What does Pasteurization mean for Oyster Mushroom Cultivation?
Growing oysters on pasteurized straw or cottonseed hulls is the highest "yield per substrate" method of oyster mushroom production. Pasteurization is a way to kill off objectionable organisms without significantly altering the chemical and physical composition of the substrate. After pasteurization, straw can be inoculated and packed into either buckets or bags for incubation and cultivation. You can use either wheat or oat straw, but avoid hay or weedy straw.
How to Pasteurize Straw
1. Materials Needed: To pasteurize straw, you will need a large vessel in which you can heat water, several water permeable bags, straw, and twine. For this blog we're talking about large-scale production, but keep in mind that you can do this on a small scale with a large pot over your stovetop.
At Field & Forest we use chopped straw. You can use unchopped straw as well, but chopped straw is easier to work with and allows for a denser pack. More straw equals more mushrooms.
We mentioned using water permeable bags- we repurpose dowel bags but you can use burlap bags, pillow cases, or anything that will allow water to pass in and out of. Stuff the bag full of straw and tightly tie twine around the opening to ensure the straw doesn't escape during pasteurization.
While you're stuffing your bags full of straw, you will want to begin filling and heating your water as this can take some time depending on the size of your vessel. Aim to keep the water temperature between 160- and 180-degrees Fahrenheit. Once the water reaches that temperature you can toss your bags of straw into the water. The straw will need to be completely submerged for proper pasteurization, so you will likely need to find a way to weigh down the bags.
Keep the bags submerged for 45-60 minutes, maintaining an internal bag temperature between 160° and 165°. Be careful about increasing the heat of the water at this time as you will want to avoid the straw temperature reaching above 180 degrees. At 190° the sugars in the straw will start to caramelize, creating excess sugars which invite green molds during incubation. A long-stemmed thermometer, such as a basic compost thermometer, can help keep tabs on the internal temperature of your bagged straw.
As soon as the 45-60 minutes is up, drain the water away and let the bags cool, preferably in the tank, or pot, for 24 hours.
The following day you can inoculate your straw. At this point you want the straw to be cooled to at least 85° before inoculating, but preferably closer to 75°. To get started you will want to choose whether you want to grow your mushrooms in bags or buckets. One small bale of straw will inoculate approximately 6 14x40 or 18x36 oyster bags or ten, 5-gallon buckets. Next, you'll need your spawn. You can either use grain or sawdust spawn, but the more popular choice is grain because it breaks up so easily.
Use an inoculation rate of 5% spawn per wet weight of the straw. This translates to 1 lb of spawn per 20 lbs of straw. Another way to think about it is you'll need about 1 lb of spawn per sleeve, or oyster bag, depending on size, or about ½ lb of spawn per 5-gallon bucket.
Before getting started, wipe down your work area with a bleach or alcohol solution in order to lower the risk of introducing any contaminants. It's also helpful to do this in a room that does not have a lot of traffic as moving bodies and open doors can move around a lot of airborne particles. Today we're going to use a bag- we're going to stuff about 5-7 handfuls of straw into the bag before adding spawn. Lightly incorporate a layer of spawn and then compress the straw as much as possible. We use a slam and compress technique in order to get rid of as much airspace as possible. Layer straw and spawn, using your fingers to tease in spawn at each layer, compressing after each addition. After filling the bag, zip tie it shut and cut small Xs every 4 to 6 inches in staggered rows.
Place your bag or bucket in a warm environment, ideally somewhere in the 70-75° range, for 2-5 weeks depending on strain and incubation temperature. During this time the mycelium will begin running throughout the bag or bucket. If you're using a sleeve, you will be able to see condensation appear after a day or two- this is good and means the mycelium is healthy. Over the next several weeks the mycelium will work its way through the bag and slowly coat the straw in white. The only exception to this is pink oyster, which will not have a heavy, white mycelium.
After 3-4 weeks you will start to see pins develop at several of the cut sites in the bag or holes in the bucket. These pins will look nothing like mushrooms at this stage, but rather just textured blobs. At this stage move the bags to a fruiting room. Fruiting rooms should be cooler than the incubation space, 65° is optimal, with a humidity of around 85-90%. Bags should also have 10-12 hours of natural or incandescent light.