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Past Issues

Harvest Issues:
Stockpiling, Moisture Content, Quality Problems and Aflatoxin

Orchard management activities continue after harvest to protect the crop from contamination and loss of quality. It has been shown that certain conditions during stockpiling can lead to concealed damage and mold growth that can reduce kernel quality and lead to food safety issues.

Various degrees of concealed damage are shown in the
group of kernels on the right. Concealed damage results from
conditions of heat and moisture in the field following harvest
and is usually revealed only after roasting. Normal kernels are
shown on the left.
Studies led by Bruce Lampinen (UC Davis Extension specialist, Plant Sciences) from 2007 to 2012 and supported by the Almond Board of California show there are clearly stockpile conditions under which the Aspergillus mold can grow with resulting aflatoxin contamination. Other quality problems can also occur that are associated with high moisture content.

Further studies show that stockpiling at a total fruit (kernels + hulls) moisture content greater than 9% is problematic. This amount of moisture within the pile can result in a relative humidity greater than 65%, which is the maximum rH for almond storage.

Storage Guidelines
As a practical guideline, do not stockpile if any one of the following moisture content/level conditions exist: Either 1) the total fruit (kernels + hulls) moisture content exceeds 9%; 2) the hull moisture content exceeds 12%; or 3) the in-shell kernel moisture content exceeds 6%.

A quick reference guide, “Stockpile Management and Aflatoxin Potential,” and chart, Moisture Level Guide for Stockpiling, have been developed by the Almond Board.

New studies show that stockpiling at a total fruit (kernels +
hulls) moisture content greater than 9% can lead to quality
and food safety issues.
Most critical is the outer portion of piles, where there can be significant temperature fluctuation, condensation on tarps and moisture accumulation. Of particular concern in these outer portions are the “green molds,” which include the Aspergillus mold that produces aflatoxin. In addition, there are several other associated quality problems, such as black molds on hulls and appearance of concealed damage in kernels. This is not a uniform problem throughout high-moisture-content piles because the equilibrium rH within the piles comes to a steady state below maximum limits recommended for storage.

Orchard Sampling Prior to Stockpiling
Crop moisture should be gauged while the crop is on the orchard floor, either before or after sweeping.

Harvested nuts and stockpiles should be sampled to account for variability in drying on the orchard floor and in windrows. Light, temperature and canopy cover will influence that variability. Prior to sweeping, representative samples should be taken across the orchard floor and along the tree row. Keep in mind that the north side of the canopy adjacent to the trunk can have moisture content 2% higher than the crop in drive rows and middles. In windrow samples, moisture at the bottom of the windrow is also typically 2% higher. Therefore, “worst-case sampling” is on the north side of the canopy along the tree row and at the bottom of windrows.

Managing Piles
The orientation and shape of the pile can also play an important role in minimizing mold growth potential. If possible, orient the long axis of the pile in a north/south direction. Condensation and mold growth tend to be worse on the north side of piles with the long axis oriented east and west. Smooth tops on the pile help minimize the concentration of condensation and resulting mold growth.

Click to hear Almond Board's Bob Curtis talk about the
importance of moisture management in stockpiles.
Tarp type and color can also play a role in minimizing temperature locations and condensation. Consider a white-on-black tarp, particularly for piles with higher moisture content.

If piles are stacked too wet, it is important to open them up in the daytime, when the relative humidity is lower, and close them at night, when it is high. Stockpiles should be formed on a firm surface, preferably raised slightly to avoid water puddling around edges.

Concealed Damage
Concealed damage can significantly impact quality and reduce grower returns in years with late harvests and/or early rains. Following roasting, the kernel interiors turn darker than undamaged nuts, and flavor can be bitter. In extreme cases, kernel internal color and flavor are altered before roasting. Prolonged moisture at both elevated temperatures (above field temperatures) and even at ambient temperatures can create this condition.

The orientation and shape of a pile can play an important
role in minimizing mold growth potential. If possible, orient
the long axis of the pile in a north/south direction.
Current ABC-funded research led by UCCE farm advisor Franz Niederholzer in Colusa County is helping us better understand the field conditions that contribute to concealed damage as well as mold and management practices in the field that can reduce these risks.

If untimely rain at harvest is anticipated, this new information can help determine when growers and handlers should take action to prevent mold development and concealed damage, particularly in more vulnerable late-harvest varieties or under other scenarios for delayed crop maturity.

Harvest Guidelines
Among other things, the research is establishing a threshold for the specific kernel moisture content, temperature, and the various time and field management scenarios that will lead to the appearance of concealed damage, and refine guidelines for when action should be taken under different field, handling, temperature and moisture scenarios.

Some Key Guidelines:

  • If rain is forecast, don’t shake. After a rain, wet nuts dry faster on the tree than on the wet orchard floor.
  • If rain is forecast and nuts are harvested but too wet to pick up, blow them away from the tree trunks but don’t windrow. Rain-wetted nuts on the orchard floor often are very difficult to blow, as they tend to stick to muddy soil.
  • Condition (“Drop chute”) windrowed nuts, certainly after, and even before rain. Removing leaves and other trash helps the nuts dry faster. Current research shows conditioning the crop even before rain and under normal dry conditions facilitates drying.

A newsletter article written by Niederholzer (Sacramento Valley Almond News, July 2012), “Coping with Rain at Harvest,” contains a table illustrating grower options across a range of harvest conditions. This newsletter article and table can be viewed on the ABC website: Stockpiling.

Trials in 2011 and 2012 looked at five conditioning treatments — before rain and three, five and eight days after simulated rainfall at various levels of kernel and hull moisture. This research found that conditioning the crop both before and after the simulated rain produced the most rapid drying.

The take-home message is that conditioning works. Conditioning prior to rain can help facilitate drying after the rain occurs, and if that rain doesn’t come, conditioning in and of itself can help dry the crop and put you ahead of the game.

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