Almost everyone who is reading this checks the weather at night before going to bed and then again first thing in the morning. You ask yourself will it be raining or will it be sunny? Will it be cold or maybe hot? But if you are a wheel of cheese, one of the things you really want to know is the dew point.
Why, you ask, would a piece of cheese want to know what the dew point is? Let me explain.
There is a unit of measure in food science called water activity. The value of water activity is the partial vapor pressure of the wheel of cheese. But before understanding water activity we need to understand vapor pressure. A simple example is when you heat up a pot of water, you increase the vapor pressure of the water in the pot. When the vapor pressure of the water in the pot becomes greater than the vapor pressure of the water in the air, water vapor moves from the pot (evaporates) which is at higher pressure than the vapor pressure of the air around it. Think of what happens when you have a hole in your tire and the pressure in the tire is greater than outside the tire. The pressure wants to equalize, and before you know it, both the pressure inside and outside the tire are the same and you have a flat.
One more point before we get back to that wheel of cheese: vapor pressure and dew point are related.
If we know the dew point, we can know the vapor pressure. Conversely, if we know the vapor pressure then we can know the dew point.
So why does our wheel of cheese care what the dew point is? Because dew point has a direct impact on the loss of our cheese's water. The lower the vapor pressure in the air around it (aka dew point) as compared to the vapor pressure of the wheel, the faster the water will leave the wheel in the form of water vapor. So what really makes our wheel of cheese happy is controlling the dew point of the room where it ages. With just the right balance between the vapor pressure of the wheel and the dew point in the aging space, a happy state of comfort for the cheese can be accomplished.
With proper control of dew point in an aging space, moisture can be removed in a controlled manner by either cooling a surface (such as an evaporator coil) below the dew point of the air in the room, which will condense water and remove it from the air, or by adding moisture to the air in the form of water vapor. I would like to point out that steam will directly introduce moisture in the form of (sanitary) water vapor while foggers or misters will not. Foggers and misters simply introduce water droplets that then need to be converted from a liquid to a gas, which takes energy, which there is not much of to spare in the air in a properly designed aging space.
So the next time you see your cheese isn't happy, look at your local weather. Note what the dew point is outside as compared to the desired dew point in the room where your cheese is living (which should probably be in the 47F to 49F range). You will find, depending on your location, the dew point outside might be significantly lower, particularly in the winter months, which will cause the moisture in your room (which is at a higher vapor pressure inside) to travel right through all the nooks and cracks in the construction of the walls and ceilings of your aging space. Also, keep in mind that wall materials that can absorb moisture do not stop vapor from traveling right through them. The opposite will happen if the dew point outside is higher than the desired dew point in your room. This is common in the summer months or areas in the south. At these times the vapor pressure outside is higher than in the room and there will be the constant flow of moisture from outside to inside.
To a wheel of cheese, a poorly constructed aging space that is vapor permeable is like a house with a leaky roof to you and me - it makes us unhappy and uncomfortable. Unfortunately, water vapor is invisible so we can't see it like drips of water, but your wheel of cheese can certainly feel it. Remember, a properly designed aging space and aging room control system can address these issues and keep the vapor pressure constant year round no matter the climate conditions.