Spent Grains in Mushroom Growing

Spent brewer’s grain offers a tantalizing opportunity for developing circular economies. It has all sorts of uses after the brewing process, including feeding livestock, making compost, or producing energy. But this post will focus on a use that is often overlooked: using spent grain to grow mushrooms.

While the fermentation process in brewing strips the grain of its sugars, it is still high in proteins and cellulose. This makes it an appealing growing medium for fungi, as they feed primarily on cellulose. While the overall nutrient content depends on the type of grain used, there typically is enough to fuel fungi. The brewing process also brings the grain to temperatures in the 140-160 degree Fahrenheit range, a range similar to what is needed to pasteurize growing media for fungi.

One challenge with using spent brewers grain in growing mushrooms is that the longer the spent grain sits out after brewing, the more likely it is that the grain will accumulate bacteria and spores from other types of fungi that could out compete a desirable fungi (such as mushrooms). If a mushroom grower can collect the spent grain directly after the brewing process, they can skip the often costly and labor intensive process of pasteurizing the growing media. Obviously there are logistical challenges in doing this…unless of course the mushroom operation is co-located with a brewing operation.

Last summer Plant Chicago’s Urban Agriculture Specialist, Jessica Zeiger, and her intern, Mike Smith, set out to take advantage of both such operations being under the same roof. They performed an experiment to see how well spent grain from Whiner Beer Co. could be incorporated into the mushroom growing process. They inoculated the spent grain with Pleurotus djamor (commonly known as the pink oyster mushroom), and waited for the results.

Mike Smith
Mike Smith preparing a petri dish

To their surprise, they found that the mycelia (the rooting structure of a mushroom) colonized the spent grain in 5 days, compared to 10-15 days for the more traditionally used rye berries. That’s 2-3 times faster! This myceliated spent grain (also known as ‘spawn’) was then mixed with sterilized sawdust and woodchips to make four ‘fruiting blocks,’ from which they were able to harvest 1.5 pounds each. Once the spent grain is ‘spent’ again through the mushroom growing process, it can further be used as an excellent bulking agent for composting.

Pink Oyster Spawn
Pink Oyster mycelium colonizing spent brewer’s grain

So what does this mean and how does it scale?

When mycelia is grown out from a petri dish, it is transferred to a small jar of rye berries then to a 5-lb bag of rye berries. That 5-lb bag can then be used to inoculate four fruiting blocks as above, or one 5-ft long fruiting log. The log would produce on average about 60 lbs of oyster mushrooms. With oysters selling at $8/lb, that equals $480 of product per log. Replacing the 5-lb bag of rye berries with spent grain saves $15. (The amount saved would be even higher if the grower was purchasing pre-made spawn bags.) If the grain is fresh and not needing to be sterilized like the rye berries would, additional electricity would be saved plus approximately 3 hours of staff time to sterilize. In total this would save approximately $33 for every log, or an increase of almost 7% to the bottom line!

For a farm producing 540 lbs of mushrooms per months (9 logs), that’s over $3,500/year!

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