“We all use color to make judgment calls on whether food products are of good quality. But we use color in so many more instances. Farmers use color to help decide whether his or her crop is ready to harvest. And you can use it to determine how your cooking or production process is progressing, e.g. shrimps that turn pink during cooking.
Since color can be subjective, scientists have developed ways to make this more objective. Measuring color is harder than you might think. For instance, you have to take into account how and which light falls on your color and whether the surface is glossy or matte. Scientists have had to develop methods to overcome these challenges, and nowadays we have several technologies available to help us measure color objectively. We’ll have a look at how they work and how you can use them for analyzing foods.
When you measure color, you need to step away from terms such as red, burgundy, purple, or lavender. Those are too descriptive. Instead, there are ways to quantitatively (in numbers) describe color. By using these harmonized and standardized systems we all speak the same ‘color language’. There are several of these systems. For instance, you might be familiar with hex codes that describe digital colors. The dark orange color on this website for instance is also known as #f26e3f, the light orange as #f2a04e.
Colors are wavelengths
The color that we see is electromagnetic radiation.
As you may have noticed, electromagnetic radiation encompasses a wide range of waves and possible application. You may have also noticed that you can’t actually see most of these waves! The exception here are waves within the ‘visible spectrum’. These are waves with a wavelength between approximately 380 to 750nm. This visible spectrum encompasses all the colors that we can see, every color has its own wavelength. For instance, light with a wavelength of 610nm can be described as orange, around 500nm it’s cyan and 540nm is perceived as green.
Our eyes have cones
But a wavelength is not yet a color. In order for us to actually see the color, the light has to land on our eyes and be processed internally, by our eyes and brain. When light with a specific wavelength enters our eyes it will land on three different types of ‘cones’ inside our eyes. These cones capture the incoming light where each cone is optimized for a slightly different wavelength. Together, these cones send a signal to our brain, which then processes the signal into a color!
Describing color objectively
Let’s have a look at what scientists have come up with over the years that combine the science of radiation with how we humans perceive colors.
The CIE & Tristimulus values
One tristimulus system still actively used is the RGB system in which R stands for red, G for green and B for blue. You might have heard about this system since it’s commonly used for computer screens. It is not as commonly used in food research applications though.
Another common tristimulus system used to describe colors is the L*a*b* system. You will come across these numbers quite regularly within the food science world. The value for L* represents lightness. Something that is completely white will be 100 and something completely black is 0. a* is a measure for the green to red value (so how red and green something is) and b* represents blue to yellow.
Measuring color of food products
If you want to measure the color of your food there are roughly two options available: colorimeters and spectrophotometers. Both have their pro’s and cons and are best suited for slightly different situations (and budgets!).
A colorimeter is a slightly simpler device than a spectrophotometer, so we’ll get started with this one. A colorimeter works by projecting light onto your sample. It filters this light through three filters, to mimic your eyes and then analyzes how the light is reflected from your sample. As the outcome, you get a tristimulus value, this can for instance be an L*a*b* value.
Colorimeters are relatively simple devices and don’t need a lot of advanced software (or hardware) to operate. They are generally the cheaper option of the two, but they also don’t provide you with as much data. Because of the way a colorimeter works, by always using one and the same light source, it might not be able to pick up on all differences between colors.
If you need a bit more information about your color you might need a spectrophotometer. A spectrophotometer works according to the same principle as a colorimeter: it shines a light on a sample and then analyzes its reflections. However, a spectrophotometer can do a more advanced analysis of the sample due to a few subtle changes.
First of all, it can work with different light sources, instead of just one. This can be important for some samples which might look similar under one light source, but are different under another.
Also, it doesn’t use those three light filters a colorimeter does. Instead, it can completely analyze the spectrum of the light, showing exactly which wavelengths are reflected from your sample.
Lastly, another convenient property of some types is that it can beam light onto a product under several different angles. This is especially relevant when you’re trying to analyze the color of very uneven samples.
Generally speaking, when you’re doing research on your food and really want to understand color differences, you’d want to use a spectrophotometer. These devices used to be large and bulky, but nowadays they’ve shrunk in size (and costs!) making them more accessible.
Goal of your measurement
As with any analysis you might do, you should always have a clear goal in mind. It is of no use to just collect a lot of data and then try to figure out what you can do with it. A few questions you could consider are:
- Is sample A the same as sample B? Or, similarly, are all my product made in the factory of the same color?
- A reason for looking into this question might be to ensure that your product always looks the same. Or it might be to check whether certain changes you’ve made to your product have impacted the color.
- Do my samples have a color of a specific pre-determined spectrum?
- If you’re making products that need to be of a very specific color, for instance, because of the identity of a brand, you want to make sure you hit that exact color.
- How does my color change over time?
- If the color of your product isn’t stable over shelf life you might want to investigate just exactly what is changing. This can then help you to determine what the cause of these changes might be.
As with any analytical technique, you might want the fanciest device, but if you can’t fit it in your workflow, there’s no use for it. Consider where you want to do your measurements and how fast a measurement needs to be completed. Do you have well-trained people available to do the analysis, or should just about anyone be able to run it. This is also a good time to consider your budget, both for the equipment itself as for possible costs required to run it.
Last but not least, you need to have a look at your sample. No matter how fancy the device, food can still be pretty complicated to measure properly.
Color homogeneous vs. heterogeneous
Is the color of your sample very homogeneous or more heterogeneous? If your sample is homogeneous measuring the color tends to be pretty straightforward. However, if your product has a lot of different colors (which, in all honesty, is the case for a lot of food products!) measuring color becomes a lot more complicated.
Think of an apple that’s ripening, it might have, yellow and some green areas. Which area do you decide to measure? The yellow one, the green one, all of them? What does the information that you get out of such an analysis even teach you?
Reflectance vs. Transmittance
Another aspect to think about is whether the food lets light through or not. If a food lets light through, this could be colored vitamin water for instance, you need to use a technique that uses this. You can use transmittance tests. During such a measurement light is beamed through a sample. The light that comes through the sample is then analyzed. Not all colorimeters can do these types of measurements, so would be something to look out for.
In other cases, when the light can’t pass through (e.g. an orange or a piece of bread) you will need to use a reflectance method. In this case, light is bounced onto a sample, and the device measures what is reflected by the food or drink.”
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