Nonenzymatic Browning in Pear Juice Concentrate at Elevated Temperatures

The effect of temperature and soluble solids (°Brix) on nonenzymatic browning in pear juice concentrate was determined by following absorbance at 420 nm (A₄₂₀) over the temperature range of 50–80°C. Browning could be modeled as a zero order rate process with rates of 22.2 × 10⁻⁴ (45.2 °Brix), 36.9 × 10⁻⁴ (55.4 °Brix), 53.5 × 10⁻⁴ (65.1 °Brix) and 107 × 10⁻⁴ (72.5 °Brix) A₄₂₀· min⁻¹ at 80°C. Temperature dependence was described by the Arrhenius relationship with an average activation energy of 21.9 kcal · mole⁻¹. Formol titration indicated a 20% loss of amino acids during heating 4.4 hr at 80°C and no loss of carbohydrates was observed after any heating period.     

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.     

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.     

Kinetics of ascorbic acid degradation and colour change in ground cashew apples treated at high temperatures (100–180 °C)

Ascorbic acid (AA) degradation and colour changes, measured by the lightness index (L*), were determined in cashew apples (at low dissolved O₂ concentrations) heated at high temperature (100–180 °C) in a hermetically sealed cell. A nonisothermal method was developed to estimate thermal degradation kinetics. The results showed that reaction kinetics during heat treatments were well represented by first‐order reactions. The temperature dependence of the kinetic constants was described by an Arrhenius type equation. The activation energy (E_a) for AA degradation and lightness index were 94 ± 3 and 98 ± 3 kJ mol^?1, respectively. The reaction rate constant at 140 °C for AA degradation (64 × 10^?5 ± 3 × 10^?5 s^?1) was twice that for the lightness index change (33 × 10^?5 ± 2 × 10^?5 s^?1). Results allow generating temperature profiles of heat processes that would help preserve the AA of cashew apples as well as control the colour formation during high‐temperature processes.     

Gamma irradiation as a means to eliminate Listeria monocytogenes from frozen chicken meat

Cells of Listeria monocytogenes ATCC 35152 were sensitive to gamma irradiation in phosphate buffer, pH 7.00 (D₁₀, dose required for 10% survival—0.15 kGy) at 0–5°C. The cells showed higher radiation survival when irradiated under frozen condition, with a D₁₀ of 0.3 kGy. The protection offered by shrimp/chicken/kheema homogenates (100 g litre^?1) was evidenced by even higher D₁₀ values (0.5 kGy) at both 0–5°C and cryogenic temperature. Boneless chicken meat samples were artificially inoculated with L monocytogenes ATCC 35152 cells at low (5 × 10³) colony-forming unit (cfu) g^?1 and high (5 × 10⁶ cfu g^?1) concentrations and irradiated at 1, 3, 4, 6 kGy doses under cryogenic conditions. The efficacy of the radiation process was evaluated by detecting L monocytogenes during storage at 2–4°C in the irradiated samples. These studies, when repeated with three other serotypes of L monocytogenes, clearly suggested the need for a dose of 3 kGy for elimination of 10³ cfu cells of L monocytogenes g^?1 from air-packed frozen chicken meat.     

A MODEL FOR DIFFUSION of SODIUM IN GREEN OLIVES AT DIFFERENT TEMPERATURES and LYE CONCENTRATIONS

A diffusion model for a composite flat plate, consisting of a thin skin of thickness o, and a thicker flesh of thickness L, with constant flesh and skin effective diffusion coefficients (D_F and D_S) was successfully adjusted to the experimental data obtained during the debittering of green olives variety Arauco (also known as Criolla) at 15C, 20C and 25C and lye concentrations of 1.50%, 2.25% and 3.00% of NaOH. the D_F values ranged between 1.64 × 10^?10 and 9.20 × 10^?10 m²/s, and increased with increasing temperature and lye concentration. the D_S values ranged between 1.27 × 10^?12 and 5.63 × 10^?12 and also increased with increasing lye concentration and temperature. Activation Energies (Ea) of diffusion in the flesh and the skin were evaluated with an Arrhenius‐type relationship. the Ea values for the flesh were 93.2, 72.7 and 64.8 kJ/mol, and 92.7, 91.46 and 33.67 for the skin for lye concentrations of 1.50%, 2.25% and 3.00%, respectively.     

Adsorption and Desorption of Cholesterol in Continuous Supercritical Fluid Processing of Anhydrous Milk Fat

The adsorption breakthrough volume of adsorbent for cholesterol reduction of anhydrous milk fat (AMF) was determined by passing supercritical carbon dioxide (SC-CO₂) phase through an adsorption column with magnesium silicate at 40°C/241 bar. The breakthrough quantity for 76% cholesterol reduction was 2.0g fat/g adsorbent. With in-line adsorption on magnesium silicate, the cholesterol reduction in the extracts was 80 to 86%. Magnesium silicates saturated with cholesterol were regenerated with 10% ethanol in CO₂ at 40°C and 64°C/241 bar. The adsorption capacity of the magnesium silicate regenerated with 10% ethanol in SC-CO₂ at 40°C/241 bar was close to its original value. Its behavior was described by the Langmuir isotherm.     

Thermal death kinetics of adult red flour beetle Tribolium castaneum (Herbst) in canola seeds during radio frequency heating

Thermal mortalities of adult red flour beetles Tribolium castaneum (Herbst) infesting canola seeds at various moisture contents and volumes were determined after radio frequency (RF) heating (i.e. temperature between 30 and 80°C). The mortality of 92% was achieved at the end temperature of 343 K for small-volume (1.96 × 10^?4 m³, 0.250 kg) seeds, and the mortality of 99% at 333 K for large-volume (1.77 × 10^?3 m³, 2.26 kg) seeds. Regardless of sample volume, the thermal mortalities of the test insects increased significantly after the seed temperature reached 333 K (60°C). The kinetic parameters of the thermal death of the adult T. castaneum were estimated using inverse simulation. The ordinary differential equation-based kinetic model with the Arrhenius temperature-dependent reaction rate constant was solved using the fourth-order Runge–Kutta method. The kinetics followed first-order reaction with the activation energy of 100 kJ/mol. Good agreements were observed between the mortalities predicted using the kinetic model and the experiments (R² = 0.972–0.987) except for the small-volume seeds at 11% MC (11 g/100 g raw materials) (R² = 0.741). The predicted lethal times (s) to achieve 95 and 99% mortalities using the kinetic model agreed well with those determined from the experiments.     

Degradation Kinetics of Anthocyanins in Sour Cherry Juice and Concentrate

The effects of temperature and soluble solids on degradation of anthocyanins in sour cherry concentrate were determined over temperature ranges (-18 to 37)°and 50 to 80°C. Anthocyanin degradation could be modeled as a first-order rate reaction, with rates of 33.97 × 10^minus;3. hr^minus;1 (15°CBrix), 59.19 × 10^minus;3. hr^minus;1 (45°CBrix) and 97.14 × 10^minus;3. hr^minus;1 (71°CBrix) at 80°C. Temperature dependence of reaction was described by the Arrhenius relationship. Activation energy for a solids content of 15-71°C Brix ranged from 16.37-19.14 kcal.mole^minus;1 with an average of 17.45 kcal.mole^minus;1     

Modelling of air drying of fresh and blanched sweet potato slices

Effects of blanching, drying temperatures (50–80 °C) and thickness (5, 10 and 15 mm) on drying characteristics of sweet potato slices were investigated. Lewis, Henderson and Pabis, Modified Page and Page models were tested with the drying patterns. Page and Modified Page models best described the drying curves. Moisture ratio vs. drying time profiles of the models showed high correlation coefficient (R² = 0.9864–0.9967), and low root mean squared error (RMSE = 0.0018–0.0130) and chi‐squared (χ² = 3.446 × 10^–6–1.03 × 10^–2). Drying of sweet potato was predominantly in the falling rate period. The temperature dependence of the diffusion coefficient (D_eff) was described by Arrhenius relationship. D_eff increased with increasing thickness and air temperature. D_eff of fresh and blanched sweet potato slices varied between 6.36 × 10^–11–1.78 × 10^–9 and 1.25 × 10^–10–9.75 × 10^–9 m² s^–1, respectively. Activation energy for moisture diffusion of the slices ranged between 11.1 and 30.4 kJ mol^–1.     

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⁺⁺.     

Moisture Adsorption Characteristics of Solar-Dehydrated Mango and Jackfruit

The moisture adsorption isotherms of solar dehydrated mango and jackfruit were determined at temperatures ranging from 30 °C to 50 °C. The equilibrium moisture content (EMC) of mango and jackfruit increased sharply as the temperature increased at water activity (a _w) above 0.6 and 0.8, respectively. However, there were no clear isothermal intersection points observed at higher a _w and temperatures. The EMC of solar dehydrated jackfruit showed the isothermal characteristics between types II and III. In contrast, dehydrated mango followed the characteristic type III adsorption isotherms due to high total soluble solids content of 67.8 °Brix and total sugars of 14.21 g/100 g fresh mango. Estimated parameters and fitting ability of three isotherm models were also evaluated. The Guggenheim-Anderson-Boer (GAB) model gave the best fit to the experimental EMC data. The GAB monolayer moisture contents (m _o) of mango and jackfruit ranged from 11.1–10.0 % and 4.7–3.4 %, respectively. Specific surface area of active binding sites (S) was calculated based on the m _o values. The S value of dehydrated mango was 2.5 to 2.8 times larger than jackfruit. The maximum net isosteric heat (q _s) of sorption of solar dehydrated mango and jackfruit were determined as 19.5 and 33 kJ mol^?1, respectively, and q _s decreased significantly at high moisture.     

Purification and Characterization of Polyphenol Oxidase from Hemşin Apple (Malus communis L.)

The polyphenol oxidase (PPO) enzyme was purified and characterized from Hem?in Apple (Malus communis L.), which was organically grown in Hem?in, in the Rize province of Turkey. Enzyme (PPO) activation was determined with catechol substrate. Apples were homogenized with homogenate buffer (pH 8.5). This process was followed by precipitation with (20–80%) saturated solid (NH₄)₂SO₄ and dialysis. Finally, purification with DE52-Cellulose ion-exchange and Sephadex G-25 columns was performed. Experiments were performed at an optimum pH (5.5) and optimum temperature (30–40°C). The kinetic and thermal parameters Km (3.40 mM), Vmax (333.3 EU/mL.min), Ea (3.57 kcal), ?H (2.968 kcal/mol), Q₁₀ (1.33), k_cat (24.57 min^?1) and V₀ (7.2x10³ mM^?1.min^?1) were assessed. Additionally, the effects of Mg²⁺, Pb ²⁺, Fe²⁺, Fe³⁺, Cd²⁺, Cu²⁺, Zn²⁺, Co²⁺, Al³⁺, Mn²⁺ and Na⁺ on enzyme activity was recorded, and the IC₅₀ values, K_? values and inhibition types were determined.     

Effect of blanching on the diffusion of glucose from potatoes

Glucose losses from potato slabs into the water during blanching at 60, 70, 80 and 90 °C were studied after 10, 20, 30, 40, 50 and 60 min. In general, an increase of temperature during the considered times produced a higher glucose loss. The apparent diffusion coefficients (D) for glucose losses from potatoes were calculated under the blanching conditions. The D-values were found to be in the range of 3 × 10^?10 to 8 × 10^?10 m² · s^?1 and could be related to temperature by an Arrhenius equation, having an activation energy of 29.1 kJ/mol. The results will allow the prediction of mean glucose concentration in potato tissues, and hence the overall losses incurred, after a given blanch treatment in this time and temperature range.     

Thermal Degradation Kinetics of Pyridoxine Hydrochloride in Dehydrated Model Food Systems

The thermal degradation kinetics of pyridoxine hydrochloride were examined using a dehydrated model food system designed to simulate a ready-to-eat breakfast cereal. This study was carried out to provide information useful in estimating the thermal losses of pyridoxine hydrochloride in the processing of breakfast cereals and other low moisture dehydrated food systems. Portions of the model system which were fortified with pyridoxine hydrochloride were toasted in a conduction oven at 155°, 170°, 185°, and 200°C for a minimum of six heating times at each temperature. Pyridoxine (PN) content was determined in the heat treated model systems by high performance liquid chromatography (HPLC). For each heat treatment, the loss of pyridoxine could be described by a first order kinetics model. The first order rate constants for 155°, 170°, 185°, and 200°C were 1.74 × 10^?2 min^?1, 5.22 × 10^?2 min^?1, 16.88 × 10^?2 min^?1, and 48.95 × 10^?2 min^?1, respectively. The calculated Arrhenius activation energy was 29.8 Kcal/mol. In comparing the HPLC method to the standard microbiological assay, the HPLC assay gave lower PN values for the toasted model system. To explain this discrepancy, HPLC fractions were collected and analyzed by the microbiological assay. No significant vitamin B₆ activity was found in any fraction other than that containing the PN peak. It is possible that the milder extraction procedure used in the HPLC assay is less capable of recovering forms of PN which may become bound during the toasting process. These potentially bound forms may or may not be biologically available.     

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.     

Separation and thermal characterization of ionically and tightly cell-wall-bound pectin methylesterase from cucumbers (Cucumis sativus)

Thermal characteristics of cell-wall-bound pectin methylesterase (PME) from cucumbers were determined. Heat inactivation of PME followed first-order reaction kinetics. Biphasic inactivation curves indicated the presence of heat-labile and heat-stable fractions. Inactivation of PME accelerated above 65??°C. The z values between 70??°C and 80??°C were 7.6??°C and 9.0??°C for heat-stable fractions of ionically (IPME) and tightly cell wall bound PME (TPME), respectively. Temperature optima for the initial activities of IPME and TPME were 65??°C and 60??°C, respectively. However, for longer periods of activity determination, both IPME and TPME showed a temperature optimum at 50??°C. The E _a values for initial activities were 5?kcal mol^–1 (20–65??°C) and 7.2?kcal mol^–1 (20–60??°C) for IPME and TPME, respectively.     

The effect of gel composition on the adsorption of invertase on poly(acrylamide/maleic acid) hydrogels

The effects of external stimuli such as pH of solution, temperature, substrate concentration of solution and storage stability on the invertase adsorption capacity of poly(acrylamide/maleic acid) [P(AAm/MA)] hydrogels, synthesized by gamma irradiation of ternary mixtures of AAm/MA/Water, were investigated. The adsorption capacities of the hydrogels were found to increase from 4.0 to 13.3 mg invertase/g dry gel with increasing amount of MA in the gel system, while P(AAm) gel adsorbed only 3.1 mg invertase/g dry gel. Kinetic parameters were calculated as 20.6 mM for K_m and 6.44×10⁻⁵ mol/dm³ min for V_max for free enzyme and in the range of 23.6–57.7 mM for K_m and 8.62×10⁻⁵–1.05×10⁻⁴ mol/dm³ min for V_max, depending on the amount of MA in the hydrogel. Enzyme activities were found to increase from 50.0 to 74.0% with increasing amount of MA in the gel system and retained their activities for one month storage. The enzyme activities, after storage at 4°C for one month, were found to be 21.0 and 50.0–74.0% of the initial activity values for free and adsorbed enzyme, respectively. The optimal pH values for free and adsorbed enzymes were determined as 4.56 and 4.56–5.00. The optimum temperature for free and adsorbed enzymes was 55°C. Adsorption studies show that, not only gel composition but also the stimuli, temperature and pH of the solution, play important roles on the invertase adsorption capacity of poly(AAm/MA) hydrogels.     

Influence of ultrasound on mass transport during cheese brining

 The effect of ultrasound on mass transfer during cheese brining has been investigated. The rate of water removal and NaCl gain increased when ultrasound was applied in comparison with brining performed under static or dynamic conditions, suggesting that ultrasound improves both external and internal mass transfer. A simple diffusional model was developed to simulate mass transport during acoustic brining. Model parameters were estimated using experimental data from acoustic brining experiments carried out on cheese cylinders of 1.7×10^–2m radius and 3×10^–2 m height at different temperatures (5, 15 and 20  °C). Effective water (D _W) and NaCl (D _S) diffusivities estimated using the proposed model ranged from 5.0×10^–10m²/s and 8.0×10^–10 m²/s at 5  °C to 1.3×10^–9m²/s and 1.2×10^–9 m²/s at 20  °C. Both D _W and D _S varied with temperature according to the Arrhenius equation. Through the proposed model, water losses and NaCl gains of the experiments used in the parameter identification were accurately simulated (average %var=98.2%) and also of two additional acoustic experiments carried out under different conditions of temperature (10  °C) and sample size and geometry [parallelepiped of (6×2.5×1.25)×10^–2 m] to those used in the parameter identification (average %var=98.4%). Received: 22 September 1998 / Revised version: 20 November 1998     

Low‐field NMR: A tool for studying protein aggregation

Low‐field nuclear magnetic resonance (NMR) spin–spin relaxation (T₂) measurements were used to study the denaturation and aggregation of β‐lactoglobulin (β‐LG) solutions of varying concentrations (1–80 g L^?1) as they were heated at temperatures ranging from ambient up to 90 °C. For concentrations of 1–10 g L^?1, the T₂ of β‐LG solutions did not change, even after heating to 90 °C. A decrease in T₂ was only observed when solutions having higher concentrations (20–80 g L^?1) were heated. Circular dichroism (CD) spectroscopy and fluorescence tests using the dye 1‐anilino‐8‐naphthalene sulfonate (ANS) on 0.2 and 1 g L^?1 solutions, respectively, indicated there were changes in the protein's secondary and tertiary conformations when the β‐LG solutions reached 70 °C and above. In addition, dynamic light scattering (DLS) showed that protein aggregation occurred only at concentrations above 10 g L^?1 and for heating at 70 °C and above. The hydrodynamic radius increased as T₂ decreased. When excess 2‐mercaptoethanol was added, the changes in both T₂ and the hydrodynamic radius followed the same trend for all β‐LG protein concentrations between 1 and 40 g L^?1. These observations led to the conclusion that the changes in T₂ were due to protein aggregation, not protein unfolding. Copyright © 2007 Society of Chemical Industry