THERMAL INACTIVATION OF PEA FLOUR LIPOXYGENASE

Lipoxygenase (LOX) was determined in pea flour (9.6% moisture) samples which had been exposed to 90–130°C for 5–45 min, and in crude aqueous extracts (5%; pH 6.7) of previously unheated pea flour which had been exposed to 40–80°C for 5–30 min. The rate of thermal inactivation of pea LOX was shown to follow first-order reaction kinetics. The rate constants, k(min^?1), ranged from 0.005 to 0.252 for the flour, and from 0.068 to 0.267 (60–80°C) for the crude extract. The respective thermodynamic values were: for the energy of activation (E_a), 126.2 and 64.6 kJmole^?1; for the enthalpy of activation (ΔH^?), 123.1 and 61.8 kJmole^?1; for the free energy of activation (Δ^?), 118.0 and 101.4 kJmole^?1. The greater thermostability of LOX in the pea flour under dry-heat conditions, and the observed differences in the E_a, ΔH^? and ΔS^? values, may be accounted for by its possible complexation with other macromolecules and by the structure of the water surrounding the native enzyme.     

Kinetics of colour development in aqueous fructose systems at high temperatures

Non‐enzymatic browning was studied in aqueous model systems containing fructose and aspartic acid, glutamic acid or asparagine at three different soluble solid concentrations (150, 300 and 450 g kg⁻¹), which were heat treated for different times at four temperatures (85, 90, 95 and 100 °C). Various indicators were used to evaluate the extent of the Maillard reaction: spectrophotometric measurements at 280 and 420 nm (A₂₈₀ and A₄₂₀) and CIE (Commission Internationale de l'Eclairage) parameters L* (lightness), a* (redness), b* (yellowness) and ΔE* (colour difference, which is expressed as [(Δa*)² + (Δb*)² + (ΔL*)²]^1/2). The pH and the formol index were also determined. The indicator for colourless intermediate formation, A₂₈₀, showed that the kinetic constant values increased with increasing heat treatment temperature, while the influence of soluble solid concentration was not clear. After a short induction period the data were well described by assuming zero‐order kinetics. Glutamic acid turned out to be the most reactive amino acid, while asparagine was the least reactive. With reference to brown pigment formation, A₄₂₀ and ΔE*, glutamic acid was the least reactive amino acid. In all the experiments assayed, except for glutamic solutions with fructose concentrations of 150 and 300 g kg⁻¹, the data were best correlated with combined kinetics. © 2000 Society of Chemical Industry     

Desorption isotherms and thermodynamic properties of sweet corn cultivars (Zea mays L.)

The desorption isotherms and thermodynamic properties of two cultivars of sweet corn were obtained during the drying process of these products. The isotherms were determined by a dynamic method for various temperature and humidity conditions. Equilibrium moisture content (X_eq) data were correlated by the Guggenheim–Anderson–de Boer model and an artificial neural network (ANN) model. These models were fitted to the experimental data. The X_eq for corn grain increased with an increase in the relative humidity at fixed temperature and decreased with an increase in temperature at a constant relative humidity. The experimental data were analysed by a thermodynamic approach to obtain the isosteric heat of desorption (ΔH), differential entropy (ΔS), activation energy (E_a) and Gibbs free energy (ΔG). The ΔH and ΔS increased with a decrease in moisture content, while ΔG decreased exponentially with an increase in X_eq. The Arrhenius equation was used to obtain E_a values, with Supersweet corn having higher E_a.     

Optimisation of hydrolysis of purple sea urchin (Strongylocentrotus nudus) gonad by response surface methodology and evaluation of in vitro antioxidant activity of the hydrolysate

BACKGROUND: Hydrolysates prepared from sea urchin (Strongylocentrotus nudus) gonad by enzymatic treatment showed strong 1,1‐diphenyl‐2‐picrylhydrazyl radical scavenging activity and reducing power. RESULTS: Hydrolysis of S. nudus gonad by the commercial protease papain was optimised for maximum degree of hydrolysis (DH) and trichloroacetic acid‐soluble peptide index (TCA‐SPI) using response surface methodology. Results showed that the optimal conditions were the following: temperature of 48.83 °C, pH of 6.92, enzyme‐to‐substrate ratio of 3143 U g^?1, and substrate concentration of 83.5 g L^?1. Under these conditions, a DH of 27.96 ± 0.54% and a TCA‐SPI of 57.32 ± 0.63% were obtained. The hydrolysate prepared in the optimal conditions was fractionated by an ultra‐filtration system and the resultant fraction below 10 kDa was found to effectively scavenge hydroxyl radical (EC₅₀ = 13.29 ± 0.33 mg mL^?1) and hydrogen peroxide (EC₅₀ = 16.40 ± 0.37 mg mL^?1), inhibit lipid peroxidation (EC₅₀ = 11.05 ± 0.62 mg mL^?1), chelate Fe²⁺ (EC₅₀ = 7.26 ± 0.44 mg mL^?1), and protect mice macrophages against death induced by tert‐butyl hydroperoxide. CONCLUSION: Hydrolysates prepared from S. nudus gonad have the potential to be applied as natural antioxidant agents. Copyright © 2012 Society of Chemical Industry     

Beta-Galactosidase of Streptococcus cremoris H

Beta-galactosidase (Beta-D-galactosidegalactohydrolase, EC 3.2.1.23) of Streptococcus cremoris H was partially purified by ammonium sulfate fractionation. Only 10% of the protein was recovered as enzyme protein and more than 50% of the enzyme in the crude extract was lost. A 2.6-fold purification only was achieved. The enzyme was most active at 65°C and recorded an optimum pH at 7.0. Km and V_max with ortho-nitrophenyl beta-D galactopyranoside as the substrate were recorded as 0.384 mM and 12.6 μmoles/mg protein/min. Manganese ions activated the enzyme. The enzyme was strongly inhibited by Hg⁺⁺, Ca⁺⁺, Ni⁺⁺, and Ag⁺ as well as parachloromercuribenoate.     

Ultrasonic-Assisted Extraction of α-Tocopherol Antioxidants from the Fronds of Elaeis guineensis Jacq.: Optimization, Kinetics, and Thermodynamic Studies

This article presents the first report on the extraction and quantification of α-tocopherol from the fronds of the oil palm (Elaeis guineensis Jacq). In this study, the optimization, kinetic, and thermodynamic data of α-tocopherol extraction by sonication are presented. Response surface methodology coupled with central composite design was used to optimize the experimental conditions for α-tocopherol extraction. Three independent variables, namely sample/solvent ratio (1:20–1:40 g/ml), extraction temperature (30–50 °C), and extraction time (20–50 min) were studied. For optimum conditions of 39.31 °C (~40 °C), 50 min, and 1:23.63 g/mL (~1:20 g/mL), total tocols and α-tocopherol optimal concentrations were 346.49 μg/g oil palm fronds (OPFs) by dry weight (DW) and 28.41 μg/g OPF DW, respectively. The effects of extraction temperature and tocol concentrations on the extraction kinetics and thermodynamic parameters were also studied. From the mass transfer rate equation, the kinetic and thermodynamic data obtained from the ultrasonic-assisted extraction (UAE) of α-tocopherol from OPF were activation energy, E _a (104.6 kJ mol^?1), UAE rate constant, k (6.886?×?10^?3 min^?1), ΔH (+0.818 kJ mol^?1), ΔS (+27.22 J mol^?1 K^?1), and ΔG (?8.52 J mol^?1). According to this study, the UAE of α-tocopherol from OPF is endothermic, irreversible, and spontaneous.     

Storage stability of the natural colourant from Justicia spicigera microencapsulated in protective colloids blends by spray‐drying

Muitle (Justicia spicigera), a Mexican native plant, produces a purple aqueous extract (MAE) because of its anthocyanin content. The aim of this work was to microencapsulate MAE by spray‐drying in two different protective colloids blends in a 1:1 weight ratio: gum Arabic‐mesquite gum (GA50‐MD50) and mesquite gum‐maltodextrin DE10 (MG50‐MD50), yielding the microcapsules M_GA50‐MD50 and M_MG50‐MD50. The minimum integral entropy of the microcapsules was determined at 20, 35 and 40 °C, and the resulting water activities (a_W) were 0.555, 0.592, 0.627 for M_GA50‐MD50 and 0.581, 0.587, 0.648 for M_MG50‐MD50, respectively. These a_W temperature sets were considered as the most adequate conditions for achieving maximum storage stability of the microcapsules. Total anthocyanin content (TAC) and total colour change (ΔE) suffered considerable degradation at all storage conditions, but that degradation was significantly inhibited by encapsulating MAE in the biopolymers blends especially that made up by MG50‐MD50.     

Thermal inactivation kinetics of peroxidase and lipoxygenase from fresh pinto beans (Phaseolus vulgaris)

 Thermal inactivation kinetics of crude peroxidase (POX) and lipoxygenase (LOX) in fresh pinto beans were studied over the temperature range of 55–90°C. The inactivation of both enzymes followed first-order kinetics. The biphasic inactivation curves for POX indicate the existence of several isoenzymes of varying heat stability. In the temperature range of 55–70°C, the activation energies (E _a) of POX were 46.5 kcal·mol^–1 for the heat-labile portion and 37.6 kcal·mol^–1 for the heat-stable portion. On the other hand, the LOX enzyme had an E _a value of 42.26 kcal·mol^–1 at 55–75°C and 49.1 kcal·mol^–1 at 55–90°C. Received: 28 July 1997 / Revised version: 16 October 1997     

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     

Thermal inactivation kinetics of peroxidase and lipoxygenase from fresh pinto beans (Phaseolus vulgaris)

 Thermal inactivation kinetics of crude peroxidase (POX) and lipoxygenase (LOX) in fresh pinto beans were studied over the temperature range of 55–90°C. The inactivation of both enzymes followed first-order kinetics. The biphasic inactivation curves for POX indicate the existence of several isoenzymes of varying heat stability. In the temperature range of 55–70°C, the activation energies (E _a) of POX were 46.5 kcal·mol^–1 for the heat-labile portion and 37.6 kcal·mol^–1 for the heat-stable portion. On the other hand, the LOX enzyme had an E _a value of 42.26 kcal·mol^–1 at 55–75°C and 49.1 kcal·mol^–1 at 55–90°C. Received: 28 July 1997 / Revised version: 16 October 1997     

Detection of Free Radicals in Beer Oxidation

Short-lived radicals produced during the incubation of beer at 60°C were detected by electron spin resonance (ESR) spectroscopic analysis using N-tert-butyl-α-phenylnitrone (PBN) as a spin trapping reagent. The hyperfine splitting (hfs) constants were 15.7 ± 0.2 G and 3.2 ± 0.2 G for α_N and a^β_H, respectively. The hfs constants of PBN adducts produced by Co⁶⁰γ irradiation were 15.6 ± 0.3 G and 3.1 ± 0.2 G for α_N and α^β_H in beer and 15.7 ± 0.3 G and 3.3 ± 0.3 G for α_N and α^β_H in double distilled water. It is proposed, therefore, that short-lived radicals produced during the incubation of beer at 60°C may be hydroxyl radicals.     

Kinetics of Thermal Inactivation of Peroxidase and Color Degradation of African Cowpea (Vigna unguiculata) Leaves

Cowpea leaves form an important part of the diet for many Kenyans, and they are normally consumed after a lengthy cooking process leading to the inactivation of peroxidase (POD) that could be used as an indicator for the potential shelf life of the vegetables. However, color degradation can simultaneously occur, leading to poor consumer acceptance of the product. The kinetics of POD in situ thermal (for thermal treatments in the range of 75 to 100 °C/120 min) inactivation showed a biphasic first‐order model, with Arrhenius temperature dependence of the rate constant. The kinetic parameters using a reference temperature (T_ref) of 80 °C were determined for both the heat‐labile phase (k_ref = 11.52 ± 0.95 × 10^?2 min^?1 and E_a of 109.67 ± 6.20 kJ/mol) and the heat‐stable isoenzyme fraction (k_ref = 0.29 ± 0.07 × 10^?2 min^?1 and E_a of 256.93 ± 15.27 kJ/mol). Color degradation (L*, a*, and b* value) during thermal treatment was investigated, in particular as the “a*” value (the value of green color). Thermal degradation (thermal treatments between 55 and 80 °C per 90 min) of the green color of the leaves followed a fractional conversion model and the temperature dependence of the inactivation rate constant can be described using the Arrhenius law. The kinetic parameters using a reference temperature (T_refC = 70 °C) were determined as k_refC = 13.53 ± 0.01 × 10^?2 min^?1 and E_aC = 88.78 ± 3.21 kJ/mol. The results indicate that severe inactivation of POD (as an indicator for improved shelf life of the cooked vegetables) is accompanied by severe color degradation and that conventional cooking methods (typically 10 min/100 °C) lead to a high residual POD activity suggesting a limited shelf life of the cooked vegetables.     

Atomic force microscopy study of the antimicrobial activity of aqueous garlic versus ampicillin against Escherichia coli and Staphylococcus aureus

BACKGROUND: In this study the effects of ampicillin and aqueous garlic extract on Escherichia coli (ATCC 9637) and Staphylococcus aureus (ATCC 25923) were compared. Atomic force microscopy (AFM) was used to study the possible mechanisms of membrane disruption. RESULTS: Ampicillin disrupted the cell membrane of E. coli, inducing pores and cell leakage. Aqueous garlic extract also induced leakage from the cell membrane in E. coli, but no pores were observed. The trend in Young's modulus for E. coli was E_native ≈ E_age > E_amp. In contrast, S. aureus incubated with low ampicillin (≤50µg mL^?1) and garlic (≤50 mg mL^?1) concentrations showed no significant changes in surface morphology compared with the untreated bacterium. The trend in Young's modulus for S. aureus was E_native ≈ E_age ≈ E_amp. CONCLUSION: The trend E_native ≈ E_age for E. coli and S. aureus supports the hypothesis that the compounds in garlic show intracellular activity. This proof‐of‐concept study of the aqueous crude isolate of garlic points to the feasibility of further AFM investigations to compare the antimicrobial properties of various pure thiosulfinate isolates found in garlic. Copyright © 2009 Society of Chemical Industry     

A Kinetic Approach to Evaluate the Structure‐Based Performance of Antioxidants During Lipid Oxidation

Thermal kinetic parameters of fish oil oxidation, as affected by o‐hydroxyl, o‐methoxy, and alkyl ester derivatives of p‐hydroxybenzoic acid in various concentrations (0.02% to 0.16%) at 35 to 55 °C, were calculated. The average extent of increase (2% to 10%) in the values of free energy of activation, ?G⁺⁺, as well as the average extent of change in the Arrhenius equation parameters, including activation energy (E_a, –40.5% to 13.6%) and frequency factor (A, –55.0% to 4.3%), could be employed well to show structure‐based performance of antioxidants. Temperature coefficient (T_C) and Q₁₀ number, which were considered as the quantitative measures of thermal sensitivity of the lipid system, showed changes from –40.3% and –27.2% to 13.5% and 11.5%, respectively, in the presence of the antioxidants.     

Winged bean lipoxygenase—Part 2: Physicochemical properties

Winged bean lipoxygenase (linoleate: oxygen oxidoreductase EC 1.13.11.12) isoenzymes FI and FII were isolated and purified according to the method of Truong et al. (1980).FI and FII were both highly specific for linoleic acid. They exhibited optimal activity at pH 6·0 and 5·8, respectively at 30°C. An activation energy of 4·5 kcal mol^?1 was calculated for this lipoxygenase within the temperature range of 30–50°C.At 0·075% Tween 20, FI and FII had K_m values for linoleic acid of 0·44 and 0·37 × 10^?3M, respectively, compared to 0·4 × 10^?3M for the crude enzyme. Maximal activity was obtained at 1·6 × 10^?3M. Higher levels of Tween 20 inhibited the lipoxygenase activity.Both isoenzymes had identical average molecular weight of 80 000 daltons by gel filtration and SDS gel electrophoresis.FI and FII isoenzymes were strongly inhibited by Hg⁺⁺, Mn⁺⁺, Mg⁺⁺ and Fe⁺⁺⁺ and activated by Zn⁺⁺, Co⁺⁺ and Fe⁺⁺. A difference in the degree of inhibition or activation was observed between FI and FII response. Ca⁺⁺ inhibited both FI and FII but the former was more sensitive to Ca⁺⁺. KCN also inhibited the two isoenzymes.Among the antioxidants tested, butylated hydroxytoluene and butylated hydroxyanisole most effectively inhibited both FI and FII at only 10^?6M. Sulphydryl reagents such as iodoacetamide and dithiothreitol have little effect on the lipoxygenase isoenzyme activity.The lipoxygenase isoenzymes were more stable at neutral pH. The enzyme in the crude extract and especially in situ was more stable to heat treatment.     

Modelling osmotic dehydration of lemon slices using new sweeteners

Lemon slices were osmotically dehydrated using the following healthy sweeteners as osmotic agents: tagatose, isomaltulose, oligofructose and aqueous extract of stevia. A kinetic study using a Fickian approach was performed, which also analysed the changes in water activity, total mass, mass of water and mass of soluble solids in lemon slices. The results showed that the greatest value of effective diffusivity (D_e) in osmodehydrated lemon slices was obtained from a combination of oligofructose and stevia [D_e = (10.2 ± 0.3) × 10^?9 m²·s^?1]. However, the level of water activity (a_w) reached with this syrup was the highest (a_w = 0.978 ± 0.004 after 1440 min) meaning that the product might be less stable. Additionally, isomaltulose favoured the total mass loss, whereas tagatose did the opposite. Finally, the syrup recommended for dehydrating lemon slices would be a combination of tagatose, oligofructose and aqueous extract of stevia as its D_e was similar to the value obtained when only oligofructose and stevia were used, but a_w values were lower.     

Migration of antimony from PET trays into food simulant and food: determination of Arrhenius parameters and comparison of predicted and measured migration data

Migration experiments with small sheets cut out from ovenable PET trays were performed in two-sided contact with 3% acetic acid as food simulant at various temperatures. The fraction of diffusible antimony (Sb) was estimated to be 62% in the PET sample under study. Apparent diffusion coefficients of Sb in PET trays were determined experimentally. Measurement of migration between 20 and 150°C yielded a linear Arrhenius plot over a wide temperature range from which the activation energy (E _a) of 188?±?36?kJ?mol^?1 and the pre-exponential factor (D ₀) of 3.6?×?10¹⁴?cm²?s^?1 were determined for diffusing Sb species. E _a was similar to previously reported values for PET bottles obtained with a different experimental approach. E _a and D ₀ were applied as model parameters in migration modelling software for predicting the Sb transfer in real food. Ready meals intended for preparation in a baking oven were heated in the PET trays under study and the actual Sb migration into the food phase was measured by isotope dilution ICP-MS. It was shown that the predictive modelling reproduces correctly experimental data.     

Bioactivities of essential oil and extract of Thymus argaeus,Turkish endemic wild thyme

BACKGROUND: Thymus argaeus Boiss. & Bal. (Lamiaceae), an endemic plant species of Turkey known as wild thyme, is traditionally used as a spice and a wild tea in the Inner Anatolia region of Turkey. In this study the composition of the essential oil and the antimicrobial and antioxidant effects of the methanolic extract and essential oil of T. argaeus were determined. RESULTS: The main components of the essential oil were linalool (499 g kg^?1), α‐terpineol (150 g kg^?1), linalyl acetate (97 g kg^?1) and thymol (94 g kg^?1). The total phenolic, flavanol and flavonol contents of the extract were 83.31 ± 0.59 mg gallic acid equivalent g^?1, 6.26 ± 0.00 mg catechin equivalent g^?1 and 28.81 ± 0.21 mg rutin equivalent g^?1 respectively. The antioxidant activities of the extract and essential oil determined by the 2,2‐diphenyl‐1‐picrylhydrazyl radical‐scavenging method were 830.18 ± 0.42 and 20.47 ± 2.3 mg g^?1 respectively. The antimicrobial activities of the extract and essential oil against 13 bacteria and two yeasts were studied by the agar diffusion method. The micro‐organisms most sensitive to the essential oil were Aeromonas hydrophila and Pseudomonas aeruginosa, while the micro‐organism most sensitive to the extract was P. aeruginosa. CONCLUSION: Only the extract of T. argaeus could be used as a natural antioxidant, while both the extract and the essential oil could be useful as natural antimicrobial agents in food preservation. Copyright © 2009 Society of Chemical Industry     

Kinetics of degradation of sorbic acid in aqueous glycerol solutions

Degradation of sorbic acid in aqueous glycerol solutions at pH 4·0 over the a_w range 0·71–1·00 and the temperature range 40°–60°C was found to follow first-order reaction kinetics and to conform to the Arrhenius equation. Activation energy values obtained were 5·8 kcal mol^?1 and 7·8 kcal mol^?1 for systems at 0·80 a_w with and without added Co⁺⁺, respectively. The rate of sorbic acid degradation was observed to increase with decreasing a_w (i.e. increasing glycerol concentration). The presence of added Co⁺⁺ decreased the rate of sorbic acid breakdown at any particular a_w or temperature. Browning of sorbate solutions during storage was markedly inhibited by Co⁺⁺.     

Kinetics of Crude Peroxidase Inactivation and Color Changes of Thermally Treated Seedless Guava (Psidium guajava L.)

Thermal treatment of seedless guava (Psidium guajava L.) cubes was carried out in the temperature range of 80–95 °C. The kinetics of peroxidase inactivation and color changes due to thermal treatments were determined. Peroxidase inactivation followed a first-order kinetic model, where the activation energy was 96.39 ± 4 kJ mol⁻¹. Color was quantified in terms of L, a, and b values in the Hunter system. The color changes during processing were described by a first-order kinetic model, except total color difference which followed a zero-order kinetic model. The temperature dependence of the degradation followed the Arrhenius relation. The activation energies (E _a) for L, a, b, and total color difference (ΔE) were 122.68 ± 3, 88.47 ± 5, 104.86 ± 5, and 112.65 ± 5 kJ mol⁻¹, respectively. The results of this work are a good tool to further optimize seedless guava thermal treatment conditions.