Thermal Degradation Kinetics of Chlorophyll in Pureed Coriander Leaves

Coriander leaves are widely used in cooking throughout the world. Thermal degradation kinetics of chlorophyll a, b, and total chlorophyll in coriander leaf puree was investigated at varying levels of pH (4.5–8.5) and processing temperature (80–145°C). Coriander puree at pH 4.5 was processed at 80° to 100°C, whereas that at pH 5.5 to 8.5 was processed at 105° to 145°C. Chlorophyll degradation followed first-order reaction kinetics. Good agreement was found between estimated and experimental chlorophyll retention in all cases (R ² > 0.80). Activation energies ranged from 6.57 to 96.00 kJ/mol. Reaction rate and activation energy data indicated that chlorophylls were more stable at alkaline pH. Transition state theory was applied to estimate the enthalpy, entropy, and Gibbs free energy of activation. Enthalpy of activation (ΔH ^#) ranged from 3.46 to 91.99 kJ/mol, whereas entropy of activation (ΔS^#) ranged from −0.265 to −0.047 kJ/(mol K). The overall free energy change was 107.55 kJ/mol. Results indicated that, the compensation effect did not exist for chlorophyll degradation in coriander puree during thermal processing.     

Thermal Chlorophyll Degradation Kinetics of Mint Leaves Puree

Thermal degradation kinetics of chlorophyll ‘a’, ‘b’ and total chlorophyll in mint leaves puree were investigated as function of pH (4.5–8.5) and processing temperature (80–145°C), respectively. Mint puree was processed at 80°to 100°C at pH 4.5, while that at pH 5.5 to 8.5 was processed at 105°to 145°C. Chlorophyll degradation followed the first order reaction kinetics. Good agreement was found between estimated and experimental chlorophyll retention in all cases (R² > 0.86; MRQE < 0.27). Activation energies ranged from 6.45 to 47.67 kJ/mol. Reaction rate and activation energy data indicated that chlorophylls were more stable at alkaline pH. Transition state theory was applied to estimate the enthalpy, entropy and Gibbs free energy of activation. Enthalpy (ΔH^#) ranged from 3.14 to 44.66 kJ/mol, while entropy (ΔS^#) ranged from ?0.157 to ?0.266 kJ/(mol K). The overall free energy change was 105.76 kJ/mol. Results indicated that, the compensation effect did not exist for chlorophyll degradation in mint puree during thermal processing.     

A comparative study of polyphenoloxidases from taro (Colocasia esculenta) and potato (Solanum tuberosum var. Romano)

Taro (C. esculenta) is a staple food in many tropical regions. A comparative study of crude polyphenoloxidases from taro (tPPO) and potatoes (pPPO) was carried out to provide information useful for guiding food processing operations. Crude PPO was prepared by cold acetone precipitation using ascorbic acid as antioxidant. The PPO content of taro acetone powder was 770±17 units (mg protein)⁻¹ as compared with 3848±180 units (mg protein)⁻¹ in potato acetone powder. The pH-activity optimum was pH 4.6 for tPPO and pH 6.8 for pPPO. Both enzymes retained >80% activity after incubation at pH 4.5–8 but there was rapid activity loss at pH < 4. The temperature-activity optimum (T_opt) was 30°C for tPPO and 25°C for pPPO with 75 and 27% of their respective maximum activity retained at 60°C. Both tPPO and pPPO were irreversibly inactivated by 10 min heating at 70°C. The activation enthalpy (ΔH^#) and activation entropy (ΔS^#) for tPPO heat-inactivation were 87.4 (±0.1) kJ mol⁻¹ and −56.2 (±4) J mol⁻¹ K⁻¹, respectively. For pPPO, ΔH^# was 59.1 (±0.1) kJ mol⁻¹ whilst ΔS^# was −141 (±4) J mol⁻¹ K⁻¹. The apparent substrate specificity was established from values V_max/K_m as: 4-methylcatechol>chlorogenic acid>dl-dopa>catechol>pyrogallol> dopamine>>caffeic acid for tPPO. There was no detectable activity towards caffeic acid. The substrate specificity for pPPO was: 4-methylcatechol>caffeic acid>pyrogallol>catechol>chlorogenic acid >dl-dopa>dopamine. According to the order of inhibitor effectiveness (sodium metabisulphite>ascorbic acid>NaCl≈ (EDTA), there was a significant lag-phase before increases occurred in the absorbance at 420 nm. Preincubation of PPO with inhibitors increased the extent of inhibition, indicating a direct effect on the structure of the enzyme.     

Pressure effects on the hypervalent meat pigment ferrylmyoglobin

 Acid-catalyzed autoreduction of ferrylmyoglobin [MbFe(IV)=O] decreased with increasing hydrostatic pressure corresponding to a volume of activation of ΔV ^# = +7.2±0.8 ml mol^–1, as determined for pressures of up to 250 MPa at 15°C in acidic 0.16 M NaCl solution. For the non-catalyzed autoreduction, a significant increase in compressibility for the non-protonated heme pigment, corresponding to a compressibility coefficient of activation of Δκ^# = +(4.0±1.1)×10^–8 ml mol^–1 Pa^–1, moderates the effects of pressure, although the volume of activation determined at 30°C was comparable, i. e. ΔV ^# = +7.5±2.0 mol mol^–1. At pH relevant for meat, the effect of pressure on the transformation of ferrylmyoglobin to metmyoglobin [MbFe(III)] was approximately half that of the same pressure on the transformation of oxymyoglobin to metmyoglobin, when compared on a logarithmic basis. This result finds a surprising parallel in the temperature-dependence of the same acid-catalyzed processes to yield metmyoglobin. Received: 1 December 1997     

Biochemical characteristics and thermal inhibition kinetics of polyphenol oxidase extracted from Thompson seedless grape

Polyphenol oxidase (PPO) was isolated from Thompson seedless grape (Vitis vinifera ‘Thompson Seedless’), and its biochemical characteristics were studied. The PPO showed activity to catechol and D, L-DOPA, but not towards monophenol l-Tyrosine, diphenols guaiacol and caffeic acid, and triphenols pyrogallic acid and gallic acid. Apparent Michaelis–Menten constant (K _m) and maximum velocity of the reaction (V _max) values were 45.0 ± 0.05 mM and 500.0 ± 15.3 OD_400 nm/min for catechol, and 34.6 ± 0.03 mM and 384.6 ± 11.7 OD_478 nm/min for D, L-DOPA, respectively. The obtained similar specificity values of V _max/K _m ratio of catechol and D, L-DOPA indicated their similar affinity to Thompson seedless PPO. The most effective inhibitor was l-cysteine, followed in decreasing order by ascorbic acid, sodium metabisulfite, EDTA, NaCl, and citric acid. It was discovered that metal ions of Mg²⁺ and Cu²⁺ increased, while Zn²⁺ and K⁺ reduced the PPO activity. Sugars showed inhibition on the PPO activity, with higher effect by sucrose and lower effect by fructose and glucose. Optimum pH and temperature for grape PPO activity were 6.0 and 25 °C with 10 mM catechol as substrate. The enzyme was heat stable between 10 and 25 °C, but showed significant activity loss at temperatures higher than 40 °C and completely inactivation at 70 °C for 10 min. Thermal inactivation of PPO showed a first-order kinetic with an activation energy (E _a) of 146.1 ± 10.8 kJ/mol at pH 6.0.     

Kinetic characterisation and thermal inactivation study of polyphenol oxidase and peroxidase from table grape (Crimson Seedless)

Polyphenol oxidase (PPO) and peroxidase (POD) were extracted from a table grape (Crimson Seedless) using Triton X-114 and characterized using spectrophotometric methods. Both PPO and POD were activated by acid shock. However, in the presence of the anionic detergent sodium dodecil sulphate (SDS), PPO was activated whereas POD was inactivated. The enzymes were kinetically characterized and both followed Michaelis–Menten kinetics, although with different values of their kinetic parameters. The V_m/K_m ratio showed that Crimson Seedless grape PPO presents a similar affinity for 4-tert-butyl-catechol (TBC) whether activated by acid shock (0.018 min⁻¹) or SDS (0.023 min⁻¹). With regards to POD, the K_m and V_m values for 2,2′-azinobis(3-ethylbenzothiazolinesulphonic acid) (ABTS) were 0.79 mM and 1.20 μM/min, respectively. In the case of H₂O₂, the K_m and V_m value were 0.4 mM and 0.93 μM/min, respectively. PPO and POD showed similar thermostability, losing >90% of relative activity after only 5 min of incubation at 78 °C and 75 °C, respectively. In addition, PPO´s activation energy was similar to that obtained for POD (295.5 kJ/mol and 271.9 kJ/mol, respectively).     

Determination of some biochemical properties of polyphenol oxidase from Emir grape (Vitis vinifera L. cv. Emir)

Polyphenol oxidase (PPO) was extracted from Emir grapes grown in Turkey and its characteristics in terms of pH and temperature optima, thermal inactivation, kinetic parameters and potency of some PPO inhibitors were studied. The optimum pH and temperature for grape PPO were found to be 4.2 and 25 °C respectively using catechol as substrate. K_m and V_max values were found to be 25.1 ± 2.72 mmol L⁻¹ and 0.925 ± 0.04 OD₄₁₀ min⁻¹ respectively. Of the inhibitors tested, the most potent was sodium metabisulfite, followed by ascorbic acid. The thermal inactivation curve was biphasic. Activation energy (E_a) and Z values were calculated as 251.4 kJ mol⁻¹ (r² = 0.996) and 8.92 °C (r² = 0.993) respectively. Copyright © 2006 Society of Chemical Industry     

Polyphenol oxidase inactivation and vitamin C degradation kinetics of Fuji apple quarters by high humidity air impingement blanching

In this study, polyphenol oxidase (PPO) and vitamin C were used as the indicators of enzymes and nutrients to evaluate the apple quality during high humidity air impingement blanching (HHAIB) process. The PPO can be completely inactivated within 7 min at 90–120 °C and can retain relatively more vitamin C in the case of PPO fully inactivation. PPO inactivation followed zero‐order kinetics model at 90 and 100 °C, and followed first‐order fraction model at 110 and 120 °C. Activation energy (E_a) of PPO inactivation was between 11.61 and 13.66 kJ mol^?1 by Arrhenius equation. Vitamin C degradation under all processing temperatures was well described by first‐order model and its E_a value was 26.69 kJ mol^?1. Therefore, the HHAIB process was proved to be an effective pretreatment for Fuji apple quarters to inactivate PPO fast and meanwhile to maintain produce quality.     

Mass Transfer Coefficients for the Drying of Pumpkin (Cucurbita moschata) and Dried Product Quality

The aim of this work was to determine the mass transfer properties of pumpkin (Cucurbita moschata) exposed to air drying. The drying temperatures tested ranged between 30°C and 70°C, and the kinetic behavior was studied in this temperature band. The samples were analyzed in terms of moisture content, acidity, proteins, lipids, and crude fiber, both in the fresh state and after drying. From the chemical analyses made, it was possible to conclude that drying induces some reductions in acidity, lipids, fibers, and proteins. As to the influence of the drying temperature on the process, it was observed that a temperature rise from 30°C to 70°C led to a 70% saving in drying time. The results obtained by fitting the experimental data to the kinetic models tested allowed concluding that the best model for the present case is Henderson–Pabis, and the worst is Vega–Lemus. Furthermore, in this work, it was possible to determine the values of the diffusion coefficient at an infinite temperature, D _e⁰, and activation energy for moisture diffusion, E _d, which were, respectively, 0.0039 m²/s and 32.26 kJ/mol. Similarly, the values of the Arrhenius constant and the activation energy for convective mass transfer, respectively, h _m⁰ and E _c, were also calculated, the first being 3.798 × 10⁸ m/s and the latter 86.25 kJ/mol. These results indicate that the activation energy for convective mass transfer is higher than that for mass diffusion.     

Characterization of Sultaniye grape (Vitis vinifera L. cv. Sultana) polyphenol oxidase

Polyphenol oxidase (PPO) was extracted from Sultaniye grapes grown in Turkey, and its characteristics in terms of pH and temperature optima, thermal inactivation, kinetic parameters and potency of some PPO inhibitors were studied. Optimum pH and temperature for grape PPO were found to be 3.4 and 30 °C, using catechol as substrate. K_m and V_max values were found to be 44.5 ± 5.47 mm and 0.695 ± 0.0353 OD₄₁₀ min^?1, respectively. Four inhibitors were tested in this study and the most potent inhibitor was sodium metabisulphite, followed by ascorbic acid. From the thermal inactivation studies in the range of 65–80 °C, the half‐life values of the enzyme ranged between 2.6 and 49.5 min. Activation energy (E_a) and Z values were calculated to be 208.5 kJ mol^?1 (r² = 0.9544) and 10.95 °C (r² = 0.9517), respectively.     

Kinetics of thermal degradation of vitamin C in marula fruit (Sclerocarya birrea subsp. caffra) as compared to other selected tropical fruits

The kinetics of the thermal degradation of vitamin C of marula, mango and guava pulp at different heat treatments at temperature ranging from 80 to 150 °C were investigated. For temperatures lower than 125 °C, the ascorbic acid in marula pulp was about 15 fold more stable to heat than the ascorbic acid in mango and guava pulp. The results showed that a simple first order degradation model could not describe the vitamin C degradation as biphasic behaviour was observed. Therefore the model was transformed in a two-fraction model in which the vitamin C content is divided in relatively stable and instable fractions. Marula had a low k_d1,100°C of 7.2 × 10^?3 min^?1 compared to k_d1,100°C of 1.2 × 10^?1 min^?1 for guava and 1.3 × 10^?1 min^?1 for mango. Guava had the highest activation energy, E_a of 58 kJ/mol, followed by mango with 39 kJ/mol and then marula with 29 kJ/mol.     

High Pressure Inactivation of Polyphenoloxidases

Pressure stabilities of polyphenoloxidases (PPO) from apples, avocados, grapes, pears and plums were determined at pH 6-7. These PPOs differed in pressure stability, but all were rather pressure-stable. Inactivation of PPO from apple, grape, avocado and pear at room temperature (25°C) became noticeable at 600, 700, 800 and 900 MPa respectively, and followed first-order kinetics. Plum PPO was not inactivated at room temperature by pressures up to 900 MPa. For the two most pressure-stable PPOs, we investigated whether pressure stability would be reduced by the simultaneous application of mild heat. In case of plum PPO, activity reduction was detectable at 900 MPa and 50°C. Further temperature increase resulted in increase of the inactivation rate constant (E_a 63 kJ/mol). In case of pear PPO, temperature increase up to 35°C resulted in a 3-fold reduction of the inactivation rate constant. Only at higher temperatures, increase of the inactivation rate constant with increasing temperature was noted (Ea 120 kJ/mol).     

A comparative study of inactivation of peach polyphenol oxidase and carrot polyphenol oxidase induced by high‐pressure carbon dioxide

The inactivation of polyphenol oxidase (PPO) in peach juice and PPO in carrot juice was investigated by high‐pressure carbon dioxide (HPCD), and their inactivation kinetics was analysed and compared. The temperature was 35–55 °C, the pressure was 5–15 MPa under HPCD condition. Results showed that HPCD enhanced the inactivation effect of the temperature on the two PPOs. The inactivation kinetics of peach PPO was well fitted to a first‐order kinetic model, of carrot PPO to a fraction‐conversion model as a function of temperatures or pressures. Susceptibility of the rate constant k of peach PPO was not altered and of carrot PPO was lessened to the temperature, but the susceptibility of the rate constant k of peach PPO and carrot PPO to the pressure was not changed when the pressure was >8 or 12 MPa, indicating the presence of a threshold pressure.     

Pulse Duration and Efficiency of Soft Cellular Tissue Disintegration by Pulsed Electric Fields

The effect of pulse duration on efficiency of disintegration of apple tissue by pulsed electric fields (PEF) was studied. The samples (26-mm diameter, 10-mm height) were treated by PEF at electric field strength E between 100 and 400 V/cm, pulse duration t _i of 10, 100, 1,000 μs, inter-pulse duration Δt of 100 μs and different number of pulses n. Both the degree and the time evolution of tissue damage were quantified by electrical conductivity disintegration index Z and characteristic damage time τ, respectively. The samples exposed to the same PEF treatment time nt _i showed noticeably higher disintegration efficiency for larger pulse duration. The synergism of PEF and thermal treatment with temperature T (20–50 °C) was demonstrated. The Arrhenius dependence of τ(T) for PEF treatment at E = 100 V/cm gave the decreasing activation energy W as a function of t _i, (Q ≈ 164 kJ/mol at t _i = 10 μs, Q ≈ 109 kJ/mol at t _i = 100 μs and Q ≈ 66 kJ/mol at t _i = 1,000 μs). Textural relaxation data supported the higher damage efficiency for longer pulse duration.     

Inactivation of peroxidase and polyphenol oxidase in red beet (Beta vulgaris L.) extract with continuous high pressure carbon dioxide

The inactivation of peroxidase (POD) and polyphenol oxidase (PPO) in red beet extract (RBE) with continuous high pressure carbon dioxide (HPCD) was investigated. HPCD treatment at 7.5 MPa (55 °C, 30 min) resulted in a reduction of their activities by approximately 73% and 93%, respectively. Compared with thermal treatment, continuous HPCD treatment reduced the decimal reduction time (D) of POD and PPO from 555.6 min to 55.9 min and 161.3 min to 32.1 min, respectively. The inactivation process could be described by first-order kinetics (r² > 0.70, p < 0.05); D values declined when temperature increased and continuous HPCD at 7.5 MPa and 55 °C resulted in the highest reaction rate constant (k value; smallest D value). The activation energy of the inactivation was reduced by HPCD treatment from 92.5 kJ/mol to 69.8 kJ/mol and 57.1 kJ/mol to 49.5 kJ/mol for POD and PPO, respectively. Continuous HPCD treatment had little effect on the antioxidant capacities of RBE samples.     

Drying and rehydration behaviors of green bell peppers

In this study, the drying behaviors of green bell peppers were examined in convection dryer. The study was carried out for 3 drying temperatures (55, 65, and 75°C) and for pre-treated samples with ethyl oleate solution against control samples. The pre-treated samples dried faster than the control ones. Drying time decreased with an increase of drying temperature. Rehydration ratio of the pre-treated samples was higher than control samples. Moisture transfer from green bell peppers was described by applying the Fick’s diffusion model and the effective moisture diffusivity (D _eff ) was calculated. The D _eff values for pre-treated and control samples varied between 0.705 and 2.618×10⁻⁹ m²/sec. Activation energy values for moisture diffusion ranged from 41.67 and 52.99 kJ/mol. Drying data was fitted to 4 thin-layer drying models, namely, Lewis, Henderson and Pabis, logarithmic, and Page. The best model, which best represented the green bell peppers drying, was logarithmic.     

Change of polyphenol oxidase activity, color, and browning degree during storage of cloudy apple juice treated by supercritical carbon dioxide

The effects of supercritical carbon dioxide (SCCO₂) treatment of 8, 15, 22, and 30 MPa for 60 min at 55 °C on polyphenol oxidase (PPO) activity, color, and browning degree in cloudy apple juice during storage at 4 °C for 4-weeks were investigated. The SCCO₂ treatment had significant effects on inactivation of PPO and the least residual activity of PPO was 38.50% at 30 MPa. The restoration of PPO residual activity after SCCO₂ treatment was also observed, which was dependent on the pressure level. A greater reduction of lightness L and a minor increase of redness a of cloudy apple juices after SCCO₂ treatment occurred. Moreover, the total color difference (ΔE), which was significantly less than that of untreated sample, was decreased by enhancing the pressure level. The changes of lightness L and browning degree A during storage were well fitted to a first-order kinetic model. The rate constants of k _L and k _A of samples subjected to SCCO₂ treatment reduced from 4.75×10⁻² to 0.42×10⁻² and 37.19×10⁻² to 8.02×10⁻², respectively, when pressure increased from 0 MPa (untreated sample) to 30 MPa.     

Study of Ca<Superscript>2+</Superscript>-ATPase Activity and Solubility in the Whole Kuruma Prawn (<Emphasis Type="Italic">Marsupenaeus japonicus</Emphasis>) Meat During Heating: Based on the Kinetics Analysis of Myofibril Protein Thermal Denaturation

Protein denaturation is considered to be the main cause of physicochemical changes in prawns during heating. However, no studies have been analyzed the kinetics of protein denaturation and the relationship between the degree of denaturation and chemical changes. Therefore, we investigated the changes in Ca²⁺-ATPase activity, protein solubility, and total sulfhydryl content of whole prawn meat during heating by determining the thermal denaturation kinetics of the proteins. Activation energies (E _a ) for the denaturation of myosin (183.2 kJ/mol) and actin (178.8 kJ/mol) were obtained by non-isothermal differential scanning calorimetry analysis. Using the kinetic parameters, the distribution of protein denaturation was predicted in whole prawns under arbitrary heating conditions. The results revealed an uneven distribution of the protein denaturation in prawns that was dependent on the heating conditions. Ca²⁺-ATPase activity decreased with increasing heating times at 51 or 85 °C and was strongly related to the average degree of protein denaturation. The results of protein solubility analysis suggested that hydrogen bonds, hydrophobic interactions, and ionic bonds changed with protein denaturation. The number of ionic bonds was reduced, while hydrogen content was enhanced at both temperatures. Hydrophobic interactions increased gradually at 51 °C (p?<?0.05). At 85 °C, hydrophobic interactions increased notably at first (p?<?0.05); however, as heating continued, no significant changes were observed (p?>?0.05). Our results indicate that the extent of protein solubility is significantly correlated with the average degree of protein denaturation during the heating process.     

Kinetics of green asparagus ascorbic acid heated in a high-temperature thermoresistometer

 Thermal degradation of green asparagus ascorbic acid in high-temperature short-time conditions was studied by heating in a five-channel computer-controlled thermoresistometer. Ascorbic acid was heated to between 110  °C and 140  °C and the degradation kinetics were analyzed assuming that two different inactivation mechanisms were occurring, one aerobic and the other anaerobic. The two reactions followed first-order kinetics, with E _a=12.3(2.0) kcal/mol and k _125  °C=47.0(3.0)×10^–3 s^–1 for the aerobic oxidation, and E _a=6.1(1.4) kcal/mol and k _125  °C=4.1(0.2)×10^–3 s^–1 for the anaerobic degradation. Received: 30 January 1998 / Revised version: 11 June 1998     

Sorption Isotherms of Barnyard Millet Grain and Kernel

The moisture sorption isotherms of grain and kernel of barnyard millet (Echinochloa frumentacea) were determined at 20, 30, 40, and 50 °C. A gravimetric static method was used under 0.112–0.964 water activity (a _w) range for the determination of sorption isotherms. The models were compared using the coefficient of determination (r ²), reduced chi-square (χ ²) values, and on the basis of residual plots. In grain, modified Chung–Pfost (r ² > 0.99; χ ² < 0.7) and modified Oswin (r ² > 0.99; χ ² < 0.55) models were found suitable for predicting the M _e –a _w relationship for adsorption and desorption, respectively. Modified Henderson model was found to give the best fit (r ² > 0.99 and χ ² < 0.55) for describing the adsorption and desorption of the kernel. The isosteric heat, calculated using Clausius–Clapeyron equation, was varied between 46.76 and 61.71 kJ g⁻¹ mol⁻¹ at moisture levels 7–21% (d.b.) for grain and 47.11–63.52 kJ g⁻¹ mol⁻¹ at moisture level between 4% and 20% (d.b.) for kernel. The monolayer moisture content values ranged from 4.3% to 6% d.b. in the case of adsorption of barnyard millet grain and 5.2–6.6% d.b. in the case of desorption at the temperature ranges of 50–20 °C. The monolayer moisture values of barnyard millet kernel ranged from 4.4% to 6.67% d.b. in adsorption and 4.6% to 7.3% d.b. in desorption in the temperature ranges of 50–20 °C.