Postharvest Procedures

Early Harvest

Testing of clones in the early harvest Tri-State and Regional Trials involved French frying samples at harvest only, following the same procedure as that used in the late harvest trials. In addition to French frying and chipping, culinary and quality characteristics of clones from the Red/Specialty Trial were evaluated after oven-baking, microwaving and boiling.


Four- to six-ounce tubers were selected for the cooking protocols described below. After cooking, each tuber was halved from stem to bud end. One half was immediately tasted and evaluated on a scale from 1 to 5 (5 is best) for texture, flavor, tuber center and skin. The remaining half was set aside for 30 minutes and after-cooking-darkening was then graded on a 1 to 5 scale based on a color chart that we developed for white- and yellow-fleshed clones (1 = excessive graying, 5 = no discoloration).


Oven Baking - Tubers were pierced twice with a fork on each side and baked at 400oF for 1 hour.


Boiling - Tubers were cooked in a sieved double-boiler for 1 hour after coming to a boil.


Microwaving - Tubers were pierced twice with a fork on each side and cooked for 10 minutes at the outer edge of a microwave oven (high setting). The tubers were then turned over and moved to the center of the microwave where they were cooked for an additional 10 minutes. Four-tuber samples from each of two clones (8 tubers total) were cooked simultaneously.


Chipping - Tubers were cut longitudinally from stem to bud end. One side was used to make French fries as described below. The other half was sliced 0.05 inch thick. The first slice was discarded to insure uniform thickness of the sample. The samples (12 tuber sample) were then rinsed with water and fried in 375̊F vegetable oil for 2 minutes. The chips were then put on paper towels to drain. When all the samples were completed, chip color was graded using the Potato Chip/Snack Food Association (PC/SFA) color chart (1 = light, 5 = dark).


Late Harvest

Testing of clones in the late harvest trials involved the following postharvest quality evaluations. As soon as possible after harvest, tuber specific gravity and fry color (Photovolt readings) were measured on 12 tubers from each clone. Clones designated as fresh processing were French fried and Photovolt readings compared at harvest only. Additional tubers of each clone were placed in storage at 40, 44 and 48oF. Tubers stored at 48oF were evaluated for bruise potential, soft rot susceptibility, consumer acceptance of French fries, and cooking time in October and November. Reducing sugar content and French fry color were assessed in early December. The extent of sprouting was recorded in late December. Tubers stored at 44oF were also evaluated for sugar accumulation in December. Storage of tubers at 40oF until mid December was done to determine the "cold-frying" potential of clones. Fry

color and reducing sugar content were assessed in these tubers but the results are not reflected in the final numerical rating for each clone (see below).


Statistical Analysis

Least significant difference (LSD) values are included in the tables to facilitate evaluation of differences in fry color (Photovolt readings) and specific gravity among clones. Any two means whose difference is greater than or equal to the LSD value are considered to be significantly different. LSD values allow comparisons of the performance of any two clones with regard to a particular characteristic, such as fry color.



Specific gravity 1998 - 2004 was measured on a 12-tuber sample from each clone prior to storage by the weight in air/weight in water method and values were transformed into a 5-point scale as shown below. These same tubers were then used for French fry quality evaluation.

The new scale is:

  • 5= 1.083 - 1.088
  • 4= 1.081 - 1.082 & 1.089 - 1.091
  • 3= 1.080 & 1.092 - 1.093
  • 2= 1.078 - 1.079 & 1.094 - 1.095
  • 1= 1.076 - 1.077 & 1.096 or higher
  • 0= 1.075 or lower

  • French fries - were processed by frying tuber slices (3/8" x 1 1/8") in 375oF oil for 3.5 minutes. Fry color was measured with a Photovolt meter within 3 minutes of frying. A Photovolt reading of 19 or less was considered unacceptably dark. The stem and bud end Photovolt readings were reported along with the USDA color class (see below). A difference of 9 Photovolt units or more between bud and stem end constitutes non-uniform fry color. A point was either added or subtracted from the total score, based on the uniformity of fry color. A (+) or (-) symbol was included with the Photovolt rating to indicate that a point was added or subtracted during tabulation of the total score. The USDA color classes assigned to French fries were based upon Photovolt readings of the darkest ends (usually the stem end) and are for information only; they were not used in determining the final rating.


    Photovolt readings/USDA color


    Rating/Av. Photovolt reading




    5 =

    41 or higher




    4 =

    36 thru 40




    3 =

    31 thru 35




    2 =

    25 thru 30




    1 =

    20 thru 24




    0 =

    19 or less



    Taste panels - were used to determine the consumer acceptance of French fries from each clone. To date, WSU’s Human Subjects Board has not given clearance for taste-testing transgenic material. Hence, all of the clones evaluated by the taste panels were produced through classical breeding techniques. Slices (3/8" x 3/8") from tubers stored at 48oF were fried in 375oF oil for 4.5 minutes. Approximately 20 untrained panelists rated the fries on a 1 to 5 (5=best) scale for taste, texture, internal flesh color and weak units (limpness). The average rating of the four fry characteristics is reported and was used in calculating the total rating score for each clone.


    Reducing sugar content - of tuber stem and bud ends was determined on a percent dry weight basis. Reducing sugars were assayed in tubers stored at 44 and 48oF and percent values were transformed into a 5-point scale as shown below. Sugar scores contributed to the final rating of each clone.

                                               5 = 0.9% or lower

                                               4 = 1.0 through 1.49%

                                               3 = 1.5 through 1.9%

                                               2 = 2.0 through 2.49%

                                               1 = 2.5% or higher


    Calculation of Total Score - The overall postharvest rating for each clone is equal to the sum of the individual ratings for each of the following quality characteristics:


    Quality Parameter

    Max. Rating*

    Fry color prior to storage (0-5)


    Specific gravity (0-5)


    Taste panel (avg of 5 pts for taste, texture, internal flesh color and limpness of cooked fries). (1-5)


    After-storage (~60 days) fry colors & reducing sugars for tubers stored at:


    48F fry color (0-5)


    48F Reducing sugars (1-5)


    44F fry color(0-5)


    44F Reducing sugars (1-5)


    Maximum postharvest rating =


    *all characteristics rated from 0-5 or 1-5 as indicated. A rating of 5 is best. **fry color can get +1 for uniformity (see explanation below)

    **With regard to fry color, uniformity from bud to stem end is also assessed. The fry color ratings will gain or lose a point, depending on uniformity. For example, if the difference between stem and bud end fry color is <9 photovolt reflectance units, indicating highly uniform fry color, then a point is added to whatever fry color rating score was given to that sample. On the other hand, if the difference between stem and bud end fry color is ≥9 photovolt reflectance units (non-uniform fry color), then a point is subtracted from whatever score was given to that sample. Hence, it is possible for a clone to receive 38 possible points.


    Bruise potential - For each clone, 12 tubers were warmed to room temperature for one day. Each tuber was then held under a device that dropped a 4-oz weight from a height of 23". Each tuber received 4 such impacts, 2 on the stem end and 2 on the bud end. After 24 hours, the tubers were peeled and the percentage of impacts resulting in a blackspot or shatter bruise was calculated.

    Soft rot index - Bacterial soft rot susceptibility was determined by wounding the stem and bud ends of room-temperature tubers, inoculating them with the soft rot bacterium, Erwinia carotovora var. carotovora, and then incubating the tubers (6 tubers per clone) for 24 hours at 72oF in a mist chamber. The percent, by fresh weight, of tissue lost due to rot is reported.

    Reconditioning potential - Reconditioning ability of tubers stored at 40̊F for approximately 60 days was determined by subsequently storing the tubers at 60̊F for 21 days. The change in fry color over the reconditioning interval provides a relative measure of the reconditioning potential for each clone.

    Sprouting - The degree of sprout development in tubers stored at 40 and 48oF was assessed after all other tests had been completed (usually late December). The percentage of tubers that sprouted and the average sprout length per tuber were recorded for 15 tubers of each clone.


    Tuber shape characteristics - The lengths and widths of up to twenty five 8- to 10-ounce tubers from each clone were measured and length:width (L/W) ratios were reported. This was done to reveal the effects (if any) of growing location on tuber shape and to estimate the yield (% by number) of ≥3-inch long fries for each clone. Fry yields were calculated based on algorithms that we developed to relate tuber shape (L/W) to the number and weight of fries. The following table reflects these relationships.

    Visual Shape


    L/W Ratio

    Percentage of French Fries (≥3 in.)

    (by weight) (by number)
























    A L/W ratio close to one indicates that the tuber is round and not ideally suited for French fry production. A ratio in the 1.5 to 1.75 range is representative of a more oblong, blocky tuber, such as Russet Burbank, which is desirable for processing. A typical L/W ratio for Russet Burbank is about 1.80. A schematic illustrating the relative sizes of potatoes having various ratios is included in the postharvest sections for the Tri-State and Regional Trials. Blocky tubers result in high French fry yield with less waste.




    For evaluation of long-term storability, tubers were held at 48oF until late December and were then transferred to 44oF. The tubers were processed into French fries, and reducing sugars were measured in Late April or Early May of the following year. Tubers were not reconditioned prior to frying. Results from clones that were advanced from the Tri-State to the Regional Trial are reported in the Regional Trial section.