Thesis (MScAgric)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Shrivelling and friction discolouration (FD), as postharvest disorders, negatively influence
the marketability and potential shelf life of pears. By investigating the contributing factors in
each of the disorders, the potential involvement of handling and storage variables were
determined. This allowed for a better understanding of the responsible factors that create
susceptible environments for these disorders to occur.
From the moment that pears are harvested they lose weight by means of transpiration and, to
a lesser extent, respiration. When excessive losses are experienced, the fruit will appear
shrivelled and the marketability and shelf life are negatively influenced. By minimizing the
rates of weight loss, the occurrence of shrivelling among pears during the postharvest
handling can be lessened. The periods that proved to be most conducive to shrivelling (during
a simulated postharvest handling duration) were where temperatures above 0 °C were
experienced. These short periods proved more perilous for shrivelling than lengthy storage
durations at low temperatures. This influenced the transpiration rate in such a way that the
driving force accelerated the rate of weight loss in all the cultivars that were studied.
Removing field heat from fresh produce and maintaining the cold chain reduces the driving
force behind the transpiration of the pears.
In all the cultivars studied, ‘Packham’s Triumph’, ‘Beurrè Bosc’ and ‘Forelle’, smaller and
less mature fruit were more inclined to appear shrivelled. The surface area to volume ratio is
fundamental in determining the rate of weight loss. This was most evident in ‘Beurrè Bosc’.
Although no reproducible results could be obtained from the morphological studies, literature
has attributed this phenomenon to the composition and quantity of the cuticle layer.
Reduction of weight loss was obtained by sealing of the fruit stem. This obstructed water
movement from the fruit through the xylem conducting tissue to the surrounding atmosphere.
Not only did the stem appear greener and fresher, but less weight loss and subsequent shrivel
was noticeable in the treated fruit. This effect was most evident in ‘Packham’s Triumph’ and
‘Beurrè Bosc’, but not in ‘Forelle’. ‘Forelle’ typically has a very short, thin stem in
comparison to the other two cultivars.All the cultivars showed visual shrivel symptoms after 11 days at 18 °C. Rate of weight loss
was the lowest in ‘Packham’s Triumph’, but due to its prominent dimensions, it appeared
shrivelled before any of the other cultivars. ‘Beurrè Bosc’ lost weight at the highest rate
(0.42%.day-1).
As fruit injury, in the presence of oxygen, is inevitable, the oxidative enzymatic browning of
pears will always be troublesome. This defensive mechanism partially prevents the infection
of the fruit where epidermal cells are injured. To minimize FD, impact and frictional forces
need to be lower during both harvesting and handling practices.
A laboratory scale method was developed through which reproduceable treatments could be
performed, thereby subjecting the fruit to industry related friction, rather than impact, injury.
By assessing the discolouration in terms of both extent and intensity, the influence of
variables could be determined on both ‘Packham’s Triumph’ and ‘Doyenne du Comice’
pears. As also found in practice, ‘Doyenne du Comice’ proved to be far more susceptible to
FD than ‘Packham’s Triumph’, although the activity of the enzyme, polyphenol oxidase
(PPO) was found to be higher in the latter.
Although no significant difference was found between the FD encountered at fruit
temperature of 3 °C and 15 °C, discolouration was greater at the higher temperature. This
might be attributed to a greater degree of water loss, lower cell turgidity or higher enzyme
activity. Thus, fruit taken from storage and sorted directly thereafter will exhibit less FD. The
contribution of condensation forming on the fruit, acting as lubrication, cannot be ignored.
Such fruit, with high turgor pressure, might again be more susceptible to bruising which will
only be revealed well after the injury. Since enzymes, which include PPO, catalyse
biochemical reactions, the availability of sufficient substrate most probably regulates the
extent of this biochemical discolouration.
Harvesting at optimum maturity and preventing any unnecessary friction will most definitely
reduce the occurrence of FD. The ultimate challenge remains to optimize sorting and packing
conditions without compromising on fruit quality. / AFRIKAANSE OPSOMMING: Verrimpeling en friksie verbruining (FV), as na-oes defekte, het ’n negatiewe invloed op die
bemarkbaarheid en potensiële raklewe van pere. Hierdie studie het die bydraende faktore vir
elkeen van die defekte ondersoek. Daar is gepoog om die effek van verskillende hanteringsen
opbergingsmetodes op bogenoemde defekte te bepaal. ‘n Beter begrip is verkry van die
oorsaaklike faktore wat bydra tot die ontstaan van die betrokke defekte.
Direk na die oes van pere, begin die vrug gewig verloor as gevolg van veral transpirasie.
Oormatige verliese sal lei tot ’n vrug wat verrimpeld voorkom, met ‘n negatiewe invloed op
die bemarkbaarheid en raklewe daarvan. Die voorkoms van die verrimpeling van pere tydens
die na-oes hantering van die vrugte, kan verminder word deur vermindering van die tempo
van gewigsverlies. Die periodes waartydens die verrimpeling veral voorgekom het, (soos
gevind in ’n gesimuleerde na-oes hanteringsmodel) was wanneer temperature bo 0 °C
ondervind is. Sulke kort periodes was meer geneig om aanleiding te gee tot verrimpeling as
die verlengde periodes van opberging by lae temperature. Sulke periodes van hoër
temperature het gelei tot versnelde transpirasie en ’n versnelde tempo van gewigsverlies in al
die kultivars wat ondersoek is. Die transpirasietempo van pere kan verlaag word deur die
verwydering van veld-hitte en deur die streng behoud van die koue-ketting.
In al die kultivars wat ondersoek is, ‘Packham’s Triumph’, ‘Beurrè Bosc’ en ‘Forelle’, is
gevind dat die kleiner en minder volwasse vrugte meer geneig was tot verrimpeling. Die
oppervlak area tot volume verhouding is krities in die bepaling van die tempo van
gewigsverlies. Hierdie bevinding was die prominentste in ‘Beurrè Bosc’. Alhoewel geen
beduidende resultate verkry kon word van die morfologiese studies nie, is daar verskeie
verwysings in die literatuur wat hierdie verskynsel toeskryf aan die samestelling en
hoeveelheid van die kutikula laag.
Vermindering van gewigsverlies is verkry deur verseëling van die vrugtestingel. Hierdie
tegniek het gelei tot ’n blokkering van die watervloei van die vrug na die omgewing deur die
xileem weefsel. Verseëling van die stingel het dit groener en varser laat voorkom, en het ook
’n merkbare vermindering in gewigsverlies en die daaropvolgende verrimpeling tot gevolg
gehad. Die effek van stingel-verseëling was die prominentste in ‘Packham’s Triumph’ en‘Beurrè Bosc’. Dit was minder duidelik in ‘Forelle’ wat tipies gekenmerk word deur ‘n baie
korter, dun stingel in vergelyking met die ander twee kultivars.
Al die kultivars het makroskopiese verrimpeling getoon na ‘n opbergingperiode van 11 dae
by 18 °C. Die tempo van gewigsverlies was die laagste in ‘Packham’s Triumph’ alhoewel dit
eerste verrimpeld voorgekom het. Dit kan toegeskryf word aan die spesifieke afmetings van
hierdie betrokke kultivar. ‘Beurrè Bosc’ het vinnigste gewig verloor (0.42%.dag-1).
Aangesien die besering van vrugte, in die aanwesigheid van suurstof, onvermydelik is, sal
oksidatiewe, ensiematiese verbruining van pere altyd problematies wees. Hierdie
verdedigingsmeganisme voorkom tot ’n mate die infeksie van die vrug wanneer epidermale
selle beskadig word. FV kan beperk word deur die vrugte tydens oes en hanteringsprosedures
so min as moontlik bloot te stel aan impak en friksie kragte.
’n Laboratorium-model is ontwikkel ter nabootsing van die omstandighede in die industrie.
Die vrugte is aan friksie, eerder as impak, onderwerp, soos ondervind in die industrie. Die
omvang asook die intensiteit van die verbruining is gemeet in beide ‘Packham’s Triumph’ en
‘Doyenne du Comice’ pere. Op hierdie wyse kon die invloed van die onderskeie
veranderlikes in elke kultivar bepaal word. Alhoewel die ensiematiese aktiwiteit van die
polifenol oksidase ensiem (PFO) die hoogste in ‘Packham’s Triumph’ was, is gevind dat
‘Doyenne du Comice’ veel meer geneig was tot FV as ‘Packham’s Triumph’. Hierdie
bevinding bevestig die verskynsel soos in die praktyk gevind.
Alhoewel geen betekenisvolle verskil gevind is tussen FV by vrug temperatuur van 3 °C en
15 °C nie, was daar meer verbruining by die hoër temperatuur. Hierdie verskynsel kan
toegeskryf word aan ’n groter mate van waterverlies, laer sel turgiditeit en hoër ensiem
aktiwiteit. Dus sal vrugte wat direk na opberging gesorteer word, minder FV toon. Die bydrae
van die kondensasie wat op die vrug vorm, en as ’n smeermiddel dien, kan nie geïgnoreer
word nie. Sulke vrugte met hoër turgiditeit, mag egter meer vatbaar wees vir kneusing, wat
egter eers ’n geruime tyd na die kneusing tevoorverskyn mag kom. Aangesien ensieme, wat
PFO insluit, as katalis dien in biochemiese reaksies, sal die beskikbaarheid van voldoende
substraat, na alle waarskynlikheid die omvang van die biochemiese verkleuring reguleer.Die oes van pere tydens optimum volwassenheid, en die voorkoming van onnodige friksie sal
definitief die voorkoms van FV verminder. Die uitdaging is steeds om sortering- en
verpakkingstegnieke verder te verfyn sonder om ’n negatiewe invloed op vrugtekwaliteit te
hê.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16264 |
| Date | 04 1900 |
| Creators | Burger, G. E. (Gerrit Erasmus) |
| Contributors | Huysamer, M., Jacobs, G., University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | x, 146 leaves : ill. |
| Rights | University of Stellenbosch |
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