by Dan West
Making “Masters Mix” fruiting blocks (the details)
We’re making Fungus Food!!
5-pound Fruiting Block Recipe Demystified!
Before we start, if at any point you feel like making your own mushroom fruiting blocks isn't for you, you're in luck. We sell Grain & Master all-in-one Mushroom Fruiting Blocks. They are sealed, sterile, professionally crafted dual substrate blocks that are ready to be inoculated with your favorite mushroom liquid culture. Ok, lets begin!
Many species of gourmet and medicinal fungi require sterilized sawdust media with amendments to produce mushrooms and achieve high success rates. One recipe that works well is a 50/50 mix of hardwood sawdust and soy hulls, known as “masters mix.” It works well for many mushrooms and can produce large yields. You can find hardwood sawdust easily at home improvement stores in the form of hardwood fuel pellets. Soy hulls can be purchased in feed stores or online. Soy hulls come in pellet form or coarsely ground. We like to use pellets since they are easy to store and don’t create a mess. Here is a masters mix recipe that works well.
Masters Mix Recipe for a 5 lb. fruiting block
- Mushroom grow bag with .5-micron or .2-micron filter patch (see more on appropriate size below)
- Hard wood fuel pellets (HWFP) (.85 lbs.) or (13.6 oz)
- Soy hull pellets (.85 lbs.) or (13.6 oz)
- Gypsum (17 grams) or (.6 oz)
- 1,400 millimeters of water (chlorine free and pH neutral or slightly basic water)
The Details
Let’s take it from the top and start with the mushroom grow bag. It should be autoclavable, have a .2-micron or .5-micron filter patch, and be the correct size for your block recipe. Some species of fungi such as shiitake, need more time to colonize substrate before they are ready to fruit. To decrease chances of contamination, use mushroom grow bags with a .2-micron filter patch. This helps prevent contamination working its way through the filter during long colonizing times, and helps prevent water loss through evaporation. For many fast colonizing gourmet species that fruit more quickly, .5-micron filter patches work great and are preferred because they allow more gas exchange which can boost growth for some species. Do your homework and find out which bags work best for the specific species of fungi you want to grow.
HWFPs and Soy Hull pellets need to be rehydrated for some time before you can mix and bag them. Soaking them overnight works well. Mix in the gypsum powder slowly to help prevent clumps and uneven distribution once the pellets are fully hydrated. Gypsum adds calcium to the substrate which is beneficial for mycelial growth. Continue mixing the substrate until it has an even consistency and all the pellets are broken up. Once everything looks ready to go, STOP. Did you test your substrate moisture content? We highly recommend doing this when getting a new batch of fuel pellets and soy hulls since the moisture content can vary slightly. You’ll also get more control so you can dial in the exact percentage of moisture your mushrooms will love. Gourmet species like a range between 55 % and 65% moisture content. Starting at 60% moisture is a good goal and can be adjusted to develop bigger yields. Blocks that are too dry or wet can have greater contamination problems and/or decreased yields, so experiment with your species.
Calculating percentage moisture: Follow these steps to calculate your substrate percent moisture. You will need a polypropylene, glass, or porcelain container. Place the empty container on the scale and zero it out by hitting the tare button. Then place your five to ten-gram sample substrate in your container and weigh it again. Record this weight as your total sample weight. Microwave the sample in 1-minute increments, measuring and recording its weight in-between each cooking session. Once your sample is getting dry, decrease cooking time to 30 seconds. When your measurements don’t change in-between cooking sessions that means your sample is completely dry; you have your end weight. Here is how you calculate percent moisture.
Total sample weight = Weight of hydrated substrate sample (5 or 10 gram samples work, but make sure your scale goes out two decimals to the hundredths spot for an accurate reading)
Weight of evaporated water = total sample weight minus the sample weight after heating or end weight
(Weight of evaporated water / Total sample weight) x 100 = % moisture content
Example: (5.62 gram end weight/10 gram total sample weight ) x 100= 56.2% moisture content
Before we move on to bagging and sterilizing, let’s chat about creating custom recipes for your masters mix blocks. The recipe we provided above is conservative, considering the number of other ingredients that could be added. Adding other ingredients could decrease colonization times and boost mushroom yields while decreasing chances of contamination. Our recipe calls for 17 grams of gypsum. This is a 2% by dry weight addition but could be increased up to 10%. Depending on which species of mushroom you are growing, increasing the amount of gypsum above 2% may help or hurt. Experiment! Here is a list of some other ingredients you can use to make custom masters mix recipes. Keep in mind, there are seemingly endless possibilities of what you can feed your fungus.
Other useful ingredients: Sesame or Soybean oil (1 tablespoon/gallon of substrate), fulvic acid (1 tablespoon per gallon of water), sesame meal (up to 4% of dry substrate weight), worm castings (5 to 15% by volume), used coffee grounds, wood ash, pickling lime, and the list goes on. Wood ash and pickling lime are alkalizing agents. The rest of the other ingredients add some level of nutrition and/or boost growth. Experiment and see what works best for your mushroom culture, substrate, and growing environment.
Now your substrate is ready to bag! Put the desired amount of substrate in your fruiting bag, in this case, 5 lbs., and make sure no substrate gets stuck to any of the plastic on the inside of the bag above the filter patch. These bags won’t seal up if there is a bunch of junk where you are folding the bag flat. Push in the gussets and pinch the side corners together. You are trying to shape the top of the bag above the substrate back to its original packaged configuration. Don’t leave any voids or air spaces inside the bag because they will expand, distorting your bag during pressure cooking and can prevent a good seal; we’ll mention that in a minute. Fold down the flattened top. When blocks are pressure cooked, they will release some gasses and moisture out of the open end of the bag even though it’s folded over tightly. When the pressure cooker cools down, the gasses will contract, ideally creating a vacuum inside the pressure cooker and the bag. Sometimes this doesn’t happen very well, and the pressure equalizes. This is where contamination can get in. As the pressure decreases, air starts to flow back into the pressure cooker. If your bags didn’t seal well within the pressure cooker as it cools, they can also suck up un-clean air due to the vacuum, causing contamination before you even get started; so fold carefully. Here’s what bag folding looks like.
Pressure cooking: Pressure cooking substrate is necessary to kill competitive organisms that are on the material. Most wood loving gourmet mushroom species need a sterile environment to grow properly for mushroom production. 5 lb. masters mix blocks need to be pressure sterilized for 3 hrs. at 15 psi, approximately 250° F. Start the timer when the right temp and pressure are achieved. It’s necessary to have a large pressure cooker that can hold 5 lb. blocks. You’ll need at least a 20-quart pressure cooker to sterilize more than 1 x 5 lb. block. For reference, the 41 quart All American pressure cooker can hold 9 x 5 lbs. blocks comfortably. There is some misinformation out there stating that sterilizing for 1.5 hours will sufficiently sterilize your substrate every time. It’s true, it can work, but it can fail too. Fruiting blocks take a long time to heat up in the center during sterilization due to their mass and insulating properties. Don’t waste all your hard work making blocks and not sterilizing them properly. Sterilize for 3 hours! Large pressure cookers take a long time to cool down. You can speed up the process by pointing a fan at the pressure cooker for several hours. The last picture above shows the bottom 3 blocks sitting upright with their folded tops and filter patches pushed up against the outside wall of the pressure cooker. The rest of the blocks are upside down with their folded tops pushed against the outside wall of the pressure cooker too. Use every safety precaution while using pressure cookers. You don’t want a fruiting block bag to expand and block the pressure release. Placing a ceramic plate, ball of tinfoil, mason jar lid ring, or empty mason jar on top, and sometimes in-between, your fruiting blocks. They all work well for weighing down the top of masters mix bags and taking up empty space where bags could expand. Now it’s time to inoculate your fruiting blocks with your favorite mushroom species.
Inoculate your 5 lb. fruiting blocks with ½ lb. of colonized grain spawn using your best sterile technique. Generally, this is done in front of a HEPA laminar flow or inside of a still air box to prevent contamination from entering the bag. Inoculating with ½ lb. of grain spawn achieves a 10% inoculation rate of your fruiting block and is a good place to start. Grain spawn adds a significant amount of nutrition to the block, so over-inoculating is possible and can have negative effects on the success of your grow. The two main problems with over-inoculating your fruiting blocks is contamination and mutations of fruit bodies. Some species of mushrooms don’t like a diet that is too rich. For example, shiitake will exhibit mutated mushroom fruit bodies that are less desirable if their diet is too rich.
After inoculating your fruiting blocks, the tops of the bags need to be sealed. Ideally this is done in front of a laminar flow from a HEPA filter as well. It can also be done inside of a still air box. The best way to seal bags is up to you. We use an impulse heat sealer, but they are expensive. Other options include, zip ties, tape, or ½ inch clear tubing that has been cut down the length of the tube on one side and slid over a small fold at the open end of the bag to prevent air from getting in. You can get creative with this part. The goal is to prevent any unfiltered air from entering the top of the bag. Now your fruiting substrate is ready to be inoculated by grain spawn or directly injected with a liquid culture. Once you have your recipe dialed-in, you can expect multiple flushes of beautiful mushrooms!
If this is your first attempt at making your own masters mix blocks and you don’t have a laminar flow hood or still air box to create a sterile environment, we recommend using filter patch bags that have a build-in self-healing injection port. It’s super easy to inoculate this type of block. All you do is take your favorite liquid culture syringe and inject 2 cc to 6 cc directly through the inoculation port. Make sure to wipe down the injection port with rubbing alcohol before you get started to prevent contamination. Easy peasy!
Ok, you’ve read this entire article and you’re thinking about giving this a try, but don’t really want to make your own fruiting blocks. You’re in luck! You can buy Grain & Master all-in-one Mushroom Fruiting Blocks from us. They are sealed, sterile, professionally crafted dual substrate blocks that are ready to be inoculated with your favorite mushroom liquid culture.
Want to learn more about growing mushrooms? Learn a lot more about growing mushrooms at ccmushrooms.com. Need liquid culture to grow the mushrooms of your dreams? We sell professional quality commercial mushroom liquid culture.
We look forward to helping you become great mushroom growers!
Until next time; enjoy and “always grow culture.”
