TECHNICAL |
INCORPORATING COLD CHAIN |
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detector on a Phenomenex ® column ( Rezex RCM – Monosaccharide ). The concentration of individual sugars was determined by comparison with authentic sugar standards .
Hexanal and pentanal determination using Gas mass spectrophotometry ( GC / MS ) Fruit volatiles were determined according to Obenland et al . ( 2012 ) with slight modification . Briefly , mesocarp tissue ( 20g ) was cut and homogenised with 40ml saturated NaCl for 1 min . The addition of NaCl was to limit the formation of volatiles in order to figure out the actual volatiles during the time of sampling . Volatile compounds were analysed using coupled Varian 3800 gas chromatography ( Varian Palo Alto , California , USA ) and Varian 1200 mass spectrometry ( GC-MS ). The GC was equipped with an Alltech EC-WAX column of 30 m x 0.25 mm internal diameter x 0.25 μm film thickness ( Alltech Associates Inc ., Deerfield , Illinois , USA ). Helium was used as the carrier gas at a flow rate of 1ml / min . Compound identification was carried out using the NIST05 mass spectral library and comparisons with retention times of chemical standards , as well as comparisons between calculated Kovats retention indices and those published in the literature . Clean chromatoprobe traps were run in GC-MS as controls to identify background contamination .
Data analysis The data collected was analysed using statistical software using GenStat 18.1 . Standard error values were calculated where a significant standard deviation was found at P ≤ 0.05 between individual values .
RESULTS AND DISCUSSION Fruit respiration The fruit was respiring at a slower rate for the first 14 d during cold storage and started to increase at a faster rate towards the end of the cold storage ( Fig . 1 ). In the current study , a gradual increase in the rate of respiration was observed during postharvest storage , indicating that the rate
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of avocado fruit respiration increases as the fruit ripens or senesces .
For instance , in ‘ Gem ’, combined effects of coating and ozone exposure treatments had a significant effect on fruit respiration with fruit displaying the slower rate ( 170 ± 55 mg / kg /. h ) compared to untreated fruit ( 220 ± 60 mg / kg ./ h ). Coated fruit with two times ( 14d , 21d ) and three times ( 7d , 14d , 21d ) ozone exposure , significantly reduced fruit respiration rate to ( 160.0 ± 40.8 mg / kg ./ h ) and ( 130.0 ± 20.3 mg / kg /. h ), respectively .
There were differences in fruit oxidation during the storage period . In avocado , typically a climacteric fruit , the increase in respiration rate , which is triggered by ethylene accumulation , is accompanied by a complex of biochemical changes resulting in fruit softening . The results observed herein are similar to those reported in other studies on avocado fruit , where respiration have been shown to increase with time and the rate reduced by coating treatments ( Jeong , Huber , & Sargent , 2002 ; Jeong , Huber , & Sargent , 2003 ). Minas et al . ( 2014 ) reported Ozone delays ripening and inhibits ethylene production and respiration rate of Kiwi fruit . They further stated that upon transfer to 20 ° C after 2 months , control fruit exhibited a typical climacteric rise in ethylene production , as opposed to ozone treated fruit , which consistently showed basal levels of ethylene production .
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Fig . 1 . Effect of ozone on coated as well as uncoated avocado fruit CO 2 production during postharvest 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 ethylene production In ‘ Gem ’ avocado , ethylene production rate was significantly reduced ( p < 0.05 ) in fruit coated with CMC and ozone treatments ( Fig . 2 ). Between the two treatments ozone and control , the treatment consisting of edible coating and ozone had the lowest ethylene levels throughout cold storage and shelf-life . At shelf-life , the ethylene production rate reached 11.8 mg kg−1 fruit for the control treatment while it was 3.5 mg kg−1 fruit for the edible coating and ozone . The increasing pattern of ethylene production observed at shelf life was due to the positive correlation between temperature and the rate of metabolic reactions in fruit tissues ( Tesfay & Magwaza , 2017 ). However , in the current study , fruit treated with edible coating and ozone did not have a similar rise in ethylene production at shelf life , indicating that these treatments suppressed metabolic reactions and delaying tissue sensitiveness towards ethylene concentration in the storage room .
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Fig . 2 . Effect of ozone on ethylene production of coated as well as uncoated fruit . 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 mass loss Coating treatments had a significant influence on mass loss of avocado fruit during postharvest storage . Similarly the uncoated fruit exposed to ozone also had significant differences with untreated control fruit . In this case fruit mass loss may be reduced due to slowing down of fruit biological process , it has possibly been occurred due to chemical reaction between ozone and ethylene . Higher fruit mass loss was observed in untreated control compared to treated fruit ( p < 0.01 ) ( Fig . 3 ).
Further , and perhaps very important , was the rate of mass loss over the storage period . The greatest mass loss was observed during the first ten days of postharvest storage . In fact , most moisture loss and thus mass loss could be expected to occur during the initial cooling of the fruit ( Wills , McGlasson , Graham , & Joyce , 1998 ; Bower & Magwaza 2004 ). Fruit moisture loss was consistently declining for all treatments during cold storage as well as during ripening stage at ambient conditions .
The most efficient mass loss control was in the fruit coated with a combination of coated and ozone exposure times . Untreated control fruit continued losing water into the cold room atmosphere . This is due to the cold room atmosphere continually being dried as it passes through the cooling coil and the resultant deficiency of water being replaced down the concentration gradient from the fruit to the atmosphere ( Bower & Magwaza , 2004 ). Observed moisture loss was mostly due to moisture loss by transpiration , which accounts for about 90 % of total mass loss in fruit ( Magwaza et al ., 2013a ).
This is in accordance with the literature stating that moisture loss from the fruit results from vapour pressure deficit between the less saturated atmosphere and the
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