One of the most important factors of success in any industry is the quality of the products you sell. This is particularly true of the food industry. Spectroscopy allows us to examine the chemical composition of food items, which gives us important insights into their quality.

What is spectroscopy?

In short, spectroscopy allows us to see the chemical composition of an object by seeing how it reacts with electromagnetic radiation. So how does this specifically work?

Broadly, when light interacts with a surface, it can either be absorbed, transmitted (goes straight through), or reflected. It is worth noting that when light is absorbed, it can also be re-emitted. The crucial thing here is that different frequencies of light can have different interactions when hitting the same material (i.e. some light could be absorbed while some could be transmitted). This means that different frequencies can be observed in different locations: light that would usually be transmitted can be seen directly behind the material while light that was reflected can now be observed from a different angle. Therefore, if we can find a way to spread this light out, we can see how different frequencies interact.

This is obviously very useful because when we shine white light on an object, as certain wavelengths are absorbed and re-emitted, we can create an emission spectrum by observing which wavelengths are re-emitted. Conversely, we can create an absorption spectrum, observing wavelengths that aren’t. As different materials absorb different frequencies, we are able to understand the chemical composition of a material by observing these spectra. Nowadays, we have spectra libraries that detail the spectra of various molecules which we can compare to the spectra we observe, making it easy to determine which materials are present.

Now that we understand the key principles behind spectroscopy, we can examine how a spectrometer works. A spectrometer typically observes light that is dispersed through diffraction gratings, refraction, or another method of dispersal. If we were to use a diffraction grating, we can use the double-slit formula: d sinq = ml, where d is the distance between slits, q is the angle from the centre, m is the order of interference and l is the wavelength of the light. So when the light is re-emitted from an object, we can observe the emission spectrum using a spectrometer that can then calculate which wavelengths have been re-emitted. Using this information, we can then work out the chemical composition of the object.

NIRS

Instead of using visible light, Near-Infrared Spectroscopy (NIRS) uses the near-infrared region on the electromagnetic spectrum (780nm – 2500nm). NIRS allows us to look at the molecular bonds present in an object. We use the same principles as before – we pass infrared light through the material – but this time we focus more on which wavelengths of infrared light are absorbed as opposed to emitted. So how do we know what is being absorbed?

In order to this, we use overtones and combinations. Molecular bonds have vibrational frequencies (so-called ‘fundamentals’). Overtones operate in a similar way to harmonics where there is a series of absorptions for each fundamental. Combinations occur when near-infrared energy is shared between fundamental absorptions. We can use these overtones or look at the combinations to plot a graph where we should observe peaks at certain wavelengths. This gives us the spectra detailed earlier and again we can use these to identify certain molecular groups present in the substance we are examining.

Applications in the food industry

What makes spectrometry so useful is that it doesn’t damage or alter the material examined, making this a non-destructive food analysis method. This means that we can use spectroscopy to see how much fat, water, and protein (among various other substances) are present in food items. This is immensely important as the chemical composition of food has a massive impact on taste/quality as well as potential uses for the food (for example salmon with less fat might be ideal for smoking while salmon with high fat content is good for making sushi).

One of the advantages of NIRS, in particular, is that it has a high penetrative power and so we can see inside the foods much more. While this means we lose some fine detail, this is useful for examining larger quantities. From this, we can broadly see the percentages of different substances in large amounts of food, giving us useful information about their quality. In addition, as it uses reflected energy, we don’t need to put much effort into preparing the sample for analysis.

Obviously, with foods, they are not just one type of molecule and so the spectra/graphs we get are slightly more complex and may not always fit with existing libraries to identify the chemical composition of a material. Therefore, we need statistical techniques, common sense, and machine learning to identify the substances present in the food sample. Unfortunately, the mathematics of this is not fully understood as the field is relatively new and untouched (it was largely dismissed for a few decades and even somewhat today). However, using these processes, we’re still able to get very useful results.

By using spectroscopy to create visual diagrams of the composition of foods, manufacturers can not only assort food to be used for different purposes (or be thrown away for being low quality) but also to perform statistical analysis and refine stages of the manufacturing process in the era of big data.

References

McQuarrie and Simon. 2020. Physical Chemistry: A Molecular Approach. Libre Texts. 

Davies, A. An introduction to near infrared (NIR) spectroscopy. https://www.impopen.com/introduction-near-infrared-nir-spectroscopy [retrieved 1st February 2020]

Calibre. What is NIR and how is it used in food testing?. https://www.calibrecontrol.com/news-blog/2019/10/1/what-is-nir-and-how-is-it-used-in-food-testing [retrieved 30th January 2020]

Crook, S. Gordon, T. 2019. Spectroscopy explained – with Crooked Science and USyd Kickstart, https://www.youtube.com/watch?v=cMCzA9rqJy8

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