|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:
38 F 67%
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 CHLORIDE 32.80%
POTASSIUM CARBONATE 43.16%
MAGNESIUM NITRATE 52.89%
SODIUM BROMIDE 57.57%
POTASSIUM IODIDE 68.86%
SODIUM CHLORIDE 75.30%
POTASSIUM CHLORIDE 84.34%
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.