Measuring Processed Tomato Products Using the ColorFlex

Color is often used as an indicator of quality and freshness for food products, including tomato products (such as ketchup, paste, sauce, and juice) for which the perception is "the redder the better." It has become important for tomato processors to be able to evaluate and grade their products based on color. These translucent samples require special handling when being evaluated either visually or instrumentally. The color of a translucent sample changes when the path length is changed, so that path length must be fixed and sufficient to render the sample effectively opaque. Ambient light may also affect the sample's appearance and should be minimized.

A HunterLab ColorFlex 45/0 spectrophotometer can be used to measure the reflectance of processed tomato products in a glass sample cup covered with the opaque cover. This is the method advocated by HunterLab for the measurement of tomato products. In fact, the United States Department of Agriculture has officially qualified the ColorFlex 45/0 for evaluating the color of tomato products.

The ColorFlex

The Application

Tomato products have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.

• These liquids and semi-solids must be measured through the bottom of a clear glass sample cup in order to be effectively made solid.

• They are translucent — not opaque — and will be sensitive to ambient light, path length changes, and small differences in the optical configuration of the instrument. Using a constant sample thickness and an opaque cover will minimize these effects.

  Note: If inter-instrument agreement is a concern when measuring tomato products, all the instruments used for these measurements MUST be the same model to minimize measurement differences.

Recommended Color Scale	Hunter L, a, b as a full color descriptor
Recommended Color Indices	Fresh Tomato Color Index, Tomato Juice Score, Tomato Paste/Puree Score, Tomato Sauce Score, Tomato Catsup Score, Tomato a/b Ratio. (Note that a special instrument configuration, HunterLab Part Number CFLX-Tomato, or a special software option, HunterLab Part Number EZMQC-Tomato/CFLX, are required in order to display these indices.)
Recommended Illuminant/Observer	C/2°

Measurement Method

1. Configure your software or the instrument firmware to read using the desired color scale, index, illuminant, and observer.
2. Orient the instrument with the port facing up. Replace the regular port insert with the special port insert for the sample cup (included as part of HunterLab Part Number CFLX-Tomato).
3. Standardize the instrument using the black and calibrated white standards that come with the instrument.
4. Center the tomato standard tile that is included with CFLX-Tomato or EZMQC-Tomato/CFLX at the sample port.
5. Measure the tile as a standard. The instrument will make and average three readings for the standard and display its color values.
6. Hitch the standard just read to the assigned values for the tomato standard tile.
7. Stir or shake the sample, if necessary, to homogenize it to its usual level, but do not introduce additional air. Tomato paste samples ONLY should be diluted with distilled water to a Brix of 85° before measurement. Then fill the 2.5-inch glass sample cup (included as part of HunterLab Part Number CFLX-Tomato) with the tomato sample.
8. Your goal is to have the sample appear smooth and opaque through the glass bottom of the sample cup.
9. Place the sample cup at the instrument port.
10. Cover the sample cup with the opaque cover (included as part of HunterLab Part Number CFLX-Tomato). The opaque cover prevents any ambient light from outside the instrument from leaking into the detector.
11. Take a single reading of the sample through the bottom of the sample cup. Associate this sample with the hitched standard. The instrument will automatically make and average three readings for the sample and display the color values adjusted using the hitch offset established for the tomato standard tile. If the sample is uniform in color (such as ketchup) or the tolerances are wide, one reading may be sufficient to characterize the sample color. For non-uniform samples or to ensure a higher level of measurement repeatability, average several readings (three for samples such as stewed tomatoes) with sample replacement for a single color measurement representing the color of the batch. Scoop or pour the sample out of the sample cup, refill it, and measure again several times from the same batch. Average the readings together for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
12. Record the average color values for the sample batch.