The Yeast Fermentation Method

The fourth alternative method for rendering straw is biological. Straw can be biologically treated using yeast cultures, specifically strains of beer yeast, Saccharomyces cerevisiae. This method, by itself, is not as effective as those previ ously described, but has achieved limited success.

First a strain of beer yeast is propagated in 50 gallons (200 liters) warm water to which malt sugar has also been added. Recommended rates vary. Usually a 1-5% sugar broth is concocted. Fermentation proceeds for two to three days undisturbed in a sealed drum at room temperature (75° F„ 24° C.). Another yeast culture can be introduced for secondary, booster fermentation that lasts for another 24 hours. After this period of fermentation, chopped straw is then forcibly submerged into the yeast broth for no more than 48 hours. Not only do these yeasts multiply, absorbing readily available nutrients, which can then be consumed by the mushroom mycelium, but metabolites such as alcohol and anti-bacterial by-products are generated in the process, killing competitors. Upon draining, the straw is inoculated using standard procedures.

Another method of submerged fermentation uses the natural resident microflora from the bulk substrate. After 3-4 days of room temperature fermentation, a microbial soup of great biological complexity evolves. The broth is now discarded and the substrate is inoculated. Although highly odoriferous for the first two days the offensive smell soon disappears and is replaced by the sweet fragrance of actively growing mycelium. I hesitate to recommend it over the other procedures described here.

The outcome of any one of these methods greatly depends on the cleanliness of the straw being used, the water quality, the spawn rate, and the aerobic state of the substrate during colonization These methods generally do not result in the high consistency of success (> 95%) typical with heat pasteurization techniques. However, with refinement, these simple and cheap alternatives may prove practical wherever steam is unavailable.


Choosing the "best" type of cropping container depends upon a number of variables: the mushroom species; the cultivator; and the equipment/facility at hand. White Button growers typically grow in trays made of either wood or metal. Facilities designed for grow-iqg Button mushrooms (.Agaricus species) encounter many difficulties in their attempts to adapt to the cultivation of the so-called exotic I mushrooms. For instance, most Oyster mushrooms have evolved on j the vertical surfaces of trees, and readily form eccentrically attached stems. These species, with few exceptions, perform poorly on the horizontal trays designed for the Agaricu^x%lustry. Because Oyster mushrooms require healthy exposure the darkened, dense-

packed tray system gives rise to unnatural-looking, trumpet-shaped Oyster mushrooms. This ;s not to say that Oyster strains can not be j grown en masse in trays. However, many Oyster strains perform bet- ; ter, in my opinion, in columns, vertical racks, or bags. After taking into account all the variables, cultivators must decide for themselves the best marriage between species and cropping container. Please consult Chapter 21 for specific recommendations for the cultivation of each species.

Figure 152. Wooden tray culture of the Button Mushroom (Agaricus brunnescens) at a commercial mushroom farm.

Figure 152. Wooden tray culture of the Button Mushroom (Agaricus brunnescens) at a commercial mushroom farm.

dling of substrate mass-filling, transporting cropping and dumping. The Dutch are currently using tray culture for the cultivation of Button, Shiitake, Oyster and other musbroom species-Tray culture easily accepts a casing layer. Casing layers are usually composed of peat moss, buffered with calcium carbonate, and applied'directly to the surface of a myceliated substrate. Button mushroom production excels from the application of a casing layer, whereas it is debatable whether yields from the wood decomposers are substantially affected. Those using trays andnoiapplying a casing must take extra precaution to ensure the necessary mici -climate for piimordia formation. Tnis can be accomplished by covering the trays with either a perforated layer of plastic or breathable, anti-condensate films The plastic is stripped off. depending upon the spec' js, at the time of, or soon after, primordia formation Fog-like envi

Growing mushrooms in trays is the traditional method of cropping, first developed by the Button mushroom (Agaricus) industry. Trays range in size from small 2 ft. x 3 ft. x 6 in. deep which can be handled by one person to trays 6 ft. x 10 ft. x 12 in. deep which are usually moved into place by electric or propane-powered forklifts. For years, trays have been constructed of treated or rot resistant wood. More recently, polycarbonate and steel trays have been introduced with obvious advantages. Both types are designed to stack upon each other without additional struc tural supports

Trays allow for the dense filling of growing room (up to 25% of the volume) and because Agaricus brunnescens, the Button Mushroom, is not photosensitive, no provisions are made for the equal illumination of the beds' surfaces The main advantage of tray culture is in the har.

ronments typ^ -ally ensue until the primordia have firmly set

In North America, tray culture for Oyster mushrooms was perfected by Davel Brooke-Webster (1987) This method utilizes a perforated plastic covering over the surface of trays. Since many Button mushroom farms are centered around tray technology, the replace ment of the casing layer with a sheet of perforated plastic allows the cultivation of both species at the same facility. Holes (1-2 inches dia.) are punched evenly through a 8 ft. roll of plastic, before application.The plastic sheeting is stretched overthe trays, < lirectly after iiocu-lef,on of Oyster spawn into pasteurized wheat straw. The plastic barrier prevents 98% of the evaporation that would otherwise occur had the inoculated straw remained exposed Even with the plastic covering, humidity within the growing room should remain relatively high so that the straw exposed to the air d; rectly below the holes does not "pan" or die back. A sure sign that the growing room humidity is too low is when brown zones of dry straw form around each puncture site while the remainder of the substrate is wh 'te with mycelium.

An advantage of the Brooke-Webster technique is that: bouquets of equal weight are produced simultaneously on the same trays so widely used by the Button mushroom (.Agaricus brunnescens) industry A disadvan tage of tray culture is that equal exposure of light overthe surface of each tray, when tightly stacked upon one another, is difficult. (When providing light to tightly packed trays, the fix tures are usually mounted on the underside of the tray imme(,: itely above the fruiting surface. Most cultivators remove the heat-generating ballasts to a remote location ana re-capture the heat into their air ci culation system.) When lighting is insufficient in Oyster mushroom cultivation, stems elongate while caps remain

Figure 153. Dutch-made, rust resistant, metal trays used for the cultivation of the Button musnrocm

— Tnl '------— -7^nprfnrated Dlastic anow for the simultaneous emergen« m e 57. H irizontal trays cove-d with perioratedplastica.iow cultivated is the


Figure 158. The Phoenix Oyster, Pleurotus vulmonarius fruiting from a wall formation of stacked bags.

underdeveloped, causing abnormal, fluted or trumpet shaped mushrooms.

The key to the success of this method lies with strains which form bouquets of mushrooms site-specifically at the holes in the plastic. Ideally, primordial clusters hosting multiple mushrooms form at each locus (See Chapter 14. Features for Evaluating & Selecting a Mushroom Strain. )

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