Abstract:
This research studied the effects of combined High Hydrostatic Pressure (HHP) and Dense- Phase Carbon Dioxide (DPCD) treatment on inactivation of fruit enzymes: peroxidase (POD), polyphenoloxidase (PPO) and pectinmethylesterase (PME) of feijoa, inactivation of microorganisms in their growth media and sensory properties of treated feijoa products. The changes in pure PPO secondary structure, size and inactivation kinetics were studied. Feijoa puree, pure mushroom PPO (E.C.1.14.18.1) and microorganisms (Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae) in growth media, were treated with no CO2 (HHP), or saturated with CO2 at 0.1 MPa (HHPcarb), or with 0.1 to 2.4%w/w of CO2 injected into the package (HHPcarb+CO2) and treated with HHP at 200-600MPa, 20oC, for 1-13min and stored at -20oC or 4oC for 1 month. Optimisation for feijoa enzyme activity reduction was performed using response surface methodology. Treated samples were analysed for colour, enzyme activity, secondary structure, size, microbial viability and sensory attributes. In feijoa, increasing CO2 decreased PME activity, pressure influenced PPO and POD activities and increasing time reduced all enzyme activities. The optimal process conditions of 13min, 600MPa, pH=3, CO2 saturation, resulted in 74%,71% and 54% residual enzyme activity (REA) of POD, PPO and PME respectively. HHPcarb+CO2 significantly decreased REA of pure PPO to 30-12% after 1-9min, whereas HHP had no effect on activity. Alpha-helix fractions were reduced by 10-20% in HHP and 20-30% in HHPcarb+CO2. Protein sizes of HHPcarb+CO2 were 5-6 folds larger than that of Control and HHP. Inactivation kinetics of HHPcarb+CO2 followed Weibull, Peleg and 2-fractional models. No restoration in activity, structure and size were observed during storage. HHPcarb+CO2 had additional reduction of 2 logs in E.coli (400MPa, 4 min), >6.5 logs in B.subtilis (starting at 200 MPa, 4 min) and 1.5 logs in yeast (250MPa, 4 min) compared to HHP. HHPcarb+CO2 did not alter the appearance and colour, while affecting the texture and flavour of unsweetened feijoa samples, but that was masked by sucrose addition. There were no differences in preference between HHPcarb+CO2 and Fresh in sweetened products. Addition of CO2 in HHP has synergistic effects on enzyme and microbial inactivation, which can improve product quality.
