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⁺⁺.     

Photocatalytic degradation of acid blue 80 in aqueous solutions containing TiO2 suspensions

The photocatalytic degradation of the anthraquinonic dye Acid Blue 80 in aqueous solutions containing TiO2 dispersions has been investigated. The process has been monitored by following either the disappearance of the dye (via HPLC) and the formation of its end-products (via IC, GC, and TOC analysis). Although a relatively fast decolorization of the solutions has been observed, the mineralization is slower, and the presence of residual organic compounds was evidenced even after long term irradiation, confirming the relevant stability of anthraquinone derivatives. The identification of various unstable intermediates formed after low irradiation times was performed by HPLC-MS, allowing us to give insight into the early steps of the degradation process which mainly involve C-N bonds breaking and substrate hydroxylation. Complete and relatively fast mineralization of the substrate was achieved by irradiating the semiconductor dispersions in the presence of added K2S2O8.     

On-line monitoring of stevioside sweetener hydrolysis to steviol in acidic aqueous solutions

Stevioside, a potent sweetener obtained from leaves of Stevia rebaudiana (Bertoni), is the glycone of steviol. However, despite its natural origin, there has been concern about stevioside toxicity due to hydrolysis to the carcinogenic steviol. To approve it as an additive, the FAO/WHO committee on food additives has required further information about hydrolytic stability of the steviol glycoside in acidic foods and beverages. In this study, aqueous solutions of stevioside at different pH values were monitored in real time by direct infusion ESI-MS. Owing to the high speed and sensitivity of ESI-MS monitoring, fast hydrolysis of the stevioside molecule to steviol in aqueous acidic solutions was observed, particularly in acidic juices.     

Inhibition of citral degradation in an acidic aqueous environment by polyoxyethylene alkylether surfactants

Citral is a flavour component widely used in food and cosmetic industries, but is chemically unstable and degrades over time in aqueous solutions due to acid-catalysed and oxidative reactions leading to loss of desirable flavour. The present study reveals the effect of non-ionic micellar solutions of Brij30 and Brij35 on the extent of solubilisation and stabilisation of citral. The rate of chemical degradation of citral in acidic aqueous solutions was found to be highest, which was subsequently reduced significantly within these studied surfactant systems, suggesting protection of citral from an acidic environment once it is incorporated into the micelles. The work concludes that polyoxyethylene alkylether surfactants with lower HLB value, less dense hydrophilic corona and more hydrophobic core volume are efficient in solubilising and stabilising citral against an acidic environment.     

Kinetics and mechanisms of radiolytic degradation of nitrobenzene in aqueous solutions

Steady-state radiolysis experiments were performed to gain insight into the kinetics and mechanisms of nitrobenzene (NB) degradation in aqueous solutions. The degradation of NB under gamma-ray irradiation followed pseudo-first-order kinetics, and the pseudo first-order rate constant and the initial G value of NB decomposition were functionally related to both the initial NB concentration and the irradiation dose rate. Under oxidative conditions, complete mineralization of NB was achieved, whereas no total organic carbon reduction was observed under reductive conditions. The radiolytic products of NB under various conditions were identified using FTIR and GC-MS analyses. The mechanisms behind the radiolytic degradation of NB under both oxidative and reductive conditions were proposed schematically in light of the degradation products observed. In addition, calculations based on ab initio molecular orbital and density functional theory provided support for the proposed mechanisms and the preferred pathways among all the possible reactions.     

番茄酱加工中还原型Vc与氧化型Vc变化的研究

研究了在番茄酱加工过程中Vc含量的变化,结果表明:经过蒸汽热烫后的番茄中Vc含量比热水热烫后的番茄中Vc含量要高;番茄打浆前加入0.08%TBHQ能有效控制Vc损失;真空浓缩有利于保持番茄酱Vc,通过正交试验得其最优条件为:真空度0.055kPa、加糖量2.0%、温度65℃;高压杀菌比常压杀菌更有利于番茄酱Vc的保存。     

微酸性电解水对溶液体系中有机磷农药的降解机制及途径分析研究

目的研究对硫磷、甲基对硫磷和乐果对微酸性电解水的敏感性,检测其降解产物并推断降解过程。方法以对硫磷、甲基对硫磷和乐果标准品单一组分为研究对象,通过调节反应时间、微酸性电解水有效氯浓度和有机磷农药的初始浓度,采用气相色谱-质谱联用法检测有机磷农药的降解产物。对有机磷农药在微酸性电解水主要活性成分次氯酸和羟自由基作用下的降解途径进行了分析。结果在反应时间30 s下,有效氯浓度为5.66 mg/L时,对硫磷、甲基对硫磷和乐果的降解率均能达到100%。并检测到对硫磷和甲基对硫磷的直接降解产物对氧磷和甲基对氧磷。结论微酸性电解水能够有效降解有机磷农药,并对HOCl和·OH共同作用于有机磷农药这一降解途径做出假设。该研究对于电解水在消除果蔬有机磷农药残留的应用具有一定的理论意义。     

EXAFS study on the reactions between iron and fulvic acid in acid aqueous solutions

Iron(III) competes with trace metals for binding sites on organic ligands. We used X-ray absorption fine structure (EXAFS) spectroscopy to determine the binding mode and oxidation state of iron in solutions initially containing only iron(III) and fulvic acid at pHs 2 and 4. EXAFS spectra were recorded at different times after sample preparation. Iron was octahedrally configured with inner-sphere Fe-O interactions at 1.98-2.10 A, depending on the oxidation state of iron. Iron(III) formed complexes with fulvic acid within 15 min. Iron(III) was reduced to iron(II) with time at pH 2, whereas no significant reduction occurred at pH 4. No signs of dimeric/trimeric hydrolysis products were found in any of the solution samples (<0.45 microm). However, the isolated precipitate of the pH 2 sample (>0.45 microm) showed Fe...Fe distances, indicating the presence of tightly packed iron(III) trimers and/or clusters of corner-sharing octahedra. It is suggested that the binding mode of iron(III) to fulvic acid at low pH may be phase-dependent: in solution mononuclear complexes predominate, whereas in the solid phase hydrolyzed polynuclear iron(III) complexes form, even at very low pH values. The observed pH dependence of iron(III) reduction was consistent with expected results based on thermodynamic calculations for model ligands.     

Copper-flavonoid complexes in acidic solutions

Summary. Amperometric titration was used to detect complexing of cupric ions by flavonoids in solutions more acid than pH 4.5. Strongest complexing ability was shown by the 3-hydroxy-4-carbonyl system although the 5-hydroxy-4-carbonyl system also complexed with copper in acid solution. the o -diphenolic grouping did not form complexes under the conditions used. Complexing ability was enhanced by the presence of hydroxyl groups in conjugation with the carbonyl group. Quercetin was the only flavonoid tested capable of complexing copper at pH 2.3. the significance of these results to the antioxidant action of flavonoids in fruit juices is discussed.     

Optimisation of process parameters for the degradation of caffeic acid in sugar solutions

Fenton oxidation has been used to study the degradation of caffeic acid (CA), a colour precursor in sugar cane juice, in water and sucrose solutions. Central composite design and response surface methodology were used to evaluate the relationships between % CA degradation and the process parameters (i.e. CA and sucrose concentrations, pH, reagent dosages, reaction temperature and time). A quadratic polynomial model was developed for CA degradation and indicated that the initial sucrose and CA concentrations significantly affected the amount of CA degraded. Numerical optimisation was used to determine the optimum process parameters. It showed that in water at 35 °C, 80% CA was degraded at pH 5.49, 0.72 mm Fe²⁺ and 9.44 mm H₂O₂. However, in a synthetic sugar solution under similar conditions to that of factory mixed cane juice containing 13 mass% sucrose, only 61% of CA was degraded.     

Kinetics and mechanism of degradation of dichlorvos in aqueous solutions containing reduced sulfur species

Reactions of dichlorvos with five reduced sulfur species (hydrogen sulfide, bisulfide, thiosulfate, thiophenol, and thiophenolate) were examined in well-defined anoxic aqueous solutions to investigate their role in its degradation. Reactions were monitored at varying concentrations of reduced sulfur species over pH range to obtain the second-order reaction rate constants. Experiments at 25 degrees C demonstrated that degradation of dichlorvos promoted by bisufide, thiosulfate, and thiophenolate were of much greater importance than hydrolysis under the experimental conditions in our study. In contrast, hydrogen sulfide and thiophenol were not effective in the degradation of dichlorvos. The activation parameters of the reaction of dichlorvos with bisulfide, thiosulfate, and thiophenolate were also determined from the measured second-order rate constants over a temperature range of 12-50 degrees C. The relative reactivity of the reduced sulfur species decreases in the following order: PhS- > HS- approximately equal to S2O3(2-). When the second-order rate constants at 25 degrees C are multiplied by the environmentally relevant concentration of the reduced sulfur species, predicted half-lives of dichlorvos ranged from hours to days. The results indicated that reduced sulfur species could play a very important role in the chemical fate of dichlorvos in coastal marine environments.     

Applications of ultrasound in NAPL remediation: sonochemical degradation of TCE in aqueous surfactant solutions

Surfactant-enhanced pump-and-treat technologies increase the efficiency of nonaqueous-phase liquids (NAPLs) removal from soils. However, high concentrations of surfactants in groundwater impose severe limitations to water treatment. In this paper, we explore the applicability of ultrasonic irradiation as an alternative method for surfactant recovery and contaminant degradation. The combined effects of temperature, initial substrate concentration, and concentration of added surfactant (sodium dodecyl sulfate, SDS) were analyzed for the sonolysis of trichloroethylene (TCE) in batch experiments at an ultrasonic frequency of 500 kHz and 77 W/L applied power density. In the range of 5-30 degrees C, TCE sonolysis becomes faster at higher temperatures, both in the absence and in the presence of surfactant. This indicates that gas-phase pyrolysis prevails over other chemical reactions in the liquid phase. Inhibition of TCE sonolysis was observed in the presence of surfactant at all SDS concentrations. Changes in the initial TCE concentration (from 250 microM to 1.2 mM) showed no effect on the degradation rates in the presence of SDS. For surfactant levels below its critical micelle concentration (cmc), the inhibition of TCE sonolysis exhibited a highly nonlinear dependence with increasing SDS concentration. A correlation was observed in this range between the relative inhibition of sonolysis and the decreasing surface tension of the solutions. Above the cmc up to an SDS concentration of 5%, the reaction rate decreased less markedly. Micellar sequestration of the contaminant seems to be the main reason for this additional inhibition. Bubble growth prior to collapse may incorporate some of the TCE dissolved in the micelles through their adsorption in the expanding bubble walls, thus partially overcoming the scavenging effect due to micellar entrapment of the contaminant.     

空气对水果加工中还原型Vc和氧化型Vc含量的影响

研究了切分及打浆工序对猕猴桃、葡萄等六种水果中的还原型Vc和氧化型Vc含量的影响。试验结果表明,水果在切分和打浆过程中产生了氧化型Vc,尤其是香蕉梨和葡萄,果浆中氧化型Vc占总Vc的比例高达79.43%和65.31%。空气是造成水果加工中还原型Vc损失的主要因素,六种水果中还原型Vc损失率最高的是香蕉梨,为58.75%,最低的是桃子,为49.72%。     

A comparison of the degradation products formed in aerated, aqueous alpha-D-glucose solutions by ultrasound and gamma rays

The degradation products formed in aerated aqueous alpha-D-glucose solutions by cleavage of the carbon skeleton by ultrasound and gamma irradiation are compared. Common products present for both types of irradiation are: D-glyceraldehyde, D-glyceric acid, hydroxy-malondialdehyde, 2,3-dihydroxy-4-oxo-butanoic acid, tetrodialdose, D-arabinoic acid and D-xylose. 2,3,4-Trihydroxy-5-oxo-pentoic acid and D-arabinose could be detected only by radiolysis and evidence for the formation of 2-hydroxy-3-oxo-pentoic acid was only obtained in sonolysis. These fragmentation products are supposed to be formed by reactions of the glucose-peroxyl radicals with HO2-radicals. Based on the reactions of peroxyl radicals given in the literature, a mechanism is proposed which leads to the observed products.     

Copper-catalysed oxidation of linoleic acid in buffered aqueous solutions. I—Effect of ascorbic acid

Ascorbic acid at a concentration of 10^?3 M was a more effective pro-oxidant in the presence of low copper concentrations than at high concentrations, particularly in the initial stages of oxidation or in the lower pH range (5·5–4·7). This suggested a much more complex role for ascorbic acid than that of copperreducing agent. Reaction of the ascorbic acid radical with oxygen and consequent formation of free HO' radicals was considered to be the most likely initiation reaction resulting from the oxidation of ascorbic acid to dehydroascorbic acid by copper. Dehydroascorbic acid was also found to be an effective pro-oxidant in the presence of copper.     

Kinetics and modeling of the Fe(III)/H2O2 system in the presence of sulfate in acidic aqueous solutions

This work examined the effect of sulfate ions on the rate of decomposition of H2O2 by Fe(III) in homogeneous aqueous solutions. Experiments were carried out at 25 degrees C, pH < or = 3 and the concentrations of sulfate ranged from 0 to 200 mM ([Fe(III)]0 = 0.2 or 1 mM, [H2O2]0 = 10 or 50 mM). The spectrophometric study shows that addition of sulfate decreased the formation of iron(III)-peroxo complexes and that H2O2 does not form complexes with iron(III)-sulfato complexes. The rates of decomposition of H2O2 markedly decreased in the presence of sulfate. The measured rates were accurately predicted by a kinetic model based on reactions previously validated in NaClO4/HClO4 solutions and on additional reactions involving sulfate ions and sulfate radicals. At a fixed pH, the pseudo-first-order rate constants were found to decrease linearly with the molar fraction of Fe(II) complexed with sulfate. The model was also able to predict the rate of oxidation of a probe compound (atrazine) by Fe(III)/H2O2. Computer simulations indicate that the decrease of the rate of oxidation of organic solutes by Fe(III)/H2O2 can be mainly attributed to the complexation of Fe(III) by sulfate ions, while sulfate radicals play a minor role on the overall reaction rates.     

The amount of L-dehydroascorbic acid on total-vitamin C of vegetable products (author's transl)]

Spectrophotometric determinations of vitamin C were carried out on different kinds of vegetable products in raw state, after steam-cooking and freezedrying. Indicators used were in part 2,6-dichlorophenol-indophenol (DIP) and in part N-bromosuccinimide (NBS) and starch iodide, which at the same time allows a determination of other reducing interfering substances. The amount of L-dehydroascorbic acid (DAS), which together with L-ascorbic acid (AS) becomes vitamin C or total-vitamin C (VC), proved to be of special interest. During homogenisation of the vegetable samples in oxalic oxid solution AS was not oxidized. Other precautions like cooling or on oxygen-free atmosphere were not required. DAS was not found in raw vegetable products or after cooking. A high proportion of interfering substances especially when estimating VC was considered the cause for the normally measured amount of DAS. In freeze-dried samples of vegetable products a small amount DAS could be identified depending on the freezing process.