High pressure effects on myrosinase activity and glucosinolate preservation in seedlings of Brussels sprouts
Research output: Book/Report › Ph.D. thesis › Research
During consumption of Brassica vegetables the myrosinase-glucosinolate system plays a dual role;on one hand is the health-improving effect, but on the other hand an undesired bitterness taste.Application of high pressure (HP) can be used as a potential tool to balance the good taste and goodhealth by controlling the activity of myrosinase. The seedlings of Brassica oleracea L. var.gemmifera were used as a confined vegetable in this project. They constitute a well-defined andhomogenous plant matrix with simple cell structures suitable for mechanistic studies of themyrosinase-glucosinolate system and the entire seedlings can be HP processed, thereby avoidingautolytic reactions from cutting the plant. Seven-day-old seedlings were found suitable for HPprocessing and the parallel determination of residual enzyme activity as well as degradation of thenatural enzyme substrates present in the matrix were used to describe the impact of HP includingpressure level, temperature and time on the myrosinase-glucosinolate system. The role of water(content and activity) in HP processing on Brussels sprouts seedlings and the behavior ofmyrosinase-glucosinolate system was also investigated.The activity of myrosinase in the seedlings showed different dependency of pressure than puremyrosinase from 200 MPa to 800 MPa at 5 °C for 3 min. Applying first-order kinetic to determineactivation volumes revealed a linear relationship from 400 to 600 (ΔV# = −19.04 mL/mol) and 450–600MPa (ΔV#=−37.79 mL/mol) for seedlings and purified myrosinase, respectively. Hence, thepressure effects on the myrosinase activity in the seedlings are affected less or differently than theinactivation rate of purified myrosinase. The complex relationship between myrosinase inactivation,glucosinolate degradation, and cell permeability was reinforced by the pressure treatment at 800MPa, where the glucosinolate degradation due to pressure-induced disruption of the plant cell seemsto be partly counter-acted by myrosinase inactivation. Moreover, the results indicated that merepressure-induced degradation of glucosinolates is unlikely and that the degradation is, in fact, aresult of a fast interaction with the active enzyme myrosinase.A response surface methodology approach was used to further investigate the combined effect ofpressure level (100–600 MPa), temperature (30–60 °C) and time (3–10 min) on this complexmyrosinase-glucosinolate system in Brussels sprouts seedlings. The effects on residual myrosinaseactivity, intact glucosinolate concentration, and profile and concentration of individualglucosinolate differed markedly according to combinations of pressure, time and temperature.Interestingly, the highest preservation of glucosinolate (85% of the untreated level) was obtainedafter HP treatment at 600 MPa, 60 °C, 10 min due maximum inactivation of myrosinase. On theother hand, the highest preservation of myrosinase activity compared to untreated seedlings wasafter treatment at 100 MPa, 30 °C, 3 min and 10 min due to low degree of cell permeabilization.Microscopic investigations supported the explanation that HP induces changes to the plant cellstructure, like loos of turgidity of the outer cell layers in the hypocotyl and shrinkage of cytoplasm,resulting in the observed pressure-induced enzymatic degradation of the glucosinolates.Brussels sprouts seedlings with different water content (wc=4.8-89.4 %) and water activity(aw=0.17-0.97) were pressurized from 200-800 MPa (5 ºC and 3 min). The myrosinase activity inthe dry seedlings (wc=6-14 %) was not affected by pressure, as well as the glucosinolate content.But water adsorption (duration of 5 days) prior to HP processing resulted in a decreased initialactivity due to changed plant cell permeability. For the moist seedlings (wc=45-89 %), the highwater availability resulted in both pressure-induced myrosinase inactivation and productinactivation upon reaction with glucosinolate (decreasing content) due to enhanced cellpermeability. It was found that HP treatment (up to 800 MPa) of dry seedlings (water content below6 %) will preserve both active myrosinase and intact glucosinolate.
|Publisher||Department of Food Science, Faculty of Science, University of Copenhagen|
|Publication status||Published - 2018|