Thermal Destruction of Pteroylglutamic Acid in Buffer and Model Food Systems

First order rate constants (k) and activation energies (Ea) of pteroylglutamic acid (PteGlu) were determined between 100 and 140°C in citrate buffer (pH 3–6) and model food systems. The rate constants (hr^-1) in citrate buffer ranged from 0.145–2.36 at pH 3; 0.074–0.840 at pH 4.0; 0.021–0.207 at pH 5.0, and 0.015–0.133 at pH 6.0. Corresponding Ea values were 22.6, 19.5, 17.8, and 16.8 kcal/mole at pH 3, 4,5, and 6, respectively. In an apple juice model (pH 3.4), the rate constants ranged from 0.063–0.822 and in tomato juice model (pH 4.3), from 0.056–0.723. Ea values in apple juice an tomato juice were 20.0 and 19.7 kcal/mole, respectively.     

Control of Enzymatic Browning in Processed Mushrooms (Agaricus bisporus)

Control of polyphenol oxidase (E.C. 1.14.18) activity by the use of citric acid was investigated. The enzyme was inactivated at pH 4.0 and was stable to 10 min exposures at 25°C in the pH range 4.0–8.0. At pH 6.5 the enzyme was active at 45°C but not at 70°C and thermal inactivation followed pseudo first-order kientics. At pH 6.5 the activation energy (E_a) for enzyme inactivation was 41.1 Kcal/mole while at pH 3.5 two rate constants and hence two values for E_a were observed. Between 0–5 min E_a for inactivation of polyphenol oxidase was 8.7 Kcal/mole and >5 min Ea was 21.8 Kcal/mole.     

Kinetics of (6R,S) 5-formyltetrahydrofolic acid isobaric–isothermal degradation in a model system

Isothermal and isobaric–isothermal degradation kinetics of (6R,S) 5-formyltetrahydrofolic acid (5-CHOH₄PteGlu) was studied at different pH levels in the temperature range from 80 to 110 °C and different pressure–temperature combinations (100–800 MPa and 30–70 °C). The thermal degradation of 5-CHOH₄PteGlu followed first-order kinetics at all pH levels (in the range of pH 3–9.2) studied. 5-CHOH₄PteGlu at pH 7–9.2 was quite stable up to 5–7 h of heating at 80 °C but less stable in more acidic conditions (pH 3.38 and 5). Pressure–temperature degradation of 5-CHOH₄PteGlu could be adequately described by a first-order reaction. In the whole pressure–temperature area studied, the degradation rate constants of 5-CHOH₄PteGlu were enhanced by increasing pressure or increasing temperature. Significant reductions in 5-CHOH₄PteGlu concentration were noticed when pressures above 400 MPa were combined with temperatures above 40 °C. A model to describe the effect of combined pressure and temperature on the 5-CHOH₄PteGlu destruction rate constant was presented.     

Thermal Stability of Folic Acid and 5-Methyltetrahydrofolic Acid in Liquid Model Food Systems

The effects of temperature, iron and ascorbate fortification, and oxygen concentration on the stabilities of folic acid (FA) and 5- methyltetrahydrofolic acid (5-CH₃-THF) were examined in liquid model food systems. Small retort pouches were used as reaction vessels and the model systems were processed at 100°C, 120°C, and 140°C. A cation-exchange procedure was devised to provide sample extract purification before quantitation using high-performance liquid chromatography (HPLC). FA and S-CH₃-THF were found to be very stable under these conditions. Lactose, protein, iron and ascorbate were all found to be capable of reducing the oxygen partial pressure within the model systems, thereby enhancing the stability of FA and 5-CH₃-THF. FA and 5-CH₃-THF degradation was not zero or first order but possibly second order as a result of limiting oxygen concentrations.     

The effect of different folate forms on denaturation of bovine folate binding protein

Denaturation temperatures (T_max) of folate binding protein (FBP) complexed with various folate derivatives were studied using differential scanning calorimetry. Surface plasmon resonance technique was used to elucidate the effect of heat treatment, i.e., pasteurization and UHT treatment, of FBP on FBP content and its binding capacity to folate. The folate derivatives studied were (6S)5-HCO-H₄folate, (6S)5-CH₃-H₄folate and pteroyl-l-glutamic acid (PteGlu). The results showed that different folate forms affected the heat denaturation temperature of FBP differently. Apo-FBP underwent an endothermic transition with a maximum at 60.5 ± 0 °C. After ligand binding, the maximum of the denaturation shifted with a transition maximum at 72.4 ± 0.3 °C for (6S)5-HCO-H₄folate and 78.7 ± 0.5 °C for (6S)5-CH₃-H₄folate. The highest temperature shift was observed for PteGlu with maximum transition temperature at 83.7 ± 0.2 °C. Pasteurization temperatures did not eliminate the binding capacity of FBP regardless of folate form bound, whereas the UHT-treatment did.     

Voluntary food fortification with folic acid in Spain: Predicted contribution to children’s dietary intakes as assessed with new food folate composition data

The Spanish market offers a significant number of folic acid (FA) voluntarily fortified foods. We analysed FA and (6S)-5-methyltetrahydrofolic acid ((6S)-5-CH₃-H₄PteGlu) content in ready-to-eat cereals (RTEC) (n = 68) and cow’s milk (n = 25) by a previously validated affinity chromatography–HPLC method. Contribution to potential FA intakes for children aged 2–13 years, was assessed using food consumption data from a representative nationwide study, folate Recommended Dietary Intakes (RDI), and Upper Levels (UL). Results showed that at all food fortification levels obtained, fortified products provided more than tenfold FA than (6S)-5-CH₃-H₄PteGlu. For RTEC, the high fortification level provided 6–21%, per serving, of RDI and ?32% of ULs at 90th percentile of RTEC consumption (P90). Milk products fortified at the higher level reached on average 54–136% of RDI per serving and only exceeded UL at P90 of milk consumption in children aged 2–5 years.     

Aspartame Degradation Kinetics as Affected by pH in Intermediate and Low Moisture Food Systems

Kinetics of aspartame degradation in low to intermediate moisture systems were evaluated by incorporating aspartame into agar/micro-crystalline cellulose model systems. They were prepared at pH 3, 5, and 7, equilibrated after freeze-drying to three water activities (0.34, 0.56, and 0.66), and stored at 25 to 45°C. Aspartame in such systems was most stable at pH 5 and became less stable as pH decreased or increased. As the molar buffer salt concentration increased, the rate of aspartame degradation increased. The activation energy ranged from 23 to 36 kcal/mole.     

Standardisation of HPLC techniques for the determination of naturally-occurring folates in food

The aim of this work was to evaluate current in-house HPLC procedures for the determination of naturally-occurring folates in food, and to identify problem areas for further improvement. Five intercomparison studies were completed over the period 1990–1997 in which nine participants from six countries took part. Through careful validations and detailed discussions held at evaluation meetings, possible biases and sources of systematic error have been identified and reduced. The use of ascorbic acid and nitrogen flushing during extraction, sample clean-up using strong anion exchange columns, spectrophometrically calibrated standards and fluorescence detection are all recommended. Both in-house hog kidney and human plasma deconjugase enzymes gave similar results to the circulated common hog kidney enzyme which was prepared from fresh pig’s kidneys. The most consistently reported values were for 5-CH₃H₄-PteGlu, and to a lesser extent, for H₄PteGlu. Four candidate reference materials (CRM 121, wholemeal flour; CRM 421, milk powder; CRM 485, lyophilised mixed vegetables, and CRM 487, lyophilised pig’s liver) have been proposed with both indicative values (mean ± uncertainty) for 5-CH₃H₄-PteGlu in CRM 421 (0.25; ± 0.02 mg/kg) and CRM 485 (2.14; ±0.42 mg/kg), and information values (mean; range) for 5-CH₃H₄-PteGlu in CRM 121 (0.04; 0.03–0.08 mg/kg) and CRM 487 (2.6; 1.9–3.8 mg/kg). Certified values are also given for total folate by microbiological assay: CRM 121 (0.50; ±0.07 mg/kg), CRM 421 (1.42; ±0.14 mg/kg), CRM 485 (3.15; 0.28 mg/kg), and CRM 487 (13.4; 1.3 mg/kg). Average recovery of 5-CH₃H₄-PteGlu, added prior to extraction and deconjugation, was 91% (84–95%) for the four CRMs. The average within- and between-laboratory variations were 6 and 15% for the determination of 5-CH₃H₄-PteGlu by HPLC, and 9 and 18% for the determination of total folate by microbiological assay. These CRMs will be used for quality control of folate measurements for nutritional labelling, and validation of new techniques. Further methodology work is required for the HPLC analyses of folate forms other than 5-CH₃H₄-PteGlu.¹     

Pressure-Thermal Kinetics of Furan Formation in Selected Fruit and Vegetable Juices

Furan, a potential carcinogenic compound, can be formed in array of processed foods. The objective of this study was to conduct kinetic studies in pineapple juice and assess the interactive effects of pressure (0.1 to 600 MPa) and temperature (30 to 120 °C) on furan formation. Additional experiments were carried out in tomato, watermelon, cantaloupe, kale, and carrot juice to understand the influence of matrix and juice pH. Furan was monitored in raw (control) and processed samples by automated headspace gas chromatography mass spectrometry, and quantified by calibration curve method with d₄-furan as internal standard. The data were modeled using zero-, first-, and second-order equations. The zero-order rate constants (k _T,P ), activation energy (E _a ), and Gibbs free energy of activation (ΔG ^?) of furan formation in thermally processed (TP; 90–120 °C) pineapple juice were found to be 0.036–0.55 μg/kg/min, 98–114 kJ/mol, and 173.9–180.5 kJ/mol, respectively. Furan concentration was negligible and close to the detection limit (0.37 μg/kg) after pressure treatment (600 MPa at 30 °C) of juice samples. For similar process temperatures, the rate constants of pressure-assisted thermally processed (PATP; 600 MPa at 105 °C) pineapple juice were lower than that of TP samples. Furan formation was influenced by juice matrix and pH. On the other hand, PATP markedly suppressed furan (0.7 to 1.6 μg/kg) in these selected juices. In conclusion, furan formation increased with process temperature and treatment time, while pressure treatment at ambient temperature did not promote its production. Furan formation in TP fruit juices was also influenced by juice matrix and pH, but these were not the significant factors for PATP-treated juices.     

Flow Properties of Low-Pulp Concentrated Orange Juice: Effect of Temperature and Concentration

Three 65° Brix low-pulp concentrated orange juice (COJ) samples, between −19 and 30°C were shear-thinning (pseudoplastic) fluids with negligible magnitudes of yield stress. The simple power law model fit welt the shear rate-shear stress data. For one sample, the Powell-Eyring model also described the data well. The Arrhenius model described the effect of temperature on the apparent viscosity and the consistency index of the power law model. The activation energy of flow (E_a) was 10.7 ± 0.2 kcal/g mole. The models of Harper and El Sahrigi and Christiansen and Craig were suitable for describing the combined effect of temperature and shear rate. The magnitude of E_a decreased with decrease in concentration. Apparent viscosity and K increased exponeitially with concentration.     

Changes in Muscadine Grape Juice Quality During Cold Stabilization and Storage of Bottled Juice

Muscadine grapes (Vitis rotundifolia, Michx.) of two cultivars (Noble and Carlos) were washed and extracted. The resulting juice was cold stabilized for 0, 7 and 60 days at 2°C, then treated, bottled, pasteurized and stored at 2°C and 24°C for 0, 4, 8, and 12 months. Carlos juice was lower in phenols and pH and higher in acidity than Noble. The high total anthocyanins and phenols in the Noble juice caused a significant loss in pigment during cold stabilization. Dilution of juice with 40% water and adjustment of the sugars was beneficial to flavor and color of Carlos, yet 40% dilution was too high for Noble juice. Treatment of either juice with CaCO₃ increased pH and decreased acidity. During 12 months storage, the Carlos juice became darker due to browning, while the Noble juice became lighter by losing pigment, especially at 24°C.     

Kinetics and equilibria of tea infusion: Part 7—The effects of salts and of pH on the rate of extraction of caffeine from Kapchorua Pekoe Fannings

The rates of extraction of caffeine from sieved Kapchorua PF (600–710 μm) have been measured at 80°C with a range of aqueous salt and buffer solutions of ionic strength 0·11 mol dm⁻³. The first-order rate constants and the half-times of infusion showed no trend with pH when buffers from pH 3·0 to pH 8·3 were employed. The rate constants decreased on the addition of common salts like NaCl, KCl and CaCl₂ but increased in the presence of electrolytes such as Bu₄NCl that contain large ions. The results cannot be interpreted by changes in osmotic pressure although Donnan effects may be involved. Close parallels were found between the rate constants and the solubilities of caffeine in electrolyte solutions at 25°C. In particular, the values of both properties rise appreciably in the presence of species containing aromatic or other organic rings with which caffeine molecules associate.     

Kinetics of Ascorbic Acid Loss and Nonenzymatic Browning in Orange Juice Serum: Experimental Rate Constants

The kinetics of ascorbic acid loss and nonenzymatic browning in clarified orange juice (serum) were investigated in an anaerobic environment from 70.3 to 97.6°C and from 11.7 to 80.6Brix. Data were fitted to firstorder kinetic models. Rate constants of ascorbic acid degradation in serum were not different from rate constants in whole juice. Activation energies were ±30 kcal/mol and largely independent of solids concentration. Rate constants of browning pigment formation were 30–50% greater in serum. Activation energies were 19–25 kcal/mol and increased slightly with solids concentration.     

Patulin Adsorption Kinetics on Activated Carbon, Activation Energy and Heat of Adsorption

The kinetics of adsorption of patulin on activated carbon were studied at different initial patulin concentrations (100–400 ppb) for the temperature range 20–80°C. Apparent adsorption rate constants (k_aapp) were changed from 1.07 × 10^?3 to 1.86 × 10^?3 g^?1 min^?1 while the temperature increased from 20 to 80°C. For equilibrium adsorption curves; the Langmuir model was attempted and model parameters (K and Q°) were obtained for different temperatures. Energy of activation and heat of adsorption were determined in a batch adsorption system (E_a= 2.02 kcal/mol and ΔH = 2.24 kcal/mol). The adsorption occurred endothermically and by physical mechanisms.     

Water Activity and Temperature Effects on Nonenzymic Browning of Amino Acids in Dried Squid and Simulated Model System

At constant temperatures (5 to 45°C) browning rate patterns of 22 amino acids, induced during dehydration in presence of glucose and celite, pH 6.4, Aw 0.60 to 0.85, were classified into three types: 1. (lysine, histidine, and glycine) showed high rates over a broad a_w range. 2. (Cysteine and methionine) had intermediate browning rates and a pattern of multiple maxima. 3. Amino acids (glutamine, valine and tryptophan) had low browning rates and showed a single sharp browning maximum. Squid mantle had broad browning maxima over the intermediate a range resembling type 1, which comprised 75% of the total free amino acids in squid. Browning of squid was accelerated at ≥ 23°C, with a_w 0.60 to 0.90. Activation energy of the model systems was 10 to 35 kcal/mole, and Q10 were 1 to 8.     

USE OF IMMOBILIZED LACTASE IN PROCESSING CHEESE WHEY ULTRAFILTRATE

Enzymatic hydrolysis of lactose in cottage cheese whey ultrafiltrate was investigated. Lactase of A. niger was immobilized on an alumina-silica catalyst support by the linking agent tolylene-2, 4-diisocyanate. The resulting immobilized enzyme preparation had an activity of 3 standard international units of lactase per gram at pH 4 and 37 °C. The optimum pH and temperature for hydrolysis of lactose by immobilized lactase were 3.5 and 50°C, respectively. Immobilization of the enzyme resulted in reductions of 1.1 pH units and 15°C in optimum pH and temperature, respectively. The two constants of the simple Michaelis-Menten rate expression were obtained from Lineweaver-Burk plots of the initial reaction rate data obtained at 37°C. Estimated values for V_max and the apparent K_m were 7.8 (μmoles/min-g) and 0.26 (M), respectively. Inhibition by the product galactose was measured by studying the hydrolysis reaction in a batch reactor. The inhibition constant K_i was estimated from batch reactor data to be 0.005 and 0.053 (M) at 35 and 50°C, respectively. Activation energies of 8.1 and 6.4 (kcal/gmole) were obtained for the immobilized and soluble enzyme reactions, respectively. The behavior of the batch reactor as measured in terms of a plot of conversion versus time was essentially the same for both conventional and deionized whey ultrafiltrate.     

Accelerated Kinetic Study of Aspartame Degradation in the Neutral pH Range

The degradation of aspartame in solution as a function of temperature (70–100°C), buffer concentration (0.01–0.1M phosphate), and pH (6–7) was studied in order to estimate losses during thermal processing and storage of aseptic milk-based drinks. Prior data have been mostly on acid carbonated beverages. First order rate constants were obtained in all the conditions with activation energies in the range 14–20 kcal/ mole. An increase in both pH and buffer concentration caused an increase in rate of loss. These data were used to predict losses that would occur during pasteurization and sterilization conditions. Experiments at 4 and 30°C showed significant losses would occur during 4 and 30°C temperature storage and extrapolation from high temperatures predicted faster degradation rates than those found.     

Application of gas phase surface discharge plasma with a spray reactor for Zygosaccharomyces rouxii LB inactivation in apple juice

The effects of the gas phase surface discharge plasma system with a spray reactor on yeast in 12, 36, and 60°Brix apple juice were evaluated. Results indicated that the number of Zygosaccharomyces rouxii LB cells in 0.5 L of 12°Brix apple juice was reduced by 5.60 log₁₀ after discharge plasma treatment for 30 min. When the soluble solids content increased from 12 to 60°Brix, yeast inactivation was decreased by 2.55 log₁₀. After treatment, the pH level of the apple juice only slightly increased, and the titratable acidity and concentration of reducing sugars remained unchanged. The total color differences between the plasma-treated and untreated samples in 12, 36, and 60°Brix apple juice were 9.53, 7.97, and 3.86. Meanwhile, the concentrations of alcohols, esters, aldehydes, and ketones—the main components of volatile compounds showed no significant change (p < 0.05).Industrial relevanceInactivation of microorganisms using nonthermal plasma technology can reduce food costs and minimize changes in food products and is therefore an alternative to heat-based technology. The proposed research showed that the gas phase surface discharge plasma treatment with a spray reactor can effectively reduce Zygosaccharomyces rouxii in apple juice. In addition, the effect of plasma treatment on the color and volatile compounds of the apple juice were determined. This study provided a technical support for the application of plasma technology in apple juice processing.     

Determination of Oxygen Solubility in Liquid Foods Using a Dissolved Oxygen Electrode

A rapid method for determining oxygen solubility in foods was introduced. Fructose, glucose, sucrose, orange juice, apple juice, grape juice, grapefruit juice, lemonade, and tomato juice had similar oxygen solubilities at comparable °Brix readings. The equation: In [ppm O₂] = 2.63 ? 0.0179 (°Brix) ? 0.0190 (°C) estimated to within 5% the oxygen solubility of sugar solutions and fruit juices at temperatures between 4°C and 40°C. At likely food concentrations, citric acid, ascorbic acid, and NaCl reduced oxygen solubility by less than 10%. Tests for component interactions were also conducted. There was no measurable synergism or antagonism between fructose, glucose, and sucrose with or without organic acids.     

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⁺⁺.