The Impact Of Pre-treatment On Phenolic Compounds Of Litchi Pericarps

The prosperity of litchi fruit processing has resulted in the production of large volumes of pericarps/peels, seeds and pulp as by-products with notable amounts of bioactive phenolic compounds. Recovery of these bioactive phenolic compounds for formulation of value-added products for human consumption is considered to be an attractive opportunity. However, despite litchi pericarps being composed of high content of bioactive phenolic compounds, it is highly perishable due to the high activity of the endogenous enzymes such as polyphenol oxidases(PPO) and peroxidase(POD) which catalyze the oxidation of the phenolic compounds. This factor can hinder utilization of litchi pericarps in the recovery of phenolic compounds especially during peak harvest and processing season if no initiatives taken to pre-treat and preserve the pericarps for further utilization.To expand exploitation of litchi pericarps, numbers of experiments were conducted to test the hypothesis that treatment of litchi pericarps could serve as a significant step in stabilizing and improving recovery of the phenolic compounds from litchi pericarps.In experiment 1, the inactivation of the polyphenol oxidases(PPO) and peroxidase(POD) through blanching was considered a suitable approach to understanding the behavior of PPO and peroxidase POD under different blanching conditions for the efficient design of blanching treatments. To achieve this objective, kinetics modeling of thermal inactivation of POD and PPO was performed within temperature regime of 70 °C to 90 °C for 30 s to 15 min. Linear first order decay model-Arrhenius equation, nonlinear fractional conversion and Weibullian-Log-logistic models were evaluated. The PPO and POD indicated the existence of isoenzymes with different thermal stabilities and the fractional conversion model was proved capable of enhancing goodness of fit of first order kinetic and Weibull models. The log-logistic model demonstrated a better fit of the data when compared with the Arrhenius equation. Integration of fractional conversion with Weibull-log logistic distribution to form modified Weibull(MWeLL) model, better described the kinetics of inactivation of PPO and POD in the selected range of temperatures and time, therefore, could be appropriate for predictive purposes.In experiment 2, the effect of different treatment methods on the stability of bioactive phenolic compounds of litchi pericarps were investigated. Fresh litchi pericarps from huaizhi, feizixiao, guiwei and nuominci cultivars were open air dried, combined steam blanched for 3 min and hot air dried at 60 or 80 oC, and hot air drying of unblanched pericarps at 40, 60, 70 and 80 oC until equilibrium weight was reached. The total phenolic compounds of the litchi pericarps from the four cultivars were reduced by 24% to 30% after sun drying for seven days. Hot air drying at 40 oC decreased the total phenolic content by 12% to 37%, while drying above 60 oC, notable loss of total phenolic compounds of over 43% was observed in all cultivars. The combination of steam blanching and hot air drying at 60 oC, improved the recovery of the total phenolic compounds by 2% to 19%. However, hot air drying of steam blanched pericarps at 80 oC reduced the total phenolic compounds by 28% to 49% and flavonoids by 32%-59%, due to thermal degradation. Steam blanching inactivated the oxidative enzymes such as PPO and POD, and induces structural change in the litchi pericap matrices in a short time, thus enhanced release of entrapped contents. FRAP and DPPH radical scavenging capacity of the treated pericarps from the four cultivars were significantly correlated(r >0.769, p<0.01) with the total phenolic compounds.In experiment 3, the influence of pectinase, tannase and β-glucosidase enzymes treatment on the release and transformation of phenolic compounds for enhanced bioactivity of the extracts was evaluated. The enzymes’ optimal incubation time and temperature were 180 min and 35 oC. Enzyme concentration was optimal at 3% of dry sample weight for pectinase and 1% for both tannase and β-glucosidase. Total phenolic compounds recovered from exhausted litchi pericarps(extraction cycle three-E3) after pectinase treatment increased by 1.5-fold with DPPH activity increase of 1.3-fold compared with without pectinase treatment samples. Pectinase was proved effective in hydrolyzing litchi pericarp cell matrices than β-glucosidase and tannase. Total phenolic compounds increased significantly by 59.7 to 157 mg GAE/g DW, flavonoids from 22.08 to 49.6 mg RE/g DW and proanthocyanidins from 20.7 to 46.3 mg/g DW(p<0.05) after enzyme treatment under the optimized conditions.Gallic acid,(-)-Epicatechin,(+)-Catechin, procyanidin A2 and rutin were the major phenolic compounds identified. Quercetin(1.85 mg/g DW and 1.46 mg/g DW) was recoved in enzyme blends of pectinase-β-glucosidase(PB) and pectinase-β-glucosidase-tannase(PBT) produced extracts. The concentrations of(-)-Epicatechin increased by 1.86-fold,(+)-Catechin(1.46-fold), procyanidin A2(1.4-fold), procyanidin B2(1.28-fold) and rutin(3.9-fold) pectinase treated samples respectively. Enzyme treatment of litchi pericarps promoted the liberation and possibly transformation of the phenolic compounds. The extract produced by three enzyme blend formulation(pectinase, β-glucosidase and tannase-PBT) had the highest phenolic compounds content(2.6-fold) which can be attributed to the effectiveness of the enzymes to hydrolyze litchi pericarp wall matrices and liberate entrapped as well as glycosidically bound phenolic compounds.Ferric reducing antioxidant power(FRAP) and DPPH scavenging capacity are well-known assays of accessing the antioxidant capacity of plant extracts. The enzyme produced extracts had a significant(p<0.05) increase in FRAP and DPPH radical scavenging capacity compared to control sample(without enzyme treatment). The overall range of FRAP values observed was 60.27 μmol TE/g to 77.66 μmol TE/g。The FRAP activity was significantly correlated(r=0.837, p<0.01) to the content of phenolic compounds in the extracts.The enzyme blend treatment PBT had the lowest DPPH EC50 value(2.3-fold). The DPPH radical scavenging activity was significantly correlated to the phenolic compounds in the extracts(r=-0.901, p<0.01). The efficacy of enzyme produced litchi extracts as a natural antihypertensive was evaluated by inhibition of the angiotensin-I-converting enzyme(ACE) in-vitro. Significant(p<0.05) increase in anti-ACE capacity was observed in the enzyme produced extracts. The ACE inhibition activity of the litchi extracts ranged from 20.31% to 50.77%. The results recorded for ACE inhibition capacity presents potentials of incorporating litchi pericarps extracts in the formulation of natural ACE inhibitors.Therefore, the findings of the study indicated that combination of steam blanching and drying and enzyme treatment may improve recovery and biological activity of litchi pericarps extracts which can be used as an ingredient in the development of functional products for human use.