processes Review A Study of Catechin Photostability Using Photolytic Processing Jeu-Ming P. Yuann 1, Shwu-Yuan Lee 2, Meei-Ju Yang 3, Shiuh-Tsuen Huang 4,5, Chien-Wei Cheng 1,* and Ji-Yuan Liang 1,* 1 Department of Biotechnology, Ming-Chuan University, Gui-Shan 33343, Taiwan; [email protected] 2 Department of Tourism and Leisure, Hsing-Wu University, New Taipei City 24452, Taiwan; [email protected] 3 Tea Research and Extension Station, Taoyuan 32654, Taiwan; [email protected] 4 Department of Science Education and Application, National Taichung University of Education, Taichung 40306, Taiwan; [email protected] 5 Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung 40200, Taiwan * Correspondence: [email protected] (C.-W.C.); [email protected] (J.-Y.L.); Tel.: +886-3-3507001 (ext. 3772) (C.-W.C.); +886-3-3507001 (ext. 3958) (J.-Y.L.) Abstract: Catechin exhibits numerous physiological characteristics. In this study, we determined the photosensitivity of catechin to various lights under alkaline conditions, and the mechanisms by which catechin generates free radical species and polymerizes via a photoreaction. In addition to this, the application of catechin photolysis was investigated. A solution of catechin is transparent, but turns yellowish under blue light illumination (BLI) in neutral or weak alkaline solutions. When catechin is •− subjected to BLI, a dimeric catechin (proanthocyanidin) and a superoxide anion radical (O2 ) are generated in a photolytic reaction. When ascorbic acid or gallic acid is added to catechin and the •− mixture is subjected to BLI at alkaline pH, fewer catechin dimers and less O2 are produced, because both acids inhibit the photosensitive oxidation of catechin. When AlCl3 is added to catechin and the mixture is subjected to BLI at pH 8, a photolytic reaction is suppressed by AlCl3, and AlCl3 acts as Citation: Yuann, J.-M.P.; Lee, S.-Y.; a catalyst for the disconnection of proanthocyanidin during photolysis. Under alkaline conditions, •− Yang, M.-J.; Huang, S.-T.; catechin generates O2 via photosensitive oxidation, which suppresses the growth of Acinetobacter Cheng, C.-W.; Liang, J.-Y. A Study of baumannii (A. baumannii) by at least 4 logs, and deactivates its multi-drug-resistant strain. This study Catechin Photostability Using shows that catechin photolysis is a process of oxidation, and that it can be safely applied as a tool for Photolytic Processing. Processes 2021, environmental applications. 9, 293. https://doi.org/10.3390/ pr9020293 Keywords: aluminum; catechin; light; proanthocyanidin; tea Academic Editor: Dariusz Dziki Received: 11 January 2021 Accepted: 29 January 2021 1. Introduction Published: 3 February 2021 Phenolic compounds are secondary metabolites within plants, and most are polyhy- Publisher’s Note: MDPI stays neutral droxyl examples. Catechin is a member of the plant polyphenols, and it mainly exists in with regard to jurisdictional claims in chocolate, grapes, green tea, and wine. The polyphenol content in green tea leaves is high, published maps and institutional affil- at approximately 10–30% by dry weight [1]. In green tea leaves, 80% of all polyphenols iations. are catechin compounds, and these feature anti-oxidation, anti-aging, and anti-bacterial activities [2–4]. Polyphenols are composed of an aromatic component that is attached to one or more hydroxyl groups, including diverse phenolic compounds [5]. Catechin is a flavin compound, and its main frame is attached to five hydroxyl groups, Copyright: © 2021 by the authors. as shown in Figure1. Depending on the C3 position of catechin and the mechanism by Licensee MDPI, Basel, Switzerland. which esterification occurs in gallic acid, there are two types of catechin-like compounds: This article is an open access article non-esterified catechins (non-gallate-type), including catechin and epicatechin (EC), and es- distributed under the terms and terified catechins (gallate-type), including epigallocate catechin (ECG) and epigallocatechin conditions of the Creative Commons gallate (EGCG) [6]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Processes 2021, 9, 293. https://doi.org/10.3390/pr9020293 https://www.mdpi.com/journal/processes ProcessesProcesses2021 2021, ,9 9,, 293 x FOR PEER REVIEW 22 of 1212 Figure 1. The structure of catechin. Figure 1. The structure of catechin. Catechin can be easily oxidized, as it is not chemically stable in an aqueous solution [7]. Catechin can be easily oxidized, as it is not chemically stable in an aqueous solution During the oxidation process, a significant change in pH causes a catechin-containing bev- [7]. During the oxidation process, a significant change in pH causes a catechin-containing erage to undergo a non-enzymatic browning reaction [8]. Research has also reported that beverage to undergo a non-enzymatic browning reaction [8]. Research has also reported green tea catechins are unstable, and are degraded almost entirely after a few minutes in anthat alkaline green solutiontea catechins (pH >are 8), unstable, but are relatively and are stabledegraded in acidic almost conditions entirely (pHafter < 4)a few [9]. Theminutes radical-scavenging in an alkaline solution ability of (pH catechin > 8), but compounds are relatively is a feature stable ofin tea,acidic but conditions tea beverages (pH are< 4) unstable [9]. The whenradical-scavenging bottled during ability manufacturing, of catechin becausecompounds catechin is a feature is unstable of tea, [10 but]. The tea oxidativebeverages brown are unstable reaction when of green bottled tea during catechin ma cannufacturing, affect the tastebecause of the catechin tea [8]. is Ascorbic unstable acid[10]. isThe often oxidative added tobrown bottled reaction tea in of order green to preventtea catechin oxidation can affect and thethe browningtaste of the reaction tea [8]. ofAscorbic the polyphenols; acid is often however, added to the bottled taste istea changed in order by to theprevent addition oxidation of the and acid, the and brown- thus sodiuming reaction carbonate of the ispolyphenols; added in order however, to adjust the thetaste pH is changed and maintain by the the addition flavor [of4, 11the]. acid The, inhibitionand thus sodium of the non-enzymatic carbonate is added browning in order reaction to adjust in beverages the pH containingand maintain catechin the flavor com- pounds[4,11]. The is important inhibition for of the the tea non-enzymatic industry. In this browning study, we reaction determined in beverages the mechanism containing for photosensitivitycatechin compounds in catechin, is important based onforthe the instability tea industry. of catechin, In this study, which we was determined demonstrated. the mechanism for photosensitivity in catechin, based on the instability of catechin, which 2.was Photoreaction demonstrated. of Catechin 2.1. Effect of Light Quality on the Photoreaction of Catechin 2. PhotoreactionWhen produced of Catechin using a process that does not involve fermentation, the catechin compounds in green tea remain almost intact. Research reported that there is a color change2.1. Effect when of Light green Quality tea is on left the inPhotoreaction a lit room orof Catechin in the dark for 72 h. A sample in the lit roomWhen had a produced darker color, using but a sampleprocess inthat the does dark not showed involve no significantfermentation, change the incatechin color; therefore,compounds light in chemicallygreen tea remain affects thealmost ingredients intact. ofResearch green tea reported beverages that [12 there]. is a color changeProanthocyanidin when green tea is composedleft in a lit room of catechin. or in the Hayashi dark for et 72 al. h. noted A sample that in the the proantho- lit room cyanidinhad a darker in fresh color, purple but ricea sample is sensitive in the to dark light irradiation,showed no andsignificant that the change color of in purple color; ricetherefore, darkens light because chemically an internal affects molecularthe ingredients bond of is green formed tea whenbeverages proanthocyanidin [12]. is oxidizedProanthocyanidin [13]. Catechin is alsocomposed sensitive of catechin to ultraviolet. Hayashi (UV) et light, al. noted which that changes the proantho- its struc- ture.cyanidin Forest in etfresh al. studiedpurple rice the UV-inducedis sensitive to epimerization light irradiation, of EC, and and that showed the color that of catechin purple photolysisrice darkens undergoes because reversiblean internal photoisomerization molecular bond is to formed produce when EC [14 proanthocyanidin]. is oxidizedMuch [13]. of the Catechin chemistry is ofalso catechin sensitive photolysis to ultraviolet is illustrated (UV) by light, the generation which changes of reactive its Bstructure. ring quinone Forest methide et al. studied intermediates the UV-ind [15].uced A study epimerization reported that of EC, the Cand ring showed within that the flavanolscatechin photolysis of catechin undergoes and EC dissolved reversible in photoisomerization methanol can be opened to produce after EC irradiation [14]. by exposureMuch to of ultraviolet the chemistry C for of 20 catechin h [15]. photolysis Shi et al. alsois illustrated noted that by catechinthe generation and EC of are re- sensitiveactive B ring to ultraviolet quinone methide B illumination, intermediates which causes[15]. A these study two reported compounds that the to C form ring yellowwithin productsthe flavanols [3]. Theof catechin illumination and EC of