Can Hot Water Effectively Pasteurize Substrate?
Yes, hot water can effectively pasteurize mushroom substrate by killing harmful micro-organisms. The process involves immersing the substrate in hot water for 60-90 minutes and keeping the substrate core temperature range between 140F – 170F.
Effectiveness of Hot Water for Pasteurizing Mushroom Substrate
The hot water technique is a very effective method for pasteurizing mushroom substrate because it is easy to execute and requires minimal equipment. This allows reliability and control with our substrate preparation. In this process, we kill off about 80% of the existing microbes that are bad for the substrate. The remaining 20% are made up of organisms that pose little competition for our mushrooms and actually help to protect our substrate.
What is pasteurization?
Pasteurization is the process of killing most of the competing organisms in the substrate. Unlike sterilization, it doesn’t kill all the organisms, but leaves about 20% of the heat-resistant microbes that actually aid in protecting the substrate from harmful contaminants. These pose little to no threat for the mushrooms and act to improve your chances of successful mushroom cultivation.
Pasteurization is an important part of preparing your substrate so that your mycelium can feed on the nutrients without having mold, bacteria or other fungi feed takeover.
What are some of the negative aspects of Hot Water Pasteurization?
The main drawbacks of pasteurizing with hot water are the energy and time it takes to achieve. While bringing a small stock pot to temperature may not be a big deal, it can take many hours to properly heat water when scaling up to even a 55-gallon drum for medium-large scale grows. This also introduces risk of burns and injury and requires the cultivator to be present to monitor the process. With longer times to pasteurize also come higher energy costs and fuel bills.
- Risk of Injury
- High Energy/Fuel Costs
- Longer pasteurization times with scaling up grows
Which substrates are the best to pasteurize with the hot water method?
Typically, pasteurization is best suited for non-supplemented, agricultural waste substrates. It maybe more difficult to apply this technique to substrates like sawdust. We can apply the hot water method to
- Straw (wheat, oat, barley)
- Manure
- Coconut Husks (coco coir)
How to pasteurize straw substrate with the hot water method
To pasteurize the substrate with hot water, we will need to bring the water surrounding the substrate between 140F-170F, and maintain this temperature for at least 1 hour. It is important not to exceed the 170F so that we can preserve the good microbes. If the temperature gets too high, it will kill too many micro-organisms and be at risk to more contamination.
Step-by-Step Guide: How to Pasteurize Straw substrate with Hot Water
The nice thing about this method, is you probably have everything you need at home already. This process isn’t so rigid with equipment, so use what you have available. Things like cooking thermometers and boiling containers are fairly cheap.
Materials:
- Large pot
- Polypropylene Container
- Aluminum Foil
- Rubber bands
- Cooking Thermometer w/alarm
- Canning rack or spacers for bottom on pot
Step 1: Choosing your substrate vessels and Filling
- Food grade polypropylene containers will work well. Glass jars can also be used but create more chance hazard if they break.
- Fill container and pack down.
Step 2: Hydrating the Substrate
- Pour hot water into substrate and let set for 1-2 hours
- Drain excess water from substrate materials after soak. We need to get the substrate to “Field Capacity”. This means we should be able to squeeze it with our hands and only get 1-2 drops of water.
Step 3: Prepping for the Container
- Create foil lid for your jar(s) and place rubber band around the rim to seal
- Insert the cooking thermometer into one of the jars, placing the thermometer in the middle of the substrate mass.
*We want to measure the internal temperature of the substrate, not the water!
Step 4: Prepping the pot
- Line the bottom of the pot with spacers: pressure cooker rack, jar lids, etc.
- Place your container(s) inside the pot, setting the on top of spacers or a rack. The substrate jars should not be touching the bottom of the pan directly.
- Fill the pot with water until it covers approximately 1/3 of the substrate jars.
Step 4: Heating the Substrate
NOTE* We want to measure the internal temperature of the substrate, not the water!
- Set the cooking thermometer to alarm at 140F and begin to heat water. As the water begins to boil, it may move your jars around. If necessary, place a weighted object, such as a bowl or place to prevent the jars from tipping over.
- Once temperature reaches 140F, turn of the stove heat and set a 60-minute timer on your phone.
- Set your cooking thermometer to 170F. During the course of the 60 minutes, make sure the temperature stays within this range of 140F – 170F. If the temp drops below 140F, turn the burners on the get back in the threshold. It the temperature exceeds 170F, remove from the pot to cool.
Step 5: Cooling
- Spread substrate out on clean surface to cool
- You will have about 2 weeks so inoculate the substrate before competing organisms start growing.
What are some common issues with the hot water pasteurization method?
Some people make the mistake of measuring the temperature of the water when we need to focus on the internal temperature of the substrate. This will take much longer to get hot enough to achieve pasteurization. Otherwise, you might see contamination grow during the incubation phase.
How to Scale Up the Boiling Method
A benefit use using the boiling technique to pasteurize substate is that it is easily scalable. This can be done using a steel 55-gallon drum and a heat source. A propane stove or electric coil can be fashioned onto the steel drum so we can boil the water. The drum will act as the pot and we will need to create a basket that we can use to pack in the substrate and submerge into the drum. Chicken wire can be used to create a basket to hold the substrate material and accomplish this.
Comparing Hot Water Pasteurization to Other Methods
Using hot water to pasteurize is usually is a good place to start if just dipping your toes into the mushroom growing world. If growing mushrooms at home, it may be one of the best methods available. Other pasteurization methods such as steaming generally take a lot longer to accomplish and steaming may be better suited to medium to large scale grows. Alternatively, there are many chemical solutions available that might offer lower effort, lower energy cost but the cultivator is left with having to dispose of the chemicals.
In conclusion, hot water pasteurization is an effective and accessible method for preparing mushroom substrate, particularly for home growers. By maintaining the substrate core temperature between 140F and 170F for 60-90 minutes, this technique efficiently eliminates harmful microorganisms while preserving beneficial microbes. While it requires energy and careful monitoring, its simplicity and reliability make it a popular choice. Despite the potential risks and energy costs, the ability to use readily available household items and the ease of scaling up make hot water pasteurization a valuable method for mushroom cultivation. By following the step-by-step guide and understanding the limitations, you can achieve successful pasteurization and enjoy a thriving mushroom harvest.