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United States Department of Agriculture

Agricultural Research Service

PecanNutQuality
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    PECAN NUT QUALITY

    Nut quality characteristics are based on 15 nut samples taken from each tree at harvest. Five nuts are destructively measured, the others are photographed using standard methods and maintained as a voucher specimen. The procedure follows the sequence outlined below:

    Nut length, width, and height are measured on each of 5 nuts, using digital calipers reading in mm.

     Measuring pecan nut length, width and height

    Nut weight in grams. Nuts are individually weighed on a Mettler top loading digital scale. Divide the number of grams in a pound (453.59) by the gram weight of a nut to determine the number of nuts in a pound.

    Nut volume is determined from buoyancy. Buoyancy is "the power to float or rise in a fluid". The density of water changes with temperature. At 20C(68F) (temperature recommended by Dodge), water has a density of 0.99823 g/cc, which is rounded to 1 for our purposes. At 25C (77F) water has a density of 0.99707 g/cc, which is also close enough. The temperature should be constant across all samples measured. As noted by Dodge, "buoyancy is taken to be the difference between the total water displacement of the sample and the displacement by the immersed parts of the nuts as they float. This latter is equivalent to the weight of the nuts. Hence the formula used in determining the buoyancy of the sample is: Total displacement in ml (=gm.) minus Weight of Sample in grams equals Buoyancy of Sample in grams. The total displacement of the sample can be calculated from the following formula: Weight of Sample in grams plus Buoyancy in grams equals the Total Displacement (volume)in ml (=gm.)."(Dodge, F. N. 1944. A method of measuring the degree of kernel development of samples of pecan. J. Amer. Soc. Hort. Sci. 45:151-157.)

    We measure buoyancy using the apparatus shown below. A Mettler PM600 digital scale is mounted on a stand. A hole in the upper surface of the stand allows a chain to be attached to the bottom of the scale. A wire rack (or a perforated plastic container, for multiple nut samples) is hung from the chain and into a basin of water. The weight of the apparatus is registered on the scale. Use the tare to zero the scale. Place a single nut in the water under the rack, so that it floats up under the rack. The lift or buoyancy of the nut is registered as a negative number on the scale. Record the buoyancy as a positive number. To determine the nut volume, add the buoyancy to the nut weight.

     

    Kernel weight in grams. Nuts are cracked using an Inertia Nut Cracker and carefully shelled. Kernel weight is divided by the whole nut weight to calculate percent kernel.

    Kernel color is rated on 1-6 scale based on the closest match when compared to Munsell color chips. Chips progress uniformly through color space, descending in hue and value at a constant chroma. In the diagram shown in the Photography methods below, chips are arranged left to right from 1 to 6.

     Class  Class Name  Hue  Value  Chroma
     1  Light Cream  22.5 (2.5Y)  8  4
     2  Cream  20.0 (10YR)  7  4
     3  Golden  17.5 (7.5YR  6  4
     4  Light Brown  15.0 (5YR)  5  4
     5  Reddish Brown  12.5 (2.5YR)  4  4
     6  Dark reddish brown  10.0 (10R)  3  4

    For further information concerning the six point USDA pecan kernel color standards, see Thompson, Grauke and Young, J. Amer. Soc. Hort. Sci. 121(3):548-553. 1996.

    Photograph nuts. Samples are arranged on blue sweater cloth, around a centimeter scale with color rating chips. Cultivar name, inventory information (orchard, row and tree) and year of sample collection are printed to the left. The shade of blue in the cloth is opposite brown on the color wheel, and offers chromatic contrast. The Munsell color chips are from the Soil Color Series and provide a standard for color correction.

     Organization of nut samples for photography

    L. J. Grauke
    USDA ARS Pecan Breeding
    10200 FM 50
    Somerville, TX 77879
    979-272-1402

Last Modified: 5/24/2007