Combined effect of ultrasound treatment and phytohormones on the accumulation of bioactive compunds in broccoli florets
Aguilar Camacho, Miguel
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In the past years, postharvest abiotic stresses such as wounding, modified atmospheres, UV-radiation, and the exogenous application of ethylene (ET) and methyl jasmonate (MJ) have been studied as an effective tool to improve the accumulation of bioactive compounds in fresh produce. In this context, previous reports in literature suggest that non-thermal processing technologies such as ultrasound (US) activate the biosynthesis of nutraceuticals in crops by a similar mechanism exerted by wounding stress. However, there is still limited information in literature regarding emerging technologies and their effect on the biosynthesis of bioactive compounds. Therefore, the present thesis evaluated the immediate and late response to US treatment (20 min, frequency 24 kHz, amplitude 100 μm) on glucosinolates, isothiocyanates, phenolic compounds and ascorbic acid in broccoli florets. Moreover, the combined effects of US with exogenous MJ (250 ppm) and/or ethylene ET (1000 ppm) on the content of bioactive compounds were also evaluated during storage (72 h at 15 ºC) of broccoli florets. As an immediate response to US treatment, the extractability of glucosinolates such as glucoraphanin (795%), 4-hydroxy glucobrassicin (153%), and glucobrassicin (78.6%) increased as compared with the control (CT), and induced the detection of gluconasturtiin (0.30 mmol/kg) not identified in the control. Furthermore, US treatment increased the extractability of 1- sinapoyl-2-feruloylgentiobiose (57.23%) as compared with the control, whereas the concentration of the other phenolic compounds remained unaltered. Similarly, the content of total ascorbic acid remained unchanged immediately after US treatment. The enhanced extractability of some bioactive compounds was attributed to cell disruption by US treatment. During storage, gluconasturtiin and 4-methoxy glucobrassicin, which were not detected in CT samples before storage, showed accumulation at 24 h (0.69 mmol/kg) and 72 h (0.29 mmol/kg) of storage, respectively. On the other hand, the content of glucoraphanin and 4- hidroxy glucobrassicin, which were immediately increased by US treatment, remained stable during storage in US samples. However, the high increased content of gluconasturtiin and 4- methoxy glucobrassicin at 24 h and 72 h of storage in CT samples, was not detected in US samples. The combined application of MJ and US in broccoli florets, induced a synergistic effect on the accumulation of 4-hydroxy glucobrassicin, glucoerucin, gluconasturtiin and neoglucobrassicin at 72 h of storage, showing 187.1%, 111.92%, 755.9% and 232.8%, respectively, higher content as compared with US 72 h samples. When ET was applied on US samples, it impeded the accumulation of gluconasturtiin at 24 h of storage, whereas the accumulation of 4-methoxy glucobrassicin (0.52 mmol/kg) was slightly induced at 72 h. Interestingly, when the three stressors where applied together (US+MJ+ET) the synergistic effect of US and MJ observed on the accumulation of 4-hydroxy glucobrassicin, glucoerucin, gluconasturtiin and neoglucobrassicin at 72 h of storage was repressed. Regarding with phenolic compounds the application of US increased the accumulation of 5-O-CQA (70.6%) and 1,2,2-TSG (56.4%) at 72 h and 24 h of storage, respectively, whereas the content of CA (29.8%) decreased at 48 h, as compared with CT samples. Similar to glucosinolates, after 72 h of storage, US and MJ showed a synergistic effect on the accumulation of 3-O-CQA, 1-S-2-FG, and 1,2,2-TSG, showing 73.4%, 56.0% and 136.7%, respectively, as compared with 72h CT+MJ samples. Interestingly, the application of ET did not modify the effect exerted by US on the accumulation of individual phenolic compounds. Likewise, the application of the three stressors in broccoli florets (US+MJ+ET samples) inhibited the accumulation of 3-O-CQA, 1-S-2-FG, and 1,2,2-TSG observed in US+MJ samples stored for 72 h. In general, the ascorbic acid content decreased in most samples during storage. Samples treated with US alone did not show significant differences in the ascorbic acid content during storage when compared with CT samples. However, when MJ+ET were applied, the content of total ascorbic acid was significantly reduced in CT+MJ+ET and US+MJ+ET samples after 72 h of storage by 53.4% and 86.6%, respectively, as compared with CT 0h samples. Based on the results herein obtained, the application of US can be an effective tool to enhance the extractability of certain glucocosinolate and pheniolic compounds in broccoli. Moreover, due to the synergistic effect observed on the accumulation of bioactive compounds, the combined application of US and MJ could be an effective approach to yield higher levels of glucosinolates and phenolic compounds in broccoli during storage. Additionally, recommendations for future investigations such as different US conditions, individual isothiocyanates analysis and analysis of gene expression were proposed in order to better understand the synergistic effect excerted by US and MJ on the accumulation of bioactive compounds in broccoli florets.
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