Thin monolithic slow-release devices for optimum in-package preservation of export table grape varieties

Opperman, Willem Jacobus (2002-03)

Thesis (PhD)--Stellenbosch University, 2002

Thesis

ENGLISH ABSTRACT: Prototypes of a new polymer S02 gas-generating sheet for the control of Botrytis cinerea during the post-harvest storage of table grapes, were developed and manufactured for evaluation using a pilot scale production plant. Attention was paid to the appearance of the sheet, in order to make it technologically efficient as well as aesthetically acceptable to both industry and consumers. The storage quality of semi-commercial export consignments of various cultivars table grapes packed with the monolithic thin-film polymer S02 slow release sheet, was evaluated and compared to results obtained using the locally manufactured Uvasys S02 sheet. The following were investigated: the efficacy of the new polymer sheets in controlling storage decay, the stage at which S02 damage is manifested on table grapes, the level of S02 damage associated with different S02 concentrations, whether S02 damage is manifested more readily at a particular position on the bunch, and the possible effect of an increase in storage temperature, from an initial storage at -O.5°C to 10°C, on the levels of S02 bleaching. Results showed that the new polymer S02 sheet compared favourably with the existing, commercially available Uvasys S02 sheets. The exact S02 concentration required for effective decay control varied for different cultivars, as well as for the different types of grape packages. The S02 concentration incorporated within the sheet was shown to be lower for grapes packed in non-perforated bags, and slightly higher for those in perforated bags. Differences between cultivars occurred with regard to the level of control and the levels of S02 damage. Levels of S02 damage were also significantly affected by the storage period and temperature fluctuations. No significant differences in the levels of decay development and S02 damage were observed in relation to the orientation of the bunches in the carton. The extent of damage incurred to grape tissue by the absorption of S02 gas was determined by low-temperature scanning (LTSEM) and transmission electron microscopy (TEM) techniques. LTSEM and TEM micrographs of areas damaged by S02 gas revealed that exposure to S02 gas may lead to plasmolysis and the loss of cellular fluids. Although damage to the cell walls, cell wall structures and cell membranes, caused by S02 gas, was more prominent in the tissue layers nearer to the fruit surface, damage also occurred to a lesser extent in deeper tissue layers. S02 gas release-rate studies of polymer S02 sheets containing various concentrations Na2S205 revealed that levels of S02 gas emitted depended largely on the levels of Na2S205 incorporated into the sheets. Higher levels of S02 gas were released with the polymer sheets of higher concentrations Na2S205. The release curve for the commercial Uvasys S02 sheet was very different to that of the polymer sheets, with much higher levels of S02 gas emitted initially by the Uvasys S02 sheet compared to the polymer sheets, while the polymer sheets emitted low levels of S02 gas for longer periods compared to the Uvasys S02 sheet. The manufacturing process and the pilot scale production plant that was developed and constructed was successfully used to manufacture polymer S02 generating sheets that are technically sound and efficient, and aesthetically acceptable to industry. The efficacy of such sheets, regarding levels of decay control and S02 damage, was similar to that obtained with the presently available, commercially used Uvasys S02 sheet.

AFRIKAANSE OPSOMMING: 'n Nuwe polimeriese S02-gasvrystellingsvel vir die beheer van Botritis cinerea gedurende die na-oes opberging van tafeldruiwe is ontwikkel en vervaardig. 'n Nuwe loodsaanleg is spesiaal vir hierdie doel ontwerp en gebou. Aandag is geskenk aan die voorkoms van die velle aangesien dit belangrik is dat die nuwe velle beide tegnologies effektief en esteties aanvaarbaar moet wees vir die sagtevrugtebedryf en verbruikers. Die opbergingskwaliteit van semi-kommersiële uitvoerbesendings van verskeie kultivars tafeldruiwe, verpak met die nuwe monolitiese S02- gasvrystellingsvelle, is bepaal. Die volgende is ook bepaal: die effektiwiteit van die nuwe polimeriese velle, die stadium waarby S02-skade op die druiwe duidelik word, die vlak van S02-skade wat met verskillende konsentasies S02-gas geassosieer is, die moontlike invloed wat 'n toename in temperatuur (vanaf -0.5° tot 1DOC) op die verbleiking deur S02 sal hê, en of die S02-skade by voorkeur in 'n sekere posisie op die druiwe sal plaasvind. Die nuwe S02-vel het baie goed vergelyk met die kommersieël beskikbare Uvasys S02-vel. Die S02-konsentrasie benodig vir die effektiewe beheer van Botritis cinerea beskadiging het egter van kultivar tot kultivar verskil. Die keuse van die tipe verpakking, geperforeerd of ongeperforeerd, het ook 'n rol gespeel. Die konsentrasie S02-gas benodig vir effektiewe beheer was laer wanneer die druiwe in die nie-geperforeerde sakke verpak was. Vlakke van S02-skade is ook noemenswaardig beïnvloed deur die opbergingsperiode en variasies in temperatuur. Daar was geen duidelike verskil in die ontwikkeling van bederf en S02-skade ten opsigte van die posisie van die trosse in die karton nie. Die mate van S02-skade aan vrugweefsel is deur middel van laetemperatuurskandeerelektronmikroskopie (LTSEM) en transmissieelektronmikroskopie (TEM) bepaal. Daar is bevind dat die blootstelling aan S02 moontlik tot plasmolise en die uitlek van sellulêre vloeistof kon lei. Alhoewel S02-skade aan die selwande en membrane meer prominent in die weefsel naby die oppervlak van die vrug was, het skade ook in die onderliggende lae plaasgevind. Die vlakke van vrygestelde S02-gas het grootendeels afgehang van die konsentrasie natriummetabisulfiet in die velle. Die S02-vrystellingskurwe van die nuut ontwikkelde polimeriese S02-velle het baie verskil van dié van die Uvasys vel. Laasgenoemde lewer aanvanklik 'n hoë konsentrasie vrygestelde S02-gas vir 'n kort periode, gevolg deur baie lae S02 vlakke daarna, terwyl eersgenoemde 'n laer aanvanklike S02 vrystelling het, gevolg deur vergelykederwys hoër S02 konsentrasies daarna. Die ontwikkelde vervaardigingsproses en die loodsaanleg wat daaruit voortgevloei het is dus suksesvol aangewend om goeie polimeriese S02- vrystellingsvelle te vervaardig. Hierdie velle is tegnies effektief vir die beheer van Bofrifis cinerea gedurende die na-oes verpakking van tafeldruiwe en is esteties aanvaarbaar vir die Suid Afrikaanse sagtevrugtebedryf.

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