observation the following was determined:

Placing this salt solution and a water source inside a small cheese cave that fits in a crisper will yield a stable 78% RH at 55 degrees. The range will vary somewhat based on temperature.


Typical Measured Values were:
TEMP        RH
38 F         67%
42F          73%
50F          78%
71F          80%       

An 11 gallon cooler was also used, the salt solution was scaled up to ¼ cup salt with about a tablespoon of water and a corresponding sized cup of water. A stable 80% RH was maintained for 5 days at 71F.

Adding a 1 gallon jug of ice to this arrangement, lowered the temp from 71F ambient to 50F in less that an hour, after 24 hours the temp was 54F.  Throughout this period, the RH was stable at 65%, likely due to condensation along the surfaces of the ice supply.

Using a small tupperware cheese cave, with the salt solution, and water source; sealed and placed inside the empty cooler, then adding the 1 gallon ice jug to the cooler; results in a stable 78%RH in the cheese cave, the ice keeping the cave at approximately 50-55F for about 48 hours.

For those thinking "Oh heck, I just use a crumpled up, damp paper towel, and I do fine." I would point out that due to the variability of exactly how you crumple your paper towel and lay it in your cheese cave; you may end up with RH anywhere between 40 and 90+ %. The above, however, gives a fairly precise method, with minimal fuss or monitoring.

This is further info Steve sent me before his final conclusions:

Salt Generated Humidity

By Steve Murtaugh (from various sources)

When placed in an enclosed volume, salt solutions will (given enough time) generate a certain, predictable humidity. The undissolved salt will absorb water, while water will evaporate from the solution. At a constant temperature, these rates will be constant, and thus a constant humidity can be generated. There must be both saturated solution and undissolved salt present for this to work.

A saturated solution at a stable temperature and pressure has a fixed composition and a fixed vapor pressure. Thus, at constant temperature, no matter how much salt and how much water are present, the (RH) relative humidity that is produced is fixed, just as long as both the water and the solid phase are present. So, unless the water dries up, or the salt is made so wet that it liquefies, a predetermined humidity can be produced.

It is convenient for us that a solution of ordinary salt mixed with water (preferably distilled water) produces a predictable humidity over a wide range of temperatures. The humidity created, with ordinary salt (Sodium Chloride) and water, is 75.29% at an ideal temperature of 77 degrees Fahrenheit. The temperature of the room is not critical for our purposes. For example, the RH is quite stable even with large variations: Salt solution at 59 degrees Fahrenheit will produce 75.61% RH and at 86 degrees Fahrenheit the RH is 75.09%.

Other salts can produce many different RH levels.

SALT BATH                   PUBLISHED RH AT 25°C
LITHIUM BROMIDE                  6.37%
LITHIUM CHLORIDE                11.30%
POTASSIUM ACETATE             22.51%
MAGNESIUM NITRATE           52.89%
SODIUM BROMIDE                  57.57%
POTASSIUM IODIDE                68.86%
SODIUM CHLORIDE                75.30%
POTASSIUM SULFATE             97.30%


The hygrometer must not get wet or be in contact with the salt solution, this will contaminate the reading and possibly damage the hygrometer.

It may take up to 12 hours for the humidity to stabilize.


murtsm said...

I would note that the "ice" aspect was for my particular foam cooler and was somewhat of a side line to the water/salt humidity control. Other than periodic additions of water to bring it back to a slush stage, no other maintenance is required. I have currently scaled this up to an 8 cubic foot (2'x2'x2'), wooden cheese cave with consistent results.

murtsm said...
This comment has been removed by a blog administrator.
murtsm said...

Having non-waxed cheese in the cave can affect the humidity, depending on the moisture content of the cheese. Real world observations, while making cheese, could get the humidity up to 93%. Removing the Salt solution and the water solution, then lowered the RH. at times to just the level needed. This more usual at higher temperature aged (i.e. Swiss) cheeses.

If humidity doesn't quite come up to the level needed, it can be supplemented by using a simple disperser, such as a crumpled up paper towel, however a better control would consist of a small rack supporting a paper towel which wicks water from the water source. The size of the towel can be used as a control (small towel, less moisture transferred).

Shelly said...

Hi Steve,

Thank you for your research ! I'm a new cheese maker and have a cool unheated room which is the right temperature but the humidity is too low. Place to put this technique in place to age my small 1lb :-) cheeses.

mamasam said...

This is very good factual data. Can someone translate this for my feeble mind into steps I should take? More of a "do this" type of note? Thanks.

Anonymous said...

Excellent study and presentation. I've had a problem with temp and humidity control, and am limited for space, but have plenty of large coolers. The addition of the frozen jug opens up all sorts of possibilities. Thank you for sharing all of this.

Ray Kruse
Buffalo, KY

Anonymous said...

Hi Steve,

Thanks so much for all your sharing!

Can I scale this up for my wine cooler which is my cheese cave?


redrose said...
A Question?
I have a refrig.converted to a cheese cave. I have trouble with constant humidity. How do I convert your formula to this large size?

Murtsm said...

There is no real hard and fast rule, that I've determined. Other than:
Larger size=larger supply.
I would try somewhere around 2 cups water and a 2 cup size salt solution. Mix the salt with water to just get a "slushy snow" texture.
Due to the larger size of a fridge, you will likely need to add water to the water supply periodically and maybe to the salt supply as well. Just check it all when you go to turn your cheeses.