Mycelium is the vegetative state (think root system) of the fungal life cycle. The fungal lifecycle consists of spore, mycelium, and mushroom. Mycelium is the stage that produces a mushroom, but it is not a mushroom. Just look at some images in Google.
Myceliated grain or grain spawn or mycelium biomass produces the seed for growing fresh mushrooms. Many “mushroom” ingredient suppliers have now adopted it as an inexpensive way to produce a “mushroom” supplement, yet in reality, it is not a mushroom product. It is a grain-based mycelium product similar to tempeh.
The process involves taking a plastic bag of sterilized, cooked grain (rice, oats, sorghum, etc.), injecting it with a fungal culture (reishi, chaga, shiitake, etc.), and allowing the mycelium to grow out on the grain. Once the mycelium is fully grown out on the grain, it is harvested, and the mycelium with the grain is dried and turned into a powder. This produces a product that does not contain any mushrooms, though the package may say “mushroom” on it, and it has a very high starch content (upwards of 70%) because of the residual grain ending up in the final product. Because of all this added grain, the amount of fungal material in the product is meager, leading to low active compounds like beta-glucans.
Myceliated grain is a cheap way to make “mushroom” supplements that are not actually mushrooms. In reality, when you compare the number of active compounds in the product, it ends up being a costly product. To identify myceliated grain products, look for wording like “mycelium,” “myceliated brown rice,” “mycelial biomass,” “full-spectrum,” “whole lifecycle” in the supplements panel and marketing materials. These are good indicators of grain-based mycelium.
Products made entirely from mushrooms, like Real Mushrooms, typically have 5-10x more beta-glucans than mycelium on grain products.
For more info check out How to Identify Myceliated Grain