Transport Phenomena in Pomegranate Fruit: Mechanisms of Weight Loss and Control Strategies

Lufu, Robert (2020-04)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The revived interest in this ancient fruit Pomegranate (Punica grantum L) has resulted in increased production, consumption and intensified research owing to its health and nutritional benefits. Pomegranates are considered luxurious fruit that sell well in the higher market segment and there has been a growing demand for high quality, healthy and exotic fruit both for fresh use and for processing into juices and other products. However, the fruit is classified as highly perishable and specifically being prone to moisture loss, irrespective of its thick rind and tough leathery outer skin. Water loss can easily cause a huge financial loss to the industry through direct loss of marketable fresh weight and the associated diminished commercial value of affected fruit. Therefore, the overall aim of this research was to investigate the mechanisms of weight loss in pomegranate fruit by characterising associated structural and quality changes in the fruit, quantifying the water transport properties of fruit tissues, developing a water transport model and to assess various techniques to control fruit water loss. The research methodology followed a multifaceted approach which included the application of imaging and computational techniques in combination with several laboratory experiments. The experimental studies established that ‘Wonderful’ and ‘Herskawitz’ pomegranate cultivars are more susceptible to water loss than ‘Acco’ due to differences in aril-peel ratio and moisture content. The thick non-edible peel (rind) is the major source of water loss compared to the edible portion (arils). Overall, mean fruit water loss was 0.31, 0.34 and 0.26 g cm⁻² for ‘Wonderful’, ‘Herskawitz’ and ‘Acco’, respectively, after 16 d in shelf storage at 23 ⁰C and 58 % relative humidity (RH). Analysis of fruit micro-structures showed that the arils (edible portion) are protected against excessive moistures loss by the inner epidermis membrane (white membrane) covering the aril sacs. This membrane had a lower water permeability (0.14 × 10⁻ ¹¹ kg m⁻¹ pa⁻¹ s⁻¹) compared to the exocarp (1.31-1.43 × 10⁻¹¹ kg m⁻¹ pa⁻¹ s⁻¹) and mesocarp (13.51-16.37 × 10⁻¹¹ kg m⁻¹ pa⁻¹ s⁻¹) peel tissues under ambient conditions (23 ⁰C and 58 % RH). Transpiration is the major process of water loss in pomegranate fruit; however, this study showed that water loss due to the respiratory process contributed up to 35 at high 93 % RH. The qualitative studies on fruit peel microstructure using scanning electron microscopy (SEM) identified a higher count of lenticels, larger lenticel size and a generally low peel thickness on the top and mid spatial locations along the fruit surface as compared to the bottom locations. Furthermore, X-ray examination revealed higher porosity in the exocarp than in the mesocarp peel fractions and that porosity increased from bottom to middle to top locations in the exocarp fraction. This suggests that the pomegranate fruit is more susceptible to moisture loss at the top and mid locations compared to the bottom location. In addition, a distinctively bright waxy cuticle was identified on the surface of the peel using confocal laser scanning microscopy (CLSM). A decreasing profile of the waxy cuticle thickness, increased fragmentation of waxy cuticle, widening and deepening of micro-cracks and general decrease in peel thickness were observed during fruit storage. These micro-structural features predispose the fruit to increasing water loss during storage. The study has also demonstrated the use of non-invasive technology such as magnetic resonance imaging (MRI) to assess transient water profiles across intact fruit. Furthermore, the study developed and validated a water transport model that can be used to study water loss and control strategies in future. Assessing fruit weight loss control strategies proved that surface waxing is a potential environmentally friendly solution for minimising water loss in pomegranates. However, modelling of gas transport is required for accurate optimisation of surface waxing application. Future research should also consider coupling of water transport model and mechanical deformation model to account for shrinking (also referred to as shrivelling, wilting) of the fruit due to moisture loss.

AFRIKAANSE OPSOMMING: Die herleefde belangstelling in hierdie antieke vrugtegranaat (Punica grantum L) het gelei tot verhoogde produksie, verbruik en intensiewe navorsing weens die gesondheids- en voedingsvoordele daarvan. Granate word beskou as luukse vrugte wat goed verkoop in die hoër marksegment en daar is 'n groeiende vraag na gesonde en eksotiese vrugte van hoë gehalte, sowel vir vars gebruik as vir verwerking van sappe en ander produkte. Die vrugte word egter as baie bederfbaar geklassifiseer en is spesifiek geneig tot vogverlies, ongeag die dik skil en taai leeragtige buitenste vel. Waterverlies kan maklik tot 'n groot finansiële verlies lei vir die bedryf, deur direkte verlies aan bemarkbare vars gewig en die gepaardgaande verminderde kommersiële waarde van die geaffekteerde vrugte. Die primêre doel van die navorsing was gevolglik om die meganismes van gewigsverlies by granaatvrugte te ondersoek deur gepaardgaande strukturele en kwaliteitveranderinge in die vrugte te karakteriseer, die watervervoer eienskappe van vrugweefsel te kwantifiseer, 'n watervervoermodel te ontwikkel, en waterverliesbeheertegnieke toe te pas en assesseer. Die navorsingsmetodiek het 'n veelvlakkige benadering gevolg deur beeldmodaliteite en rekenaartegnieke toe te pas gepaard met velerlei laboratorium eksperimente. Die eksperimentele studies het vasgestel dat kultivars 'Wonderful' en 'Herskawitz' meer vatbaar is vir waterverlies as 'Acco' as gevolg van verskille in die skil-verhouding en voginhoud. Die dik nie-eetbare skil is die primêre bron van waterverlies in vergelyking met die eetbare gedeelte (arils). Die algehele gemiddelde waterverlies was 0.31, 0.34 en 0.26 g cm⁻² respektiewelik vir ‘Wonderfull’, ‘Herskawitz’ en ‘Acco’ na 16 d in rakberging teen 23 ⁰C en 58 % relatiewe humiditeit (RH). Analise van die vrug se mikrostruktuur dui dat die arils (eetbare gedeelte) beskerm word teen oormatige waterverlies deur die inwendige epidermis membraan (wit membraan) wat die aril beskermhulsels bedek. Hierdie membraan het 'n laer waterdeurlaatbaarheid (0.14 × 10¹¹ kg m⁻¹ pa⁻¹ s⁻¹) in vergelyking met die epikarp (1.31-1.43 × 10¹¹ kg m⁻¹ pa⁻¹ s⁻¹) en mesokarp (13.51-16.37) × 10¹¹ kg m⁻¹ pa⁻¹ s⁻¹) skilweefsels onder ambiënt toestande (23 ⁰C en 58 % RH). Transpirasie is die primêre proses van waterverlies by granaatvrugte; hierdie studie dui egter aan dat die asemhalingsproses bydra tot die verlies van vrugte tot by 35 teen hoë 93 % RH. Die kwalitatiewe studies oor die skilmikrostruktuur, met behulp van skanderingselektronmikroskopie (SEM), het 'n hoër aantal lentiselle, groter lentiselgrootte en 'n algemeen lae skildikte op die boonste en middelste ruimtelike oppervlakareas van die vrugsfeer geïdentifiseer, in vergelyking met die onderste oppervlakareas van die vrug. Verder het ʼn X-straalondersoek 'n hoër porositeit in die epikarp geopenbaar as in die skilfraksies van die mesokarp en dat die poreusheid van onder na middel na bo in die epikarp-fraksie toegeneem het. Dit dui daarop dat die granaatvrugte meer vatbaar was vir vogverlies op die boonste en middelste oppervlakareas in vergelyking met die onderste oppervlakarea. Daarbenewens is 'n kenmerkende helder, wasagtige kutikula op die oppervlak van die skil geïdentifiseer met behulp van konfokale laser skandeer-mikroskopie (CLSM). ʼn Afnemende profiel van die wasagtige kutikula-dikte, 'n verhoogde fragmentasie van die wasagtige kutikula, die verbreding en verdieping van die mikro-krake en 'n algehele afname in die dikte van die skil is tydens die opberging van vrugte waargeneem. Die studie het ook die gebruik van nie-indringende tegnologie, soos magnetiese resonansie beelding (MRI), gedemonstreer om die kortstondige waterprofiele oor ongeskonde vrugte te assesseer. Verder het die studie ʼn watervervoer model ontwikkel, gevalideer en suksesvol gebruik in die voorspelling van waterverlies van krimp toegedraaide, voering verpakte, gewas en ongewasde granaatvrugte. Hierdie model kan in die toekoms gebruik word om waterverlies en beheerstrategieë te bestudeer. Die studie beklemtoon oppervlakwas as 'n moontlike omgewingsvriendelike oplossing vir die vermindering van waterverlies in die vrugte. Modellering van gasvervoer is egter nodig vir die akkurate optimalisering van die toediening van die oppervlakwas. Toekomstige navorsing kan ook die koppeling van die watervervoermodel en meganiese vervormingsmodel oorweeg om die inkrimping van die vrug as gevolg van vogverlies te verantwoord.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108398
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