The Wetness Within: Managing Moisture at Harvest and within Stockpiles

Delivering a quality crop to the huller requires attention to detail in many areas including field drying, stockpile layout and materials used for tarping stockpiles.   Recent research by Drs. Bruce Lampinen (Extension Specialist, UC Davis Plant Sciences Department) and Themis Michailides  (Professor, UC Davis Plant Pathology Department) provides important guidelines for post-shake handling, pickup and stockpiling of nuts.

The following are a few take-home points from their recent research report.  Some are no-brainers, others are not so obvious:

  • Don’t deliver wet nuts.
    • Check nuts under the tree (wetter) and in the tree row (driest) when deciding if nuts are ready for pickup.  Nuts on the orchard floor closer to the trunk and under the most shadow have as much as 2% higher moisture level than nuts out in the tractor aisle where there is maximum sun/heat.  If pickup timing is based on checking just in the driest location on the orchard floor, you may start too early.  Nuts with a water activity* of >0.58 (kernel + hull water content generally less than 9) are safe to stockpile.
    • In areas of the orchard where mites, disease, and/or water stress produced significant leaf loss at shaking, consider conditioning the nuts.  Blow out the trash and move the nuts out into the more sunlit portion of the orchard floor in the tractor aisle.  Conditioning nuts helps dry nuts wet by rain while on the orchard floor in windrows or scattered after shaking.
    • Think North – South, not East –West.  Build your stockpiles on a North-South alignment, not East-West.  There is less shading and overall condensation in a N-S stockpile than a E-W stockpile.
    • When covering stockpiles, use white on black tarps with the white facing up.  This keeps temperatures under the tarp and condensation down.  Clear tarp materials produced the largest day to night temperature swings and the most mold growth under the tarps compared to white or white on black tarp materials in UC studies.
    • For long-range planning:
      • Maintain no more than 80% light interception in the orchard (Put another way, no more than 20% shadow on the orchard floor.)  This provides a balance between light capture for crop production and drying power on the orchard floor.
      • Wherever possible, plant an orchard with rows oriented North-South.  This allows even light energy distribution on each side of the tree, so nuts can dry more uniformly.

*Water activity is a measure of the “free” water available for physical or chemical reaction.  It’s the most accurate measurement in predicting the risk of microbial growth.  One way of measuring water activity is to determine the relative humidity in the head space in a sealed container above the material being tested once equilibrium has been established.   There is a general relationship between product moisture content and water activity of the nuts.  However, water activity is not equal to moisture content.  More on this in a later post…

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