The potential of dynamic controlled atmospheres and possible mechanisms in mitigating superficial scald in Apples cv. 'Granny Smith'

["Apple industry -- South Africa", "Apple -- scald", "Granny Smith -- Postharvest crop losses", "Dynamic controlled atmosphere (DCA)", "Apples -- Storage -- Diseases and injuries", "UCTD"]

Mditshwa, Asanda ; Opara, Umezuruike Linus

Stellenbosch University

Others  :  https://scholar.sun.ac.za:443/bitstream/10019.1/97690/1/mditshwa_potential_2015.pdf

瑞士|英语

来源: Stellenbosch University

【 摘 要 】

ENGLISH ABSTRACT: The development of a postharvest method for controlling superficial scald,maintaining quality and reducing postharvest losses of 'Granny Smith' apples is essential inmaintaining the competitiveness of the South African apple industry. Previously, the SouthAfrican apple industry relied on diphenylamine (DPA) for controlling scald disorder;however, increasing consumer concerns and reductions in maximum residue levels (MRLs)have highlighted the urgent need for alternative control strategies. Currently, there is noeffective non-chemical method for controlling superficial scald for South African appleproducers. The overall aims of this study were (a) to examine the potential of dynamiccontrolled atmospheres (DCA) in controlling superficial scald in apples, and (b) to investigatethe mechanism of action of DCA in controlling scald, should it be effective.To get a deeper understanding of superficial scald etiology and physiologicaldynamics of apples, studies in paper 2 and 3 were conducted. In paper 2, studies onantioxidants contents and phytochemical properties of apples harvested at pre-optimal andoptimal maturity were conducted. Significant increases in fruit antioxidant capacity andascorbic acid concentration occurred with increasing maturity. Fruit harvested at optimalmaturity had lower total phenolic contents compared to pre-optimal maturity. Phenoliccompounds including catechin and quercetin were also higher in pre-optimal compared tooptimal maturity. In paper 3, an attempt was made to classify apples with different levels ofscald severity based on metabolomics analysis. The results showed that ethylene, α-farnesene, 6-methyl-5-hepten-2-one (MHO) and reactive oxygen species (ROS) increasedwith scald severity but declined in severely scalded fruit. Discriminant analysis successfullyclassified fruit based on scald severity. Ethylene, ROS and lipid peroxidation were identifiedas the major contributors in separating the five scald severity levels studied.Studies in paper 4 focused on whether DCA is effective in controlling superficialscald. The minimum period for the exposure of fruit to DCA before an extended shipmentperiod of 10 weeks was also investigated. The results showed that DCA was highly effectivein controlling scald for both pre-optimal and optimal harvested fruit. The results furtherdemonstrated that DCA stored fruit can be shipped for 6 weeks; however, extending theshipping period up to 10 weeks might lead scald development and undesired fruit quality.Fruit stored in DCA before shipment generally had higher flesh firmness and ground colour.It was also shown that DCA inhibit scald by retarding the accumulation of scald-associated metabolites such as α-farnesene and MHO. Paper 5 focused on the impact of DCA on ROS,antioxidant capacity and phytochemical properties of stored apples. Using principalcomponent analysis, two clusters which could be identified as DCA and RA stored fruit werenoticed. Compared to RA stored fruit associated with higher ROS and lipid peroxidation,fruit stored at DCA was characterized by higher contents of ascorbic acid, total phenolics andantioxidant pool.The research reported in paper 6 investigated the efficacy of repeated application ofDCA on apples with high scald potential. During the marketing season, an unexpecteddemand of fruit often leads to the opening and resealing of storage chambers. Thus, theefficacy of a repeated DCA treatment after an interruption period at RA was investigated.Fruit were stored for up to 16 weeks in DCA with a 14 d interruption in RA at -0.5 °C, 95%RH. The results showed that efficacy of DCA was not significantly affected by theinterruption. However, the development of 1% scald after 4 months of storage could be aneconomic setback for fruit producers. In paper 7, the influence of DCA on aroma volatileswas assessed. DCA stored fruit had significantly lower total amount of volatiles detectedcompared to fruit stored in RA. Notably, the production of 1-butanol, 1-hexanol and 1-hexenolby fruit stored in DCA were only 42%, 38% and 39%, respectively, of the amountsdetected in the RA. The known characteristic flavour of 'Granny Smith' apples was attributedto the production of ethyl-2-methylbutyrate, ethyl hexanoate and hexyl acetate. Thecontribution of these three aroma volatiles was higher with increasing storage duration.In paper 8, the research identified effective variables that could be used to developprediction models for superficial scald incidence in harvested 'Granny Smith' apples.Stepwise multiple regression found MHO, antioxidant capacity (FRAP), ascorbic acid andlipid peroxidation to be the best combination of predictive variables for scald. Aftervalidation, this combination gave a good prediction of scald incidence (R2 = 0.94). Theidentified variables proved to be effective regardless of fruit maturity status. The results fromthis thesis provide an alternative non-chemical postharvest technology for the South Africanapple industry. The study further provides insights on the mechanism of action of DCA incontrolling scald and maintaining fruit postharvest quality of 'Granny Smith' apples. Overall,the results contained in this thesis will be very instrumental in future optimisation of DCAtechnology in the apple industry, and provides a valuable guide for improved the storage ofapples susceptible to superficial scald.