Reinvent Pasteurization

Pasteurization is one of the more mild forms of thermal processing that kills pathogens in milk and increases its safety and shelf life.  Modern pasteurization uses high temperature and short time to kill pathogens in the milk. It kills off the most heat-sensitive pathogens while retaining the qualities of milk that consumers expect. This amounts to heating the milk to 161° F (72° C) for 15 seconds by running the milk through a tube of which the size and diameter takes the milk exactly 15 seconds to pass. Since that may be a bit more sophisticated than possible with primitive technology, you can also batch pasteurize the milk by heating it to 154.4°F (63° C) in a large container and holding it at that temperature for 30 minutes while it is stirred continuously. Americans would find it strange to walk into a store and purchase a bottle of milk that has been sitting on the shelf unrefrigerated for months but in Spain this is a common occurrence. The milk has been ultra-high temperature pasteurized by heating it to 280°F (138°C) for a minimum of two seconds. Milk pasteurized in this manner may be stored unrefrigerated for 6-9 months (using sterile packaging of course).

Pasteurization is one of the more mild forms of thermal processing and it works by denaturing the enzymes in the bacteria. Canning of food works similarly. It wasn’t until the early 1800s that we figured out how to preserve food by canning. If you are reinventing canning, you will need sturdy, air tight containers in which the food could be heated. How long and to what temperature the food needs to be heated depends upon the acidity of the food. Low acid foods must be heated more than high acid foods because the acid helps to denature the enzymes of the bacteria. You will need a pressure canner to sufficiently heat some foods. An open pot of water won’t get hotter than 212°F (100 °C) no matter how long you boil it. A closed pot in which the pressure can build allows you to heat the food to temperatures of 250°F (121°C ) for the 20 to 100 minutes it will take to kill bacteria in low acid foods. The most worrisome danger of canning is not killing C. botulinum spores. C. botulinum is a bacterium that thrives in the low oxygen, high moisture environment of the can. It releases a toxin that is among the most lethal toxins in the world. The toxin is odorless, colorless and tasteless. An amount equivalent to a quarter of a grain of sand (75 nanograms) is enough to kill a person. The good news is that, while the C. botulinum can survive boiling water, the toxin it produces breaks down with temperature. Heating canned food to a typical cooking temperature of 176°F (80°C) for 10 minutes before consumption can greatly reduce the risk of illness. Personally, I would make sure that the food was cooked enough to kill C. botulinum in the first place. So in a nutshell, canning is using heat and acid to reduce pathogens to a level that allows the food to be eaten years later (assuming proper storage – meaning cool temperatures and a good seal on the jar).

 

Build an evaporative refrigerator – no moving parts, no electricity

Zeer Pot Construction
How to build a Zeer pot
Zeer Pot Construction

What is an evaporative refrigerator?

It is a device that uses the evaporation to cool a space that can be used to store food or water. An illustration of one version is shown at right.   This version was created by putting one earthenware pot inside another, filling the space between them with sand and then wetting the sand.  Food is stored in the inner pot which can be covered by a wet towel or a lid.  As the water evaporates from the sand, it cools the inner pot and its contents. The whole thing is easy to build and to operate and, best of all, it is inexpensive.  An added bonus is that the water used for cooling does not need to be potable. That sounds perfect for third world applications and it is most often in this context that the device is discussed. 

How does it work?

The short answer:

When a liquid evaporates it loses energy.   This is why sweating cools us down.   The sweat on our skin evaporates and it takes a bit of energy from our skin with it.  This loss of energy provides us with a cooling effect.

The more thorough explanation for why evaporation causes cooling:

Temperature is a measure of the average kinetic energy of the molecules in a substance.  Sometimes the molecules near the surface of a liquid collide just right and they provide one molecule with enough kinetic energy to overcome the intermolecular forces in the liquid and it escapes (evaporates) from the liquid.  As more and more of these high kinetic energy molecules escape, the remaining molecules have, on average, lower and lower kinetic energy.  This is the very definition of cooling – lower average kinetic energy.  Evaporation slows down as the liquid gets cooler because the molecules have less average kinetic energy and they don’t collide ‘just right’ as often.  So evaporation will only cool the liquid so much before the process slows to a stop.

Is there a cooling limit?

The short answer:

Evaporative cooling is limited by the current wet bulb temperature.    No matter how much liquid evaporates or how fast it evaporates the minimum temperature that can be reached by direct evaporation is the wet bulb temperature.   A wet bulb temperature is an indication of the amount of water vapor (relative humidity) in the air.  

The more thorough explanation:

Imagine water in a jar where the higher energy water molecules near the surface of the liquid are escaping into the air above the water.   If we put a lid on the jar, the water molecules will continue to accumulate and sometimes the collisions of these molecules will result in the molecule leaving the air and returning to the liquid water.  If the jar is left alone, eventually the activity will equalize.  The average energy of the molecules leaving the liquid will equal the average energy of the molecules returning to the liquid.  This equilibrium is what we would call 100% relative humidity.  The energy lost to evaporation is equal to the energy gained from condensation.  You can determine how close you are to the equilibrium point by taking the wet bulb temperature of the air and comparing it to the dry bulb temperature of the air.  If the wet bulb temperature is lower, it indicates that there is still room for molecules to transfer from the liquid into the air. 

Let’s express the limits in a more visual manner

These charts show the minimum temperature that can be achieved by direct evaporation for a given temperature and relative humidity.   These numbers came from a psychrometric chart.   The chart lists the wet bulb temperature for various conditions. If you managed to achieve 100% efficiency with your evaporative refrigerator, this would be the coldest it could get.

Evap in F

Evap in C

So if it is 90°F (32.2°C) outside and  the relative humidity is 50% then coldest a direct evaporative refrigerator could possibly get is 74°F (23.3°C).   That is much better than no cooling at all but far less than what we expect with our modern day refrigerators which keep food at 40°F (4.4°C) or below.    On the other hand, the device is inexpensive, easy to build and operate. 

I use the word ‘direct’ evaporative cooling because there exists another type of evaporative cooling called indirect cooling.  It involves feeding an evaporative cooler air that has already been chilled by another evaporative cooler.  So it is possible to get evaporative coolers to temperatures below that of the wet bulb.  At that point, we are talking about a much more complex device and not a simple pot in pot refrigerator.   

 

What about using different designs or materials?

Let’s look at the various parts of the evaporative refrigerator:

The liquid: Water is the most practical liquid to use for evaporation.  It is readily available and it removes a large amount of heat per kilogram of evaporation as compared to other liquids.  For a practical refrigerator, it is really the only liquid that makes sense. 

Container materials: The container for the food can be any material that conducts heat.   Materials that conduct heat better will cool down faster but the will also heat up faster when the evaporation stops.   It is probably a good idea for the container to be waterproof so that the food is saturated with the water that is used for cooling.   The outer container exists just to hold water for evaporation and to provide as much surface area for cooling as possible. That means is needs to be porous so that the water can seep through it and evaporate.

Material to hold the liquid:  Sand is often used because it is inexpensive and it has a large surface area.  But it could be sponges, charcoal or another material.  The most important features are that it holds lots of water and provides surface area for evaporation. 

Shape: The shape of the container should have lots of surface area as compared to its volume.   The rate of evaporation is driven by factors such as temperature, air flow and the surface area.  Surface area is the only factor that can be controlled as the device is being built.  The idea is to maximize surface area and minimize internal volume.  The tetrahedron meets that criteria but it isn’t as practical to build as the more spherical shapes. 

Size: The larger the container becomes the worse its surface area to volume ratio gets.  This means there is less surface area (less cooling) per volume of cooled area.  This chart from Wikipedia shows that a cube with sides of 1 meter would be much better than a cube with sides of 10 meters.  So it would be better to have several small devices than one large device in terms of performance of the cooler.

SA/V Graph

 

 

 

 

 

 

 

 

 

 

 

How to get the best performance:

Often people will suggest putting them in the sun so that the water evaporates more quickly.  That will work as long as the additional cooling from evaporation is greater than the temperature gain of the device sitting in the sun.  Probably more often than not, placing the device in the shade (preferably in a nice breeze) will be the best way to improve performance.  If you can elevate the device to get evaporation on all sides that would be even better.  The device cannot run out of liquid or the cooling stops.  That means it either needs a large water reservoir or it will need to be filled frequently.

Other ideas about burying them or placing them inside another pot are sure ways to slow down or stop evaporation.   At some point, trying to improve the performance of the device starts interfering with the utility and durability of the device.  The evaporation rate is largely controlled by the temperature, relative humidity and wind speed.   

Final Word:

Evaporative coolers are a neat idea and when used in the correct environments they can provide much needed cooling for very little cost.  Just make sure your expectations are reasonable.  They will very rarely manage to keep temperatures as cool as our modern refrigerators.  Warmer temperatures and less storage space are part of the trade-off for their low cost and simplicity. 

Busting myths about the Zeer pot

Zeer Pot
Zeer pot example – Image credit: Adam from ModernBushman.com
Zeer Pot

If you don’t already know, a Zeer pot is a type of evaporative refrigerator.  It is a device that keeps being rediscovered even though various versions of the device have been used for thousands of years. I am going to call them evaporative refrigerators since they operate by evaporation and are used like a refrigerator.  Every new blog entry or YouTube video about them seems to spread misinformation and sometimes outrageous claims.  Let’s clear some things up.

  1.  It can keep a temperature of 6°C  (43°F) from this article about how a British student re-invented the device

    Technically this claim isn’t false but it is misleading.  Cooling depends upon outside temperature and relative humidity.  The table below lists the various ways the device could get to 6°C (43°F). The bottom line is that it requires desert like conditions of 10-30% and it can’t be warmer than 18.3°C (65°F) outside to get the device to cool to that temperature.   This fact isn’t mentioned.  There are probably a thousand videos on YouTube with various claims on performance, but they all have to obey the laws of physics. (More on performance limitations in this article).

    Conditions under which an evaporative refrigerator could reach 6°C (43°F)
    Outside Temperature Relative Humidity
    20.5°C (69°F) 0%
    18.3°C (65°F ) 10%
    15.3°C (59°F ) 20%
    13.3°C (56°F ) 30%
    12.2°C (54°F ) 40%
    10.5°C (51°F ) 50%
  2.  Using acetone or alcohol or another liquid will yield better results.

    It might be fun to do as a party trick but for daily use it would be more expensive than using electricity to run a modern refrigerator.   The rate of evaporation is the key to cooling performance in these devices, so faster evaporating liquids would help to improve performance, but it also matters how much energy they remove as they evaporate.  Acetone removes 29% less energy than the same amount of water.  It would probably make up for that with its faster evaporation rate. The bottom line is that it is going to be very expensive to use acetone for any long term cooling.  

  3. Instead of using clay pots, try metal pots or cloth bags or whatever.

    Because there are so many versions of this device, there are lots of ways to get to the same results.   The most important thing to remember is that the wet bulb temperature is a minimum temperature to which they can cool no matter how they are constructed.   It doesn’t matter if you use charcoal or sponges instead of sand or if you use a bag instead of an outer pot.  Anything that increases evaporation is good.  More detail explained in this article.

  4. Putting the Zeer pot in the sun will aid in evaporation and make it colder

    That will work as long as the additional cooling from evaporation is greater than the temperature gain of the device sitting in the sun. Probably more often than not, placing the device in the shade (preferably in a nice breeze) will be the best way to improve performance. Since you cool yourself the way a Zeer pot does, by evaporation, ask yourself if you feel cooler sitting in the sun or in the shade.

  5. Bury the Zeer pot, put it in your root cellar or inside another Zeer pot to increase cooling

    Those are sure ways to slow down or stop evaporation. If you can elevate the device to get evaporation on all sides, that would be  better. Unless your root cellar is also a wind tunnel?

Bottom line:

 

Evaporative coolers are a neat idea and when used in the correct environments they can provide much needed cooling for very little cost.  Just make sure your expectations are reasonable.  They will very rarely manage to keep temperatures as cool as our modern refrigerators.  Warmer temperatures and less storage space are part of the trade-off for their low cost and simplicity.