Objective: To determine the effect of bran particles on color and ash content of wheat flour

Color is a sensory attribute of food that indicates quality and acceptability. In the flour milling industry, the color of flour is used as an indication of bran contamination. Color may be determined using a simple color chart where foods are visually matched against a target color, or by instrumentation. In this lab we will be using instrumentation in the form of a colorimeter. The device is more objective compared to the color chart method. Using a colorimeter, color is reported in L*, a* and b* values (See figure below).

Color measurements. Image Source: www.micro-epsilon.co.uk

L* values (also called value) indicate degree of lightness from black to white (range = 0 – 100). Negative b* is blue, positive b* is yellow, negative a* is green and positive a* is red. The a* and b* values range from -127 to +127. They can be used to calculate hue and chroma (also called saturation).

Hue = tan-1(b/a)

Chroma = (a2 + b2)0.5

Hue is what we usually refer to as color. So red, green, blue etc are actually hue rather than simply color. Color is more complex, comprising of not only hue but also variations in lightness and intensity. Chroma refers to how intense the hue is. The diagram below illustrates the differences between lightness (L* value), hue and chroma.

Intensity, Hue and Chroma. Image source: https://www.harding.edu/gclayton/color/topics/001_huevaluechroma.html

Note that lightness (L*) changes from top to bottom, hue changes as you move around the center, and chroma changes as you move outward from the center. The calculated hue (in degrees) will give you an indication of what the “color” will look like. For example a hue of 0o will be red. See next figure.

Hue angles. Image Source: www.quackit.com/css/color/values/css_hsl_function.cfm

Method

Five samples of wheat flour with increasing concentration of wheat bran have been provided.

  1. Transfer each flour sample to five disposable petri dishes (in duplicate). Fill to half the size of the dish
  2. Pat the petri dish lightly on the table to make sure that the flour is evenly distributed at the bottom
  3. Measure the color using the Konica Milota calibrated colorimeter provided, by centering the petri dish on top of the colorimeter
  4. Press the trigger to measure and record your readings
  5. Collect 5 g of sample (in duplicate) from each flour and determine their ash content

Lab Question

  1. Explain the relationship between the color of wheat flour and ash content
  2. Draw a graph showing L* values versus ash content. Add a trendline and include the r2 value and the equation of the line
  3. Calculate the expected ash content if the L* value is 85.8
  4. What is the disadvantage, if any, in having high bran contamination in wheat flour?
  5. How can flour millers prevent bran contamination in wheat flour?

Author

  • Dr. Courtney Simons is a food science professor. He holds a Bachelor of Science in Food Science and a Ph.D. in Cereal Science from North Dakota State University.

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