Sulfur Dioxide （SO₂） Accumulation In Postharvest Table Grapes And Its Effects On Anthocyanin Degradation And Flavor

In the table grape storage and transportation industry,sulfur dioxide（SO₂）plays an important role as a commercialized preservative.Even with the continuous development of grape fresh-keeping technology,SO₂is still irreplaceable in the grape industry.In recent years,issues,such as bleaching of grape peel and off-flavor,caused by improper use of SO₂ have attracted extensive attention to researchers.In this study,four major table grapes cultivars planted in Xinjiang province were used as research objects.The relationship between pedicel structure and the SO₂ accumulation in different parts of grape cluster were investigated.The pathways for diffusion and the sites of accumulation of SO₂ in postharvest grapes after fumigation with SO2were also clarified.The types and content differences of anthocyanins in grape berry peel during storge were analyzed by liquid chromatography-tandem mass spectrometry（LC-MS/MS）.An in vitro simulated system was established to study the kinetics of anthocyanin degradation by SO₂in order to speculate on possible degradation pathways.Meanwhile,gas chromatography-mass spectrometry（GC-MS）was used to analyze the changes of volatile compounds in grape berries after excessive SO₂ fumigation treatment during storage.The aim of this research was to investigate the mechanism of grape peel bleaching and flavor alteration in table grape caused by excessive SO₂ treatment.The main research contents and results were as follows:（1）The pathway of SO₂ diffusion and the site of SO₂ accumulation in grapes were clarified.Four varieties of table grapes cultivated in Xinjiang were treated by excessive SO₂ and the accumulation of SO₂ in different parts of grapes were measured.The results showed that SO₂ accumulation in the pedicels of different grape varieties was significantly higher than that in the rachises and berries.Stomata,lenticels,and slits distributed on the surface of pedicels were the pathways for SO₂ entry into the pedicels of grapes.The morphological structure of grape pedicels did not have a significant effect on SO₂ accumulation,whereas pedicel water content and lenticel area were the main factors responsible for the differences in SO₂ accumulation in pedicels of the four table grape varieties studied.The accumulation of SO₂ may reduce the ability of the siliques to transport assimilated products,and the accumulated SO₂may enter the berries through non-vascular pathways.（2）The process changes of species and contents of anthocyanins in peels of two varieties of table grapes treated by various concentrations of SO₂ were examined by high performance liquid chromatography tandem mass spectrometry（HPLC-MS）.The SO₂ fumigation treatment with different concentrations could reduce the contents of anthocyanins in the peels of both Redglobe and Kyoho grapes.Peonidin-3-O-glucosid and cyanidin-3-O-glucoside were two main components of anthocyanins in the peel of Redglobe,which accounting for 31.68%and 18.27%of the total anthocyanin content,respectively.On the other hand,malvidin3-O-glucoside was the major component of anthocyanins in the peel of Kyoho grape.The ratio of malvidin3-O-glucoside was 18.61%of the total anthocyanin content.（3）The degradation kinetics of three main anthocyanins in the peel of table grape were established.The degradations of cyanidin-3-O-glucoside,malvidin-3-O-glucoside and peonidin-3-O-glucoside by SO₂ under different temperature and concentrations were investigated both in vivo and in vitro.The results showed that the degradation of the three anthocyanins all followed the first-order degradation kinetic model.With the increase of concentration of SO₂,the degradation rates（k）also increased under 25°C.The thermodynamic results showed that the degradation reaction was the slowest and k_0℃was the lowest under 0℃.The anthocyanins degradation mechanism speculated in this study was:with the entry of SO₂ to the grape,the p H value in fruit was changed,which leading to the changes of stability and structure of anthocyanins.The unstable anthocyanins reacted with SO₂ result in bleaching peel.（4）The influence of excessive SO₂ on volatile compounds of table grapes during storage were analyzed by gas chromatography mass spectrometry（GC-MS）.There were 14 acids,7 alcohols,11 aldehydes,5ketones and 6 terpenes in Redglobe grape and in Kyoho grape,13 esters,9 aldehydes,10 alcohols,6 terpenes,11 acids and 6 ketones were detected.Aldehydes and alcohols were the most influential in all detected volatile compounds of Redglobe grape by excessive SO₂ treatment,however esters had the greatest impact on flavor of Kyoho grape.Ethyl acetate,2-hexenal and 2-hexenol were three major volatile compounds influencing the flavor of Kyoho grape,while in Redglobe grape there were only 2-hexenal and 2-hexenol as major volatile compounds affecting the flavor.The contents of total volatile compounds decreased gradually with the increase of storage time.The reduction of volatile compounds was delayed by moderate SO₂ treatment and accelerated by excessive SO₂ treatment.