Thesis (MScAgric)--Stellenbosch University, 2008. / ENGLISH ABSTRACT: Fruit quality is the most important factor that determines prices for the fruit in the
international markets. Although different consumers perceive quality differently there are
quality variables that are always associated with poor quality by all consumers. In table
grapes (Vitis vinifera L.) these variables may include overall appearance, stem condition,
SO2 damage, decay, berry browning and shatter. The presence of these quality defects
negatively affects prices and most often results in quality claims.
Cooling is the most widely used method to reduce the postharvest loss of fruit quality. In
South Africa, most deciduous fruits including table grapes are forced air cooled to a
statutory pulp temperature of –0.5°C prior to shipping in an effort to preserve quality,
thus ensuring good market prices for the fruits. Despite these efforts, there are still quality
claims from the markets and this reduces the returns to the growers.
The objectives of this research were to: (i) see if cooling time can be reduced by cooling
to higher pulp temperatures of 1.5°C and 3°C without causing quality losses, thus
improving the throughput of the cold rooms; (ii) see if the problem of berry browning can
be alleviated by cooling grapes to higher pulp temperature, and (iii) see whether pallet
positioning in the cooling tunnels and reefer container affect quality.
The trends showed better quality when ‘Victoria’ and ‘Regal Seedless’ were forced air
cooled (FAC) to pulp temperatures of 1.5 °C and 3 °C as opposed to –0.5 °C. There were
no economic losses associated with pre-cooling grapes to pulp temperatures of 1.5 °C and
3 °C. There were no significant differences in berry browning related to pre-cooling
treatments. However, cooling time was reduced significantly. In most of the cooling
tunnels and reefer containers used in this trial, grape quality results showed no significant
differences between the positions in the stack and in reefer containers. However, in cases
where there were significant differences, the middle and the rear positions showed better
grape quality in terms of stem condition (dry and brown stems) than the front position
(near fan) in both the pre-cooling stack and reefer containers. The trends showed that the front is cooler than the back of the pre-cooling stack. The pulp
temperature differences between the front and rear positions in the reefer container were
as high as 1.23 °C. The trends also showed that the bottom layers of the pallets were
cooler than the top layers in the reefer container.
FAC to 3°C resulted in a constant reduction in percentage electrolyte leakage after 4
weeks of storage at –0.5°C, while FAC to 1.5°C, -0.5°C and static room cooling (control)
in some cases showed an initially low electrolyte leakage followed by an increase in
leakage after 4 weeks of storage.
FAC grapes to higher pulp temperatures of 3°C and 1.5°C could reduce the cooling time,
thereby improving the throughput of cold rooms. There was no clear evidence to suggest
that browning was due to pre-cooling practices. Both preharvest and postharvest
conditions need to be further investigated to better understand the problems of browning
in white table grapes. / AFRIKAANSE OPSOMMING: Vrugkwaliteit is ʼn kritiese faktor in die bepaling van pryse op die internasionale markte.
Alhoewel daar variasie voorkom tussen verbruikers in wat vrugkwaliteit is, bly sekere
aspekte altyd onveranderd. Ononderhandelbare kwaliteit aspekte in tafeldruiwe (Vitis
vinifera L.) sluit die algemene voorkoms, toestand van die trosstingels, SO2 skade,
bederf, korrel verbruining en los-korrels in. Indien enige van die kwaliteit-defekte
voorkom het dit ʼn negatiewe impak op die prys en lei gewoonlik tot gehalte eise.
Verkoeling word algemeen gebruik om die verlies van na-oes kwaliteit te verminder. Die
meeste sagtevrugte geproduseer in Suid Afrika (insluitend tafeldruiwe) ondergaan
geforseerde verkoeling tot ʼn statutêre pulptemperatuur van -0.5°C, voor verskeping.
Ondanks hierdie maatreëls om hoë pryse te verseker, is daar steeds kwaliteiteise in die
mark wat lei tot ‘n laer inkomste vir produsente.
Die navorsing het dus ten doel gehad om : (i) te bepaal of die tyd van verkoeling
verminder kan word, indien na hoër pulptemperature van 1.5°C en 3°C verkoel kan word,
sonder ‘n verlies in kwaliteit en sodoende die deurvloeitempo van die koelkamers
verhoog; (ii) om te bepaal of die voorkoms van korrelverbruining verlaag kan word
indien tot hoër pulp-temperature verkoel word, en (iii) laastens om te bepaal of posisie
van die palet in die verkoelingstonnel en verskepingshouer ʼn invloed het op
vrugkwaliteit.
Tendense toon dat ‘Victoria’ en ‘Regal Seedless’ kwaliteit beter was indien verkoel tot
pulptemperature van 1.5°C en 3°C in vergelyking met -0.5°C. Daar was geen
ekonomiese verliese waargeneem indien die hoër verkoelingstemperature gebruik is nie.
Alhoewel daar geen betekenisvolle verskille in korrelverbruining voorgekom het tussen
temperatuur behandelings nie is die verkoelingsperiode verkort. In die meeste van die
verskepingshouers, asook in posisies tydens geforseerde verkoeling is daar geen
betekenisvolle verskille waargeneem nie. In die gevalle waar daar egter wel betekenisvolle verskille voorgekom het, het die middel en agter posisies beter
vrugkwaliteit gehad as die voorste posisie tydens verkoeling asook houerverskeping.
Die palette aan die voorkant (naby die waaier) het as ʼn algemene tendens laer
temperature as in die agterkant van die verkoelingstonnel. Verskille in pulptemperature
tussen palette in die voor en agterkant van verskepingshouers was so hoog as 1.23°C. Die
temperatuurdata het uitgewys dat die onderste laag kartonne neig om by ‘n laer
temperatuur te wees as die boonste lae kartonne tydens houerverskeping.
Geforseerde verkoeling teen 3°C het gelei tot ‘n afname in persentasie elektrolietlekkasie
na 4 weke van verkoeling teen -0.5°C. Terselfdertyd het geforseerde verkoeling tot 1.5°C
en -0.5°C asook statiese verkoeling (kontrole) in sekere gevalle gelei tot ‘n laer
aanvanklike uitlek van elektrolietlekkasie, gevolg deur ʼn verhoging na 4 weke opberging.
Geforseerde verkoeling van tafeldruiwe tot pulptemperature van 1.5°C en 3°C verkort die
verkoelingstyd en verhoog dus die deurvloeitempo in die verkoelingskamers. Daar was
gedurende die studie geen duidelike bewyse gevind dat korrelverbruining voorkom as
gevolg van verkoelingspraktyke nie. Beide voor en na-oes praktyke sal verder ondersoek
moet word om die invloed daarvan te bepaal op die verbruining van wit tafeldruiwe.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/21440 |
| Date | 03 1900 |
| Creators | Ngcobo, Mduduzi E. K. |
| Contributors | Huysamer, M., Jacobs, G., Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | x, 140 leaves : ill. |
| Rights | Stellenbosch University |
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