Vis-NIR Spectroscopy for Mandarins

Mandarin is among the fruits for which the application of VIS-NIR spectroscopy techniques has been widely tested by scientists. It has been proven to be suitable to assess dry matter, total soluble solids, titrable acidity, and firmness with great accuracy. There are many applications for this internal data to determine the maturity, health, and quality parameters of mandarins, providing benefit to stakeholders at all levels of the supply chain.

NIR Spectroscopy

Spectroscopy measures absorption, transmission, and emission of light by matter. Radiation with higher wavelengths in the visible and infrared spectra are used as a tool for structural analysis. This technique can precisely identify chemicals and atoms present even in minute quantities.

Matter responds to different wavelengths of light. Organic compounds with bonds between H-O, H-N, and H-C are most sensitive to wavelengths in the near-infrared (NIR) spectrum, which lies just beyond red visible (Vis) light.

The use of Vis-NIR spectrum for food analysis is well established for not only identifying, but also quantifying compounds. Vis-NIR spectroscopy (Vis-NIRS) is used in measuring several parameters.

These chemical to light interactions are highly specific. Compounds within a fruit type will interact only with certain wavelengths. It has thus been necessary to find which parameters can be estimated by NIR, the wavelengths to which they respond, and their applications.

Vis-NIRS can replace numerous tests and equipment, as the technique can measure several parameters. This is one of its greatest advantages. To capitalize on its versatility, the technology has been miniaturized so that it can fit into small handheld devices. This crucial step has brought the technology directly to the fields, storehouses, packinghouses, and transport facilities where it is needed.

Among the products on the market, Felix Instruments’ F-750 Produce Quality Meter is one of the most established and trusted tools. It is small, easy to use, and gives precise measurements in seconds, allowing farmers and suppliers alike, to analyze thousands of fruits in a day. The tool can estimate total soluble solids, dry matter, titratable acidity, and color. The F-750 has starter models for numerous commodities, including mandarins, as well as easy instructions to create further models.

Mandarins

Citrus fruits, including mandarins (Citrus reticulata), are non-climacteric. That is, they do not produce ethylene to ripen. Fruits accumulate soluble sugars, become softer, and change color as they ripen. Their ripening process must occur completely on the tree and cannot happen post-harvest.

In mandarins, the parameters that have been successfully detected by Vis-NIRS so far are

• Total soluble sugars (TSS)
• Dry matter (DM)
• Titrable acids (TA)
• Firmness

Harvest-Only Ripe Mandarins

Figure 1: Total soluble sugars (TSS) and titrable acidity (TA) were accurately predicted in mandarin cultivars Page and Miho, by Vis-NIRS, Antonucci et al., 2011. (image credits: Food Bioprocess Technol 4, 809–813 (2011). https://doi.org/10.1007/s11947-010-0414-5)

Maintaining a high quality of mandarins at harvest is vital for the whole supply chain. Since TSS is correlated to consumer taste, it is one of the factors used to judge harvest readiness of mandarin fruits. TA is also included, as there are specified levels of acids that are permitted for fruits meant for export.

Variability in TSS and TA can be high, even within a cultivar, farm, or the same tree. There can be several reasons for the differences in ripeness:

• The time of blooming and fruit set can differ within a tree, so all fruits are not of the same age, maturity, or ripeness.
• The position of the fruit on the tree can also influence the ripening process in mandarins.

Farmers need to base harvest not only on visual characteristics, but also on internal parameters, such as TSS and TA; see Figure 1. Before harvest, they should test many fruits several times during ripening. Non-destructive and portable Vis-NIRS is a viable alternative to time-consuming and destructive testing by refractometers and titrations.

Farmers can improve the quality of fruits they market and get premium prices by fine-tuning their harvest time. Farmers can easily and rapidly analyze intact fruits still on the tree. They can stagger harvest and pluck only ripe fruits so that each batch is of the right and highest possible quality.

Moreover, farmers can make necessary changes to their agricultural strategies once they know the quality of fruits from each location, tree, and position.

Monitoring Rind Breakdown Disorder

Figure 2: Spotting due to rind breakdown disorder in mandarins, Magwaza et al., 2012. (Image credits: Postharvest Biology and Technology (74): December 2012, Pages 1-10. https://doi.org/10.1016/j.postharvbio.2012.06.007)

Mature mandarins are susceptible to rind breakdown disorder. Earlier believed to be a fungal attack, scientists now think it is a physiological disorder of the fruits brought on by abiotic factors. It can occur preharvest and postharvest depending on the variety

Preharvest

Preharvest rind breakdown disorder has been observed on Satsuma mandarins in California when fruits turn from green to orange. It occurs when wet weather is combined with sharp drops in temperature, leading to the release of rind oil and cell collapse under the cuticle. Usually, the exposed side of fruits is affected throughout the canopy.

Postharvest

Postharvest rind breakdown disorder occurs in ‘Nules Clementine’ and many other varieties. Preharvest fruit development conditions and certain postharvest management practices can lead to this problem.

Fruits that have grown in shaded portions of the tree without enough sunlight often see the disorder 3-5 weeks after harvest. NIRS has shown that fruits on the outer side of the trees have more dry matter, carbohydrates, and phenolic acids than fruits from inner canopies. These fruits also have a thinner rind.

Post-harvest practices like excessive or lengthy degreening by ethylene, postponing cooling, and prolonged cold storage can also cause this problem.

Packing houses and farmers should cull fruits that already have, or could develop, rind breakdown disorder. Spoiled fruits are easy to recognize by spotting. Small and light-colored fruits tend to be prone to rind breakdown disorder, so they should be culled also.

Early detection of rind breakdown disorder is also possible with Vis-NIRS by using total carbohydrates as the parameter.

Monitoring Quality

TSS, DM, and TA are the quality parameters that are widely used in quality control of mandarins around the world at various stages of the supply chain.

Packing houses usually test fruits before shipping to ensure that the minimum TSS standards are met and that TA is not above prescribed levels.

With NIRS it is possible to predict internal eating qualities of mandarins with TSS and DM. However, by using the full visible and NIR range, scientists were able to also predict other quality parameters such as TA and firmness in addition to TSS and DM.

Best Practices

To optimize the quality of mandarin to meet customer satisfaction, there are several measures that farmers and suppliers can adopt.

Recommendations for Farmers

Farmers can consider the following suggestions:

• Thinning of the canopy is advised, as this allows enough sunlight throughout the canopy, leading to fruits that have higher levels of carbohydrates, phenols, and dry matter. These fruits will also have a thicker rind. All these factors will help prevent the development of rind breakdown disorder postharvest.
• Thinning of fruit load will eliminate the occurrence of small fruits. Fewer fruits can accumulate more TSS and DM, resulting in higher quality and a lower chance of developing rind breakdown disorder.
• Check for calcium and magnesium in leaves after fruit set and during fruit development. If deficiency is detected, apply foliar sprays to prevent rind breakdown disorder.

Recommendations for Suppliers

Suppliers can control the quality of their mandarins by maintaining quality and preventing the occurrence of pathogens and other problems. To prevent rind breakdown disorder, packinghouses should

• Carefully monitor the use of ethylene for degreening mandarins.
• Store mandarins in cold rooms as soon as possible, but avoid keeping them in storage for excessively lengthy periods.

Portable Vis-NIRS Tools

For quality, health, and maturity checks of mandarins, it is not just the use of NIR, but the whole Vis-NIR range that is recommended in spectroscopy. People can get analysis results in real-time by using portable Vis-NIRS and can make decisions on the go. Fruit production can be improved by increasing the percentage of high-quality produce and reducing waste. In this way, all stakeholders can increase their returns on investment.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature image courtesy of Motion Array

Sources

Adaskaveg, J.E. (2005). Etiology and management of rind breakdown of mandarins. Retrieved from https://www.citrusresearch.org/wp-content/uploads/9-2005-ANR-Adaskaveg-Etiology.pdf

Antonucci, F., Pallottino, F., Paglia, G. et al. Non-destructive Estimation of Mandarin Maturity Status Through Portable VIS-NIR Spectrophotometer. Food Bioprocess Technol 4, 809–813. https://doi.org/10.1007/s11947-010-0414-5

Codex Alimentarius Commission. (2011, May 9-13). Joint FAO/WHO food standards programme codex committee on food labelling- Justification regarding the use of ethylene for the ripening of fruit. Retrieved from http://www.fao.org/tempref/codex/Meetings/CCFL/ccfl39/fl39_07e.pdf

Cronje. P., Huysamer, M., & Barry, G. (2011). Postharvest rind breakdown (RBD) of ‘Nules Clementine’ mandarin: Symptom development and factors affecting incidence.  Postharvest Biology and Technology, 60(3): 192-20.doi: 10.1016/j.postharvbio.2011.01.009

Gómez, A. H., He, Y., & Pereira, A. G. (2006). Non-destructive measurement of acidity, soluble solids and firmness of Satsuma mandarin using Vis/NIR-spectroscopy techniques. Journal of Food Engineering, 77(2), 313-319. doi: 10.1016/j.jfoodeng.2005.06.036

Guthrie, J. A., Walsh, K. B., Reid, D. J., & Liebenberg, C. J. (2005). Assessment of internal quality attributes of mandarin fruit. 1. NIR calibration model development. Australian Journal of Agricultural Research, 56(4), 405. doi:10.1071/ar04257

Guthrie, J. A., Reid, D. J., & Walsh, K. B. (2005). Assessment of internal quality attributes of mandarin fruit. 2. NIR calibration model robustness. Australian Journal of Agricultural Research, 56(4), 417. doi:10.1071/ar04299

Magwaza, L. S., Opara, U. L., Terry, L. A., Landahl, S., Cronje, P. J., Nieuwoudt, H., . . . Nicolaï, B. M. (2012). Prediction of ‘Nules Clementine’ mandarin susceptibility to rind breakdown disorder using Vis/NIR spectroscopy. Postharvest Biology and Technology, 74, 1-10. doi:10.1016/j.postharvbio.2012.06.007