Zero Waste in the Tree Nut Industry: Adding Value to Coproducts

William J. Orts, Delilah F. Wood, Zach McCaffrey, Allison Flynn and Bor-Sen Chiou USDA ARS Western Regional Research Center, 800 Buchanan Street, Albany, CA 94710.

Our research team at the USDA has been working with the industry to convert coproducts into useful commercial products – with some success. But achieving zero waste is not easy given the industry’s success over the past 35 years (Figure 1a, b). Walnut, almond and pistachio production has increased tremendously since 1980; however, traditional markets for coproducts – the cattle industry in California – have not kept pace. Changing market needs for coproducts are spurring innovation for high value end-uses.  

FIGURE 1: US tree nut (a) and biomass (b) production increases since 1980. (Source:  https://www.ers.usda.gov/data-products/fruit-and-tree-nut-data/fruit-and-tree-nut-yearbook-tables/, Fruit and Tree Nut Yearbook Tables, USDA, Economic Research Service, October 2018. Biomass calculations: walnut, Carl Eidsath, California Walnut Board; pistachio, Michael O’Banion, Wonderful Company; almond, Almond Alliance; hazelnut, Oregon Hazelnuts Food and Beverage Company, ttps://www.facebook.com/OregonHazelnuts/posts/what-percentage-of-a-hazelnut-is-the-actual-nut-as-opposed-to-the-shell-when-we-/564191627019980/; pecans, various internet searches.

Energy from Shells and Tree Prunings
Billions of pounds of biomass are generated annually, thus products that consume huge volumes of feedstock are crucial to achieving zero waste. Sustainable energy from biomass (biofuels) or bio-derived electricity consume vast amounts of biomass but have proven to be both promising and elusive. USDA researchers derived “green” technologies to convert shells into ethanol to displace gasoline when corn-based ethanol was taking off.  Corn ethanol, which now displaces >10% of the US gasoline supply, is viable because ~80% is easily fermentable.  But shells contain ~40% fermentable sugar. Robust technologies exist to convert shells into ethanol, but we haven’t found a traditional fermentation process that can compete with corn to displace petroleum given historically low oil prices. 

Ethanol production has been revitalized by Aemetis and LanzaTech. They are building a $158 million plant in Riverbank, California to convert orchard prunings and nut shells to biofuel. Aemetis gains efficiency by using high temperature gasification (Figure 2) to create an energy-rich vapor (syngas). Syngas is cooled, cleaned and fed to a patented fermentation bioreactor that converts it to ethanol. Aemetis plans to go online in late 2019, converting under-utilized coproducts into biofuel to meet California’s renewable fuel mandates.

Biocoal: Thermal Conversion for Energy Applications
Vast amounts of tree nut coproducts could be used in generating electricity. When “coal was king”, it was easy to imagine generating electricity in coal-fired utility plants using shells and prunings as a coal substitute. Through torrefaction (Figure 2) we developed technology to generate “biocoal” – high-energy pellets that look and act a lot like coal.

Multiple torrefaction units that produced shell-derived biocoal for utility plants were introduced in California. One such mobile unit (Figure 2) generated biocoal that was fed to a utility plant for the months following harvest. Clean-air mandates pushed utility companies away from coal to solar, wind and clean-burning natural gas. Subsequently, we’re searching for products beyond biocoal for converting shells into valuable products.

Thermal Conversion to Consumer Products
One idea –using torrefied shells as an inert filler in commodity plastics – turned into a pleasant surprise. The addition of torrefied shells to recycled plastics was not inert; but rather improved its stiffness and heat stability, with an increase in softening point of 5–8°C when shells were added. Improved heat stability creates new market opportunities, such as stackable seedling trays. Trays produced with recycled polypropylene alone tend to warp in the sun. Those with torrefied shells do not. Most recently, we created shipping pallets containing 10-15% torrefied shells (Figure 2). The lower-density pallets provide lower shipping weights, and stiffer pallets.

Biochar: More Uses for Thermally Processed Shells
Products generated from torrefaction, gasification, or pyrolysis are all termed “biochar” and an entire cottage industry has developed around biochar to improve soil properties.  “Activated carbon” is essentially biochar that has been chemically- or steam-treated to make it more porous. Activated carbon from shells makes a great water filter that will compete favorably with the industry standard, activated carbon from coconut fiber, with the added advantage of local production.

 

FIGURE 2: Thermal conversion processes applied to tree nut coproducts outlining (a) the temperature regimes used (drawing courtesy Lennard Torres, USDA, ARS WRRC); (b.) a transportable torrefaction unit mounted on a trailer for on-site thermal treatments (photo courtesy of Idaho National Laboratory (INL); and (c) a pallet produced by TranPak (Fresno, CA) that contains 10-15% torrefied almond shell. Photograph used with permission from TranPak.

Almond Hulls: Sugars from Hulls and their New Uses
Almonds are unique in the industry because of the high value of their hull, the “fruit of the almond tree” (Figure 3). Hulls, with a composition not unlike apricots, are rich in easy-to-extract sugars. Human consumption is limited by their high tannin content, which make them too bitter for most people to eat (but cows love them). Our USDA team has explored whether hull-derived sugars could be introduced into bitter foods or beverages – tea or beer comes to mind – where consumers increasingly crave bitter flavors. In non-food research with the almond industry we set out to answer two significant questions; (1) can we make fuel-grade ethanol from hulls and (2) should we? We quickly came to the conclusions that, yes, we can make ethanol from hulls, akin to making peach vodka. But, again, in the face of low oil prices, the seasonality of the almond harvest, and the high capital expense of ethanol plants, it makes little sense to build a stand-alone ethanol biorefinery.

Several factors could change that landscape; (1) hull prices could drop drastically, making them cheaper option than corn (not a great outcome for the industry!), (2) sugar extraction/ ethanol production could be coupled with another existing process, and (3) hulls could be developed into a host of higher-value products.  A chance to optimize ethanol production came when a California investment group approached us wanting to create a stand-alone ethanol plant using sugar beets as feedstock. Interestingly, hulls have a higher sugar content than sugar beets and a broader harvesting/ storage window. With that we worked with this team to show that almond hulls could be processed in the same equipment as sugar beets using essentially identical protocols.  Hulls and sugar beets could potentially complement each other very well in a multi-feed ethanol plant.

Figure 3.  Coproducts from almonds. Photograph and graph are taken from https://www.almonds.com/sites/default/files/17-18_whole_crop_position_report_addendum.pdf, Almond Tree Fruit Weight, 2017/2018 Crop Year, California Almond Board of California, Supplement to July 2018 Position Report, August 2018 whose sources are: Kernel Weight: USDA Incomings received by Almond Board of California; Shell & Hull Estimations: Almond Alliance of California. The chart is

Novel Uses for “Spent Hulls”
If sugar were economically extracted from hulls, what next?  The biomass (“spent hulls”) remaining after sugar extraction from hulls is remarkably useful. One novel application for spent hulls is to grow mushrooms.  Commercial mushroom production generally requires a peat moss (called sphagnum), whose physical and chemical properties include high water-holding capacity, even pore distribution for gas exchange, and balanced minerals. Spent hulls have all that, with a water-holding capacity of >500%, numerous pores for gas exchange, and high mineral content.  Figure 4 shows mushrooms produced with a commercial mushroom producer that uses spent hulls in place of peat. 

Success
Presented above are only a few of the outlets for tree nut byproducts. All sectors of the industry will need to play their part toward reaching zero waste in tree nut production.