Horticultural Science

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Enhancement of 'Hass' avocado shelf life using ultra-low temperature shipping or 1-MCP treatment and cold chain management.
(2011) Kok, Richard Dean.; Bower, John Patrick.; Bertling, Isa.
Avocados are becoming an increasingly important crop in South Africa, where the main producing areas include Limpopo, Mpumalanga and KwaZulu-Natal provinces. The South African avocado industry faces considerable challenges including increasing competition exporting avocados, particularly to the European market. The processes involved to export avocados has markedly improved over the past two decades, however there is always room for improvement and it is necessary to remain competitive on a global scale. Issues such as fruit being partially soft on arrival, quality defects and cold chain management breakdown are still present. It is necessary to investigate new aspects of cold storage such as extending the storage period and understanding the physiological aspects involved. To improvement such issues, an investigation was conducted on ultra-low temperature shipping (1°C) as well as the use of 1-MCP; the implementation of deliberate cold chain breaks to achieve a better understanding as to the quality influences involved; an extended storage period of 56 days to assess the quality issues and benefits involved; as well as investigating the physiological aspects involved with all above treatments on 'Hass' avocados. An initial study saw early-, mid- and late-season 'Hass' avocados stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. Storage at 1°C was comparable with 1-MCP treatment for both fruit softening in storage and extending the ripening period. Storage at 5.5°C resulted in partial in-transit ripening, if 1-MCP was not used. Early-season fruit incurred the most external chilling injury but overall levels were minimal and not concerning. Mid-season fruit were the most sound in terms of quality. It is suggested that 1°C can be used as a viable economic alternative to 1-MCP for long distance shipping of 'Hass' up to 28 days. The cold chain break trial included a 24 hour delay before cold storage, a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage and a control of 28 days cold storage where no break was involved. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. It was found that cold chain breaks do influence the amount of water loss, fruit softening and days taken to ripen. Storage at 1°C did not entirely negate the effects of cold chain breaks compared with 5.5°C, but did result in fruit which were harder at the end of storage and took longer to ripen. The use of 1-MCP also had advantageous effects with respect to significantly lengthening the ripening period, even when a cold chain break occurred, compared with fruit not treated with 1-MCP. As results of the study differed in some respects to those of previous studies, it is recommended that further work be conducted to determine what fruit or pre-harvest factors affect the fruit physiological changes which take place when cold chain breaks occur. Having the option to make use of an extended storage period would be of benefit to the industry if delays occur and fruit have to be maintained under cold storage. Extended storage of South African avocados, especially at the end of the season would also allow for the option of strategically holding back fruit from the export market in order to extend the supply period. It would not only benefit export options, but would also be highly beneficial to local pre-packers, as it would reduce the need to import fruit from the Northern hemisphere production areas during the South African off-season. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 56 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The combination of 1°C with the use of 1-MCP resulted in a good shelf life as well as maintenance of internal quality and integrity. External chilling injury is of concern for early-season fruit, however, mid- and late-season fruit did not incur extensive damage. It is, therefore, advised that fruit placed in extended storage are marketed through the 'Ready ripe' program to mask any chilling injury on the 'Hass' fruit. Avocados are renown as a "healthy food" due to their nutritional value as well as containing relatively high concentrations of antioxidants. The fruit also contain high amounts of C7 sugars which can act as antioxidants. Additionally, C7 sugars and other antioxidants play important roles in fruit quality. Therefore, it is important to understand how varying storage conditions and treatments affect the levels of these physiological parameters. Treatments of cold chain break/delay included a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage, a 24 hour delay before cold storage and a control of 28 days where no break was involved. A 56 day extended storage period was also used. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The use of 1-MCP maintained higher levels of antioxidants, ascorbic acid and C7 sugars for both the 28 day and the 56 day storage periods. The 24 hour delay had a tendency to increase consumption of anti-oxidant and sugar reserves. The use of 1°C resulted in antioxidant and ascorbic acid levels decreasing while maintaining higher sugar levels. Overall, high stress imposed on fruit decreased reserves resulting in poor quality fruit. The use of 1°C and 1-MCP treatments maintained fruit quality.
Some aspects of cold storage of 'Fuerte' avocados (Persea americana Mill.) grown in the Natal midlands.
(1995) Donkin, Derek John.; Wolstenholme, B. Nigel.; Cutting, Jonathan Garth Melville.
The South African avocado industry is largely export orientated and export by sea to European markets necessitates cold storage for up to 4 weeks at temperatures around 5.5°C. Avocado fruit is subject to chilling injury which is manifested as mesocarp discolouration, and pitting and blackening of the rind. Of the South African cultivars exported, 'Fuerte' is the most susceptible to chilling injury, and accounts for > 50 % of avocado exports. A number of temperature regimes where temperature was reduced in a step-wise fashion from 8.5 or 7.5°C to 4.5 or 5.5°C during 3 to 5 weeks of storage were tested weekly throughout the 1993 and 1994 'Fuerte' harvesting seasons in the Natal Midlands (a cool mesic subtropical area), in attempt to find cold storage temperature regimes which would minimise chilling injury. No definite trends with regard to certain temperature regimes resulting in fruit with less chilling injury were evident. Overall, stepped down temperature regimes produced fruit of quality no better than storage for 5.5°C for 4 weeks. There was no significant difference in concentration of total phenolics in 'Fuerte' fruit mesocarp throughout the 1994 harvesting season (P < 0.05). Levels of ethylene evolution during 4 weeks of storage at 7.5 and 5.5°C ranged from 0 to 5 µl.kg(-l).h(-l), and peaked at 109 and 75 µl.kg(-l).h(-1) in fruit stored at 7.5 and 5.5°C respectively at room temperature on removal from cold storage. Rapid moisture removal from 'Fuerte' fruit after harvest and before cold storage by placing the fruit in glass jars to which a suction of -75 kPa was applied, resulted in increased susceptibility to external chilling injury, the severity of which was proportional to the amount of moisture removed from the fruit. Pre-storage heat treatments with a view to decreasing sensitivity of fruit to cold storage were carried out on 'Fuerte' fruit. Dry heat and warm water baths at temperatures of 36 to 40°C caused rind blackening of varying severity, depending on temperature and duration. Vapour heat treatments at temperatures of 36 to 48°C for 10 min to 48 h also caused rind blackening, with the exception of 10 min at 48°C and 1.5 and 3 h at 40°C which produced fruit of higher overall quality after 4 weeks of cold storage at 3.5°C than fruit not heat treated. These treatments however, could not be repeated in 1994 to confirm the results obtained as the harvesting season was over by the time the trial was completed.
Ultra-low temperature shipping and cold chain management of 'fuerte' avocados (Persea americana Mill.) grown in the KwaZulu-Natal Midlands.
(2011) Lutge, Andre.; Bertling, Isa.; Bower, John Patrick.
‘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.

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