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The focus of this thesis is to develop pectin-based edible films for application of fruits and vegetables to extend their post harvest shelf life. Preliminary research demonstrated that edible coatings could effectively extend the shelf life of based avocado and peach. The respiration rate, moisture loss, firmness, chemical parameters and color changed in a lower rate in coated fruits as compared with the control. Pectin-based film formulations were then evaluated to identify the proper type and concentration of pectin, lipids and plasticizers in the film. The effects of pectin, beeswax and sorbitol concentration on water vapor permeability, mechanical properties and opacity of the films were then evaluated using response surface methodology to identify appropriate levels of different components. Results of studies on film structure revealed that water vapor permeability increased by pectin and sorbitol concentration and was decreased by beeswax concentration. Mechanical properties were mainly affected by pectin and sorbitol concentration. Beeswax was the most influential factor that affected opacity which increased with increasing beeswax concentration. / In order to successfully employ these films, their adsorption behavior, thermal and thermomechanical properties were evaluated as a function of moisture content and sorbitol concentration. The adsorption behavior was strongly influenced by sorbitol concentration. Moisture content and sorbitol concentration increased the films elongation at break, but decreased tensile strength, modulus of elasticity and Tg, and increased water vapor permeability of the films. Finally, avocado was coated with a pectin-based film and the associated quality changes were evaluated during storage. From storage studies, kinetic parameters (rate constants) and activation energy were quantified to help model the quality changes in avocado quality as function of storage temperature and time. Pectin-based coating resulted in slowing down the rate of quality changes in avocado at each storage temperature. In general, most changes were well described by some form of zero or first order rate. Temperature sensitivity of rate constant was adequately described by the Arrhenius model. / A hyperspectral imaging technique was also used to gather additional tools for following quality changes associated with stored avocados. Artificial neural network (ANN) concepts were evaluated as alternated models for predicting quality changes in coated and non-coated avocados during storage at different temperature. Modeling of quality changes in avocado indicated that compared to conventional mathematical models, ANN has more feasibility to predict of these changes. Models developed for firmness, weight loss and total color difference had better fitness than respiration rate. / Finally, the effect of coating on disease severity and different properties of avocados infected by Lasiodiplodia theobromae was studied. The coated fruits demonstrated slower rate of disease progress, respiration rate, softening and color changes. Respiration rate, firmness and color parameters were sensitive to coating and disease severity, and thus these parameters could successfully used to predict fruit quality from disease in coated and uncoated avocados.

Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction. / Thesis (Ph.D.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

The South African avocado industry is export-orientated with forty percent of total production sold overseas. The avocado fruit is a highly perishable product with a relatively high rate of respiration which results in the quick deterioration of fruit quality. Good phytosanitary procedures are a necessity in ensuring good product quality. Due to the threat of pests and diseases becoming resistant to the conventional chemicals currently used to control them, there has been a great need to diversify from their usage. Silicon (Si), being the second most abundant element (28%) in the earth’s crust after oxygen, is a major constituent of many soils and has been associated with disease resistance in plants for a long time. It has been used in a number of crop species to provide resistance against pathogenic agents. In some horticultural crops Si has been found to offer protection against fungal infections by strengthening cell walls, thus making it more difficult for the fungi to penetrate and colonize the plant. The aim of this research was to investigate the effects of postharvest silicon application on the quality of ‘Hass’ avocado fruit. The specific objectives included investigating the effect of silicon on the ripening pattern as well as the metabolic physiology of the avocado fruit. Avocado fruit were obtained from two locations in the KZN Midlands (Everdon Estate in Howick and Cooling Estate in Wartburg). Fruit were treated with different forms of Si (potassium silicate (KSil), calcium silicate (CaSil), sodium silicate (NaSil) and Nontox-silica® (NTS)) at concentrations ranging from 160 ppm to 2940 ppm. After dipping for 30 minutes in the silicon treatments, the fruit were stored at -0.5°C, 1°C, 5°C or at room temperature (25°C). Energy dispersive x-ray (EDAX) analysis was then conducted on the exocarp and mesocarp tissues to determine the extent of silicon infiltration within each treatment. Firmness measurements, ethylene evolution and CO2 production were recorded as fruit approached ripening. The CO2 production of fruit that were stored at room temperature was analysed daily until they had fully ripened, while fruit from cold storage were removed weekly to measure respiration. Mesocarp tissue from each fruit was extracted using a cork borer and subsequently freeze-dried and stored for physiological analysis. The freeze-dried mesocarp tissue was then finely ground and later analysed for sugar content, total anti-oxidant capacity (TAOC), total phenolic (TP) content and phenylalanine ammonia lyase (PAL) activity using their respective assays. Statistical analyses were carried out using GenStat® version 11 ANOVA. Treatment and storage temperature means were separated using least significant differences (LSD) at 5% (P = 0.05). The experimental design in this study was a split-plot design with the main effect being storage temperature and the sub-effect being treatments. Each replication was represented by a single fruit. EDAX analysis revealed that Si passed through the exocarp into the mesocarp tissue in fruit treated with high concentrations of silicon, i.e., KSil 2940 ppm. Significant differences (P < 0.001) were observed in temperature means with regards to firmness. Fruit treated with KSil and NTS only and stored at 5°C were firmer than fruit stored at other temperatures. Fruits treated with Si in the form of KSil 2940 produced the least amount of CO2, while non-treated fruits (Air) had the highest respiration rate. Fruit stored at room temperature (25°C) produced significantly higher amounts of CO2 and peaked much earlier than fruit stored at other temperatures. Ethylene results showed that there were differences (P < 0.05) between temperature means with the highest net ethylene being produced by fruit stored at 25°C. There were also significant differences amongst treatment means (P < 0.001), with fruits treated with KSil 2940 ppm producing the least ethylene. There were significant differences (P < 0.001) in temperature means with regards to the total phenolic concentration with fruits stored at 1°C having the highest TP concentration (26.4 mg L-1 gallic acid). Fruit treated with KSil 2940 ppm had the highest total phenolic concentration whilst the control fruit (Air and Water) had the lowest. There were also differences (P < 0.05) in storage temperature means with respect to the total antioxidant capacity. Fruit stored at -0.5°C had the highest TAOC (52.53 μmol FeSO4.7H2O g-1 DW). There were no significant differences in TAOC (P > 0.05) with regards to treatment means although fruit treated with KSil 2940 ppm and stored at -0.5°C showed the highest TAOC of 57.58 μmol FeSO4.7H2O g-1 DW. With regards to the concentration of major sugars in avocado, mannoheptulose and perseitol (mg g-1), no significant differences (P > 0.05) were observed in temperature means. However, fruit stored at -0.5°C had the highest concentration of these C7 sugars compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) showing that fruit treated with KSil 2940 ppm had the highest concentration of both mannoheptulose (18.92 mg g-1) and perseitol (15.93 mg g-1) in the mesocarp tissue. Biochemical analyses showed differences (P < 0.05) in storage temperature means with respect to PAL enzymatic activity. Fruit stored at 5°C had the highest PAL activity (18.61 mmol cinnamic acid g-1 DW h-1) in the mesocarp tissue compared with fruit stored at other temperatures. There were significant differences in treatment means (P < 0.001) with regard to PAL activity. Fruit treated with KSil 2940 ppm had the highest PAL activity (23.34 mmol cinnamic acid g-1 DW h-1). This research has demonstrated the beneficial effects, particularly applications of 2940 ppm Si in the form of KSil. This treatment successfully suppressed the respiration rate of avocado fruit. Biochemical analyses of total antioxidants, total phenolics and PAL activity in the mesocarp tissue have shown the usefulness of Si in improving the fruit’s metabolic processes. The C7 sugars (D-mannoheptulose and perseitol) also seem to be more prevalent in avocado fruit treated with Si (particularly KSil 2940 ppm) than in non-treated fruit. This suggests that an application of Si to avocado fruit can aid in the retention of vital antioxidants (C7 sugars). / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2010.

There is increasing interest in special heptose carbohydrates, their multifunctional roles from a plant physiological view point in fruit growth and development as well as in the whole plant in general due to their potential in mitigating photo-oxidative injury to the whole plant system and the image of avocado as ‘health fruit’. Studies have been carried out to investigate the role of avocado heptoses, rare carbohydrates predominantly produced in avocado. Several authors have documented various research findings and speculated on multifunctional roles of avocado special sugars. However, few reports have made an attempt to elucidate the multifunctional roles of avocado heptose carbohydrates as: ‘sources of energy’, storage and phloem-mobile transport sugars, and precursors for formation of antioxidants. Assessing the avocado carbohydrates over the plant growth and development during ontogeny may, therefore, offer clues to better understand whole plant behaviour. Plant sampling was carried out over different developmental stages. Using plants grown in the light versus etiolated seedlings; sugar determinations were also done to determine what sugar is produced from which storage organs. The sugars were extracted and analysed by isocratic HPLC/RID. The embryo had 47.11 % hexose and 52.96 % heptose sugars. The seed, however, also released significant amounts of D-mannoheptulose (7.09 ± 1.44 mg g-1 d. wt) and perseitol (5.36 ± 0.61 mg g-1 d. wt). Similarly fruit and leaf tissues had significant amounts of heptoses relative to hexoses at specific phenological stages. In postharvest ‘readyto-eat’ fruit the following carbohydrate concentrations were as follows:exocarp heptoses 13 ± 0.8; hexoses 4.37 ± 1.6 mg g-1 d. wt, mesocarp heptoses 8 ± 0.2; hexoses 3.55 ± 0.12 mg g-1 d. wt), seed heptoses (only perseitol) 13 ± 1.1; hexoses 5.79 ± 0.53 mg g-1 d. wt. The results of this experiment was the first to demonstrate that the heptoses D-mannoheptulose, and its polyol form, perseitol, are found in all tissues/organs at various phenological stages of avocado growth and development. Secondly, heptoses, as well as starch are carbohydrate reserves that are found in avocado. The heptoses, beyond being abundantly produced in the avocado plant, are also found in phloem and xylem saps as mobile sugars. The study also presents data on the interconversion of the C7 sugars Dmannoheptulose and perseitol. It is deduced that D-mannoheptulose can be reduced to perseitol, and perseitol can also be oxidized to D-mannoheptulose by enzymes present in a protein extract of the mesocarp. The potential catalyzing enzyme is proposed to be an aldolase, as electrophoretic determinations prove the presence of such an enzyme during various stages of development in various plant organs. Avocado heptoses play an important role in plant growth and development and in fruit in particular. Moreover, they are reported as sources of anti-oxidants, and contribute significantly to fruit physiology if they function in coordination with other anti-oxidants in fruit tissues. To evaluate the presence of anti-oxidant systems throughout avocado fruit development, various tissues were analysed for their total and specific anti-oxidant compositions. Total anti-oxidant levels were found to be higher in the exocarp and in seed tissue than in the mesocarp. While seed tissues contained predominantly ascorbic acid (AsA) and total phenolics (TP), the anti-oxidant composition of the mesocarp was characterised by the C7 sugar, D-mannoheptulose. Among the anti-oxidant enzymes assayed, peroxidase (POX) and catalase (CAT) were present in higher concentrations than superoxide dismutase (SOD) in mesocarp tissue. Different anti-oxidant systems seem to be dominant within the various fruit tissues. Carbohydrates are the universal source of carbon for cell metabolism and provide the precursors for the biosynthesis of secondary metabolites, for example via the shikimic acid pathway for phenols. The preharvest free and membrane-bound phenols, catechin and epicatechin, are distributed differently in the various fruit tissues. Membrane-bound and free phenols also play a role as anti-oxidants, with free ones being more important. KSil (potassium silicate) application to fruit as postharvest treatment was used to facilitate the release of conjugates to free phenols via lysis. This treatment improved fruit shelf life. Western blotting also revealed that postharvest Si treatment affects the expression of enzymatic anti-oxidant-catalase (CAT). Overall the thesis results revealed that C7 sugars have anti-oxidant properties and that D-mannoheptulose is the important anti-oxidant in the edible portion of the avocado fruit. Dmannoheptulose is furthermore of paramount importance as a transport sugar. Perseitol on the other hand acts as the storage product of D-mannoheptulose, which can be easily converted into D-mannoheptulose. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.

‘Fuerte’ makes up 25% of the avocados exported from South Africa to European markets and requires shipping periods of up to 28 days and a correctly managed cold chain. A temperature of 5.5°C and expensive CA and 1-MCP treatments are currently used to delay ripening over this lengthy cold chain; however, fruit still appear on the European market showing signs of softening and physiological disorders. Increased competition on the global market and the disadvantage of a particularly long distance to the European market has challenged the South African export industry. These challenges have necessitated improved road and sea transport logistics, co-ordination with producing countries which supply fruit to European markets over similar periods as South Africa, and research into ultra-low temperature storage to possibly enable future access to new lucrative markets in the USA, China and Japan. It is also known that there are various ‘weak links’ in this cold chain and that cold chain breaks are detrimental to fruit quality, but further research into the negative effects of these cold chain breaks at ultra-low temperatures was needed. Thus, the objective of the study was to determine the potential for shipping ‘Fuerte’ avocados at temperatures of 2°C as well as determining the effects of cold chain breaks on fruit quality, throughout the growing season and possibly for an extended period of 56 days. ‘Fuerte’ avocados were harvested at three different maturity stages reflecting early-, mid- and late-season fruit, with moisture contents of 74%, 68% and 63%, respectively. Fruit were stored at 2°C or 5.5°C, treated with 1-MCP and waxed. Additionally cold chain breaks (24 hour delay and break at 14 days) were implemented. Fruit softening, mass loss, days-to-ripening, external and internal quality as well as antioxidant levels and total sugar levels were determined. The first aim was to determine whether a lower than currently used storage temperature could be a successful alternative to 1-MCP use. A storage temperature of 2°C provided good internal quality as well as reduced mass loss and fruit softening, which is related to the slightly reduced use of C7 sugars at 2°C compared with 5.5°C. Although the overall occurrence of external chilling injury was relatively low, 2°C storage caused a notably higher occurrence of external chilling injury than 5.5°C storage, particularly early in the season, but extended the days-to-ripening. Unfortunately, no correlation between the anti-oxidants in the exocarp and external damage was found. Waxing significantly reduced the external damage on fruit stored at 2°C, so much so, that the treatment combinations of ‘2°C, no 1-MCP, waxed’ showed no external chilling injury throughout the season. Further, waxing fruit at 2°C could eliminate the need for 1-MCP, delivering a product of the required shelf-life and quality. Best results were achieved for mid-season fruit stored at 2°C. Late-season fruit would potentially be the most profitable to store at this low temperature, however, body rots (anthracnose and stem-end rot) were more common in the late-season. Storage at 2°C can therefore maintain the internal quality over a storage period of 28 days and be a potential alternative to 1-MCP use as the season progresses. The effect of cold chain breaks on fruit quality was then investigated and showed that both a delay and a break in the cold chain increased mass loss and fruit softening, reduced days-to-ripening and increased external chilling injury, especially early in the season. Water loss was the main contributor to the decreased fruit quality which resulted from the delay in cooling, increasing external damage significantly, particularly early in the season. The break at 14 days had a marked effect on physiological activity of fruit during storage, seen mainly in the increased metabolic activity, resulting in increased fruit softening and water loss during storage and a decrease in C7 sugars and thus shelf-life, particularly for fruit stored at 5.5°C. Importantly, 1-MCP use and storage at 2°C reduced the effects of cold chain breaks with respect to fruit softening, however, lowering the storage temperature had a greater negating effect than 1-MCP and could be a successful alternative to the use of 1-MCP. The internal quality throughout the experiment was very good, with few internal disorders and no significant treatment effects on internal quality and C7 sugar concentrations. Overall, a break in the cold chain, before and during cold storage, resulted in a marked reduction in fruit quality. The storage temperature of 5.5°C should not be used for a 56 day storage period as it resulted in significant fruit softening during storage, even when 1-MCP was used, and resulted in significantly more external chilling injury in the mid- and late-season than at 2°C. Storage of 1-MCP treated, waxed fruit at 2°C, resulted in the best shelf-life and fruit quality, particularly mid-season fruit which had negligible external chilling injury and 100% sound fruit. Early-season fruit suffered significant external chilling injury at 2°C and late-season fruit had the highest body-rots and internal disorders at this storage temperature. Although mid-season fruit could be successfully stored at 2°C for 56 days, the use of a 56 day storage period is not recommended as a practical storage period, due to the high risk of external damage, particularly if maturity levels are not optimum and trees and fruit are not of the highest quality. Overall this thesis has shown that 1-MCP treatment can play an important role early in the season when fruit are susceptible to external damage, however, storage at 2°C results in good quality fruit and, when used in conjunction with waxing, appears to be a viable alternative to the use of 1-MCP, particularly later in the season. Further, the negative effects of cold chain breaks on fruit quality have been demonstrated and, importantly, the storage temperature of 2°C negates the fruit softening effects of these breaks, even if 1-MCP is not used. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

Avocados are one of the major food sources in tropical and subtropical regions and are an important horticultural crop in South Africa. Avocados are exported over long distances and may have storage times of up to 30 or more days at temperatures of about 5.5oC. This procedure increases the risk of poor fruit quality, including physiological disorders, early softening and postharvest disease incidence. A major component of the postharvest diseases is Anthracnose caused by Colletotrichum gloeosporioides. Anthracnose infects unripe fruit and once infected, the fungus remains dormant in the fruit until ripening begins. This leads to a problem for producers and packers, as the presence of the disease cannot be detected on the pack line, and fruit is not removed. Anthracnose control is normally done through pre-harvest treatment with copper-based fungicides. While effective such treatment needs to be repeated frequently, resulting in copper residues on the avocados. The study was conducted to investigate the effects of phosphoric acid and potassium silicate on known antifungal compounds and critical enzymes of the pathways elemental for systemic resistance inducers, so as to evaluate the potential for using them as alternatives to or in conjunction with, copper fungicides in the control of Anthracnose in avocado fruit. The study included storage temperature and time variations, to take account of the logistics in shipping avocado fruit to distant markets. Pre- and postharvest applications of phosphoric acid and potassium silicate were used, and after harvest, fruit were either ripened at room temperature (22oC) without storage or stored for 28 days at temperatures of 5.5oC or 2oC before analysis. Concentrations of phenolics, activity of the enzyme phenylalanine ammonia lyase (PAL) and a known antifungal diene were determined in the fruit exocarp. Pre-harvest treatments of phosphoric acid showed that the highest phenolic concentration was found in fruit harvested 14 days after application for fruit stored at room temperature. For fruit stored at 5.5°C it was seen that as fruit softened, phenolic concentrations increased compared with hard fruit immediately after storage, with the highest increase noted for fruit harvested 7 days after application. When comparing the three storage temperatures, phenolic concentrations were enhanced most when fruit was stored at 2°C. Postharvest treatments showed a significant increase in phenolic concentrations for potassium silicate treated fruit stored at room temperature and 2°C when determined immediately after storage. Fruit stored at 5.5°C showed an increase in phenolic concentrations as it became softer. When considering PAL enzyme activity, it was found that postharvest treatments of both potassium silicate and phosphoric acid influenced enzyme activity, with potassium silicate having greater effects. Similarly, an increase in PAL activity was noted in the pre-harvest phosphoric acid treatment harvested 14 days after application for fruit ripened immediately as well as fruit stored at 5.5°C. Fruit stored at 2°C showed the highest PAL activity for fruit harvested 7 days after application. No results were obtained in the analysis of antifungal compounds for both pre- and postharvest treatments. However, it is suggested that the antifungal diene could follow similar trends to those found for phenolics. It is concluded that applications of both phosphoric acid and potassium silicate do create changes in phenolic concentrations and the activity of the enzyme PAL which is involved in the synthesis of phenolic compounds known to possess antifungal properties. It is therefore possible that phosphoric acid and potassium silicate may be used as part of an integrated programme for Anthracnose control, and should be tested as potential alternatives for high volume copper-based fungicides. / 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.