TECHNICAL |
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fruit , which is close to saturation with water ( Cutting & Wolstenholme , 1992 ; Magwaza et al ., 2013a ). The differences between control and coating treatments recorded in the current study could be due to the nature of edible coating properties , which is hydrophilic , and it displays less resistance to water loss than the lipophilic coatings ( Kester & Fennema , 1986 ).
This observation has been anticipated in avocado fruit tissues which accumulate high oil content that has a lipophilic functional property which exhibits resistance to water loss ( Kruger , 2013 ). The results observed in the current study are also in agreement with Maftoonazad & Ramaswamy ( 2005 ) who showed that methyl cellulose retards moisture loss in avocado fruit . Higher postharvest mass loss observed in the untreated control fruit could also be explained by the removal and reorganisation of natural wax on the fruit surface which is brushed down and reorganised by standard commercial brushing and washing ( Magwaza et al ., 2013b ).
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Fig . 3 . Effect of ozone in coated and uncoated avocado fruit mass loss during postharvest storage time . Vertical bars represent standard error ( SE ) of the mean value ( n = 5 ). The SE refers to the standard deviation of each measured values from the mean value . |
Mesocarp electrical conductivity The cultivar had low EC during the first 14 d of cold storage ( Fig . 4 ). The fruit membrane only started to release more electrolytes after 14 d and increased EC exponentially during the shelflife storage at ambient condition . These observations suggest that EC of avocado fruit increases with postharvest storage time .
The treatments had a significant effect on mesocarp EC and this was consistent over storage time . Fruit with coating as well as without coating but both exposed to ozone had respective EC of ( 3.5 ± 0.2 m . mohs / cm ) and ( 5.1 ± 0.4 m . mohs / cm ) which were significantly lower than that of the control fruit ( 11.2 ± 0.8 m . mohs / cm ). The fruits with any treatments retained fruit firmness longer than other control treatments . The observed retention of firmness may suggest that these treatments resulted in better membrane integrity which improved fruit quality and shelf-life .
Similar results were reported by Azad ( 2006 ), fruit treated with any coating material retains firmness and releases less EC . The observed disrupted ion balance and electrolyte leakage is hypothesised to have resulted from ultrastructural changes in the membranes ( Lyons , 1973 ). In addition , the relative EC of the mesocarp tissue in the current study was gradually increasing during postharvest storage , suggesting a continuous and a gradual loss of cell membrane integrity .
Upon removal from cold storage , the control fruit had the lowest values of firmness .
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Fig . 4 . Effect of ozone on avocado mesocarp electrolyte leakage for coated as well as uncoated fruit during storage time . Vertical bars represent standard error of the mean value ( n = 5 ). The SE refers to the standard deviation of each measured values from the mean value .
Fruit firmness Fruit firmness decreased significantly ( p < 0.05 ) with storage period and these trends were similar for both treated and control fruit , although the rate of decrease was not the same ( Fig . 5 ). In ‘ Gem ’, the highest value of firmness was maintained by the uncoated fruit and ozone exposure ( 63 ± 4.25 N ) and the coated fruit with ozone exposure ( 78.0 ± 3.75 N ).
Upon removal from cold storage , the control fruit had the lowest values of firmness ( 51 ± 3.0 N ) ( Fig . 5 ). The softness of fruit is typical of avocado fruit resulting from the weakening of the cell wall , loss of membrane integrity , hydrolysis of cellulose and hemicellulose as well as depolymerisation of pectin and starch ( Seymour , Taylor , & Tucker , 1993 ).
The hydrolysis and depolymerisation of these components are catalysed by the action of several enzymes , including polygalacturonase , pectin methylesterase , pectatelyase , Rhamnogalacturonase and bgalactosidase ( Yaman & Bayoindirli , 2002 ; Payasi , Mishra , Chaves , & Singh , 2009 ).
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Structural changes in the integrity of cell wall are likely to result from the activities of cellulase ( Pesis , Fuchs , & Zauberman , 1978 ; Tucker & Laties , 1984 ) which decreases cellular cohesion by cell disarrangement and degradation of pectin ( Awad & Young , 1979 ). The gaseous composition of lower O 2 and higher CO 2 percentage in coated fruit and ozones is likely to limit the activities of oxidising enzymes allowing the retention of firmness during postharvest storage ( Salunkhe , Boun , & Reddy , 1991 ). Our findings are therefore in agreement with this argument and that of Jeong , Huber , and Sargent ( 2000 ) who stated that firmness of untreated avocado fruit decreased faster whereas fruit treated with both 1- MCP and wax retained firmness for a longer period . Loss of firmness during marketing negatively affect fruit quality , hence the ability of these coating treatments together with ozone to reduce the rate of softening is advantageous in extending shelflife and reducing postharvest losses .
Fig . 5 . Effect of ozone on avocado fruit softness for coated as well as uncoated fruit during storage time . Vertical bars represent standard error of the mean value ( n = 5 ). The SE refers to the standard deviation of each measured values from the mean value .
Mesocarp soluble sugars A significant ( p < 0.05 ) differences was observed in concentrations of soluble
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