In the food industry, the color of rice is often measured to evaluate the process or to check the whiteness of the final product. Rice is typically translucent and non-uniform in size. Therefore, special accessories and presentation techniques are required to provide repeatable results. In general, several samplings should be measured together in order to obtain an overall average of the color for the batch. These readings should be averaged for the final result, preferably with replacement of the sample between measurements.
A HunterLab LabScan XE 0/45 spectrophotometer in the port-up orientation with the 1.75-inch sample cup viewing port can be used to measure the reflectance of rice in a glass sample cup. This is the most common method advocated by HunterLab for the measurement of rice.
Rice has several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.
Rice grains are irregular in size and shape, requiring the averaging of several readings with replacement.
Rice is translucent — not opaque — and will be sensitive to ambient light and to small differences in the optical configuration of the instrument. Using a sufficient sample thickness and an opaque cover will minimize these effects.
Note: If inter-instrument agreement is a concern when measuring rice, all the instruments used for these measurements MUST be the same model to minimize measurement differences.
|Recommended Color Scale||CIE L*a*b* as a full color descriptor|
|Recommended Single-Number Indices||YI D1925 (2/C), YI E313 for indication of yellowness|
|Recommended Illuminant/Observer||D65/10°. C/2° may also be used.|
|1. Configure your software to read using the desired color scale, 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 (HunterLab Part Number 02-3081-14).|
|3. Standardize the instrument for the 1.75-inch area of view using the black and calibrated white standards that come with the instrument.|
|4. Scoop up rice from the sample batch and fill the glass sample cup (HunterLab Part Number 04-7209-00) to the top. The 2-inch (50-mm) sample thickness makes the translucent, irregular grains of rice effectively opaque for reflectance measurement.|
|5. Place the sample cup in the port insert so that the rice will be read through the glass bottom of the cup.|
|6. Cover the sample cup with the opaque cover (HunterLab Part Number 04-4000-00). The cover minimizes the possibility of ambient light reaching the detector through the rice sample when the instrument is in the port-up orientation.|
|7. Take a single color reading of the rice through the bottom of the sample cup. Dump, refill, and read the rice at least five times from the same batch. Average the five color readings for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.|
|8. Record the average color values for the sample batch.|