Preparing HWFP

Hard Wood Fuel Pellets, aka. HWFP

Hard wood pelletsSo what is HWFP? In few simple words, it is compact sawdust. As opposed to straw, HWFP is used for a lot of mushrooms, not only oyster mushrooms. Since wood is much denser than straw, the same size bag of sawdust compared to straw weights about three times more. This means the mycelium will take more time to colonize the entire sawdust substrate and will produce more flushes, because it has more substrate.

It is used almost the same way as straw. Where straw is being pasteurized, HWFP is being sterilized. A simple pasteurization of HWFP will result in guaranteed contamination of the substrate, so it must be sterilized. 

As with all of our blogs, we are not going to provide recipes, because it is again a matter of personal preference and depends on the items we have on hand. Most recipes are never just 100% HWFP; they are usually mostly made of HWFP supplemented with some type of cereal, bran, wood chips, lime, gypsum or other ingredients. The reason why HWFP is supplemented is that doing so results in better yields. It is also a matter of which mushroom we are growing, and that’s why we can not provide a recipe that works in all scenarios.

A HWFP bag is made of 60-90% HWFP (sawdust), 10-40% rice, soy, wheat bran but other types of bran could be used, about 0.5% to 2% gypsum and about 0.5% lime, and the percentages depend on what it is that we want to grow. These are the most used ingredients; others may be using different ingredients and again, it depends on what we can come by easily and cheaply. It is also recommended to use some wood chips that will help in the block aeration but that’s not a must. The hobbyist has to make sure that the product he is buying is from leafy, hardwood trees and not a fir type because fir trees are acidic. Also, make sure the sawdust pellets contain no anti-fungal, otherwise mycelium will not grow properly.

We recommend that anyone getting into the hobby try different recipes and select one that best suits the list of materials they can come by or the type of mushroom they want to grow.

To use sawdust substrate, it is good to have medium to large mushroom bags with filter patches. It is the only way to properly create HWFP blocks. We use medium bags to create 3kg to 4kg blocks.

Usually all the ingredients go into the bag and then we have to pour the right quantity of hot water into the bag. We allow the bag to sit for about thirty minutes for the wood to become properly hydrated, in which case the volume in the bag can triple.

After the thirty minutes, we seal the bag, not too tight but just enough for water not to get into the bag once in the pressure cooker; otherwise, the humidity levels in the bag will change, and we don’t want HWFP substrate that is soaking in water. Remember that any substrate that soaks in water will end up with contamination eventually.

The rule of thumb is to pressure cook for 2.5 to 3 hours at 15 PSI, but it all depends on the quantity of substrate we have in the pressure cooker, so if we have less, then we can target 2 hours 30 minutes at 15 PSI, and if the pressure cooker is full, then we go with 3 hours. The reason for this is that sawdust substrate is dense and the heat takes longer to penetrate, and it can take even longer if we have multiple bags in the pressure cooker. The last thing we want is to complete all of this work, not properly sterilize, and end up losing the work we have done. Therefore, it is better to sterilize a bit more than not enough.

Once the sterilization period is over, we need to wait a few hours for the bags to cool down to room temperature. Be careful here, because the outside of the bag may be at room temperature, but the inside or the middle of the substrate may still be hot. We usually allow the bags to sit over night before inoculation.

When it is time to inoculate, we add about 5-10% grain spawn for the weight of the bag. Therefore, for our usual 3kg substrate bag, we will add 150-300 grams of grain spawn. We want our bags to get colonized quickly in order to reduce chances of contamination.

We close the top of the bag by hand and shake the bag in order to mix the grain spawn with the substrate. We remove most of the air from the bag and make a block out of the substrate, but always make sure the filter patch is not covered by plastic or tape. The patch must always allow for air exchange.

It is time to store the bag in the incubation chamber. Since this is a big block of mostly wood, and it’s dense, it will take the mycelium 4 to 8 weeks to fully colonize depending again on what we want to grow. Using denser sawdust blocks will provide more flushes, but will take longer for the mycelium to fully colonize and start giving mushrooms.

Once the substrate is fully colonized, it is time to move the bag in the fruiting chamber and make one or two holes on each side of the bag.

HWFP or sawdust is a great way to produce mushrooms at home. It is easy to work with, fairly clean and smells good once in the pressure cooker. The disadvantage of sawdust is the time it takes to start giving mushrooms, but as explained earlier, it will provide more flushes than straw, and the fact that building HWFP blocks requires a sterilizer.

Please see our other blogs covering more specific subjects about the fun world of growing mushrooms.

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