The primary objective of evaluating post harvest traits is to obtain information about the potential or aptitude of intermediate and advanced clones for diverse end-uses, ranging from fresh consumption to processed products. The interpretation of these evaluations provides important information to guide potato breeding and selection programs, as well as for the recommendation of varieties for specific uses. Therefore, CIP has adopted standard procedures for determining: specific gravity and dry matter content, texture and flavor components of cooking quality, storage behavior, chipping and French frying performance, oil content, and contents of undesirable secondary products such as glycoalkaloids (protocol in preparation). These procedures serve for evaluation of advanced clonal selections, and may be adapted for the determination of parental value for post harvest parameters.

Field layout and cultural management

The experimental design is a RCBD with at least two replications. Each plot consists of 2 rows with 10 plants/row. Cultural management of the experimental area should be similar to that practiced locally for commercial potato production and/or as required for each experimental site. Field experiments should include two local varieties with recognized fresh consumption or chipping capacity as controls.

Due to the significance of environmental factors (principally temperature fluctuations, rainfall, altitude and soil fertility) and Genotype x Environment interaction on post harvest performance, it is critical to record site, management and weather conditions during the experiment. A SITE AND TRIAL MANAGEMENT DATASHEET is provided in the GUIDELINES document for this purpose. Particularly for the post-harvest evaluation trial, the following weather data should be recorded during the growing season: Temperature (including day/night fluctuations), Relative Humidity, Precipitation, Radiation and Sun-hours. It is ideal to conduct evaluations in contrasting environments, particularly if different altitudes, or different seasons are relevant to the production ecologies targeted for variety development, in order to determine stability for quality traits.

Harvest data

The attached FLOW DIAGRAM indicates how tubers from the harvest are divided among the various post-harvest quality and storage tests.

At harvest time the following data are recorded on a per plot basis:

  • Number of plants harvested.
  • Number and weight (kg/plot) of commercial tubers (> 30 mm diameter)
  • Number and weight (kg/plot) of non commercial tubers
  • Presence of tubers with external defects (cracking, misshapes, secondary growth,)
  • Observations: insect or nematode damage, greening
  • Ten tubers are taken and cut to check for internal defects (recorded as count/10).

Grading of commercial tubers per plot will include tuber between the following categories:

  • No.1 tubers between 200 and 300 g, 55 mm
  • No.2 tubers between 80 and 200 g, 30-55 mm

The following data are recorded at graded tubers:

  • Number and weight (kg/plot) of No.1 tubers
  • Number and weight (kg/plot) of No.2 tubers

A balanced random sample of 10 No.1 and No.2 tubers is taken per plot to carry out processing evaluations. Tuber samples will remain bagged and kept in the dark at room temperature for 10 days prior to evaluations.

A balanced random sample of 10 No.1 and No.2 tubers will also be taken for storage evaluations.

Laboratory Evaluations
1. Determination of Dry Matter Content

Dry matter content should be determined if practical within 24 hours after harvest to avoid post harvest changes due to shrinkage loss. This determination requires an oven and balance accurate to 0.1g.

1.- Chop 5 tubers into small cubes (approx. 500 g total, in 1-2 cm pieces), mix thoroughly, and take 2 sub-samples of 200 g (fresh weight) each. It is important to sample all parts of the tuber, as dry matter content is not uniform throughout.

2.- Weigh and record the weight of each sub-sample, and place the samples in an open container or paper bag and set it an oven set at 80°C for 72 hours or until reaching constant dry weight.

3.- Weigh the two samples after oven drying, and calculate dry matter content (DM) on a percent basis as DM = (dry weight/fresh weight) x 100.

4.- Calculate the mean DM of the two samples.

A PHQS DATASHEET (DM) is provided to facilitate calculation and data recording. Dry matter content values larger than 20% are considered acceptable.

Figure 1.
2. Determination of Specific Gravity

Specific gravity of potato tubers is relatively easy to determine and is used to estimate tuber quality. Specific gravity is an estimate of solids or dry matter content. Potatoes tubers containing high solid matter up to 25% have been shown to produce chips with high yields, absorb less oil, and have better texture than those with lower solids.

The weight-in-air/weight-in-water method is used at CIP to determine specific gravity.

A given amount of commercially sized potatoes (at least 1 kg) is placed in a metal basket, weighed in air, and then re-weighed in water. From these two measurements, specific gravity can be calculated by using the following formula:

Good quality potatoes should have a specific gravity value larger than 1.080. Potato tubers with specific gravity values less than 1.070 are generally unacceptable.

Once specific gravity is determined, solids in potatoes can be estimated by the following relationship:

% Solids = 17.54 + 199.07 (Specific Gravity – 1.0988)

Figure 2.
3. Chipping performance

Field tubers should be allowed to rest ("stabilize") at room temperature for a period of 10-15 days after harvest. Two types of evaluation may be used. One of them evaluates the overall sensorial quality of the chips, and the other is used to evaluate the degree of darkening that occurs during frying (using a color chart).

3.1. Determination of chip color (using color chart)

Tubers should be free of disease and undamaged. Peeling is not necessary. Three potato tubers per sample are cut perpendicular to the long axis, and three 0.5 mm slices of each half are used for the frying test and chip color evaluation. Potato tuber slices are rinsed in water and fried at 176°- 180° until oil bubbling ceases. Chip color is evaluated with a chip color reference chart by the PC/SFA, or on a 1-5 scale where 1 is light cream or yellow and 5 is dark brown (SFA potato chip standard color chart).

3.2. Sensorial evaluation of chips

The overall evaluation of chip quality should be made by a taste panel previously trained on the steps and importance of the test. The reliability of the information obtained depends on the evaluators’ being able to recognize differences among the samples to be evaluated. The panelists should not eat or smoke for at least one hour before the evaluations. It is recommended not to carry out tasting sessions close to lunchtime. The most suitable hours for an evaluation are 10 in the morning or 2 in the afternoon, depending on the alimentary habits and customs of the evaluating group.

The characteristics to be analyzed are color, flavor, texture, uniformity of size and shape, and general appearance. Panel members receive a specific scoring table for each set of 8-10 samples; A larger number of samples should not be evaluated in a given experiment.

3.3. Determination of yield of chips

Chip yield is determined for each potato genotype by recording the weight of the raw material (slices) used and the weight of the chips obtained. This is calculated as follows:

Figure 3.
4. Determination of oil absorption rate

The oil or shortening used in the manufacture of potato chips or other fried potato products may be one of the most costly ingredients. Excessive oiliness in fried products not only indicates a poor quality product but also the loss of an expensive ingredient. Excessive oil results in greasy or oily chips, undesirable to the consumer. Some of the most important factors affecting oil content are specific gravity, partial drying of potatoes before frying, loss of liquid by cell disruption of raw slices, thickness of slices, type of oil, temperature of oil during frying and frying time. The press method is currently used to evaluate oil content; this method requires a carver press, scale and paper towels. The procedure is as follows:

  • Grind 20 to 30 g of potato chip sample with mortar and pestle or other apparatus and thoroughly mix the sample.
  • Place paper towels in the bottom of the sample chamber to absorb the expressed oil.
  • Weigh 10g ground sample (initial weight) and place into the sample chamber.
  • Place the piston portion of the assembly on the sample and chamber
  • Place the entire assembly in the press and pump it up at a rate of one stroke every two seconds until 15000 pounds/h2 pressure is reached.
  • Allow 20 seconds for the pressure to drop off in the press and then pump it back up to 15000 pounds pressure.
  • Set the timer for 3 minutes
  • At the end of 3 minutes, release the pressure and remove the sample cake from the bottom side of the chamber using care to include all cake portions and not include any oil portions in the weighing process.
  • Weigh the sample cake (final weight)
  • Determine percent of oil content using the following formula:
        Oil content (%) = 100 – [(final weight / initial weight) x 100]

Percentages of oil content less than 25% are considered acceptable.

5. French fry test

This test is designed to duplicate the commercial process for frozen French fries. Evaluation is conducted by a trained four-person taste panel.

  • A 3-tuber sample is selected. Tubers should be free of disease and undamaged. Tubers are sliced to give strips 3/8 inch in section
  • Because the specific gravity (dry matter content) varies in different parts of the tuber and because quality is influenced by dry matter, it is recommended that slices be taken from different parts of the tuber: the center, the outer part and the intermediate region. Slices can usually be distinguished by the slant at the ends (POTATO TUBER SAMPLE FOR FRENCH FRIES). Each of four samples - one for each panel member - is made up of one slice from each of the three parts from each of the three tubers to give a total of nine strips. Each of the nine strip samples is then taken through the remaining steps:
  • Par fry in cooking oil at 193°C for 1 minute. Drain off excess oil.
  • Fast freeze and store at -7°C.
  • To evaluate, complete frying (without allowing the samples to thaw) in cooking oil at 193°C for 1½' and present to panel members.
  • To evaluate it is necessary to sort strips into inside and outside slices. External appearance and external color are scored on the whole sample. Internal color is obtained by breaking the fries and squeezing the flesh oil to allow visual examination. Observation of the flesh is also required to evaluate texture (mealiness). The texture of the strips is scored differently for the inside and outside strips (see #2 above) - since good dry matter, and therefore good texture, in a sample is most often limiting in the center strips, the score is weighted to give a higher proportion of the marks to good texture in the center strips. A score sheet is attached (PHQS DATASHEET) the values to score are written in parenthesis and the rating for each sample is the total score for the five components.

Figure 4.
6. Cooking test

Evaluation of cooking/eating quality of boiled or steamed potato tubers is performed with the aid of a trained taste panel consisting of 6-12 members. No one person should have the sole responsibility for judging a sensory characteristic. The combined judgment of several people will minimize any individual sensitivity variation or fluctuation through the day and from day to day. A numerical scoring method is described to perform flavor evaluation:

  • The selection of a tasting panel is similar to that done for chipping performance; the panel should be trained to differentiate color, texture and flavor of cooked potatoes, and local varieties of good and poor cooking quality should be used as controls.
  • A 3-6 tuber sample is selected. Tubers should be free of disease, undamaged and washed free of soil and debris. A code is assigned  to each entry and the relation recorded (it is not recommended to evaluate by known names or identifiers). If a microwave is used, wrap each in a paper towel, then wet the samples with water and drain them with the hands.
  • Cook in microwave oven (full power) for 10½' (the time recommended for 3 medium potatoes of approximately 7.5 cm diameter). At mid time, turn the potatoes to reach a uniform cooking.
  • For boiling, tubers are placed in boiling water until a pin/probe penetrates the tissue, and the average time needed for the stem and bud ends to cook is recorded.
  • Immediately after cooking, wrap the potatoes in thin aluminum foil to keep hot until evaluation by the panel.
  • At the moment of evaluation cut one tuber in half for each panelist.
  • A score sheet for evaluation for cooked potatoes is presented (PHQS DATASHEET). The values to score are written in parenthesis and the rating for each sample is the total score for the six components.

A brief description of psychological errors associated with sensorial evaluations is attached (SENSORIAL EVALUATION).

7. Sprouting and loss of weight

A sample of 10 commercial tubers in good condition is weighed and stored at room temperature in the dark. Observations begin after one month and are repeated at every 15 days intervals for three months. At each date the number of sprouts per tuber is recorded. At 3 months, the tubers are de-sprouts, and the weight of sprouts and of tubers is determined.

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