Minimizing the potential for aflatoxins in almonds is an industrywide effort, beginning in the orchard by managing navel orangeworm, which is associated with the Aspergillus fungus and aflatoxin. This includes winter sanitation to remove mummy nuts, if necessary, in-season treatments for navel orangeworm, and prompt early harvest. Aflatoxins can also be reduced at the handler by sampling, testing, and reprocessing.
A third opportunity for preventing aflatoxin is in the stockpile, where recent studies supported by the Almond Board of California have shown that certain conditions can lead to the growth of the aflatoxin-producing molds Aspergillus flavus and A. parasiticus and the resulting aflatoxin production.
Stockpiling of in-hull almonds has become a more common occurrence alongside both orchards and huller/shellers as a result of increased crop sizes and the need to extend the processing season in order to maximize the use of processing facilities.
To investigate potential for aflatoxin production in stockpiles, UC researchers Bruce Lampinen and Themis Michailides began a research project in 2007 to examine temperature and moisture conditions in stockpiles, and from that information, develop recommendations for stockpiling that will minimize the potential for growth of Aspergillus and resulting aflatoxin contamination.
Almond stockpiles in Kern, San Joaquin and Glenn counties were selected for the trials and monitored in the fall for temperature and relative humidity at four different depths – top of the pile, upper mid-pile, lower mid-pile and at the bottom edge. Conditions were monitored at one to three minute intervals throughout the storage period, which ranged from 84 to 150 days, and recorded with a datalogger.
Samples of the stored in-hull almonds were taken from each pile at the beginning of the trial, during the trial, and at the end of storage, and analyzed for moisture content, mold growth and the presence of aflatoxin.
The researchers found that there are conditions in stockpiles under which Aspergillus can grow, and that the mold growth does result in aflatoxin contamination. The most significant factor for mold growth is the moisture content of in-hull almonds when they go into storage. Stockpiling of in-hull almonds with a total fruit (hull, shell, kernel) moisture content at or above 9 percent – 9.2 percent in this study – can lead to problems, particularly on the outer areas of the piles where significant condensation builds up under the tarps and moisture accumulates. However, mold growth and resulting aflatoxin development was not uniform throughout these high-moisture-content piles because the relative humidity within the piles came to a steady state equilibrium below the maximum recommended for storage, which is 65 percent to 70 percent.
In stockpiles where the initial moisture content of in-hull almonds was low – at or below 5.7 percent total fruit moisture content – there was minimal mold growth and no aflatoxin production.
The stockpile monitoring research project will continue this year with the hopes of refining stockpiling guidelines by answering these questions:
• At what moisture content is it appropriate to stockpile in-hull almonds, and how can this be quickly determined in the field before pick-up?
• What method can be used by growers and hullers to determine the moisture content of in-hull almonds, and what is the effect of different tarp types on moisture development?
• Once stockpiles are formed, what techniques can be used to determine the relative humidity of the surface and within the pile to determine moisture status?
• What are the visual cues that would indicate a problem – for example, amount of moisture or condensation, the extent of mold growth?
• If a stockpile has too much moisture, what can be done to reduce potential mold growth, such as removing tarps during the day and covering them at night, or using a stockpile aeration system?
Until stockpile management guidelines can be refined through ongoing research, current research results suggest:
To be on the safe side until more is known, don’t stockpile in-hull almonds that have a total moisture content exceeding 6.5 percent to 7 percent. As a practical guide, kernels, shells and hulls that “snap” when bent between fingers are certainly safe.
If piles are stacked too wet, there will be condensation, pooling of water and mold growth at the outer portions (tops, sides and bottom) of the piles. Of particular concern are the “green molds,” including Aspergillus. Under these circumstances, it is important to open up the piles in the daytime when relative humidity is lower, and close them at night when relative humidity is high. In the 2007 research, ambient relative humidity ranged from about 90 percent or more at night, down to 55 percent or less during daylight hours.