Agar is an indispensable tool for professional and amateur mycologists. It plays a key role in mushroom cultivation, forming the base ingredient of growth mediums used in Petri dishes. In particular, agar allows mycologists to isolate clean cultures, store them, and eventually produce the spawn needed for the final steps of the cultivation process. It is also used for work with genetics, liquid cultures, experimental designs, and other processes.
While working with Agar is a fundamental part of the mushroom cultivation process, it is considered a more advanced procedure. It requires laboratory-like conditions and the ability to sterilize your materials using high heat and pressure. Those looking to grow mushrooms can often skip the agar stage by finding a trusted source of mushroom spawn that can be used to inoculate bulk substrates. Agar work is best suited for those willing to invest the resources, time, and effort into intimately working with fungal mycelium.
Why Grow Mushrooms On Agar?
Technically speaking, mycologists don’t grow mushrooms on agar. They grow fungal mycelium. The white spider-web growth which composes the actual body of the fungus. Production of mushrooms takes place in processes that proceed working with Agar.
Cultivating within Petri plates is practical for a variety of reasons. The two-dimensional nature of agar within Petri plates allows you to easily see your culture and any contamination that may be present. This allows you to isolate, clean, and store cultures.
For example, when isolating a fungal culture from a mature mushroom your first step is transferring it into a sterile Petri plate with an agar growth medium. This is done either with spores or live tissue cultures.
In many cases, these cultures become contaminated with other microorganisms present within the environment of the donor mushroom. To obtain a clean culture, clean fungal mycelium grown on agar can be transferred from the contaminated plate into a clean one. While it may take several transfers, this process allows mycologists to clean a culture. Once free of contamination, cultures can be transferred into the grain, liquid cultures, replicated into more plates, or stored in the refrigerator.
What is Agar?
Agar is called “Agar Agar” but it is usually just shortened to a single Agar. It is used as the main growth medium in Petri plates because of its useful physical properties. It liquefies when mixed with hot water but solidifies at room temperature. Think of Jello or Gelatin.
It is usually mixed with other nutrients, and sometimes antibiotics, to provide suitable food sources for the fungi. Agar-agar itself is a natural plant-based product derived from algae. Aside from its use in mycology and microbiology, agar is also used in some culinary traditions and vegan desserts.
How Are Mushrooms Grown On Agar?
1) Preparing The Agar
The first step in the Agar process involves preparing the growth medium. Generally speaking, these recipes require 20 grams of dry agar for every liter of water. Food sources such as Potato Dextrose, Fructose, Malt, and other sugars/starches are often added to provide nutrients for the fungi. Recipes are provided in the next section of the article.
The preparation involves heating all the ingredients together and forming a liquid solution which is then sterilized. This can be done in an autoclave or at home in a pressure cooker. Ten to fifteen minutes at 15 PSI is enough to sterilize the medium. Overcooking your medium can caramelize the sugars and make it less suitable for fungi.
2) Pouring The Agar
After cooling down to just over 100F the agar can then be poured into the plates. This process and all the following steps should be done in laboratory-like conditions. This means in a laboratory with a laminar flow-hood for professionals or a still air-box within a clean room for an amateur.
Pouring is done by pouring the hot agar solution into sterile plates to form a thin layer. Temperature is important for this step. Agar that is too cold will begin to solidify and if it’s too hot it may create condensation within the plate. After pouring, agar is left to cool and solidify. Poured plates can be stored for later use or be used immediately after cooling.
3) Cultivating Mycelium On Agar
Cultivation of mycelium on agar is done by placing a fungal culture within a Petri plate. This can come from a mushroom, grain spawn, liquid culture, or another Petri plate. Spores can also be germinated within Petri plates. Cultures are usually grown from the center of the Petri plate and allowed to expand towards the edges of the plate. When transferring mycelium out of a Petri plate, mycelium is cut using a sterile scalpel and placed into a new growth medium.
Culture can be isolated from contamination and cleaned by transferring mycelium located away from the contamination. Spores and other forms of contamination may travel along with the original mycelium so various transfers may be necessary. Before expanding from agar to grain or liquid culture your culture must be free of contamination.
Basic Agar Recipes
Below are some simple recipes for making an Agar growth medium. In general, there is a lot of flexibility with these recipes as mycelium easily adapts to its environment. Lowering the nutrient content in Agar can also be useful to reduce rates of contamination.
Grain Water Agar: 500 ml of Grain Water + 10 Grams of Agar Agar
This recipe utilizes the leftover water made from boiling grain when making grain spawn.
Malt Extract Agar: 10 Grams of Light Malt Extract + 10 Grams of Agar Agar + 500 ml of Water
Malt is traditionally used in brewing but serves as an excellent sugar source for fungi. Light malt extract is easily available at any brewery store.
Potato Water Agar: 100 Grams of Potatoes + 500 ml of Water + 20 Grams of Agar Agar + 10 Grams of Honey
This recipe is handy because potatoes are readily available and easy to get. Just cut the potatoes into 1” cubes and put them with 500 ml of water to boil. Once the potatoes are perfectly cooked, strain them from the water. Avoid overcooking them or agitating them as this will release too many starches into the water. Starchy agar will be more difficult to pour. Once strained, top off your solution to 500ml to make up for any water lost during evaporation.
Potato Flake Agar: 5 Grams of Instant Potato Flakes + 20 Grams of Agar Agar 10 Grams Of Honey + 500 ml of Water
Take all the ingredients except the agar and boil them together. If the flakes don’t fully dissolve you can run them through a filter. After filtering you can add the Agar Agar and sterilize.
Fast Growing Agar: 1 Gram of Honey + 500 ml of Water + 20 Grams of Agar Agar
This recipe is simple and encourages fast growth. It is low in nutrients which trigger the mycelium to produce fast-growing hyphae which are eagerly searching for a new food source. Fast-growing agar may not always be ideal because it reduces the time you can store a culture.
How Long Does It Take For Mushrooms To Grow On AGar?
This usually depends on the strain, the growth medium, and the ambient temperature. In most cases, a plate can be fully colonized within 2-4 weeks.