Please read the abstract in the section 00front of this document / Thesis (PhD (Microbiology))--University of Pretoria, 2004. / Microbiology and Plant Pathology / unrestricted

No description available.

No description available.

The problem of this investigation was to develop a satisfactory method by which insect infested corn could be treated continuously to eliminate the contamination normally found in the corn meal produced from the corn. An extensive literature review was compiled covering the processing of corn, the description of insects infesting stored corn, the sources of insect contamination, the tests for contamination in the corn meal, and the methods now employed or proposed for the reduction of this contamination. In connection with the fumigation of stored grain, tables were prepared showing the results of fumigations of grain insects with more than 400 chemical compounds. Those compounds which appeared to be best suited for this type of treatment of the corn, were then listed in a separate table and the properties of the compounds detailed so that a selection of the optimum compounds for experimental testing could be made. In addition to fumigation procedures, thermal procedures employing infrared radiation and dielectric heating were possible methods for killing the insects in the corn. Another procedure recommended for this purpose was the use of the mechanical force to kill the insect by means of high speed centrifugal machines called "Entoleter" Infestation Destroyers. By means of fragment analyses of samples of corn meal, it was found that the source of the corn and the care which it received during storage and handling were of primary importance in producing corn meal free from contamination by the present milling procedures. As a result of similar analyses it was found that precracking and aspirating of the corn on certain types of equipment prior to the normal grinding of the meal did not result in a reduction in the quantity of fragments present in the meal. Carbon tetrachloride and trichloroethylene were selected for possible utilization in very rapid fumigation of the infested corn. Laboratory apparatus based on the operation of the solvent-vapor degreaser for metal parts was constructed. In the resulting tests, it was found that with infested corn suspended in the vapor of boiling carbon tetrachloride or trichloroethylene the mortality of all stages of the rise weevil was 100 per cent after an exposure period of only one minute. Other tests indicated that after air drying of the grain the carbon tetrachloride did not have any deleterious effect on the baking properties of the corn. In connection with heat sterilization methods, tests were conducted which indicated that the baking properties of corn heated to temperatures below 160 °F were not impaired. Further tests were conducted which indicated that both infrared heating and dielectric heating were 100 per cent effective in killing all stages of rice weevil in infested com after an exposure period of two minutes to infrared radiation or one minute to dielectric heating. In both cases the temperature of the com did not reach 160 °F. Tests conducted with the "Entoleter” centrifugal machine indicated that the damage to the whole grain com by the action of the machine was excessive and all of the rise weevil were not killed. After due consideration of the experimental results it was decided that the system of rapid fumigation of the infested corn was the most reasonable solution to the problem of killing the insect infestation in the corn, The equipment necessary for the proposed system can be assembled with a few modifications of solvent-vapor, metal-degreasing equipment and with a suitable solvent recovery system. It is anticipated that the removal of the dead insects, larvae, eggs, and pupae can be accomplished on commercial milling equipment now available if the insects are killed prior to the processing and the reinfestation of the product is not an important factor during the processing. / Master of Science

There has been controversy over the accuracy of grading peanuts for freeze damage by United States Department of Agriculture (USDA) guidelines. Several other tests for estimating freeze damage in peanuts were investigated. The conductivity test, which measures electrolyte leakage from cells, and the tetrazolium test, a test currently used for estimating viability of seed peanuts, were adapted for assessing freeze damage. The dipicrylamine test, a qualitative spot test for potassium, was developed and used to detect potassium leakage from cells onto the surface of the cotyledon. A visual test was developed based on differences in glossiness of the flat surface of the peanut cotyledon. A freezing apparatus was designed and operated so that peanuts could be cooled at a slow, even rate to a series of low temperatures. These peanuts and peanuts exposed to a natural freeze were tested for freeze damage. The USDA visual test underestimated freeze damage compared to all of the tests used in this study. The VPI visual test differentiated damaged from undamaged peanuts using subtle differences in glossiness that are not considered by USDA visual test guidelines. Results of the VPI visual, tetrazolium, and dipicrylamine tests for peanuts exposed to the laboratory freeze treatments were not significantly different. The dipicrylamine test is a rapid test, but interpretation of color differences proved to be difficult. The tetrazolium test revealed the greatest differences between damaged and undamaged peanuts and these differences were easily interpreted. The tetrazolium test is, therefore, favored over the other tests for grading freeze damage in peanuts. / M. S.

The public concern over synthetic pesticides in foods and the environment has created an interest to find effective and safe non-fungicide means of controlling postharvest pathogens. The overall objective of this thesis was to evaluate the effect of potassium silicate, yeast antagonists and hot water dip treatment to control postharvest grey mould and blue mould of pome fruits, caused by Botrytis cinerea and Penicillium expansum, respectively. Botrytis cinerea and Penicillium expansum were isolated from infected strawberry and pear fruits, respectively. These isolates were found to be non-resistant to YieldPlus® (Anchor yeast, Cape Town, South Africa), a biofungicide containing a yeast Cryptococcus albidus. A total of 100 epiphytic yeast isolates were obtained from the fruit surface of “Golden Delicious” apples and “Packham’s Triumph” pears, and screened against B. cinerea and P. expansum. Fifteen yeast isolates reduced grey mould incidence by > 50%, when applied four hours before inoculation with B. cinerea. Similarly, seven yeast isolates reduced blue mould incidence by > 50%, when applied four hours before inoculation with P. expansum. YieldPlus® and yeast Isolate YP25 provided the best control of B. cinerea, while Isolate YP60 and YieldPlus® provided the best control of P. expansum on “Golden Delicious” apples. A mixture of YP25 and YP60 provided complete control of both B. cinerea and P. expansum, when applied to “Golden Delicious” apples before inoculation with either B. cinerea or P. expansum. Electron microscopy studies showed that yeast Isolates YP25 and YP60 inhibited the mycelial growth of B. cinerea and P. expansum, respectively. Preventative and curative application of potassium silicate resulted in reduced incidence of B. cinerea or P. expansum of “Golden Delicious” apples. Electron microscopy studies indicated that potassium silicate inhibited the growth of B. cinerea and P. expansum. Furthermore, treatment of “Golden Delicious” apples with either potassium chloride or potassium hydroxide resulted in reduced incidence of both B. cinerea and P. expansum. In vivo tests showed that the disease incidence of P. expansum and B. cinerea on “Golden Delicious” apples was reduced by hot water dip treatments at 58-60°C for 60 to 120 seconds, compared with the control fruit treated with sterile distilled water, without causing skin damage. The use of potassium silicate, yeasts (Isolates YP25 and YP60), YieldPlus® and the antagonists mixture (YP25+YP60) in combination, resulted in the control of B. cinerea and P. expansum of “Golden Delicious” apples compared with Imazalil® treated fruit. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

The susceptibility of the 'Pinkerton' avocado cultivar to mesocarp discolouration, after storage, has seriously threatened its export from South Africa. This disorder has proven to be complex, requiring a better understanding of the fruit's physiology. The purposes of this study were to identify the role of pre- and postharvest factors, or their interactions, in the development of the problem. This was done by obtaining fruit from several production areas of varying mesocarp discolouration histories (referred to as "high", "medium" or "Iow risk" areas) during the 2000 and 2001 seasons. Fruit were stored at 8, 5.5 and 2°C for 30 days, as well as ambient (20°C). Evaluations of fruit quality were made before and after storage, as well as after softening. Once removed from storage the weight loss (during storage) was determined, and fruit firmness and carbon dioxide (C02) production rates monitored daily. It was found that temperatures below the recommended shipping temperature of 5.5°C, i.e. 2°C, produced the best internal fruit quality. This was supported by the membrane integrity studies that showed less membrane stability at the warmer storage temperature of 8°C. Furthermore, remained hard during storage and subsequently had an extended shelf life. Fruit origin was also found to play a major role in browning potential, with discolouration being consistently more severe in fruit from "high risk" areas and increasing in severity as the season progressed. The rate of CO2 production was found to follow a similar trend, with rates increasing as the season progressed, and also being slightly higher in fruit from "high risk" areas. The higher CO2 production rates were thought to be related to a decrease in membrane integrity as the season progressed. While storage temperature was not found to have a significant effect on the rate of CO2 production after storage, it did affect the time taken to reach the maximum rate, with fruit stored at 2°C taking longer. Biochemical analyses to determine the concentration of total phenolics and the activity of the enzyme polyphenol oxidase (PPO) also showed that the potential for browning was initiated by preharvest conditions. While no significant differences were found between growers with regards to total phenol concentrations, the PPO activity was found to be higher in fruit from poor quality areas, and subsequently browning potential was expected to be higher in these fruit. It was, however, found that the potential for browning could be reduced by storing fruit at 2°C, as this decreased the total phenolics concentration. This evidence further emphasized the idea that storage at 2°C could be highly advantageous. Fruit mineral analysis showed that certain key elements played a significant role in the severity of mesocarp discolouration, with excessive fruit nitrogen and decreasing copper and manganese concentrations appearing to play major roles. The high fruit nitrogen concentrations were suspected to reflect fruit grown on very vigorous trees, resulting in shoots competing with fruit for available reserves. It is suggested that 'Pinkerton' of a quality acceptable to the market, can be produced by manipulating source:sink relationships, particularly through decreasing the availability of nitrogen, followed by low temperature (24° C) shipping. Future work should concentrate on manipulation of source:sink relationships, to take account of both climatic conditions and leaf to fruit ratios. The evaluation of chlorophyll fluorescence as a tool for predicting mesocarp discolouration potential in 'Pinkerton' proved to be unsuccessful in this study and future studies may require modifications to the current technique. It is suspected that differences in chlorophyll content, for example, between fruit from different origins, will have to be taken into account when interpreting results. The success of using 2°C storage to improve the internal quality on 'Pinkerton' fruit prompted further studies, during 2004, to ensure that the development of external chilling injury would not decrease the marketability of the cultivar. Low temperature conditioning treatments, prior to storage, proved to be highly successful in reducing the development of external chilling injury, thus further improving fruit quality as a whole. Preconditioning treatments consisted of fruit that were kept at either 10°C, 15°C or 20°C for 1 or 2 days before being placed into storage for 30 days at 2°C or 5.5°C. All preconditioning treatments were compared to fruit that were placed directly into storage. The effect of fruit packaging on moisture loss (as determined by weight loss) and chilling injury was also investigated using unwaxed fruit, commercially waxed and unwaxed fruit individually sealed in micro-perforated polypropylene bags with an anti-mist coating on the inside (polybags). Holding 'Pinkerton' fruit, regardless of packaging treatment, at 10°C for 2 days prior to storage at 2°C or 5.5°C significantly decreased the severity of external chilling injury. The use of polybags during preconditioning and storage showed potential in further reducing the development of external chilling injury, although the higher incidence of fungal infections in these fruit needs to be addressed. The determination of proline concentrations in fruit exocarp tissue after storage was helpful in determining the level of stress experienced by fruit that were subjected to different packaging and preconditioning treatments. In this study waxed fruit subjected to 1 d preconditioning at 10°C, 15°C or 20°C or placed directly into storage at 2°C showed very high proline concentrations and also displayed more severe external chilling injury, despite unwaxed fruit losing more weight during these treatments. The role of moisture loss thus needs further investigation. The thickness and method of wax application was thought to play an important role in the higher external chilling injury ratings in this study as waxed fruit often developed chilling injury symptoms around the lenticels and it was suspected that either the lenticels were damaged by the brushes used to apply the wax or that the lenticels became clogged thus resulting in reduced gaseous exchange. Nevertheless, the success of low temperature conditioning in reducing external chilling injury, while maintaining sound internal quality, may enable storage temperatures to be dropped even further, thus enabling South Africa to export avocados to countries that require a cold disinfestation period prior to entry to eliminate quarantine pests (e.g. fruit fly). / Thesis (Ph.D.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

Thesis (MScAgric)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Green mould is a major citrus postharvest disease caused by a wound pathogen called Penicillium digitatum. Imazalil (IMZ) is the most important postharvest fungicide that is currently registered against green mould and blue mould, given its good protective and curative control and sporulation inhibition. Imazalil can be applied in drench, dip, spray and wax coating applications. Wax coatings improve the quality of the fruit during shelf life by limiting moisture loss and providing a shiny appearance. The primary objective of this study was to investigate the application of imazalil in wax coatings in citrus packhouses to control green mould and preserve fruit quality. The first aim of this study was to evaluate the protective and curative control and sporulation inhibition of IMZ sensitive and resistant isolates of P. digitatum following single IMZ application in wax coating and IMZ application in aqueous dip as well as wax coating. For single application, Valencia orange fruit was curatively or protectively treated with a carnauba-based coating into which IMZ at 3000 μg.mL-1 was mixed. For curative treatments, fruit was treated after 24 hour's incubation. Coating was applied at 0.6, 1.2 and 1.8 L.ton-1 of fruit on a commercial coating applicator. Untreated control and fruit treated with coating only showed up to 80% infection for both sensitive and resistant isolates. Imazalil in coating proved to have better protective than curative action against the sensitive isolate, while control of the resistant isolate was poor. Imazalil residue levels increased with increasing coating load (0.85 to 1.75 μg.g-1). For double application, clementine, satsuma, navel and Valencia orange fruit were used. Curative and protective control following dip only treatment (IMZ sulphate at 500 μg.mL-1 for 45 s and 90 s) was compared with coating only treatment (IMZ at 3000 μg.mL-1 at 1.8 L.ton-1), and double application comprising dip (45 s in IMZ sulphate at 500 μg.mL-1) followed by 2000 μg.mL-1 IMZ in coating at 0.6, 1.2 and 1.8 L.ton-1. Double application resulted in improved residue loading (0.85 to 2.06 μg.g-1) compared to dip only treatments (≈ 0.49 μg.g-1). Coating only resulted in the highest residue levels (1.82 to 7.09 μg.g-1), often exceeding the maximum residue limit of 5 μg.g-1. In all treatments, poor curative and protective control of the resistant isolate (<46% and <55%, respectively) and generally no sporulation inhibition was observed. For the sensitive isolate, dip only treatments resulted in better curative control (≈ 77%) than protective control (≈ 38%). Double application showed increased protective control with increasing wax load (≈ 69%) as well as good curative control (≈ 83%). Wax coating only treatment resulted in poor curative control (≈ 26%), but good protective control (≈ 80%). Sporulation inhibition of the sensitive isolate was generally improved in treatments that included IMZ application in the wax coating. This study successfully showed the additive benefits of double application of IMZ in the wax coating following an aqueous dip application. However, despite improved control of the sensitive isolate, the resistant isolate could not be controlled. The second objective of the study was to evaluate green mould control and quality preservation following IMZ application using different coating and brush types. The inoculation and treatment procedure was similar to the single application of wax coating described above but only the sensitive isolate of P. digitatum was used. Carnauba- or polyethylene-based wax coatings, including IMZ at 3000 μg.mL-1, was applied at a dosage of 0.6, 1.2 and 1.8 L.ton-1 using commercial horsehair or synthetic brushes. Fruit was either incubated for 4 days prior to wound infection ratings or stored at -0.5oC for 26 days and 7 days shelf life. Imazalil residues on fruit increased with increasing coating loads on navel oranges (1.31 to 3.32 μg.g-1) and Valencia oranges (3.22 to 6.00 μg.g-1). Better protective (≈ 58%) than curative control (≈ 14%) was observed, with horsehair brushes resulting in lower sporulation levels (≈ 59%) than synthetic brushes (≈ 64%). Fruit weight loss and firmness loss ratios relative to uncoated control fruit were significantly higher in fruit treated with polyethylene coating compared with carnauba coating treated fruit. However, polyethylene coating resulted in shinier fruit before and after storage, whereas carnauba coating resulted in lower shine ratios. Gaseous (CO2) exchange ratios remained similar for both coatings, but higher polyethylene coating loads (1.8 L.ton-1) resulted in off-tastes similar to the uncoated control fruit and higher than the ratings for carnauba. Scanning electron micrographs revealed an amorphous crystallised natural wax layer with uncovered stomatal pores on the surface of uncoated fruit. The thickness of the applied coating layer increased with increasing coating load. Coating layers following application with synthetic brushes at 1.2 L.ton-1 appeared to be thicker than when applied using horsehair brushes. This study indicated the effects of brush type on the distribution and polishing of the wax coating on the surface of the fruit and also the importance of cautious application of coating as under or over application may lead to poor results in terms of bio-efficacy, residue loading and fruit quality. / AFRIKAANSE OPSOMMING: Groenskimmel is 'n belangrike sitrus na-oessiekte, veroorsaak deur 'n wondpatogeen, genaamd Penicillium digitatum. Imazalil (IMZ) is die belangrikste na-oesfungisied wat tans teen groenskimmel geregistreer is, gegewe sy goeie beskermende en genesende beheer en sporulasie-inhibisie. Imazalil kan in bad-, doop-, spuit- en wakslaagbehandelings toegedien word. Wakslae behou die kwaliteit van die vrugte gedurende raklewe deur vogverlies te beperk en 'n glansvoorkoms te verskaf. Die primêre doel van hierdie studie was om die toediening van imazalil in wakslaagbehandelings in sitruspakhuise vir groenskimmel en vrugkwaliteit beheer te ondersoek. Die eerste doelwit van hierdie studie was om die beskermende en genesende beheer en sporulasie-inhibisie van IMZ sensitiewe en weerstandbiedende isolate van P. digitatum, gevolg deur enkel IMZ toediening in 'n wakslaagbehandeling, en IMZ toediening in 'n doop-, asook wakslaagbehandeling, te evalueer. Vir enkelltoediening is Valencia lemoenvrugte genesend of beskermend met 'n carnauba-gebaseerde wakslaag behandel, waarin IMZ teen 3000 μg.mL-1 gemeng is. Vir genesende behandelings is vrugte ná 24 uur inkubasie behandel. Wakslaagbehandeling is teen 0.6, 1.2 en 1.8 L.ton-1 vrugte op 'n semi-kommersiële paklyn toegedien. Onbehandelde kontrole en vrugte met waks behandel, het slegs tot 80% infeksie vir beide sensitiewe en weerstandbiedende isolate getoon. Imazalil in waks het beter beskermende as genesende aksie teen die sensitiewe isolaat getoon, terwyl beheer van die weerstandbiedende isolaat swak was. Imazalil residuvlakke het met toename in wakslading toegeneem (0.85 tot 1.75 μg.g-1). Vir dubbeltoediening is clementine, satsuma, navel en Valencia lemoenvrugte gebruik. Genesende en beskermende beheer van IMZ na enkel doopbehandeling (IMZ sulfaat teen 500 μg.mL-1 vir 45 s en 90 s), is met enkel waksbehandeling (IMZ teen 3000 μg.mL-1 teen 1.8 L.ton-1) vergelyk, asook dubbeltoediening bestaande uit doop (45 s in IMZ sulfaat teen 500 μg.mL-1), gevolg deur 2000 μg.mL-1 IMZ in waksbehandeling teen 0.6, 1.2 en 1.8 L.ton-1. Dubbeltoediening het verbeterde residulading (0.85 tot 2.06 μg.g-1) tot gevolg gehad, in vergelyking met enkel doopbehandelings (≈ 0.49 μg.g-1). Enkel waksbehandeling het die hoogste residuvlakke (1.82 tot 7.09 μg.g-1) tot gevolg gehad, wat dikwels die maksimum residu-limiet van 5 μg.g-1 oorskry het. In alle behandelings is swak genesende en beskermende beheer van die weerstandbiedende isolaat (<46% en <55%, onderskeidelik), en oor die algemeen, geen sporulasie-inhibisie, waargeneem. Vir die sensitiewe isolaat, het die enkel doopbehandelings beter genesende beheer (≈ 77%) in vergelyking met die beskermende beheer (≈ 38%) tot gevolg gehad. Dubbeltoediening het verhoogde beskermende beheer met toenemende wakslading (≈ 69%) getoon, asook goeie genesende beheer (≈ 83%). Enkel wakslaagbehandeling het swak genesende beheer (≈ 26%) tot gevolg gehad, maar goeie beskermende beheer (≈ 80%). Sporulasie-inhibisie van die sensitiewe isolaat was oor die algemeen beter in behandelings wat IMZ toediening in die wakslaag ingesluit het. Hierdie studie het suksesvol die toegevoegde voordele van dubbeltoediening van IMZ in doopbehandeling en wakslaag getoon. Ten spyte van die verbeterde beheer van die sensitiewe isolaat, kon die weerstandbiedende isolaat egter nie beheer word nie. Die tweede doelwit was om groenskimmelbeheer en behoud van kwaliteit te evalueer, volgende op IMZ toediening deur gebruik van verskillende waks- en borseltipes. Inokulasie- en behandelingsprosedure was soortgelyk aan die enkeltoediening van wakslaagbehandeling soos bo beskryf, maar slegs die sensitiewe isolaat van P. digitatum is gebruik. Carnauba- of poli-etileen-gebaseerde wakslae, insluitende IMZ teen 3000 μg.mL-1, is teen 'n dosis van 0.6, 1.2 en 1.8 L.ton-1 toegedien, deur die gebruik van kommersiële perdehaar of sintetiese borsels. Vrugte is óf vir 4 dae vóór wond-infeksie evaluering geïnkubeer, óf teen -0.5oC vir 26 dae en 7 dae raklewe gestoor. Imazalil residuvlakke op vrugte het met toenemende waksladings op navellemoene (1.31 tot 3.32 μg.g-1) en Valencia lemoene (3.22 tot 6.00 μg.g-1) toegeneem. Beter beskermende (≈ 58%) as genesende beheer (≈ 14%) is waargeneem, met perdehaar borsels wat laer sporulasievlakke (≈ 59%) tot gevolg gehad het, in vergelyking met sintetiese borsels (≈ 64%). Vruggewig- en fermheidsverlies verhoudings, relatief tot onbedekte kontrole vrugte, was betekenisvol hoër in vrugte wat met 'n poli-etileenwaks behandel is, in vergelyking met carnauba-waks behandelde vrugte. Poli-etileen waksbehandeling het egter blinker vrugte vóór en ná opberging tot gevolg gehad, terwyl canauba waksbehandeling laer blink verhoudings tot gevolg gehad het. Gas (CO2) uitruilverhoudings het dieselfde vir beide waksbehandelings gebly, maar hoër poli-etileen waksladings (1.8 L.ton-1) het tot afsmake soortgelyk aan die onbedekte kontrole vrugte gelei, en hoër as die evaluerings vir carnauba. Skandeer-elektronmikrograwe het 'n amorfe gekristalliseerde natuurlike wakslaag met onbedekte stomata op die oppervlak van onbedekte vrugte getoon. Die dikte van die toegediende waks het met toename in wakslading toegeneem. Wakslae na toediening met sintetiese borsels teen 1.2 L.ton-1 het dikker vertoon in vergelyking met perdehaar borsels. Hierdie studie het die effekte van borseltipe op die verspreiding en polering van die wakslaag op die oppervlak van vrugte getoon, en versigtige toediening van wakslae onderskryf, aangesien ónder- of óórtoediening swak resultate, in terme van bio-effektiwiteit, residu-lading en vrugkwaliteit, tot gevolg kan hê. / National Research Foundation, THRIP, Citrus Academy & Citrus Research International for bursary and research funding.

South African ‘Eureka’ lemon fruit must be exposed to chilling temperatures (± 0.6°C) as a mandatory quarantine treatment against insect pests for all its overseas markets. Chilling lemon fruit at such temperatures may develop chilling injury (CI) symptoms on the flavedo. This negative effect on fruit quality reduces fruit marketability. This study evaluated postharvest factors influencing physiological, biochemical and ultra-structural mechanisms involved in alleviating CI in lemon fruit. It was hypothesised that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by maintaining cellular integrity and inducing synthesis of enzymatic and non-enzymatic antioxidants. Furthermore, fruit susceptibility to CI was associated with the source of fruit. Lemon fruit were harvested from three locations representative of moderate subtropical, warm temperate and cool subtropical environments. Harvested fruit were treated either with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, stored either at -0.5, 2 or 4.5°C for 0, 7, 14, 21, or 28 days and afterwards transferred to 23°C for a week as shelf-life simulation. Thereafter, fruit were evaluated for alterations in physiological, biochemical and ultra-structural features involved in the manifestation of CI symptoms. Chilling damage was more severe in untreated lemon fruit than in treated lemon fruit. Storing lemon fruit at 4.5°C accelerated the manifestation of CI symptoms more so than at 2°C while storage at -0.5°C delayed the manifestation of CI symptoms. Lemon fruit of moderate subtropical origin were more chilling-tolerant than lemon fruit of warm temperate and cool subtropical origin. Treatment with 10 μM MJ plus 2 mM SA significantly (P < 0.05) improved chilling tolerance in lemon fruit. This treatment effectively maintained membrane integrity, thereby retarding electrolyte leakage and membrane lipid peroxidation as well as mass loss and respiration rate. Treatment with 10 μM MJ plus 2 mM SA was also effective in enhancing the antioxidant concentrations of vitamin E and carotenoids. The production of these antioxidants could have been part of a defence system against chilling damage, reducing CI and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA enhanced the concentration of compounds involved in chilling resistance, such as proline, soluble sugars, ascorbic acid and total phenolics as well as the enzyme phenylalanine ammonia-lyase (PAL). The enhancement of the defence mechanisms may have played a role in enhancing chilling tolerance in lemon fruit. The treatment also inhibited certain enzymes involved in tissue browning, such as peroxidase (POD) which might have contributed to delaying manifestation of symptoms. Polyphenol oxidase (PPO) was found to not be a good biochemical marker of the occurrence of CI. Treatment with 10 μM MJ plus 2 mM SA appeared to be able to enhance chilling tolerance in lemon fruit by maintaining the ultra-structure of the cuticle, cell wall integrity, cell membrane of parenchyma cells of the flavedo. This treatment also preserved the mineral nutrients of the flavedo (carbon, oxygen, phosphorus, potassium, calcium, magnesium, sulphur, sodium, silicon and aluminium) during cold storage. This could have played a role in protecting the fruit against chilling stress and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA reduced ROS production, while the activity of enzymatic antioxidants such as catalyse (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and accumulation of essential proteins was enhanced. This increase in activity of enzymatic antioxidants and the presence of stress-responsive proteins in the lemon flavedo could have been directly involved in enhancing chilling tolerance. The CI symptoms were accompanied by an increase in membrane permeability, membrane lipid peroxidation as well as phospholipase D (PLD) and lipoxygenase (LOX) activity; however, treatment with 10 μM MJ plus 2 mM SA effectively reduced the membrane permeability, membrane lipid peroxidation, and PLD and LOX activity induced by the cold treatment. This could have contributed to the efficacy of 10 μM MJ plus 2 mM SA in inhibiting the manifestation of CI symptoms. Treatment with 10 μM MJ plus 2 mM SA enhanced flavedo total antioxidant capacity measured by ferric reducing ability of plasma; 2,2-diphenyl-1-picrylhydrazyl; 2,2-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) and the oxygen radical absorption capacity assays. The enhancement of antioxidant capacity in lemon flavedo could have contributed to the fruit’s chilling tolerance. Therefore, the effect of 10 μM MJ plus 2 mM SA treatment, enhancing chilling tolerance, may be attributed to its ability to enhance enzymatic and non-enzymatic antioxidants; activate essential proteins and mitigate the effect of ROS accumulation. With the use of 10 μM MJ plus 2 mM SA treatments, the South African citrus industry will be able to meet the quarantine temperature requirements for exportation of lemon fruit whilst reducing economic losses, depending on the preharvest conditions experienced by the fruit in each shipment. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Thesis (MscAgric (Horticulture))--University of Stellenbosch, 2007. / More and more markets develop around the world for South African grapes and it becomes a challenge to store grapes for longer and reach the market with superior quality. The most destructive decay fungus, Botrytis cinerea can cause huge economic losses and successful postharvest control in the table grape industry relies on SO2. This gas not only controls the fungus but also causes losses due to phytotoxicity. SO2 also creates allergic reactions amongst certain people. In modern times the focus is on food safety and governments consequently impose certain regulations and restrictions to restrict the use of chemicals and ensure “cleaner” produce. The objective of this study was to find a steriliser to reduce B. cinerea inoculum on the berry surface prior to storage,to be used in conjunction with the current method of control – the SO2 generator pad.