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Cold Water Lime Pasteurization For Growing Mushrooms

I’ll admit, pasteurizing straw in hot water with a propane burner can be a bit of a pain. You need a large burner, a full propane tank, and plenty of time to make sure the whole process goes smooth. Even more of a pain, it needs to be done outside or in a large shop- because you can’t set up a propane burner in your living room! Which leaves many new growers wondering… "Isn’t there another way to pasteurize large amounts of straw?" Yes! It’s called Cold Water Lime Pasteurization, and it’s a cheap and effective low-tech way to prepare substrates for growing mushrooms. The process is simple. You basically just soak straw for 12-24 hours in a bath of cold water that has been treated with hydrated lime. The lime will rapidly and dramatically increase the pH of the water, causing the mold spores, bacteria, and other contaminants in the straw to be killed off. Once drained, the substrate can be inoculated with your mushroom spawn. The mycelium will then be able to grow in squeaky clean straw, unhindered by other competing organisms- for at least a little while!  

What is hydrated lime?

Hydrated Lime is basically just Calcium Hydroxide, chemically known as Ca(OH)2, and is often used in industry for a whole whack of different reasons, from pickling cucumbers to treating sewage water. This white powder is known by many different names. BUT- not all Lime is created equal, and some types will not work for growing mushrooms.

✘ What Type of Lime WON’T Work?

Make sure you don’t get the type of lime that is used for treating garden soils, or buffering substrates. This is too weak! It is typically found at garden centers in pellet form or crushed stone. This is Calcium Carbonate Lime. You might see it called a bunch of different names including dolomitic lime, garden lime, quicklime, limestone, chalk, and many more. This will not be effective in raising the pH high enough, or fast enough to kill the contamination. (check out Wikipedia for a quick primer ('limer?) on pH) Also, you want to make sure that the lime you use is low in magnesium, and high in calcium. This type of lime is typically WAY high in magnesium. High concentrations of magnesium will retard the growth of mycelium, even after the substrate is drained and inoculated.

✔ What Type of Lime WILL Work?

The type of lime you want is Hydrated Lime that is low in magnesium. This white fluffy powder is manufactured by a process that treats "quicklime" with water, changing the chemical structure by converting oxides to hydroxides. Keep in mind that it is possible to find a garden variety “hydrated lime” that is high in magnesium. This will not work. Chances are, you’ll need to go to an agricultural supply store or similar to find the correct type of lime. It isn't always easy to find, so if you have trouble trying to get some, don't be surprised! The lime I used was picked up at a local UFA, and is the type typically used to treat drinking water on acreages. Image

How Does Lime Work?

Lime works by rapidly altering the pH of the water, making it extremely alkaline and killing off the contaminants in the substrate. Once drained off, the substrate has a low concentration of living contaminates, giving mushrooms the upper hand. I haven't tested it, but I'm guessing it's a combination between the substrate pH coming back down to earth after being drained AND the mycelium's natural ability to be resilient in high pH environments. Either way it works pretty well!


Hydrated Lime is potentially hazardous- so be careful when handling it. I like to wear gloves, safety glasses and a mask. Getting it on your skin can cause rashes and burns, breathing it into your lungs can cause respiratory issues, and getting it in your eyes can be especially dangerous. Always use protective equipment and don’t say I didn’t warn you!

Cold Lime Step By Step

This type of substrate treatment is easy for most people to do at home. It's even used by some small-scale commercial mushroom growing operations that want a low-tech, cheap and effective way to pasteurize straw. The overall process is very similar to growing mushrooms on straw with heat pasteurization, the only difference is to replace heat with lime.  

What You Need

  • Hydrated Lime (about 6 grams per gallon of water used)
  • Water
  • A 55 gallon drum, or large tote
  • Straw (wheat straw or oat straw)
  • Poly Tubing or Other Suitable Cropping Container
  • Spawn, about 2.5lbs per 25 lbs wet substrate.


Chop your straw into 2-3” pieces. You can do this with shears if you have a small amount, or by whipping it up with a weed-whacker in a large drum. This is not 100% necessary, but chopping the straw impacts how easily the mycelium can work its way through the straw, and makes the nutrients in the straw more accessible. The result of chopping is faster colonization, and very likely a much bigger yield.  


Add the hydrated lime to the water in a large drum or tote. A good rule of thumb is to use about 6 grams of hydrated lime for every 1 gallon of water. If you have a 55 gallon drum of water almost full (50-55 gallons of water), this equates to about 300-330 grams of lime. (doesn't have to be exact) If you don’t have a scale, you can estimate with volume and use about 1-1.5 cups lime per 55 gallons of water.  


Add your straw and soak it in the lime bath for 12-24 hours. Ensure that the straw is evenly mixed through the water bath so that it is all treated equally. Straw floats, so you might want to place something heavy on top to ensure full submersion. I like to use a cinder block and a metal grate on top of the straw to hold it down. Image


After 12-24 hours of soaking, remove the straw and let it drain for 20 minutes or so. You don’t want to leave it so long that new contaminates have a chance to settle in the substrate, but you also want to ensure that most of the water drains off so that the straw is not overly wet. Inoculated the straw at a rate of 10-15%. You can either mix it all together, or place it down in layers. One of the best options for cropping containers is poly-tubing. I like to use 14” lay flat diameter tubing, which makes straw logs that are about 9” in diameter. Make sure that you cut holes in the poly tubing to allow for the mycelium to breathe while colonizing. I like to use an arrow head and stab holes all around the log, every 4 inches or so. Make sure you also place holes in the bottom of the log so that any excess water can drain off.  


The straw should colonize relatively quick, depending on the species and the spawn rate. The mycelium should start growing right away. Place the log in a room temperature environment away from direct sunlight. In 10-14 days, the log should be fully colonized and ready to be placed in fruiting conditions. That's it! Again, it's very similar to the process for heat treated straw. Takes a little longer, but requires less work overall.  

A Recent Grow With Hydrated Lime

I just finished up a grow using Cold Pasteurization of straw and had some pretty good results! For this grow, I chose Blue Oyster (Pleurotus columbinus) and Yellow Oyster (Pleurotus citrinopileatus) because they’re both fast growing, high yielding strains that typically do well on straw substrates. I decided to fruit them outside, because the conditions were right, and because growing these species outside can produce some nice large fruits without too much hassle! First step, I chopped the straw using a weed whacker in a large decommissioned rain barrel. I then soaked the straw for about 18 hours in a 55 gallon drum full of cold hose water and about 300 grams hydrated lime. Image The straw was then piled up on a large mesh screen for about 20 minutes to drain. It’s surprising how much the color of the straw changes after the lime water bath! It appears so much more yellow/gold from the treatment. At first, I thought the straw might have been fried in some way… but it turned out to be no problem at all! Once drained, I stuffed the straw into poly tubing and inoculated it with grain spawn. Instead of mixing the spawn and straw before loading the tubes, I decided to add the spawn to the log in layers. This involves grabbing handfuls of straw, stuffing it into the log, sprinkling a thin layer of spawn, and then repeating until you run out of straw or spawn. The result is a kind of “layer cake” of spawn and straw- which, believe it or not, will colonize just about as fast as if it were thoroughly mixed. Keep in mind- If you decide to use this method, make sure that you really press down each layer as you are making it, so that there are no large air pockets or areas of loosely packed material. Once the logs were stuffed, I stabbed the outside of the log with an arrowhead, about once every 4 inches or so. This included perforating the bottom of the log so that any excess water can drain off as the log colonized. Image The logs were brought into a cool storage area, inside, so that they could colonize. After about a 7-10 days, the logs were fully colonized, and ready to be brought outside. I hung these logs in a custom-made arbor, complete with a poly windscreen to help prevent the fruits from drying out. Oysters definitely need LOTS of fresh air, but if there is a constant wind, even just a slight breeze, the fruits will inevitably dry out and abort. You can usually remedy this by placing the log near the ground, deep enough into the trees, or simply draping it in poly sheeting. The two logs fruited nicely! Image The Blue Oyster, of course, had no problems producing some nice large cluster. I did however have to get ‘em pretty early so that the bugs didn’t take over. The Yellow Oyster also fruited nice, although I didn’t manage to snag any pictures. That being said, once I thought the log was totally done, I stripped off the poly tubing and threw it in the compost pile… where it decided it wasn’t done yet! This happens often, especially with Oysters, who just like to keep fruiting and fruiting and fruiting. Image

Other Low-Tech Methods

Of course, both heat pasteurization and lime pasteurization are effective, but they aren’t the only way to treat your substrate. There are some other methods that are cheap and effective, that you might want to try for yourself.  

Anaerobic Pasteurization

Although there are a million different types of bacteria and other contaminants that can affect your grow, you can divide them into two groups- aerobic (oxygen loving) and anaerobic (oxygen hating). Basically, some contaminants need oxygen to thrive, while others need a complete lack of oxygen in order to survive. We can take advantage of this to manipulate conditions and pasteurize a substrate without heat, lime, or any other chemicals. The basic process involves soaking a substrate (such as straw or wood chips) in water, draining, and then placing in a sealed container. 5 gallon buckets work really well. If properly sealed, the aerobic contaminates will eventually die off, and the anaerobic contaminates will proliferate. Once the container is opened, and the substrate is exposed to the air, the anaerobic contaminates will die off, and there won’t be any aerobic contaminates to worry about, at least for a little while. This gives the mushroom mycelium enough of a head start to take over the substrate. This topic really deserves its own post, hopefully I’ll get to that soon!  

Laundry Detergent and Soap

Some people have had success using laundry detergent, or even just regular dish soap. I guess it makes sense, because soap is antibacterial and I’m sure laundry detergent is the same. Although I haven’t tried this myself, the process involves soaking a substrate in cold water with laundry detergent for a day or so, in a very similar fashion to cold water lime pasteurization. I imagine this might work OK for home and hobby growers, but is not something that would make much sense to do commercially. Also, I would be a little concerned about the quality of the mushrooms if the substrate is pasteurized in this way.  

Ash Pasteurization

Wood ash has also been used to successfully pasteurize substrates. It works in a similar way to hydrated lime, by raising the PH enough to kill off contaminates in the straw. Some have reported that this method actually results in the best yields of any method, and considering it is a low cost material in many parts of the world, Wood Ash Pasteurization might be the best way to go for many growers.  

Give Lime A Chance

I haven’t run any strict trials, but many growers report that hydrated lime pasteurization actually has a much higher Biological Efficiency (BE) than steam pasteurization. Some growers will actually calculate the exact amount of water needed, so there is no excess high pH water to get rid of. For example, if you had 25 lbs of dry straw, you would use about 10-12 gallons of water, most of which will be absorbed by the straw. This is much different than hot-water pasteurization, where you need to dispose of all the hot water. All this and you don’t need to wrestle hot straw, continuously monitor temperatures, and burn off expensive propane. Don't you think it's worth giving lime a shot? I sure do!
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