Moisture loss studies in Japanese plums (Prunus salicina Lindl.)

Theron, Jacobus Adriaan (2015-12)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The export of Japanese plums from South Africa is challenging. Exporting late season plums require fruit to last as long as 8 weeks in cold-storage. Prolonged storage periods can cause some cultivars to develop a shrivelled appearance due to moisture loss. Moisture loss from perishable commodities manifests mainly as shrivelling due to a loss in the turgidity of the surface cells of the fruit, or weight loss. ‘African DelightTM’ (highly susceptible to shrivel), ‘Laetitia’ (shrivel susceptible), ‘Sapphire’ (shrivel susceptible) and ‘Songold’ (not shrivel susceptible) plums were investigated by means of fluorescent microscopy for cracks and openings in the fruit peel. Only ‘African DelightTM’ had open hairline cracks in its peel, and fruit with wider cracks were associated with higher water vapour permeabilities. Open lenticels were found in the peels of ‘African DelightTM’, ‘Laetitia’ and ‘Sapphire’ plums. For ‘Songold’ no peel cracking or open lenticels were observed. The fact that the cuticle of this cultivar is mostly intact may be the reason why it is not susceptible to postharvest shrivel manifestation. The water vapour permeance of the fruit peel determines how easily fruit lose moisture. In this study it was determined to what extent fruit, trees, orchards, harvest date and cultivar contribute to the total variation in plum peel water vapour permeability. The permeabilities of ‘African DelightTM’, ‘Laetitia’, and ‘Songold’ were determined weekly from 4 weeks before harvest until post optimum maturity. Fruit to fruit variation made the largest contribution towards the total variation (> 45%), followed by harvest date (> 20%) and orchard (> 15%) effects. The permeability across all cultivars increased two-fold as fruit became over mature. The contribution of cultivar differences to fruit peel permeability varied greatly between seasons (42% in 2013/2014 and 5% in 2014/2015). Differences between cultivars may include cuticle thickness and composition, micro cracks in the peel and/or open lenticels. Current handling protocols suggest that fruit should be cooled as soon as possible after harvest, but this is not always possible. ‘African DelightTM’ plums were exposed to various handling scenarios in order to determine the handling protocol with the least risk of moisture loss. The control consisted of packaging and cooling the fruit within 6 h of harvest. Fruit quality was comparable or even better than the control when the fruit were pre-cooled to 0 °C and 15 °C for up to 72 h. High vapour pressure deficits caused fruit to lose more moisture, especially when fruit were exposed to ambient temperatures for 48 h and 72 h. It is recommended that handling protocols for plums should be followed stringently in order to reduce mass loss and shrivel manifestation. Since other studies found that silicate (Si) has positive effects on fruit quality, we applied potassium silicate preharvest to ‘African DelightTM’ trees. However, we did not find significant differences between treatments regarding crack width or crack incidence in the fruit peel, shrivel, decay, internal browning, gel breakdown or aerated tissue levels. Currently preharvest potassium silicate applications are not recommended to improve plum quality.

AFRIKAANSE OPSOMMING: Die uitvoer van die Japanese pruime uit Suid-Afrika is 'n uitdaging, omrede daar verwag word dat laatseisoen kultivars tot 8 weke in koelopberging moet bly. Lang opbergingsperiodes veroorsaak dat sommige kultivars 'n verrimpelde voorkoms ontwikkel a.g.v. vogverlies. Vogverlies uit vars produkte manifesteer hoofsaaklik as verrimpeling a.g.v. 'n verlies in die turgiditeit van die selle in en onder die vrugskil, en as massaverlies. ‘African DelightTM’ (hoogs vatbaar vir verrimpeling), ‘Laetitia’ (vatbaar vir verrimpeling), ‘Sapphire’ (vatbaar vir verrimpeling) en ‘Songold’ (nie vatbaar vir verrimpeling) pruime is ondersoek deur middel van fluoressensie mikroskopie vir krake en openinge in die vrugskil. Slegs ‘African DelightTM’ het oop haarlyn krake in sy skil gehad en vrugte met wyer krake het ʼn hoër waterdamp deurlaatbaarheid gehad. Oop lentiselle is gevind in die skille van ‘African DelightTM’, ‘Laetitia’ en ‘Sapphire’ pruime. ‘Songold’ het geen krake of oop lentiselle getoon nie. Die feit dat ‘Songold’ se kutikula meestal ongeskonde was, mag die rede wees waarom hierdie kultivar nie vatbaar vir verrimpeling is nie. Die waterdamp deurlaatbaarheid van 'n vrugskil bepaal hoe maklik vrugte vog verloor. In hierdie studie is bepaal tot watter mate vrugte, bome, boorde, oesdatum en kultivar bydra tot die totale variasie in die pruimskil se waterdamp deurlaatbaarheid. Die deurlaatbaarheid van ‘African DelightTM’, ‘Laetitia’, en ‘Songold’ is weekliks bepaal vanaf 4 weke voor die verwagte oesdatum tot die vrugte oorryp was. Vrug tot vrug variasie het die grootste bydrae tot die totale variasie gemaak (> 45%), gevolg deur oesdatum (> 20%) en boord (> 15%). Die skildeurlaatbaarheid van al die kultivars het verdubbel in die tyd van net voor oes tot die vrugte oorryp was. Die kultivar se bydrae tot die deurlaatbaarheid van die vrugskil het baie gewissel tussen seisoene (42% in 2013/2014 en 5% in 2014/2015). Verskille in skil-deurlaatbaarheid tussen kultivars kan kutikula-dikte en -samestelling, mikro-krake in die skil en/of oop lentiselle insluit. Huidige hanteringsprotokolle stel voor dat vrugte so spoedig moontlik afgekoel word na oes, maar dit is nie altyd moontlik nie. In hierdie studie is 'African DelightTM' pruime is blootgestel aan verskeie hantering scenario's om die hanteringsprotokol met die laagste risiko vir vogverlies te bepaal. Die kontrole vrugte is gepak en onder geforseerde verkoeling geplaas binne 6 ure na oes. Vrugkwaliteit was vergelykbaar of selfs beter in vergelyking met die kontrole wanneer die vrugte voorverkoel is tot 0 °C en 15 °C vir tot 72 uur. Hoë dampdrukverskille het veroorsaak dat vrugte meer vog verloor, veral wanneer vrugte aan kamertemperatuur blootgestel was vir 48 h en 72 h na oes. Dit word aanbeveel dat hanteringsprotokolle vir pruime streng gevolg moet word om massaverlies en verrimpeling te beperk. Aangesien ander studies gevind het dat silikaat (Si) ‘n positiewe uitwerking op vrugkwaliteit het, het ons kaliumsilikaat vooroes aan ‘African DelightTM’ bome toegedien. Daar was egter geen beduidende verskille tussen behandelings met betrekking tot kraakbreedte of kraakvoorkoms in die vrugskil of t.o.v. gehalte eienskappe soos die voorkoms van verrimpeling, bederf, interne verbruining, gelverval of deurlugte weefsel nie. Tans word voor-oes kaliumsilikaat spuite nie aanbeveel om pruimkwaliteit te verbeter nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97918
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