Research On The Effect And Degradation Mechanism Of Sulfur Dioxide Fumigation And Desulfurization Applied To Postharvest Litchi

Fresh litchi?Litchi chinensis Sonn.?fruit is a precious subtropical or tropical fruit with the reputation of‘the delicious fruit in the south of the Five Ridges',which originated in southern China.Both litchi cultivation area and total output of China are the first in the world,but the export volume is relatively inadequate,mainly due to the difficult preservation,the high cost and the increasing harsh and complex foreign quarantine inspection standards of postharvest litchi fruit and other reasons.Currently,sulfur dioxide fumigation treatment is the most often used method in the preservation of litchi fruit,which haseveral advantages of long aging time,low cost,and easy to industrial operation and so on.However,litchi fruits treated with SO₂ fumigation are prone to go wrong with excessive sulfite residue and quality losses.In order to reduce the amount of sulfite residues while keeping the quality of fumigating fruits,the technology and mechanism of desulfurization treatment was studyed,which can reduce the adverse effects of negative effects on litchi fruits.As an air pollutant,SO₂ will produce physiological damage on plant.The response to SO₂ and detoxification mechanism of plant has been studied in-depth in Arabidopsis thaliana,poplar and other plants.When applied to the storage and preservation of fresh fruit such as litchi,longan and grape,SO₂ had a significant preservation effect.The SO₂ content in the fruit during storage was also change with the prolongation of the storage period,but there are few studies on the SO₂ response mechanism of these fruits.Thus,it is necessary to study the metabolic mechanism of SO₂ in litchi fruit after treated with sulfur dioxide fumigation or desulfurization and to provide theoretical basis for optimizing the method of sulfur treatment.This is of great significance to the sustainable development of whole litchi industry.In this paper,the effect of desulfurization on the storage and quality of litchi fruit was studied by using Heiye?Litchi chinensis Sonn.cv.Heiye?and Feizixiao?Litchi chinensis Sonn.cv.Feizixiao?litchi fruit.At the same time,the differences in the degradation of SO₂in fruits after sulfur dioxide fumigation and desulfurization were compared.The results are as follows:1.Compared with the SO₂ fumigation treatment,the browning index,C*value,peel water content,TSS content and respiration intensity of the peel were not significantly different from those of the two varieties of litchi fruit during the storage process.The conductivity of peel and a*Value increased significantly;L*value,fruit decay rate decreased significantly.The results showed that the desulfurization treatment could reduce the adverse effect of the smoked sulfur treatment on the appearance quality of the fruit,keep the commodity value of the fruit and keep the similar storage effect with the sulfur treatment.2.Determination of sulfite residues in peel and pulp found that desulfurization treatment can effectively control the amount of sulfite residues in Heiye and Feizixiao fruits while prolonging their storage period.After desulfurization,the amount of sulfite residues in the pulp was already reached the limits of sulfite residue?10 mg/kg?of United States Food and Drug Administration?FDA?on the quality at the 8th day and decreased from11.82 mg kg and 7.35 mg/kg to 1.39 mg/kg and 1.74 mg/kg on the 48th day of storage respectively.At the same time,compared with only SO₂ fumigation treatment,fruits treated with desulfurization smell without off-flavor,and the TSS was no significant change.The quality of fruit is effectively preserved.3.when analysis several enzyme activities of SO₂ metabolic pathway and their correlation relationship with the sulfite residues of the two cultivars after SO₂ fumigation or desulfurization treatment,we found that:The activities of sulfite oxidase?SO?,sulfite reductase?SiR?,acetylserine transferase?SAT?and acetylserinelyase?OAS-TL?in the flesh of two cultivars increased,and adenosine-5'-phosphoryl sulfate reductase?APR?activity decreased first and then increased.There were significant positive correlations with the activities of SiR,SO,SAT and OASTL in the fleshy sulphite residues of two cultivars,and also significantly correlated with the APR activity in Feizixiao.These results indicate that the excess SO₂ is mainly produced by the addition of SO by increasing the oxidation process excess SO₂ after treatment was mainly removed by increasing the oxidation process,and combined with several enzymes included APR,SiR,SAT and OAS-TL of reduction process.Under the role of these enzymes,SO₂ entered the organic sulfur metabolic pathway and formed sulfuric compounds such as Cys and GSH,thereby reduces the amount of sulfite residues and relieved the SO₂ damage to the cells,and desulfurization treatment reduced the impact of SO₂ on litchi fruit.4.In the relative quantification of genes of enzymes that encode sulfite oxidase?SO?sulfite oxidase?SO?,sulfite reductase?SiR?,acetylserine transferase?SAT?,acetylserinelyase?OAS-TL?and adenosine-5'-phosphoryl sulfate reductase?APR?,there were some differences in the two litchi cultivars.The effect of SO₂ on the early storage time of Heiye pulp was down-regulated,and was up-regulated in the later stage and these genes that encode the correspondi ng proteinases did not regulate the activity of the corresponding enzymes at the transcriptional level.But it was opposite in the Feizixiao,in addition to LcSiR for increasing in pre-strorage time,the rest were all down.And only SO and SAT regulated the activity of the corresponding enzymes by upregulating mRNA levels to remove excess SO₂,while the other genes may be regulated at other levels.Whether in Heiye or Feizixiao,desulfurization treatment attenuated the effect of SO₂ on the relative expression of these genes,but did not affect the degradation of SO₂.In summary,the desulfurization treatment can reduce the harm caused by the sulfur fumigation treatment and keep the value of fruit products.At the same time,it can extend the fruit storage period while reduced the content of sulfite in the fruit.The excess SO₂ in the treated fruit was degraded and metabolized into organic matter under the action of sulfur metabolism oxidative pathway SO and reduction pathway APR,SiR,SAT and OAS-TL.