Abstract
The objective of this research was to gain a better understanding of the relationship between changes in volatiles release, colour and biochemical markers (i.e. pigment content, antioxidant levels, activity of enzymes and oxidative damage levels) during postharvest storage and mechanical damage. This study attempted to find potential volatile markers, using Proton Transfer Reaction Mass Spectrometry (PTR-MS), that were statistically correlated with biochemical and physiological changes associated with postharvest storage and mechanical damage. Multivariate statistical analysis and regression analysis using Partial Least Squares Regression (PLSR) was used to evaluate possible correlations. It is hypothesized that volatile markers in combination with biochemical analyses can be used to assess quality changes during postharvest storage and handling of broccoli.
The intensity of protonated volatile organic compounds with the mass ions m/z 33 and m/z 49 were tentatively identified as methanol and methanethiol. While both increased, methanol was the dominant volatile detected. The increase of these mass ions was observed at the onset of senescence (Day 3, 23 °C), at which point chlorophyll breakdown was initiated, resulting in colour change from green to yellow. Validations were carried out using cytokinin 6-benzylaminopurine (BAP) which delays the onset of senescence for several days. Application of BAP delayed senescence in a concentration dependent manner, with 50 ppm or above BAP slowed the rate of senescence, as determined by chlorophyll loss measurement. Application of BAP also slowed the rate of methanol release. Colour change as a marker for the onset of senescence was validated using markers of oxidative damage and defence, which are known to be key indicators of the onset and progression of senescence. Depending on the BAP concentrations, measurements of antioxidant enzymes, non-enzymatic antioxidants and markers of oxidative damage confirmed that senescence was delayed. To further determine if the release of methanol was due to major cellular disruption, which occurs during senescence, broccoli florets at different stages of senescence were subjected to severe mechanical damage and the damaged tissues were monitored for up to six hours.
In addition, broccoli florets were subjected to different temperature treatments ranging from -18 to 100 °C to simulate different types of cellular and metabolic disruption. The stage of senescence prior to mechanical damage was the critical factor determining the cellular and metabolic integrity of broccoli samples six hours following the damage event although mechanical damage accelerated the breakdown of photosynthetic pigments, the inactivation of antioxidant enzymes, the degradation of non-enzymatic antioxidants and increases in the levels of oxidative damage and methanol.
The temperature treatments (-18, 50, 70 and 100 °C) disrupted cellular metabolism as evidenced by changes in the activities of several enzymes (POD, SOD, CAT and AAO) and led to significant increases in methanol release, compared to the control (4 °C). In addition, higher levels of methanethiol were detected from samples exposed to high temperatures.
Methanol (m/z 33) and methanethiol (m/z 49) are suggested to be potential volatile markers of broccoli florets that can be analysed using PTR-MS technique. Meanwhile, colour, pigment and ascorbic acid content are proposed as markers to determine the quality of broccoli.