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

Thesis (PhDAgric)--Stellenbosch University, 2015.

ENGLISH ABSTRACT: The development of a postharvest method for controlling superficial scald, maintaining quality and reducing postharvest losses of ‘Granny Smith’ apples is essential in maintaining the competitiveness of the South African apple industry. Previously, the South African 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 no effective non-chemical method for controlling superficial scald for South African apple producers. The overall aims of this study were (a) to examine the potential of dynamic controlled atmospheres (DCA) in controlling superficial scald in apples, and (b) to investigate the mechanism of action of DCA in controlling scald, should it be effective. To get a deeper understanding of superficial scald etiology and physiological dynamics of apples, studies in paper 2 and 3 were conducted. In paper 2, studies on antioxidants contents and phytochemical properties of apples harvested at pre-optimal and optimal maturity were conducted. Significant increases in fruit antioxidant capacity and ascorbic acid concentration occurred with increasing maturity. Fruit harvested at optimal maturity had lower total phenolic contents compared to pre-optimal maturity. Phenolic compounds including catechin and quercetin were also higher in pre-optimal compared to optimal maturity. In paper 3, an attempt was made to classify apples with different levels of scald severity based on metabolomics analysis. The results showed that ethylene, α- farnesene, 6-methyl-5-hepten-2-one (MHO) and reactive oxygen species (ROS) increased with scald severity but declined in severely scalded fruit. Discriminant analysis successfully classified fruit based on scald severity. Ethylene, ROS and lipid peroxidation were identified as the major contributors in separating the five scald severity levels studied. Studies in paper 4 focused on whether DCA is effective in controlling superficial scald. The minimum period for the exposure of fruit to DCA before an extended shipment period of 10 weeks was also investigated. The results showed that DCA was highly effective in controlling scald for both pre-optimal and optimal harvested fruit. The results further demonstrated that DCA stored fruit can be shipped for 6 weeks; however, extending the shipping 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 principal component analysis, two clusters which could be identified as DCA and RA stored fruit were noticed. 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 and antioxidant pool. The research reported in paper 6 investigated the efficacy of repeated application of DCA on apples with high scald potential. During the marketing season, an unexpected demand of fruit often leads to the opening and resealing of storage chambers. Thus, the efficacy 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 the interruption. However, the development of 1% scald after 4 months of storage could be an economic setback for fruit producers. In paper 7, the influence of DCA on aroma volatiles was assessed. DCA stored fruit had significantly lower total amount of volatiles detected compared to fruit stored in RA. Notably, the production of 1-butanol, 1-hexanol and 1-hexenol by fruit stored in DCA were only 42%, 38% and 39%, respectively, of the amounts detected in the RA. The known characteristic flavour of ‘Granny Smith’ apples was attributed to the production of ethyl-2-methylbutyrate, ethyl hexanoate and hexyl acetate. The contribution of these three aroma volatiles was higher with increasing storage duration. In paper 8, the research identified effective variables that could be used to develop prediction models for superficial scald incidence in harvested ‘Granny Smith’ apples. Stepwise multiple regression found MHO, antioxidant capacity (FRAP), ascorbic acid and lipid peroxidation to be the best combination of predictive variables for scald. After validation, this combination gave a good prediction of scald incidence (R2 = 0.94). The identified variables proved to be effective regardless of fruit maturity status. The results from this thesis provide an alternative non-chemical postharvest technology for the South African apple industry. The study further provides insights on the mechanism of action of DCA in controlling 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 DCA technology in the apple industry, and provides a valuable guide for improved the storage of apples susceptible to superficial scald.

AFRIKAANS OPSOMMING: Die ontwikkeling van ’n na-oes metode vir die beheer van oppervlakkige brandvlek, die behoud van gehalte en die vermindering van na-oes verliese van die ‘Granny Smith’ appel is belangrik as Suid-Afrika kompeterend wil bly in die appelindustrie. In die verlede het die Suid-Afrikaanse appelindustrie difenielamien (DPA) benut vir die beheer van oppervlakkige brandvlek maar die groeiende bekommernis van verbruikers en die vermindering in maksimum residuvlakke het die behoefte aan alternatiewe beheerstrategieë beklemtoon. Daar is tans geen alternatiewe nie-chemiese metodes wat Suid-Afrikaanse appelboere kan gebruik om oppervlakkige brandvlek te beheer nie. Die doel met hierdie navorsing is (a) om die potensiaal van dinamies beheerde atmosfeer (DBA) en die beheer van oppervlakkige brandvlek in appels te ondersoek, en (b) om die meganisme van aksie van DBA te ondersoek, indien dit wel effektief bevind word in die beheer van oppervlakkige brandvlek in appels. Die navorsing wat in Artikels 2 en 3 opgeteken is, is gedoen om ’n dieper begrip van oppervlakkige brandvlek etiologie en die fisiologiese dinamika van appels te bekom. In Artikel 2 is die bevindings oor die chemiese kenmerke van appels wat geoes is by preoptimale en optimale oesrypehied, opgeteken. Betekenisvolle vermederinge in die vrugte se antiodatiewe status en askorbiensuur konsentrasie vind met volwassenheid plaas. Vrugte wat by optimale rypheid geoes word het ’n laer totale fenoliese inhoud vergeleke met vrugte wat by pre-optimale volwassenheid gepluk word. Fenoliese samestellings insluitende catechin en quercetin is ook hoër by volwasse vrugte. In die navorsing wat in Artikel 3 opgeteken is, is daar gepoog om appels met verskillende vlakke van brandvlek deur middel van metabolomiese analise te klassifiseer. Die resultate toon dat etileen, α-farnesene, 6-metiel-5- hepten-2-een (MHO) en die reaktiewe suurstof spesie (RSS) toeneem hoe erger die brandvlek raak maar afneem in erg gebrandvlekde vrugte. Die vrugte is suksesvol geklassifiseer volgens hoe erg die brandvlek voorgekom het deur middel van onderskeidende ontledings. Etileen, RSS en lipied peroksidasie is identifiseer as die hoof bydraers tot die onderskeiding van die vyf brandvlek vlakke wat bestudeer is. In die studie wat in Artikel 4 opgeteken is, is die fokus op of dinamiese beheeranaliese, oppervlakkige brandvlek doeltreffend kan beheer. Die minimum periode vir die blootstelling van vrugte aan dinamies beheerde atmosfeer voor ’n uitgebreide verskepings periode van 10 weke is ook ondersoek. Die resultate toon dat dinamies beheerde atmosfeer hoogs effektief is in die beheer van brandvlek beide in vrugte wat voor die optimale tyd of op die optimale tyd geoes is. Die resultate het ook getoon dat vrugte wat in ’n dinamies beheerde atmosfeer gestoor is, vir ses weke verskeep kan word; maar dat as die periode tot tien weke verleng word, brandvlek kan ontwikkel en die gehalte van die vrugte kan afneem. Vrugte wat voor verskeping in DBA gestoor is, is fermer en die agtergrondkleur is beter behou. Daar is ook getoon dat DBA brandvlek ontmoedig deur om die akkumulasie van metaboliete wat met brandvlek geassosieer word, soos α-farnesene en MHO te verminder. In Artikel 5 is die fokus op die impak van DBA op RSS, antioksidant vermoë en die fitochemiese kenmerke van gestoorde appels. Deur om die hoofkomponente te ontleed is twee groepe wat as DBA en verkoelde lug (RA) gestoorde vrugte identifiseer is, uitgeken. In vergelyking met vrugte wat onder RA toedtande gestoor is, en wat geassosieer was met hoë ROS en lipied peroksidasie, is vrugte wat in DBA gestoor is, gekenmerk deur ’n hoër askorbiensuur inhoud, totale fenoloë en oksidante. In Artikel 6 is die bevindings van ’n ondersoek na die doeltreffendheid van herhaaldelike toepassing van DBA op appels met ’n hoë brandvlek potensiaal, opgeteken. Dit gebeur dikwels dat daar gedurende die markseisoen ’n onverwagte vraag na vrugte ontstaan en dat die stoorkamers dan oopgemaak en weer verseël word. Dus is die doeltreffendheid van herhaaldelike DBA behandeling na ’n periode van RA ondersoek. Vrugte is vir tot 16 weke in DBA gestoor met ’n onderbreking van 14 dae in RA teen -0.5 °C, 95% RH. Die bevindinge het bewys dat die doeltreffendheid van DBA nie merkbaar deur die onderbreking aangetas is nie. Die ontwikkeling van 1% brandvlek na 4 maande in die stoorkamers mag egter deur die produsente as negatief beskou word. In Artikel 7, word die invloed van DBA op die aromtiesevlugtige stowwe geassesseer. Vrugte wat in DBA gestoor is, het ’n merkbaar laer totale hoeveelheid aromatiese vlugtige stowwe getoon, in vergelyking met vrugte wat in ’n RA gestoor is. Die produksie van 1-butanol, 1-heksanol en 1-heksen-ol in vrugte wat in DBA gestoor is, is egter net 42%, 38% en 39%, onderskeidelik van die wat in RA gestoor is. Die kenmerkende geur van ‘Granny Smith’ appels is die gevolg van die produksie van etiel-2- metielbutyraat, etiel heksanoate and heksiel asetaat.Die bydrae van hierdie drie aromatiese vlugtige stowwe was hoër na’n uitgebreide stoortydperk. In Artikel 8 word die navorsing beskryf wat doeltreffende veranderlikes identifiseer wat gebruik kan word om voorspellingsmodelle vir die hoeveelheid van oppervlakkige brandvlek in geoesde ‘Granny appels’ te ontwikkel. Deur stapsgewyse veelregressie is daar gevind dat MHO, antioksidant vermoë, askorbiensuur en lipiede peroksidasie die beste kombinasie is vir die voorspellende veranderlikes vir brandvlek. Nadat dit geldig gevind is, het hierdie kombinasie ’n goeie voorspelling vir die voorkoms van brandvlek (R2 = 0.94) verskaf. Die geïdentifiseerde veranderlikes was effektief vir alle vrugte, sonder inagneming van volwassenheid. Die bevindinge van hierdie navorsing verskaf ’n alternatiewe niechemiese na-oes tegnologie vir die Suid-Afrikaanse appelindustrie. Verder is nuwe insigte bekom in die meganismes van aksie van DBA wat betref die beheer van brandvlek en die behoud van na-oes gehalte van ‘Granny Smith’ appels. In die geheel sal die bevindinge van hierdie tesis bydra tot die toekomstige optimalisering van DBA tegnologie in die appelindustrie, en waardevolle riglyne verskaf word vir die verbetering van die stoor van appels wat vatbaar is vir oppervlakkige brandvlek.

Doctoral