Mushroom drying technologies

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By Dylan Gordon, May 2014

Drying wild mushrooms is the most common way of preserving them. It’s relatively quick and can be done in the field. When proper procedures are followed, the result is a high-quality product with a long shelf life, familiar and attractive to consumers.

The major task in field drying is removing the correct amount of moisture from the mushrooms, before they spoil. This involves three factors: the drying chamber; the use of heat; and, airflow.

Drying Chambers

Drying chambers can range from open air stacks of screens on which mushrooms dry in the wind, to mobile drying systems transported by trailer, equipped with fans and stoves. The type of chamber determines its mobility, which is a major concern for many harvesters and field buyers, because you need to be able to get the dryer to where you are picking. Those who pick near and dry at home may prefer a more permanent installation.

Dryer capacity is also important. If you’re buying mushrooms, you need a dryer large enough to allow you to dry everything you purchase on your busiest days of the season before it spoils. Otherwise, you’ll have to refuse to purchase mushrooms from some pickers, harming your reputation, or have product you purchase and can’t process go to waste (although freezing or fresh-marketing surplus may be an option to explore).

In general, a drying chamber consists of some sort of box or covering, at a minimum covered on top to protect from rain. It may cover all sides as well, in order to trap heat or prevent dust and debris being deposited by the wind. Drying chambers observed in the field include metal boxes, tents, tarp or plastic sheeting attached to a frame, or simple hanging racks.

Screens and Racks

Drying chambers must include some method of distributing mushrooms with airspace between them and airflow around them, usually involving wire sheeting either suspended by hanging from above, or supported by a racking system. At the most rudimentary, a drying chamber and rack could be a perforated basket perched atop a bucket in the sun. At the most advanced, they involve food-safe metal screening (not galvanized screening, which releases toxins when heated), edged with rigid strips, designed to slide in and out on the rails of a built-in rack. In the middle is a set of wire screens, tied together with string, suspended from the roof of a tent structure.

Heat

Although sun- or air-drying are options, they’re poor ones. Temperatures vary, which means drying is often uneven, and perhaps too slow or fast, harming product quality. In any case, they take a long time, and are highly labour intensive with all the turning and rotating of product, meaning you can’t process much like this. Also, neither option works in the rain, or in humid climates, so depending on it can lead to a massive loss of product if the weather turns against you.

This makes applying and controlling heat a major concern for dryer operators. Mushrooms exposed to too high a heat will cook and/or release spore, covering the product with an unsightly film that is seen as a ruining them, or destroying their texture. On the other hand, insufficient heat will severely reduce how many mushrooms can be processed.

A further complexity is the “finishing” of mushrooms. Mushrooms are said to require an application of high heat late in the drying process in order to kill any worms or eggs present. Failure to finish mushrooms is a major differentiator between amateur and quality producers, with distributors particularly cautious about dealing with pickers who air dry mushrooms and later attempt to market them directly. Whether mushrooms are finished or not can impact the price paid or whether they are bought at all.

Rather than using thermometers or applying set procedures, dryer operators rely on intuition and experience to judge appropriate heat, which is also dependent on degree of airflow. Greater airflow will allow the application of higher heat. In addition, as the moisture content of the product and the humidity of the climate can vary day-to-day, drying time and temperature can fluctuate, and one must get a feel for how the product progresses, and when it is done.

Field dryers generally use wood, propane or diesel heat. Some use solar heat. Others air dry. Each variety has its fans and detractors, and is controllable to a varying degree. The pros and cons for each type of dryer are summarized in Table 1.

Airflow

The third concern in mushroom drying is airflow, generally provided by fans, wind, convection circulation within a chamber, or a combination of these. Airflow is used to dry the mushrooms as well as to distribute and control the temperature of hot air. Although it can seem simple, airflow is a tricky component of dryer design. Depending on the placement of screens, racks, inlets, outlets and fans, air can flow evenly or unevenly within a chamber. An even flow of air is important when drying a large batch of mushrooms to minimize labour and time required, and ensure quality.

Designing Field Dryers

Designing your own field dryer is a matter of experimentation and expertise. No universal design exists in the trade, and the operation of dryers is a highly personal and often secretive matter. Because professional buyers rely more on their sense of touch and their intuition, rather than on thermometers and moisture meters, no specific rules exist available for optimal drying and finishing temperatures, for dryer designs, or for operating processes.

Instead, an overview of the typical and alternative field dryers may help you to develop your own system.

Drying Trailers

The majority of field buyers use dryers self-fashioned from cargo trailer shells. These provide a secure and ready-made chamber, but are expensive and heavy and therefore reduce the capacity and responsiveness of their operations. In these dryers, a stove and fans are installed at one end, and racks of mushrooms on screens fill the remainder. Racks can be made out of wooden strips or metal racking. In the simplest cases, a wood stove is installed in a trailer along with electric fans to circulate the air. More complex variants include the use of exhaust fans (“pulled” air said to be less turbulent, and therefore more even in heat distribution and moisture removal) and heat exchangers (generally a zig-zagging flue along one wall after leaving a firebox).

Generally, fans are generator powered, which adds substantially to the fuel, maintenance and oversight requirements of dryer operations. In one case, a belt-driven fan run by a small 2-stroke engine was observed. This configuration ran for a long time on minimal fuel and was easily serviced. Directly running fans with an engine, rather than a generator, is thus highly recommended. The practice is likely not as common because it adds an additional layer of complexity to the dryer design and fabrication, preventing the use of only off-the-shelf parts.

 

Table 1. Dryer Type Pros and Cons.

Dryer Type Pros Cons
Wood · Free (if harvested)· Non-tainting odour· Produces a dry heat

· Wood stoves readily available and not complex

· Imprecise fire control· Fire needs constant tending· Wood supply takes time or money

· Requires large, dry storage

· Wood types vary as fuel

Propane · Precise flame control· Unattended, long-duration operation· Easy fuel storage

· Minimal odour

 

· Expensive· Limited availability of fuel· Requires complicated fitting

· Propane stove more complex

· Produces a wet heat

Diesel · Widely available· Inexpensive· Precise flame control

· Unattended, mid-duration operation

· Difficult to store· Risk of tainting odour· Produces a wet heat

· Diesel stove most complex

 

Air · Free· Simple and inexpensive to setup · Impractical in moist climates· Limited capacity· Slow

· Poor quality due to varying temperature

Solar · Free· Sustainable· Odourless (aside from plastic boxes)

 

· Impractical in rain· Limited capacity· Slow

· Complex

 

Alternative Dryer Designs

A number of alternative dryer systems, other than the trailer-plus-firebox, also exist, although they are not as common.

Pre-Drying

Pre-drying is undertaken by all dryer users in cases of high volume, by necessity: as product waits to enter the dryer, it pre-dries. Buyers may fashion air dryers using screens placed in the sun, or under a tent in racks, in order to facilitate pre-drying.

Pre-drying also takes place through refrigeration. Buyers also selling fresh product who have a reefer available will store all product, subject to capacity, in the reefer, and then remove dryer-grade product when as the reefer fills or dryer empties. This is said to improve the final quality of the dried mushrooms by reducing their deterioration as they wait for the dryer, and by avoiding an initial stage of drying that is warm and moist. Some claim that reefers can be used for the entire drying process, though they are slow, prone to breakdown, and expensive to operate.

Air Drying

Air drying beyond pre-drying is common among individual pickers and also some field buyers, who erect tents for this purpose. Air drying usually uses wind for airflow, though in rare cases solar or generator-driven fans are used. In any case, it is always undertaken without a heat source. Pickers generally air dry by spreading product on screens or the dashboard of a vehicle and placing them in the sun. If rain threatens, they temporarily move the product into storage until the humidity clears.

Prolonged humidity can ruin a crop by making it impossible to dry before it rots. Even if not prolonged, deterioration from air drying can be noticeable: often air-dried mushrooms are dark brown in appearance, an indication of repeated drying and rewetting (through the absorption of the air’s humidity between drying periods, or even exposure to rain before a picker returns to camp from picking to tend to the drying product on a rainy day).

Air drying also increases the risk of mushroom theft (dryers can be locked), which also prevents it from being a sound strategy for large-scale picking except when there is someone to remain at camp.

Air drying in tents by field buyers generally involves an open-sided tent with racks suspected on ropes from the ceiling. This creates a large dryer open to the wind and relatively protected from rain, which can dry or pre-dry a substantial quantity of mushrooms. Similarly, in cases of high, hot winds, field buyers may conserve fuel by allowing the wind to run through a more standard field dryer with its fans and stove turned off.

Two additional concerns to note with air drying: First, it offers little protection from airborne contaminants, particularly dust, which is common in the gravel pits where field buying stations and camps tend to be located. Second, it prevents finishing at high temperature.

Air Drying Tents with Heaters

Some field buyers use a large tent outfitted with not only with racks but also with generator-driven fans as a large air dryer. This can be augmented with stationary propane or diesel heaters (or perhaps, even a wood stove). This method is highly portable and inexpensive, and thus can be easily scaled to meet increased demand or moved to a new place. This is a major benefit, because allocating field dryers to different harvest regions is a major problem for distributors: they are expensive and slow to move, which limits responsiveness to crop conditions. However, air drying with tents requires a good amount of experimentation to identify drying times, fan and heater placement, etc. before it can be used effectively, and the fact that it has not caught on suggests the method may be difficult to perfect.

Air drying tents can also be fabricated using clear polyethylene sheeting on a wooden frame. Sun penetration provides some heat, and a wood-fired stove in an adjacent chamber can also be used to augment. Control of temperature and airflow can be difficult with such an arrangement as well.

Solar Dryers

Solar dryers are used in rare instances for very small volume drying (perhaps 10 – 20 lbs/day). One individual used black plastic boxes that heated up in the sun, outfitted with solar-powered computer fans for airflow. Another used a glass heat collector system that relied on wind or presumed convection, with mixed success.

A final solar strategy suggested involved the use of 25+ feet of collapsible black plastic tubing or field cover, either of diameter sufficient for human entry or removable. This would create a long tunnel that, thanks to convection, would have substantial airflow while also capturing heat from the sun and protecting the product from the rain. Although this is a promising idea it has not been implemented in practice and no doubt would require substantial experimentation.

Recommendations

The development of a highly mobile field dryer, inexpensive and simple to fabricate and operate, is a complex problem. Trade-offs between equipment cost, operating cost, complexity, capacity and the ability to quickly add or move dryers needs to be made. However, based on the technologies observed, some recommendations can be made for field drying in contexts where expense is the overriding consideration.

Chamber

A canvas or plastic tent is portable and inexpensive, and does not require a towing vehicle. This is an ideal choice for an inexpensive field dryer. The tent members should be capable of supporting a substantial load of suspended racks, if standing racks are not going to be used.

Heat Source

Wood fuel and stoves should be used as a source of heat. They are widely available and understood, and easily fabricated, serviced and fueled.

Air Flow

This is the heart of the field drying problem in this instance. Wind should be used where appropriate in order to reduce complexity and cost. However, fans are a necessity to ensure prompt drying in all conditions, and to circulate and control heat. If possible, engine-powered fans are preferable, but these involve some engineering and are not easily serviced, making easy fabrication or replication of dryers difficult. Generator-powered fans, on the other hand, dramatically increase cost, require additional fuel sources, and are difficult to maintain and secure. Solar powered fans may be a good compromise but increase the risk of crop loss in times of rain and are also pricey.

One recommendation is to develop a portable fan system powered by a small engine, appropriate to operating a single drying tent. While it would not allow the simple fabrication of new dryers, it would be easily transportable in a regular vehicle.

A more complicated alternative is to explore the development of convection-based airflow within the chamber, perhaps augmented by the use of a Peltier (http://www.karlander.net/projekt/peltier-power-fan.html, http://www.instructables.com/id/DIY-Stove-Fan-for-under-50/, http://www.modernoutpost.com/gear/details/cf_ecofan.php) or a Stirling (http://www.scraptopower.co.uk/can-stirling/make-a-coke-can-stirling-engine, http://www.youtube.com/watch?v=cqmeYc8GWmA, http://www.youtube.com/watch?v=KxBug0Vzt10) fan that runs on the thermal energy output by the wood stove. It is possible that minimal air circulation, combined with the dehumidifying effect of a wood stove and the adequate venting of hot, moist air through a top vent could be sufficient to operate a dryer. Although these alternative fan designs remain complex they are more portable, inexpensive and perhaps simple to fabricate than a fan powered by a belt and engine.

Racks

A final matter of concern is drying racks. Racks require a mesh or screen as a drying surface and generally also a rigid frame to hold it. Some dryer racks are made from galvanized wire mesh which is both heavy and possibly toxic, but widely available. Plastic meshes are also a possibility if food-grade mesh with the proper temperature rating can be found.

In many tent designs, rigid frames with eyelets are connected together via ropes suspended from the roof members. However, fabricating rack stands from lumber or young trees may also be possible and more economical (in that a heavy-duty tent capable of supporting such weight from its frame need not be purchased).

Designs using less rigid frames may be possible. For instance, a long trough of plastic or wire mesh could be suspended with X-shaped supports along its length. However, this would restrict the use of vertical stacking which is a major aid to maximizing capacity.

Other rack designs could be innovated; this is another area in which some design consideration would be valuable.

A Drying System

Field drying should be considered not just in terms of field dryers, but as a complete drying system involving various equipment, aided by expertise and a quality process. Temporary tents of polyethylene or bare racks can be used to air dry product in many situations, turning to a heat- and fan-equipped drying tent in cases of high humidity or volume. If the workforce of pickers is flexible enough such that picking can be restricted or halted when air and sun drying are unfeasible, such a system could flexibly handle infinite volumes of product (subject to the labour of the dryer-tender). However, if picking labour cannot be controlled, and will continue in all conditions, the capacity of the heat- and fan-equipped dryer must be carefully considered to ensure minimal crop spoilage in case of continued harvesting despite prolonged bad climate.

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