By Allison Flynn and William Orts, Bioproducts Research Unit, Western Region Research Center, US Dept. of Agriculture; and Andy Lam, Omnis Mineral Technology, LLC
Until recently, almond hulls have been sold almost exclusively as cattle feed, prized for both its caloric value as well as its fiber. However, the almond industry has been growing at a rate of 20% per year for the last five years, 20 with an annual yield of 4.3 billion pounds of hulls. During the same period, the California drought has reduced the number of dairies in the state by more than 11%. Thus, the market demand for almond hulls has fallen significantly, creating an opportunity for new and creative uses of this versatile, renewable material.
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ALMOND HULL BYPRODUCTS AS A CASING AMENDMENT MATERIAL IN MUSHROOM (AGARICUS BISPORUS) CULTIVATION
Most commercial mushroom casing consists of peat moss (Sphagnum peat), generally neutralized with calcitic chalk (CaCO3), lime (CaOH), or beet lime, a byproduct of sugar extraction from sugar beets. The required physical and chemical properties of an effective casing are relatively specific, including high water-holding capacity, evenly distributed pore space for good gas exchange, a balanced mineral content with specific nitrogen ranges, the ability for pH to be adjusted to 7.2-8.2 and relative sterility.22
Although a wide range of casing materials have been explored, including paper pulp,12 waste paper,13 sugar cane residues14-16 and a range of other agriculturally-derived residues and coproducts,3-11 Sphagnum peat is well-suited for this application because it is naturally anti-fungal and exhibits the appropriate water-holding capacity and pH maintenance.
In addition to casing mushrooms to promote fruiting, it has been shown that mushrooms prosper with the introduction of some pseudomonad species to balance optimal fruiting yield,18,19 but that fungal infections with this method are a constant concern. From this point of view, it would be advantageous to find other casing amendments that promote fruiting growth without the risks associated with additional microbial species.
Almond hulls contain about 30% soluble sugars and 70% fiber. Approximately 80% of the world’s supply of almonds is produced in the U.S., almost entirely in California’s San Joaquin and Sacramento Valley regions.1 Until recently, almond hulls have been sold almost exclusively as cattle feed, prized for both its caloric value as well as its fiber. However, the almond industry has been growing at a rate of 20% per year for the last five years,20 with an annual yield of 4.3 billion pounds of hulls.21 During the same period, the California drought has reduced the number of dairies in the state by more than 11%.24,25 Thus, the market demand for almond hulls has fallen significantly, creating an opportunity for new and creative uses of this versatile, renewable material.
The objective of this research was to investigate the use of spent almond hulls as a casing additive for Agaricus fruiting. After removing the sugar, the spent almond hulls have many of the characteristics of Sphagnum peat. Spent hulls are fibrous with a water-holding capacity of up to 500% the original mass with both absorption and adsorption properties.1 In addition to the water-holding capacity, we hypothesize that the additional porosity will improve the peat casing layer, allowing faster mycelium colonization leading to increased yield per flush.
Almond hulls were provided by RPac Almonds in Los Banos, CA. Extraction of sugars from the hulls was carried out using aqueous extraction methods described elsewhere.1 Fibrous material was collected, dried and autoclaved to remove native microbial populations. Spent almond hulls were then milled to 8 mm (granular) and 200 mm (powder) screen mesh size and then mixed with Sphagnum peat casing formulations. Both almond hull concentrations in peat and particle size were evaluated against a 100% Sphagnum peat moss casing control.
Mushroom fungi (Agaricus bisporus), compost and lime, and Sphagnum peat moss were provided by Premier Mushrooms, Colusa, CA. Compost was laid out into traylike mushrooming pots to a depth of 6 inches. Roughly 600 grams (total dry weight) of specific casing was mixed with 1800-2000 mL of water to bring moisture content to 75-78%. Casing formulations were prepared as follows: In order to adjust the pH level to 7.8, approximately 200 grams of lime (CaO or CaOH) was added to each of the casing formulations. A casing inoculum (113 grams) of mycelia was mixed into each casing formulation before laying the casing down evenly onto the surface of the compost. The temperature and relative humidity were maintained according to industry standards.
Ventilation using a fan was provided to induce fruit-body formation when the mycelium reached the surface of the casing layer. Casing moisture content was measured throughout the experiment. To evaluate the efficacy of almond hulls in casing, mushrooms were harvested when the overall average diameter of caps reached 7 centimeters across or when the veils began to open.
RESULTS & DISCUSSION
Results are preliminary but encouraging. Water absorption of almond hulls is comparable to Sphagnum peat moss (Fig. 1). Moisture content in casings that included spent almond hulls was maintained or increased compared to the Sphagnum control (Fig. 3).
Mycelial growth and pinning were improved in casing that contained almond hulls milled to 8 mm mesh size (granular) and 200 mm (powder) when used in conjunction with peat moss (Tables 1, 2 and Fig. 4, 5). Mycelial growth in casing made with 100% granular (80 mm mesh size) showed excellent mycelial growth but pinning either did not occur or was aborted (Tables 1, 2 and Fig. 4, 5). Casing containing 20% granular spent almond hulls as an adjunct in Sphagnum peat moss doubled the fruiting mass and number (Tables 1,2 and Fig. 4-6).
The marked difference in the behavior of Agaricus growth and fruiting in casings made with spent almond hulls milled to different sizes and used in different ratios with Sphagnum peat moss merits further investigation.
There seems to be a “sweet spot” in terms of the physical size and ratio of hulls to peat moss. On the one hand, the adsorption nature of the hulls is diminished upon excessive milling, and casing made with 100% powdered hulls collapses, providing no structure for aeration. On the other hand, the improved porosity of casing with granular hulls also changes the nature of the casing. When used alone, mycelium grew quickly throughout the granular casing, but this material dries out due to wicking, and any pinning that occurred was aborted. It is conceivable that once optimized, spent almond hulls can be used to improve growth and increase yield in Agaricus production.
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