Determining the best management techniques for almond harvest and stockpiles has been the subject of several years’ research supported by the Almond Board of California. The findings from this research were compiled and presented at the 2010 Almond Conference by Butte County Farm Advisor Joe Connell and Bruce Lampinen, UC Davis Extension specialist, who led the work.
Earlier research, in the late 1970s and 1980s, demonstrated the value of timely harvest for crop quality, particularly related to insect damage. Current emphasis is on stockpiling and crop moisture content to avoid mold growth in piles, particularly the Aspergillus fungus that produces aflatoxin.
Researchers have found that stockpiling in-hull almonds at a moisture content greater than 7 percent leads to a relative humiditiy (rH) within the stockpile of more than 70 percent, which is the maximum allowable rH for almond storage. This amount of moisture is of greater risk in the outer portions of piles, where temperature fluctuation, condensation on tarps and moisture accumulation can be significant.
Molds such as Aspergillus are more likely to grow in these marginal areas than in the rest of a high-moisture-content pile, where the equilibrium rH within the pile comes to a steady state below the maximum limits recommended for storage.
As a practical guideline, the researchers suggest that almonds should not be stockpiled if either the hull moisture content exceeds 13 percent, or the kernel moisture content exceeds 6 percent.
The first step to maintain quality is to harvest at the optimum time. Shaking should begin when there is 100 percent hullsplit of the lower interior nuts. All nuts in the lower canopy should be at least at stage “b3” and beyond (see “Integrated Pest Management for Almonds, Second Edition,” page 7, for a description of hullsplit stages). At this stage of maturity, nuts in the upper-outer portions of tree canopies will be drier, all nuts will have achieved maximum dry weight of oils and carbohydrates, and there will be maximum nut removal.
Delayed shaking risks higher worm damage (and potential for aflatoxin contamination), particularly when the crop is exposed to a preharvest third generation of egg-laying by navel orangeworm.
Harvesting too early will increase the potential for ant damage because the crop will take longer to dry on the orchard floor. Early shaking also increases the potential for quality problems, such as ‘sticktight’ hulls, foreign material and damage to kernels during hulling.
A third risk is moisture as a result of rain, especially in a year when crop maturity is delayed. If rain is in the forecast, it is best to leave the crop in the tree rather than to begin shaking. If the crop is already on the ground, be sure to provide access for conditioning — blowing out leaves and debris — and for turning to facilitate drying. Nuts down the tree row should be moved toward the middles (drive rows) for access.
Sampling before stockpiling
Crop moisture should be determined while the crop is on the orchard floor, either before or after sweeping. Remember, almonds should not be stockpiled if either the average hull moisture content exceeds 13 percent, or the kernel moisture content exceeds 6 percent.
If sampling is done before sweeping, take a representative sample across the orchard floor, from trunk to middle of the drive row, and along the tree row. Typically, samples from the north side of the canopy adjacent to the trunk and along the tree row will have the highest moisture: on average, 2 percent higher moisture content than nuts in the drive rows and middles.
Samples taken after windrowing should be taken from both the bottom and the top of the windrow; moisture content of samples taken from the bottom of the windrow can be 2 percent higher than at the top.
The crop will dry slower in sections of orchards with denser canopies. If your orchard is producing above 3500 kernel pounds per acre (about 70 percent of available light is intercepted by the canopy), particular attention should be paid to facilitate in-orchard drying; e.g., hedging before harvest, conditioning and turning, to reach the acceptable kernel and hull moisture contents given above.
Several factors contribute to stockpiling that minimizes condensation, mold growth and aflatoxin production.
• Orientation and shape. Research shows that orienting the long axis of piles in a north to south direction is preferable. Condensation and mold growth tends to be worse on the north sides of piles with the long axis oriented east to west. Smoothing the tops of piles helps to minimize the concentration of moisture from condensation and resulting mold growth.
• Tarp type and color. Researchers have found that white-on-black tarps are best for minimizing temperature fluctuations and resulting condensation. Consider using this type of tarp for piles with a higher moisture content of in-hull almonds. In studies, almonds covered with white-on-black tarp were up to 40 degrees cooler than nuts under clear tarp. They also had much narrower temperature fluctuations and lower condensation. Clear tarps had the highest potential for condensation and mold growth, but can be used on dry, in-hull product. White tarps are intermediate, in terms of temperature ranges and condensation, between black-on-white and clear.
• Air circulation. If piles are stacked with higher than recommended percent of moisture, it is important to open them up in the daytime, when the rH is lower, and close them at night when the rH is high.
Three years of research reports on almond stockpiling and aflatoxin by Bruce Lampinen and Themis Michailides (plant pathologist, UC Kearney Agricultural Center) are available at the Almond Board website. Go to AlmondBoard.com/farmpress18, choose research reports, proceedings and posters, then Aflatoxin Field Research.