Sept. 11, 2020
Sept. 1, 2020
Farmers need non-destructive, accurate, rapid, and user-friendly tools to use on the farm to give them detailed information on the physical and chemical properties of crops at every stage of their growth. They also need to monitor the maturity and quality of their products. Quality control also continues beyond the farm and is necessary for the supply chain and retailing. Near infrared technology, which has been in use for the last four decades, is providing vital solutions in agriculture for these purposes. This article's goal is to give the reader a general background on NIR and, in addition, provide information on how NIR is specifically being used in different areas of agriculture.
Solar radiation has a spectrum that ranges from 290 to ~ 1,000,000 nm. Of the three bands that make up the radiation, 50% of the light that reaches the earth is made up of infrared, 42% is visible light, and 8% is ultraviolet (UV) rays (See Figure 1). While the infrared range lies between 760 nm to 1 mm, the near infrared falls between 760–1400 nm. Unlike the visual spectrum, infrared waves cannot be seen but are detected as heat.
Fig. 1. Solar spectrum composition with its component parts, Barolet et al. 2016. (Image credits: https://doi.org/10.1016/j.jphotobiol.2015.12.014)
When solar radiation passes through the atmosphere, air and pollutants scatter visual light, but the infrared spectrum can pass through it. This property of infrared is used to make remote sensing photographs of the earth's surface with satellites.
All objects and living matter emit radiation in the infrared spectrum as heat. Of all the spectrums in light, infrared is the one that is emitted most because of its longer wavelength.
Infrared radiation is absorbed, transmitted, and emitted by compounds, depending on the vibrations of the chemical bonding between component atoms or molecules. The vibrations’ frequencies determine the parts of the infrared spectrum that they absorb, transmit, or reflect. Thus, each molecule or compound has its own unique fingerprint or spectral signature.
Based on the spectral signature, it is possible to know the size, shape of atoms and molecules, and the bonds holding them together. The information on the structure of the compounds helps to identify the makeup of compounds or plants. Depending on the interaction with light, it is also possible to find out the amount of these compounds. This makes infrared valuable in studying the different compounds, as well as their concentrations.
Moreover, the infrared spectrum is not used by plants in photosynthesis or any other plant function, so there is no “interference” in the production of spectral signatures.
Near infrared (NIR) is most suitable for detecting organic compounds because bonds between carbon and hydrogen (C-H), oxygen and hydrogen (O-H), and nitrogen and hydrogen (N-H) absorb wavelengths that fall in the near infrared spectrum. Organic compounds are the major components found in living tissue, including plants, making NIR suitable for use in agriculture.
Moreover, near infrared can penetrate deeper into a sample than any other part of the infrared spectrum. NIR is the part of the IR spectrum that can identify components in a sample; whereas, the mid-infrared spectrum provides information on the structure-function relationships of compounds.
The ability of NIR to determine the physical and chemical properties of substances can be used through spot measurements and imaging.
NIR spectroscopy is incorporated in many hand-held tools and spectrometers to provide a wide range of applications, which are only increasing with time.
NIR analysis technology was developed primarily as a tool for field quality control. NIR spectroscopy can be used for the qualitative and quantitative analysis of moisture, protein, fat, starch, sugar, fibre, and ash of agricultural products.
In spot measurements, the tool directs a ray of NIR light of a specified spectrum towards the object. Depending on the compounds and their concentrations, the absorbed, reflected, and transmitted light spectrum will differ. This is recorded by the tools. In case of spectrometers, the spectral signature is used in a wide range of pre-programmed vegetation indices. Most indices use reflectance data, but some will also use absorbance data. Some examples of these include Normalized Difference Vegetation Index (NDVI), Water Band Index (WBI), Plant Senescence Reflectance Index (PRSI), etc.
The measurements are non-destructive and quick, and the tools can be used in fields, laboratories, and classrooms.
NIR spectral measurements have applications at various stages in agriculture and are discussed below.
NIR tools are adept at measuring various aspects of plant physiology that reflect the nutritional status of crops.
NIR provides information on the concentrations, deficiency or excess, of the major nutrients such as nitrogen, phosphorus, and potassium, by detecting chemical composition in leaves. NIR spectroscopy can replace cumbersome, long, and complicated analysis in the laboratory. For example, it can be used instead of the Kjeldahl Method, which is used to quantify nitrogen in plants. Instead, photosynthesis analysis by NIR spectroscopy lets farmers and agronomists know if the plants are suffering from nitrogen deficiency.
Similarly, NIR spectroscopy can detect levels of iron, magnesium, and zinc in leaves of cereal and other crops. The nutritional status of leaves has been determined successfully for apple, citrus, alfalfa, sugarcane, and root crops as well.
If the analysis shows a deficiency of any element, a farmer can provide timely nutritional supplements in the amounts required. As a result, they can also avoid over-use of fertilizers, and the resulting toxicity in plants, and reduce expenses.
There are NIR tools produced by CID Bio-Science for this purpose, such as the CI-710 Miniature Leaf Spectrometer that measures chlorophyll levels in leaves and photosynthetic rates, among many other metrics.
One of the most widespread uses of NIR spectroscopy is the measurement of dry matter to fix optimum harvest time and to monitor the quality of grains, pulses, fruits, and vegetables.
Dry matter (DM) is the total of all solids in a plant minus its water content. DM has been established as one of the most important parameters in crop production. Non-destructive measurements are necessary since the agricultural product will have to be tested several times to determine if it is ready for harvest. Small, cost-effective devices, which give rapid, easy-to-understand results, can sample even large crop areas within a day.
Felix Instruments has three quality meters based on NIR spectroscopy, which measure DM, as well as total soluble solids (TSS), titratable acidity, and external and internal colour of fruits.
NIR tools have a wide range of uses during the processing of fruits. The use of NIR tools begins on the farm.
Farmers growing fruits meant for processing, to make juices, wines, and other liquids, place a premium on sugar content. Fruits have to be harvested at the correct time to achieve the right sugar content.
Maturity and ripening are followed meticulously to fix the optimum harvest time. Among climacteric fruits, such as apple or mango, the DM content is used to fix harvest date. Later, after ripening, the DM content is used as an indicator of sugar content. In non-climacteric fruits, like berries and grapes that do not ripen much after harvest, the TSS of crops are monitored to fix harvest time. In case of blackberries, both sugar content and acidity are checked.
Sugar content is also monitored post-harvest to use fruits at a similar stage of ripeness for processing.
Water content is the parameter that is measured for several products meant for fresh consumption, such as mushrooms, vegetables, mango, banana, and strawberry.
During production of sultanas, which are produced by drying grapes, moisture content is monitored in the field to fix drying time and also in dry centers.
In all these cases, non-destructive methods like NIR tools that give instant readings are assets that assist decision making in real time. The F-750 Produce Quality Meter has been successfully used to test dry matter, TSS, acidity, color, and moisture content of various fruits, while the F-751 Mango Quality Meter can measure dry matter and brix in mango.
Composition of grains, i.e. proportions of different compounds such as proteins, starch, or fat, can differ in species, varieties, and individual plants; therefore, they are tested at harvest or post-harvest.
Crops are tested during harvest to sort them or check that they meet the required parameters.
Wheat with higher protein content fetches premium prices. Therefore, protein content in wheat is checked to sort them into different grades, either during harvest or handling. NIR spectroscopy has been used in combination with harvesters to screen large areas of crops to sort grains according to protein content at harvest time.
The post-harvest quality of many types of forage is ascertained based on the dry matter content. For example, the nutritional quality of potatoes is used to calculate the amounts of feed given to animals to ensure they have the optimum diet. Starch, sugars, and DM are measured in maize meant for the silos to ensure the quality of silage.
Analysis of oil and protein content of single seeds in rape, sunflower, cotton, and soya beans is also done with the help of NIR spectroscopy.
NIR spectroscopy also determines the levels of amino acid nitrogen in tuber mustards.
The CI-710 leaf spectrometer from Felix Instruments can be used for testing seed crop quality.
NIR spectroscopy can be used to detect diseases before visual symptoms appear in growing crops as well as in stored grains. Stress produced due to pests, diseases, and weeds will change plant composition and can also be detected by NIR spectroscopy.
Fusarium (Fusarium asiaticum and Fusarium graminearum) is a common fungal disease that afflicts barley plants, affecting the end quality of the grains and decreasing yield.
Rice blast caused by Magnaporthe oryzae reduces yield and is a health hazard for people.
Both these fungi have been successfully detected using NIR field tools and thus provide a timely and efficient means of controlling infection to limit the damage.
During post-harvest storage, rice is particularly susceptible to fungal infections. Using the wavelengths of 950 and 1650 nm, NIR spectroscopy has been used to find and quantify fungal infection of Aspergillus producing aflatoxins in rice grain. This was done by detecting the presence of aflatoxin B 1.
NIR spectroscopy has also been used to detect insect infestation and damages to cereals post-harvest.
Internal compound examination by NIR tools has been used to detect the development of toxins in seeds. For example, amino acid L-canavanine is a toxic compound that can develop in grains and reduce food intake in non-ruminants.
The CI-710 Miniature Leaf Spectrometer, from CID-Science, can be used to detect stress in plants.
Water has its own spectral signature that can be detected by NIR. Water content in leaves can be measured to let farmers and agronomists know if the plants are suffering from water stress. They can then use this information to regulate irrigation schedules.
The CI-710 Miniature Leaf Spectrometer, from CID-Science, is an example of a leaf probe that has been widely used for detecting water stress in plants and soil.
NIR spectroscopy is useful for retailers in monitoring the quality of fresh produce, as well as processed products.
Appearance, color, and texture are all very important to ensure consumer satisfaction. With quality meters, retailers can judge when the fruits are ripe enough to move goods from storage to shops.
Early detection of over-ripe produce, before the appearance of visual symptoms, can help in culling spoiled products to prevent ethylene from spreading and causing early ripening of other nearby produce.
NIR technology is also used in quality control of wide-ranging products such as honey, oils, or olives to ensure good quality of purchases and advise buying choices.
By detecting the hydrophilic phenols produced only by olives, NIR spectroscopy determines the authenticity of olive oil and checks that it is not adulterated.
The F-750 Produce Quality Meter, from Felix Instruments, can measure internal color and firmness of fruits and internal decay in fruits and vegetables. It can also be used to check liquids, like juices or wine, and test hydrophilic phenols in olive oils.
With NIR tools, quality control between the farm and shop is easy and can be performed often, as the method is quick and non-destructive.
NIR spectroscopy is useful not only for plants but also for aquatic animals, such as fish and shell fish, to determine body composition, such as protein, fat, and water, as well as minerals, vitamins, carbohydrates, and extractives.
Soils are made of minerals, water, air, organic matter, and microbes. It is difficult to separate organic matter, of which there is usually only 1-5 percent present in soil, from the mineral content. Moreover, conventional estimation methods of organic matter are long drawn and involve many elaborate steps. NIR measurements are quick and can be taken on the farm or laboratory without sample preparations.
NIR spectroscopy has also been used to test other aspects of soil pertaining to crops. Mineral composition of soils, such as nitrogen (N), phosphorus (P), potassium (K), iron (Fe), calcium (Ca), and magnesium (Mg), can be quantified to ascertain their availability for plants in fertilizer management.
Water dynamics in soil, which is determined by soil texture, is another important parameter that can be measured by NIR to advise irrigation management. However, it does require calibration for different localities.
NIR spectroscopy is used in the bio ethanol production at various stages, from the farm to the processors.
Raw materials that contain starch, cellulose, and sugars are fermented to produce bioethanol. These could be grains—such as wheat, corn, sorghum, and barley—sugar cane and beet, or grass, crop, and forest residues.
The starch or sugar content of feedstock has to be estimated before processing to know the possible ethanol yield.
If grains and residues are used, they are first crushed and treated with enzymes to convert the starch to sugars. The sugar is then fermented by yeasts to produce alcohol. Detailed chemical analysis of various components in the fermentation media, such as glucose, maltose, fructose, arabinose, lactic acid, and acetic acid, are tested to monitor the fermentation process.
NIR spectroscopy tools have been providing information on BRIX or TSS content, starch, and lignocellulose of feedstocks at harvest. During the process, it has also been used to monitor the fermentation parameters.
Scientists and agronomists use many instruments that utilize NIR spectroscopy, especially in breeding programmes and to suggest agricultural practices, due to its precision.
The right applications of fertilizer and irrigation for each variety are monitored and fixed depending on health and response, which can be monitored with spectroscopy. The CI-710 Miniature Leaf Spectrometer, for example, can be used to match species and varieties to different regimes of water and nutrition supply in cereals, vegetables, and fruits.
NIR tools are useful to determine the best varieties and crops for bioethanol production by measuring their sugar, starch, cellulose, or lignin content.
Similarly, NIR devices can be used to establish the best set of agricultural practices to improve fruit and vegetable quality.
Figure 2: “Normal color photo (right) and normalized difference vegetation index (NDVI) image (left). NDVI image was derived from two color channels in a single photo taken with a camera modified with a special infrared filter. Note that tree trunks, brown grass, and rocks have very low NDVI values because they are not photosynthetic. Healthy plants typically have NDVI values between 0.1 and 0.9.” (Image credits: @cfastie, https://publiclab.org/wiki/ndvi)
NIR spectroscopy imagery involves taking images of pixels in contiguous spectral bands, so that a radiance spectrum of a pixel is acquired, identifying the surface materials.
The imagery can provide information on the stress that plants are suffering. The water content in plants can be measured by spectroscopy imagery, based on water’s spectral signature. Spectral images also reflect changes in leaf composition due to pest and disease attacks. The chlorophyll content in leaves can be an indirect measure of nutritional deficiency in plants. Recognition of species is also easy through NIR imagery.
NIR imagery has been more commonly used for large-scale surveys than for field measurements.
NIR imagery is well established in producing satellite imagery through remote sensing. It is suitable for multispectral and hyperspectral images and can provide visual and spatial information about agricultural crops.
Satellite imagery is more suitable for research and larger organizations, like government departments, for research and policy development. Though consultants can transfer information to farmers, there will always be a time lag.
With the advent of precision farming, there has been an interest in small-scale applications of NIR imagery. NIR cameras are installed on drones to get pictures of internal differences in soil conditions, and crop health and performance of an entire field. This is still an airborne application, even if it is at field level, and needs additional equipment such as a drone. This kind of imagery is particularly suitable for identifying soil type, structure, water status, and fertility.
At present, there are only a few NIR cameras that are available for use by individuals on the field. Some DIY (do it yourself) versions are also being developed. NIR cameras can be used much in the same way as normal cameras; the only difference is that the imagery is in the NIR and not visual spectrum.
These NIR images can highlight areas with high photosynthesis, unhealthy plants, or unsuitable growing conditions (See Figure 2). The cameras give farmers and even gardeners the benefits of imagery at low costs. However, the DIY varieties at present require analysis-using software by users themselves.
The ready commercial NIR cameras can also be used to inspect the quality of fruits at purchase. The NIR images show images of bruises clearly, before they are visible to human eyes.
NIR is one of the technologies tailor-made for agriculture, not only for research but also for farmers. Devices these days are sophisticated and give rapid measurements that are easy to understand and use in real time. They are precise enough for use in research and complement existing equipment. They take the guesswork out of farm management, but at the same time do not require much expertise to use. Affordable NIR tools have been and will continue to be an important step in bringing science to farms.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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