Sept. 1, 2021
Aug. 17, 2021
Portable devices based on near-infrared and visible light spectroscopy have revolutionized grape analysis in the vineyard, supply chain, and the processing of grape products. These new tools can be used to gather rapid and precise information on the chemical composition, nutritional value, and physiology of grapes. In this article, we provide a brief overview of the many applications of NIR spectroscopy in grape production.
The species vitis vinifera accounts for 90% of the grapes grown globally.
Fifty percent of the grapes produced are used for making wine, thirty percent are eaten as fresh fruits, and the rest is processed to make raisins, juices, jams, etc.
The desired appearance, taste, and chemical composition of grapes will differ depending on its end-use.
Grapes meant for the table are large, attractive looking, thin-skinned, and have more pulp. Table grapes, especially the seedless varieties, have less flavor than wine grapes. Table grapes also have less sugar (about 17-19 Brix) and little acidity.
Grapes meant for winemaking are smaller. They have seeds, thicker skin, and less pulp, but contain more juice, and higher sugar (about 24-26 Brix) and acid content.
Many chemical components in grapes are monitored. Besides the usual total soluble solids (TSS) and titratable acidity (TA) that are measured in all fruits, chemicals that give flavor are also measured. These include anthocyanin, total polyphenols, and many minerals.
Traditionally, grape samples were collected and transported to laboratories for analysis. These methods were time-consuming, expensive, and involved the use of chemicals.
Portable devices that have miniaturized near-infrared (NIR) spectroscopy are now allowing analysis to be conducted onsite from the vineyards to processing facilities.
Figure 1: Spectra signature of a grape cluster, Fernández-Novales, et al., 2019. (Image credits: Molecules 2019, 24(15), 2795; https://doi.org/10.3390/molecules24152795)
NIR is the part of the light spectrum best suited for use in spectroscopy, and it is used to study the chemical composition and concentration of the various compounds. NIR interacts with the chemical bonds within compounds and helps in precise identification. Chemicals absorb, transmit, or reflect the light, and this interaction is measured by spectrometers; see Figure 1.
As NIR spectroscopy can detect a wide range of chemicals in grapes, a single, easy-to-use tool manages to replace an array of complicated laboratory procedures. Based on the results, which are obtained immediately, farmers, suppliers, and processers can make vital management decisions in real-time.
Grapes are non-climacteric and have to ripen on the plant. Therefore, the quality of grapes at harvest is vitally important, and the ripening process is monitored throughout maturation.
The parameters measured on the go in the vineyard are the following:
The non-climacteric grapes’ quality at harvest will determine consumer satisfaction of table grapes, wine, or raisins. Visual examination alone is not precise enough to set harvest time. In this case, internal chemical analysis by NIR is a boon.
Maturity indices that are used to set harvest time are guided by the end-use of the grapes.
The position and orientation of grape clusters on a vine will influence the chemical composition of the commodity, depending on exposure to sunlight; therefore, the NIR spectra obtained from them will also differ. Using this information, farmers can identify grape bunches for early, median, and late stage harvest. This way vineyards can improve and homogenize the quality of grapes, for all purposes.
Figure 2. “Prediction maps of the spatial variability of anthocyanins (A), total soluble solids (B), and total polyphenols concentrations (C) along with the grape ripening period (11 August to 28 September),” Fernández-Novales, et al., 2019. (Image credits: Molecules 2019, 24(15), 2795; https://doi.org/10.3390/molecules24152795)
NIR tools can be used with agricultural machines to analyze the chemical composition of grapes. Based on the analysis, vineyard owners can make spatial maps of grape quality (see Figure 2) and trace this data back to variations in soil and environmental conditions within the vineyard.
For example, the effect of tillage and weeds on the growth of vines and the composition of grapes can be detected by measuring TSS, acidity, anthocyanin, and total phenol contents in grapes.
Spatial maps of the quality of grapes can guide future decisions of farmers and help them to alter their agricultural practices to optimize conditions as needed. Any vineyard can apply these precision farming techniques by using NIR tools, with minimal cash investment.
Since grapes will not ripen or see any further quality improvements after harvest, fresh table varieties must be transported and stored at low temperatures so that they can survive adequately for several weeks. During this time there are four uses of NIR monitoring:
One of the important products made from grapes, raisins are rich in fiber, carbohydrates, and minerals like copper and iron. They also have medicinal value and help in regulating blood pressure. NIR can be used to monitor these important commodities throughout the processing and end product stages.
NIR use in winemaking is not yet widespread, even though there are many stages where its use can be incredibly beneficial, such as
The chemical composition of grapes is used to establish the authenticity and variety of grape products.
The F-750 Produce Quality Meter, manufactured by Felix Instruments - Applied Food Science, can non-destructively measure total soluble solids by BRIX, titrable acidity, and color. It has been tested for seedless grape varieties and has a starter model for Sultana Fresh Table Grapes. Models for other varieties can be easily created using included instructions. With simple operation and sturdy design, it is made to be used throughout the supply chain, giving stakeholders from the vineyard to the processing facility the quantitative edge they need.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Feature image courtesy of brando
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