Scientists discovered that cellulose autofluorescence changes in the presence of tannic acid. Discovery paves the way for the development of technology that detects tannin content in wines. As it is an optical analysis method, the technique has no need for reagents and is faster. Liquid sensor would allow production in portable fluorescence equipment. Tannin is related to different wine properties ranging from aroma and flavor to pigment fixation. Although its presence in the beverage is important, excess tannin is negative. Researchers from Embrapa Instrumentation (SP) and the Federal University of São Carlos (UFSCar) developed an aqueous solution known as liquid sensor, which allowed an important discovery: the fluorescence of cellulose nanocrystals (CNCs) has the potential to monitor tannic acid content in red wines. The solution composed of pure CNC proved to be effective in indicating the presence of tannin, a parameter that is considered fundamental by winemakers in terms of structure and organoleptic characteristics (aroma and flavor) and also helps in the stabilization of the wine color. The finding that cellulose autofluorescence changes in the presence of tannic acid paves the way for the development of optical sensors based on cellulose nanocrystals to monitor tannins, not only in wines, but in food products and other beverages. In a demonstration, as a proof of concept, the study detected the tannin molecule associated with palatability in Cabernet Sauvignon and Tannat wines. The president of the Brazilian Association of Oenology (ABE), oenologist Ricardo Morari, explains that tannins are phenolic compounds that play an important role in the composition, quality and aging potential of red wines. In addition, he says that tannins not only can fix pigments and thus contribute to color stability, but also provide structure and, when they are well integrated and balanced, complexity and elegance to wines. "The possibility of instant detection of this component through a new technology could provide winemakers with important information so that they can better understand the characteristics and composition of the product they are making, in order to extract the maximum potential and quality of this wine," says Morari, who works at the Garibaldi Winery Cooperative located in Garibaldi, Rio Grande do Sul. He reports that tannins are associated with the astringent mouthfeel of beverages like wines. It is what makes the texture of the wine dry, bitter and full-bodied in light of its astringent effect, while it also helps in the conservation of the beverage. They are present in grape skins, seeds and in the bunch stems. However, although it is an important oenological sensory attribute in the determination of wine qualities, when in excess it is considered an antinutrient. "Therefore, determining tannin content in food samples through optical analytical methods – e.g. photoluminescence – can be advantageous, since this type of procedure is fast and eliminates the need for laborious experiments and reagents, while it provides relative sensitivity and selectivity," says postdoctoral scholar Kelcilene Teodoro, who conducted the research under the supervision of Embrapa researcher Daniel Souza Corrêa. For the president of the São Carlos chapter of the Brazilian Association of Sommeliers, Mário Francisco Mucheroni, with the sensors, specialists will be able to have more information for analysis. "Who knows, maybe in the future we can also have information about the origin, seeds, skins and barrels. This separation would be fundamental for the winemaker, in the case of wines," the president states. Another advantage of the sensor being in liquid form is that, in principle, the method can be adapted for portable fluorescence instruments, which would facilitate its use in different locations. According to Teodoro, the scientific literature is scarce with regard to the exploitation of cellulose fluorescent properties, which can be used in chemical sensors. “Generally, while cellulose or nanocellulose are quite versatile, in the field of sensors it is combined with other materials that play the main role in the detection mechanism. Thus, in most applications, nanocellulose works as a support or substratum, while its inherent autofluorescence remains underexplored for sensor applications," the researcher explains. "Our goal was to expand the use of nanocellulose in the field of sensors, finding innovative applications for this material, since we found that its autofluorescence property has the potential to be used as an optical sensor, a phenomenon little reported in the literature," Corrêa reports. Picture above (wine in glasses): Edilson Fragalle Infographic: Kelcilene Teodoro Enzymatic route The study was carried out at the National Laboratory of Nanotechnology for Agribusiness (LNNA), headquartered at Embrapa Instrumentation, in São Carlos, São Paulo, with the aim to investigate the use of the autofluorescence property of cellulose nanocrystals to use it as a tool in the detection of tannins. The study integrates ongoing efforts by a group of scientists to promote new applications for sustainable cellulosic materials in the face of growing demand for optoelectronic devices in an automated scenario. The nanocrystals were obtained through an enzymatic route, which comprises the isolation of cellulose nanostructures through enzyme-mediated treatments, based on bleached rubber wood chips provided by the São Paulo State Agribusiness Technology Agency (APTA), in Colina, São Paulo. According to Teodoro, studies on the synthesis of cellulose nanocrystals (CNC - photo) from enzymatic hydrolysis indicate a trend of lower energy cost associated with experimental conditions of milder temperature and pressure, in addition to the generation of less toxic waste as opposed to acid hydrolysis with mineral acids. Thus they have large potential for sustainability in the process of obtaining nanocelluloses. Photo: Kelcilene Teodoro Potential sensor The researchers tested the applicability of the liquid sensor in beverages obtained from Cabernet Sauvignon and Tannat grapes. The detection process consisted of adding different concentrations of commercial tannic acid to the aqueous suspension of cellulose nanocrystals extracted from the rubberwood. Changes in the fluorescence spectra of the cellulose nanocrystals were monitored immediately after the tannin was mixed with the aqueous suspension. “We found that the photoluminescence from the cellulose nanocrystal aggregates could efficiently identify tannic acid (picture) in different red wines, in concentrations that can be of industrial interest. We also observed that compounds that are normally found in wine sources did not cause significant changes in the detected signal," the researcher explains. The efficiency of applying the sensor to Brazilian wine samples was validated by measuring recovery rates. “In this method, the data obtained for tannic acid in wines were compared to those recorded for tannic acid in water. The results achieved high recovery rates: close to 100% – 99.7% for Cabernet Sauvignon wine and 95.3% for Tannat wine," Teodoro says. For Corrêa, these findings suggest that optical sensors based on cellulose nanocrystals have the potential to be used to monitor the quality of food and beverage products. “Nanocelluloses are traditionally explored as mechanical reinforcement agents, but the gain in sustainability using cellulose to replace toxic, non-renewable or high-cost materials has drawn the attention of the timber and paper industries, and collaborative efforts have been started around the world to develop new environmentally friendly products,” the researcher says. The scientists underscore the additional benefits cellulose gives the sensing platform, such as its non-toxic, biodegradable, versatile and sustainable nature. Based on their sources of origin and physicochemical characteristics, nanocelluloses can be classified as cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial cellulose (BC). The research The study “Exploring the potential of cellulose autofluorescence for optical detection of tannin in red wines” was published online by the journal Carbohydrate Polymers. In addition to Teodoro and Corrêa, co-authors include the Embrapa Instrumentation researchers Maria Alice Martins and Luiz Henrique Capparelli Mattoso, and LNNA and UFSCar scholars Rafaela S. Andre, Maycon J. Silva and Rodrigo Schneider. The study was supported by the National System of Nanotechnology Laboratories (SisNANO), under the Brazilian Ministry of Science, Technology and Innovation (MCTI); the São Paulo State Research Foundation (Fapesp), the National Council for Scientific and Technological Development (CNPq), Coordination for the Improvement of Higher Education Personnel (Capes); the Network on Nanotechnology for Agricultural Research (Rede AgroNano); and Embrapa. Photo: Joana Silva