Purpose: Color is a key indication of the quality and freshness of tomato products. As tomatoes are processed, their value is graded based on color. This application note looks at the colorimetric methods for evaluating tomatoes.

Challenge: To grade tomato products objectively.

Objective colorimetric methods for tomato products are based on standards prepared by the University of California at Davis (UCD). Variables are minimized and each processor prepares their standards as defined by UCD. Using a tomato tile, the processor assigns values based on a hitch standard. This tile is then used to standardize their instrument for measurements that correlate to the tomato standards.

To simplify this process HunterLab provides a tomato tile standard calibrated based on UCDs tomato paste standards and USDA-approved formulas optimized for grading processed tomato products.

Materials

• Tomato Tile Standard
• Glass Sample Cup
• HunterLab Instrument (ColorFlex EZ, LabScan XE)
• Special Port Plate for Glass Sample Cup
• Opaque Light Shield T

Samples are prepared as prescribed by UCD and placed in special glass sample cups. These cups should be purchased from the same supplier to ensure consistent calibration values. A line is to be drawn on each cup ensure the same depth of product is measured each time. An opaque light shield is used during measurements to prevent ambient light interference.

American Tomato Tile

This tile is supplied with USDA traceable calibration and is used for hitching the instrument to the USDA soft processed tomato standard. The method to establish a hitch standard for the Tomato Paste Color Tile is as follows:

1. Change the normal 32 mm diameter port to the sample cup port (04-6622-00 Port Insert for 2.5 in (64 mm) Sample Cup).

2. Standardize the Instrument using the supplied Black Glass and Calibrated White Tile.

3. Select a setup that is not in use. If using a ColorFlex Tomato Meter, Setup #10 is usually free.

4. Change the setup parameters to the following:

• Set Up Name can be something appropriate like Tomato Color.
• Standard: Hitch
• Display: Absolute
• Average: OFF (will take just 1 reading)       
• Ill/Obs: C/2
• Color Scale: Hunter L, a, b (plain, no stars)

5. Measure the Tomato Red Tile. The values should be similar to the assigned values.

6 Adjust the read values until they match the Hunter L, a, b C/2 values assigned to Tomato Tile.

7. Return to the measurement mode and read the tomato tile as an OQ (Operation Qualification) step. It should now read very closely to the values assigned to the tile showing the        effect of the hitch.

8. Fill the sample cup (04-7209-00 Glass Sample Cup 2.5 in (64 mm) diameter) with tomato product to near the top.

9. Read the processed tomato product, record the Hunter L, a, b C/2 values for the sample as hitched to the Tomato Tile.

Method for the Grading of Tomato Products

1. Make sure the sample cup port (04-6622-00 Port Insert for 2.5 in (64 mm) Sample Cup) is in place.

2. Select the Tomato Color Setup.

3. Standardize the ColorFlex L2 using its own Black Glass and Calibrated White Tile.

4. Read the tomato tile as an OQ (Operation Qualification) step.

5. Place the tomato tile at the port and take a reading.

6. Make sure the instrument is reading within +/- 0.3 of the Hunter L, a, b values assigned to the tile. If the tile is not within those tolerances, re-set the Hitch.

7. Fill the sample cup (04-7209-00 Glass Sample Cup 2.5 in (64 mm) diameter) with tomato product.

8. Read the processed tomato product and record the Hunter L, a, b values for the sample as hitched to the selected Tomato Tile.

Summary

Consumers often judge the taste and quality of tomato products based on their appearance. Therefore, appearance measurement techniques are crucial for predicting product acceptance and valuation.

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