June 19, 2020
June 11, 2020
Spectroscopy is a sensitive technology that allows precise analysis of food and is used for quality control. Infrared spectroscopy, ultraviolet-visual, Raman, nuclear magnetic resonance, and atomic emission spectroscopies are commonly used in agriculture. The incorporation of these spectroscopies into small portable devices has increased the application of these techniques. This article will briefly highlight the use of different spectroscopic instruments in agriculture.
Spectroscopy measures the absorption, transmission, and emission of electromagnetic radiation by light and other materials based on the wavelength of the radiation. The term spectroscopy also covers the interaction of electrons, protons, and ions within a compound and with those of other compounds based on collision energy.
Spectroscopy provides precise and non-destructive analysis and is widely used as an analytical tool in different sciences.
In agriculture, spectroscopy has been used for analysis and quality control of food. As Figure 1 shows, these uses range from detecting chemical composition, microbial infection, toxins, pests, pathogens, to adulteration. Internal and external defects can also be monitored.
Figure 1: “Common quality evaluation of staple foods,” Su et al., 2017. (Image credits: DOI:10.1080/10408398.2015.1082966)
Each compound or element will respond to a particular wavelength and, based on its composition, has a unique spectral signature. So, spectroscopy is used in biological sciences to determine the composition of materials and also for quantitative analysis.
The use of optical/light spectroscopy is popular in agriculture. However, agricultural technologies that use Nuclear Magnetic Resonance (NMR) and Atomic Emission Spectroscopy (AE) can also be found.
Light is a kind of electromagnetic radiation, and the different spectra are made of varying wavelengths, frequency, and energy, see Figure 2.
In spectroscopy, the spectral ranges of infrared (IR) and ultraviolet-visual have vital agricultural applications.
Figure 2: The electromagnetic spectrum, where λ refers to wavelength. (Image credits: http://www.chem.uiuc.edu/chem103/spectroscopy/introduction.htm)
The ultraviolet-visible spectrum used for spectroscopy lies within the UV (100 nm to 380 nm) and visible (380 nm to 750 nm) range of wavelengths. In this case, it is the part of the light that is absorbed that is of interest.
UV-VIS spectroscopy is primarily used to control the quality of edible oils. There are two aspects of oil that can be tested: fat oxidation and the general color.
The light that is emitted after the absorption of UV and visible light by a fluorescent molecule or fluorophore is called fluorescence.
Fluorescence spectroscopy is widely used for quantitative analysis; it is sensitive and specific enough to detect even small concentrations of compounds. Therefore, it is used to control contaminants/toxins and for structural analysis. Fluorescence can be used in combination with other techniques, such as liquid chromatography and fluorometer.
Infrared (IR) falls within the light spectrum, stretching from 780 nm to 1 mm, and can be divided into three sub-divisions: infrared (30 µm to 1 mm), mid-infrared (5 µm to 30 µm), and near-infrared (780 nm to 5 µm).
Infrared spectroscopy is one of the most common spectroscopic methods used in agriculture. It is used in quality control for all major food groups, such as cereals, pulses, vegetables, fruits, fish, meat, dairy, and processed foods.
This wide range of food groups reacts to infrared spectroscopy, which is mostly restricted to the mid-infrared (MIR) and near-infrared (NIR) spectra.
Infrared has become a powerful technology to provide fast and non-destructive analysis in the whole supply chain, ranging from farms to retailers.
The various uses of the different sections of infrared are discussed below.
MIR detects the bonds and functional groups, as well as carbon, nitrogen, lignin. This allows for the detection of complex components. Spectroscopy uses the reflectance of mid-infrared (MIR) to study soil and food, as it provides information on structure-function relationships in quantitative analysis.
While MIR can be used alone as attenuated total reflectance (ATR), it is also often combined with the Fourier transform process (FT) and used as the diffuse reflectance infrared Fourier transform (DRIFT) process.
The main uses of MIR include studying soil and organic matter and detecting fungus contamination in food.
Near-infrared is undoubtedly the most widely used technique in spectroscopic applications for agriculture and is applied for quantitative analysis.
There are many categories of NIR spectroscopy use in agriculture:
Felix Instruments have developed many tools with NIR spectroscopy, including leaf spectrometers and quality meters.
Raman spectroscopy is described as vibrational spectroscopy. It depends on the Raman effect where incoming photons (basic units of light) interact with electrons in a compound. The photons can either lose or gain energy, depending on the levels of vibrational energy in the atoms of the compounds. The Raman effect is the difference in the pre- and post-interaction energy levels of photons. Raman spectroscopy provides narrow and specific bands as information.
However, the Raman effect is weak because it needs high concentrations of samples and is more difficult to measure.
Other spectroscopy used in agriculture is nuclear magnetic resonance (NMR) spectroscopy and atomic emission spectroscopy. These two techniques, however, require extensive instrumentation.
It is used to test soil components, plant tissues, and food products. For example, it can be used to determine the genotype of the grapes used in wine-making, their place of origin, and the soil properties of the vineyard it was grown in. As a food quality control measure, it can be used to monitor ripening, drying, and adulteration.
In combination with inductively coupled plasma, atomic emission spectroscopy is used to determine the presence of trace elements in herbal medicinal preparations, such as traditional Chinese medicines, or it can be used to detect the presence of metals and arsenic in food or wine.
Conventional methods of food analysis are destructive, time-consuming, complicated, and need infrastructure and resources. Spectroscopy allows quick and precise measurements with little or no sample preparation. As of late, this technology is being incorporated in small handheld tools to help to bring these sensitive yet powerful techniques within reach of not only scientists but also farmers and other food producers.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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