Once harvest kicks off, stockpiles of in-hull almonds become a favorite spot for Aspergillus mold as the moisture in almonds, combined with larger swings in air temperature from day to evening, result in elevated moisture levels and create a hospitable environment for the damaging mold to thrive.
Aspergillus mold produces aflatoxin that causes significant food safety concerns that can put export shipments at risk as there are stringent maximum limits for aflatoxin contamination in key markets. High moisture levels in stockpiles can also lead to black mold on hulls and concealed damage of kernels – a condition in which nuts develop off-color and off-flavors after roasting.
Proper moisture control before stockpiling, and regular monitoring and adjusting of moisture levels in stockpiled almonds can help to limit the amount of moisture build-up and prevent these issues. Prior to stockpiling, field samples should be taken either before sweeping or in the windrow. Because there can be great variability in the moisture levels of crop that is drying on the ground, growers should sample from areas where the moisture content is likely highest. The highest moisture levels prior to sweeping can typically be found in almonds on the north side of the canopy next to the trunk. For windrows, moisture levels are higher in the bottom layer of almonds. Moisture content for almonds should be below 6% for the in-shell kernel, less than 9% for the total fruit (in-hull almond) and less than 12% moisture content for hulls. If moisture exceeds these recommended levels, growers should hold off on stockpiling and take the crop to a dry area where the nuts can be turned and spread to reduce moisture. Machine drying is also an option.
Once almonds are ready for stockpiling, the orientation and shape of the stockpile plays an important role in moisture management. Stockpiles should be placed on a firm surface in an area that encourages optimal drainage – either with a raised or sloped bottom. Creating drainage channels can further prevent water or rain from affecting stockpiles. Since condensation and mold growth are typically worse on the north end of a pile, it’s recommended to position the long side of the stockpile on a north-south axis to minimize that effect. Finally, one should ensure that each stockpile has an even, flat top to reduce the number of areas where condensation can build up on the underside of the tarp.
With stockpiles in place, hullers/shellers should implement proper management practices to keep moisture in check. While covering stockpiles with tarps is important, tarp selection is just as important to keep humidity minimized. White-on-black tarps work best as they minimize temperature fluctuations. Clear tarps allow the greatest temperature fluctuations, but are fine to use on dry, in-hull almonds that are below the moisture threshold. And white tarps fall somewhere in the middle in terms of their ability to minimize temperature fluctuations. When ongoing moisture monitoring indicates that moisture levels are too high, hullers/shellers should open up the tarps during the day to allow moisture to escape and cover again at night.
Additionally, it’s important to manage trash, dust and other potential contaminants, particularly pests like insects and rodents who can spread bacteria such as Salmonella and E. coli. When disease-carrying pests are found they should be treated, as needed, keeping detailed records if fumigants are used.
The Almond Board of California (ABC) is committed to continued research into best practices around almond storage and processing. You can learn more about stockpile management in ABC’s “Stockpile Management Best Practices” guide. This resource provides valuable information on creating and managing stockpiles in ways that help our industry deliver a safe, high-quality product to the U.S. and around the world.