Postharvest application of edible coatings to reduce quality losses and prolong shelf life in plums

Thesis (MScFoodSc)--Stellenbosch University, 2019.

ENGLISH ABSTRACT: The Japanese plum (Prunus salicina Lindl.) is one of the most popular stone fruits consumed worldwide, characterised by its distinctive taste and high nutritional value. However, postharvest losses limit the economic value of exported plums, with the long handling chain of export and sale often resulting in shrivel, overripeness and decay. Whilst postharvest technologies such as low temperature storage and high density polyethylene (HDPE) bags are used to delay ripening and minimise moisture loss in exported plums, the incidence of fruit rejection on account of quality- related issues is still unfavourably high. Edible coatings have been widely reported to maintain postharvest quality in fresh produce. Coatings form a semi-permeable barrier on the fruit surface that controls moisture loss and gaseous exchange, consequently delaying ripening and extending shelf life. This study aimed to investigate the potential of edible coatings to improve the export quality of plums by controlling postharvest losses and extending shelf life. In a laboratory-scale trial, six edible coatings, four of which were experimental (alginate, chitosan, gellan gum and gum arabic) and two commercial (High shine and Sta-fresh), were screened during a simulated shipping period (-0.5 ± 2°C and 90 ± 5% RH for five weeks) and a subsequent shelf life period (20 ± 2°C and 80 ± 5% RH for 20 days) using ‘African Delight™’ plums. Gum arabic performed best out of all the coatings. At 20 d shelf life, weight loss was significantly (p<0.05) reduced from 23.62% in control plums to 5.36% in plums coated with gum arabic. Similarly, shrivel was significantly (p<0.05) reduced from 9.56% (control) to 4.91% (gum arabic), and decay was significantly (p<0.05) reduced from 7.57% (control) to 1.19% (gum arabic) at 20 d shelf life. Additionally, plums coated with gum arabic exhibited a delay in physico-chemical changes during storage, such as fruit softening, loss of acidity and darkening of the peel colour. These changes were delayed as a result of suppressed respiration and ethylene production, which consequently reduced the rate of fruit ripening. At 20 d shelf life, plums coated with gum arabic resembled control plums at 5-10 d shelf life, indicating an extension of shelf life as a result of coating application. Furthermore, a volatile analysis confirmed that coating application did not result in the formation of off-flavours. In order to validate the commercial viability of coating application and optimise coating formulation, gum arabic (GA) based coatings, including GA 2%, GA 5%, GA 10%, GA 5% + pomegranate seed oil and GA 5% + ascorbic acid, were applied to ‘African Delight™’ plums in a commercial packhouse. Commercial pack lines are generally equipped with atomizers to apply postharvest solutions such as fungicides; therefore, coating application on a commercial-scale is highly viable and would not require additional infrastructure. Fruit were subjected to real-life postharvest handling practices, and quality was assessed during a simulated shipping period (-0.5 ± 2°C and 90 ± 5% RH for six weeks) and a subsequent shelf life period (20 ± 2°C and 80 ± 5% RH for 15 days). The best performance was achieved with GA 10%, which resulted in a significant delay in physico-chemical changes during storage such as fruit softening, loss of acidity and darkening of the peel colour. These changes were delayed as a result of suppressed respiration and ethylene production, which consequently reduced the rate of fruit ripening. Plums coated with GA 10% were described by a trained sensory panel during descriptive sensory analysis as having unripe to semi- ripe sensory attributes at 5 d shelf life, compared to control plums which were characterised with a ripe to overripe sensory profile. This suggests that GA 10% could extend the shelf life of ‘African Delight™’ plums beyond the current five day end point of commercial sale. This observation was confirmed in the instrumental measurements, where fruit coated with GA 10% retained firmness (14.04 N), peel colour (L* = 34.67 and h° = 6.37) and TSS (15.60 °Brix) after 15 d at shelf life conditions. No off-flavours were detected in the sensory analysis as a result of coating application. Plums coated with GA 10% were also found to be microbially safe at 5 d shelf life, with no faecal coliforms detected and total coliforms falling within specified limits. Furthermore, coatings exhibited potential as a green replacement technology for HDPE bags. No significant difference (p≥0.05) in respiration rate was observed between coated fruit packed without HDPE and control fruit packed with HDPE bags at the end of cold storage. Thus, coatings may have created a similar modified atmosphere in plums as that created by the HDPE bags within the carton, which resulted in comparable physico-chemical changes during cold storage. Although the commercial viability and technological readiness of GA 10% as a postharvest edible coating is limited in this study, the use of gum arabic in postharvest edible coating application is promising.

AFRIKAANSE OPSOMMING: Die Japanese pruim (Prunus salicina Lindl.) is een van die mees gewildste steenvrugte in die wêreld, as gevolg van die vrug se aangename smaak en hoë voedingswaarde. Ongelukkig is na-oes verlies ‘n groot uitdaging in uitvoerpruime met lang stoor periodes, soos verrimpeling, oorrypheid and bederf. Alhoewel na-oes tegnologie soos koelopberging en Hoë Digtheid Poli-Etileen (HDPE) sakke reeds gebruik word om die rypwordingsproses en vogverlies te vertraag, is die gehalteverlies steeds ongunstig hoog. Daar word wyd berig dat eetbare bedekkings die na-oes kwaliteit van vars produkte onderhou. Hierdie bedekkings vorm 'n semi-deurlaatbare versperring op die vrugoppervlak wat vogverlies en gaswisseling beheer. Gevolglik word rypwording vertraag en die rakleeftyd verleng. Die doel van hierdie studie was om die potensiaal van eetbare bedekkings te ondersoek om die uitvoergehalte van pruime te verbeter en die rakleeftyd te verleng. Ses eetbare bedekkings waarvan vier eksperimenteel was (alginaat, chitosan, gellangom en arabiese gom) en twee van kommersiële oorsprong was (High shine en Sta-fresh) is op laboratoriumskaal beproef tydens ‘n gesimuleerde versendingstydperk (-0.5 ± 2°C en 90 ± 5% RH vir vyf weke) en daaropvolgende rakleeftyd (20 ± 2°C en 80 ± 5% RH vir 20 dae). ‘African Delight™’ pruime was gebruik om die proef uit te voer. Arabiese gom het die beste in vergelyking met die ander bedekkings presteer. Op 20 d rakleeftyd was gewigsverlies beduidend (p<0.05) verminder van 23.62% in kontrole pruime tot 5.36% in pruime wat met arabiese gom het bedek was (5.36%). Soortgelyk aan bogenoemde was verrimpeling beduidend (p<0.05) verminderd vanaf 9.56% (kontrole) tot 4.91% (arabiese gom) en bederf beduidend (p<0.05) verminderd vanaf 7.57% (kontrole) tot 1.19% (arabiese gom) op 20 d rakleeftyd. Bowendien het pruime wat met arabiese gom bedek was ‘n vertraging in fisisie-chemiese na-oes veranderinge ervaar, soos ‘n afname in fermheid, ‘n verlies in suurheid en ‘n verdonkering van skilkleur. Hierdie veranderinge was vertraag as gevolg van ‘n laer respirasie tempo en etileenproduksie wat gevolglik die tempo van vrugte-rypwording verlaag het. Pruime wat met arabiese gom bedek was op 20 d rakleeftyd was gelykstaande aan kontrole pruime van ‘n 5-10 d rakleeftyd. Na aanleidng van die bevinding, kan eetbare bedekkings dus die rakleeftyd van pruime verleng. ‘n Ontleding van die aromatiese vlugtige stowwe het bevestig dat die bedekkings nie ongewensde af geure in die pruime gevorm het nie. 'n Kommersiële proef was gedoen om te bevestig dat die toepassing van eetbare bedekkings vir die vrugtebedryf lewensvatbaar is. Bedekking formulering was geoptimaliseer deur verskillende variasies van arabiese gom (GA) te ondersoek, insluitend GA 2%, GA 5%, GA 10%, GA 5% + granaatsaadolie en GA 5% + askorbiensuur. Bedekkings was op 'African Delight™' pruime in 'n kommersiële pakhuis met 'n verstuiwer toegepas. Kommersiële paklyne is oor die algemeen toegerus om na-oesoplossings soos swamdoders aan te wend. Die toepassing van bedekkings op vrugte is dus lewensvatbaar en ekstra infrastruktuur onkostes is nie noodsaaklik vir die implimentering daarvan nie. Vrugte was onderworpe aan regte lewe na-oes hantering tegnieke, en vrugkwaliteit was tydensdeur ‘n gesimuleerde versendingstydperk (-0.5 ± 2°C en 90 ± 5% RH vir ses weke) en daaropvolgende rakleeftyd (20 ± 2°C en 80 ± 5% RH vir 15 dae) getoets. GA 10% het die beste uit al die bedekkings presteer. Die bedekking het gelei tot 'n beduidende vertraging in fisiese-chemiese veranderinge tydens opberging, soos ‘n afname in fermheid, ‘n verlies in suurheid en ‘n verdonkering van skilkleur. Hierdie veranderinge was vertraag as gevolg van ‘n laer respirasie tempo en etileenproduksie wat gevolglik die tempo van vrugte-rypwording verlaag het. Pruime met ‘n GA 10% bedekking was deur 'n opgeleide sensoriese paneel tydens beskrywende sensoriese analise beskryf as onryp tot semi-ryp, terwyl kontrole pruime beskryf was as ryp tot oorryp. Dit stel voor dat die GA 10% bedekking wel die rakleeftyd van ‘African Delight™’ pruime kan verleng vir langer as die reedsbestaande eindpunt van vyf dae rakleeftyd van kommersiële pruime. Instrumentale metings het hierdie waarneming bevestig. Pruime wat met GA 10% bedek was het na 15 d rakleeftyd hulle fermheid (14.04 N), skilkleur (L* = 34.67 and h° = 6.37) en TOS (15.60 °Brix) gehandhaaf. Geen af geure was in die beskrywende sensoriese analise opgespoor nie. Pruime met GA 10% bedekking was ook mikrobies veilig teen 5 d rakleeftyd, met geen fekale koliforme nie en met die totale koliforme binne gespesifiseerde perke. Eetbare bedekkings het ook potensiaal vertoon as 'n groen vervangingstegnologie vir HDPE sakke. Geen beduidende verskil (p≥0.05) in respirasie tempo was waargeneem tussen bedekte pruime wat sonder HDPE sakke verpak was en kontrole pruime wat met HDPE sakke verpak was aan die einde van die koelopbergingsperiod nie. Die eetbare bedekkings kon dus 'n soortgelyke atmosfeer rondom pruime geskep het in vergelyking met die HDPE sakke wat uiteindelik gelei het tot vergelykbare fisiese-chemiese veranderinge tydens koelopberging. Alhoewel die kommersiële lewensvatbaarheid en tegnologiese gereedheid van GA 10% as 'n eetbare bedekking beperk is in hierdie studie, is die gebruik van arabiese gom as ‘n na oes eetbare bedekking baie belowend.

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