Browsing by Subject "Oxidation"

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Challenges of green production of 2,5‐furandicarboxylic acid from bio‐derived 5‐hydroxymethylfurfural: Overcoming deactivation by concomitant amino acids
(2022) Neukum, Dominik; Baumgarten, Lorena; Wüst, Dominik; Sarma, Bidyut Bikash; Saraçi, Erisa; Kruse, Andrea; Grunwaldt, Jan‐Dierk
The oxidation of 5‐hydroxymethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) is highly attractive as FDCA is considered as substitute for the petrochemically derived terephthalic acid. There are only few reports on the direct use of unrefined HMF solutions from biomass resources and the influence of remaining constituents on the catalytic processes. In this work, the oxidation of HMF in a solution as obtained from hydrolysis and dehydration of saccharides in chicory roots was investigated without intermediate purification steps. The amount of base added to the solution was critical to increase the FDCA yield. Catalyst deactivation occurred and was attributed to poisoning by amino acids from the bio‐source. A strong influence of amino acids on the catalytic activity was found for all supported Au, Pt, Pd, and Ru catalysts. A supported AuPd(2 : 1)/C alloy catalyst exhibited both superior catalytic activity and higher stability against deactivation by the critical amino acids.
Determination of potentially hazardous oxidation products in cosmetics containing lanolin or 1-(1,2,3,4,5,6,7,8-octahydro-2,3,8,8-tetramethyl-2-naphthalenyl)ethanone (OTNE)
(2019) Schrack-Belschner, Sonja Miriam Irmgard; Schwack, Wolfgang
Cosmetic products are important consumer goods in the "non-food" sector, which should not have negative effects of the human health. Critical compounds, however, can be formed by the oxidation of an unsaturated organic compound.Thereby formed oxidation products with potentially adverse properties are well known from the food sector. As the oxidation of cosmetic ingredients, however, has less been studied, the oxidation of selected cosmetic ingredients with respect to the formation of potentially critical compounds was investigated within the framework of this thesis.The oxidation of cholesterol to various cholesterol oxidation products (COPs) was investigated in a first step.COPs are known from the food sector and are suspected of causing certain diseases such as arteriosclerosis.Cosmetic products have not yet been tested for COPs, although a versatile ingredient used only in cosmetic products, lanolin, contains above-average levels of the cholesterol, which is the precursor.Total COPs contents in cosmetics containing lanolin, namely lip care products, fat creams and ointments for nursing women were in the low percent range (up to 3 %) and were thus several orders of magnitudes higher than the contents found in food.The oxidation of fragrances was studied in the second part of this work.The subject is not new as the oxidation of terpenes to contact allergens has been studied in earlier studies. The oxidation of other fragrances was hardly investigated. In order to extend our knowledge in this field, the oxidation of a synthetic fragrance frequently used in perfumes, octahydro tetramethyl naphthalenyl ethanone (OTNE) was studied. Obtained results indicated that peroxides of OTNE were formed during oxidation.It was found out that the OTNE oxidation even occurs, when perfumes are stored indoors under normal temperature and light conditions. An in-vivo test showed that OTNE oxidation can be expected on the skin after application of a perfume.
Impact of oxygen on quality of white wine
(2013) Morozova, Ksenia; Schwack, Wolfgang
Today?s wine market is becoming more and more saturated. At given conditions, the understanding of the consumer needs and preferences determines the success of a wine producer. The value of white and rosé wines appreciated by the consumers lies in their fruity and fresh character. Wine oxidation is one of the major wine failures. Recently, it has been reported that up to 48% of the wines rated as faulty by judges in wine competitions exhibited off-flavours that can be linked to the erroneous management of oxygen. Wine exposure to oxygen is possible throughout the whole winemaking process and depends on process conditions and applied equipment. Oxygen can be dissolved in wine up to 8.8 mg/L at normal conditions. After dissolution, oxygen concentration in liquid phase is normally decreasing to undetectable content due to chemical reactions of molecular oxygen with other wine constituents. Wine oxidation is a complex process. The effects may vary significantly influenced by numerous factors, including a type of wine, operation, oxygen amount chemical composition of wine, pH, storage temperature, light exposure, metal content and redox state of wine. Oxygen has a potential to support positively, and subsequently to ruin aroma and colour of wine. Most white wines are negatively affected by small amounts of dissolved oxygen provoking rapid aroma loss and oxidative browning, thus decreasing attractiveness for consumers, whereas in red wines microoxygenation may help to stabilise wine colour and remove reductive off-flavours. Up to now, most of the research on wine oxidation was focused on experiments in model solutions. Although these studies deliver valuable information about oxidation mechanisms, there was, however, limited data published on real wine experiments linking analytical data and sensory analysis. Consequently, the background of the present investigations was comprehensive evaluation of the effectiveness of the various factors influencing wine oxidation in real wine medium, and finally the development of innovative strategy for quality improvement and shelf-life extension of white wines. Comparison of different methods for O2 and CO2 measurement in wine The initial part of the present work was to determine a reliable quantification method for dissolved oxygen quantification in wine medium. Since dissolved carbon dioxide present in wine may influence the oxygen concentration, its amount should also be taken into account. Thus, development of dissolved carbon dioxide determination was of major importance for the further experiments described in this thesis. The main aim of the first study was to give a review of the oxygen and carbon dioxide measurement principles and to compare several instruments for carbon dioxide and oxygen measurement in wine available in the market. For carbon dioxide determination, CarboQC, Orbisphere 3658 and agitation cylinder were used. Oxygen measurements were conducted with PreSens, OxyQC and Orbisphere 3650. Sample bottles were prepared with concentrations in the range from 0 to 2200 mg/L for carbon dioxide and from 0 to 12 mg/L for oxygen, respectively, dissolved in white wine of Müller Thurgau variety. O2 and CO2 measurements with six instruments were sequentially conducted at 20°C after 10 minutes shaking of trial bottles. Subsequently calculated concentrations of oxygen and carbon dioxide were used for correlation regression tests. From the data obtained for CO2, three investigated instruments showed good repeatable measurement results (R2=98%) in the range from 0 to 1500 mg/L. However, in the wine samples oversaturated with CO2 (>1500 mg/L at 20°C) CarboQC and Orbisphere 3658 showed significantly higher precision (R2=99%), compared to the agitation cylinder, due to carbon dioxide losses during filling of the cylinder. As for oxygen data, results obtained with each of the measurement devices (PreSens, OxyQC and Orbisphere 3650) demonstrated good correlation with the initial oxygen amount in wine samples (R2=98%) in the whole monitored range of dissolved oxygen concentrations. However, due to the flexibility of PreSens mini-sensor application inside the bottle and noninvasive measurement, this technique was found to be more advantageous, compared to the other two. Additionally, it allowed not only determination of dissolved oxygen, but also of the gaseous oxygen concentration in a bottle headspace. For these reasons, the PreSens device was chosen for further experiments. Effect of Headspace Volume and Iron and Copper Addition on Oxidation Processes in Model Solution and Riesling Wine: Chemical and Sensory Changes Since previous studies in model solutions published by other authors clearly showed the key role of iron and copper in oxidation reactions in wine, the main aim of this part of the thesis was to compare the effects of oxygen and iron and copper additions on oxygen consumption rate, sulphur dioxide and colour of bottled model solution and actual wine with similar properties. Model solution was prepared of deionised water, glycerol, (+)-tartaric acid, ethanol, gallic acid and potassium metabisulfite to reach a free sulphur dioxide concentration of 50 mg/L. 200 L Riesling wine (vintage 2010) was prepared using standard winemaking techniques and equipment, and was subsequently treated with potassium ferrocyanide to eliminate iron and copper. Model solution and Riesling wine were bottled in 500 mL bottles with and without small additions of iron (0.1 mg/L) and copper (0.05 mg/L). Oxygen concentration in bottles was adjusted using various headspace volumes (0, 50 and 100 mL for model solution, and 0, 20 and 40 mL for Riesling, respectively) full of ambient air. Iron and copper concentration, total consumed oxygen, sulphur dioxide, browning rate (E420) were monitored. Additionally, sensory analysis of bottled Riesling wines (triangle tests and descriptive analysis) was conducted after 90 and 240 days of storage. The results revealed major differences between model solution and real wine. In model solution the headspace volume and the metal addition contributed to significant changes in total consumed oxygen, colour, and free sulphur dioxide. The metal addition increased the rate of the molecular oxygen consumption and resulted in elevated consumption of free SO2. Enhanced colour was observed in all wines, where iron was added. The experiment with Riesling wine with similar parameters showed strong influence of the headspace volume. The rate of oxidative browning and oxygen consumption rate strongly correlated with the headspace volume at the bottling. In contrast to model solution experiment, addition of small concentrations of iron and copper did not contribute to the colour and oxygen consumption rate of Riesling wine. Sensory analysis showed that the wines bottled with 0, 20 mL and 40 mL HS volume became significantly different already after three months of storage at 15°C, which was proved by the triangle tests. Descriptive analysis after six and nine months of storage confirmed negative influence of headspace volume, thus proving the significance of oxygen ingress at bottling on wine quality. Moreover, traces of metals in Riesling wines even in the wines with no iron and copper addition were sufficient to initialise oxidation processes. However, since small iron and copper additions had significant impact on model solution, further studies with Riesling wine needed to be conducted. The Impact of Headspace Oxygen and Copper and Iron Addition on Oxygen Consumption Rate, Sulphur Dioxide Loss, Colour and Sensory Properties of Riesling Wine For further investigation of the impact of iron and copper on white wine oxidation, Riesling wine was bottled with the addition of 1 mg/L of iron and 0.5 mg/L of copper, which correspond to average iron and copper concentrations in wines of Baden-Württemberg. Oxygen concentrations were determined, as previously reported, by the headspace volume in the bottle (0 mL, 10 mL and 20 mL) full with ambient air. In contrast to the previous experiment, addition of 1 mg/L of iron and 0.5 mg/L of copper had significant influence on the oxygen consumption rate, on the loss of free SO2 during storage, and on the sensory changes in wine. Addition of iron and copper significantly catalysed the oxygen consumption. Free sulphur dioxide loss was found to be proportional to the total consumed oxygen after bottling. Moreover, in all wines with iron and copper addition free sulphur dioxide decay was significantly elevated compared to wines with no iron and copper addition. Although colour changes were not observed in wines after 90 days of storage, significant sensory changes were detected. Both oxygen and iron and copper addition made an impact on sensory evaluation of wines. At low oxygen concentration (0 mL headspace) metal addition had positive effect resulting in elimination reduced aromas. In contrast, for 10 mL and 20 mL headspace, wines with iron and copper addition showed lower scores in fruity, citrus, tropical aromas, and elevated scores in untypical aging and had pronounced oxidised character. The results of the study indicate that, in the case of Riesling wine, excessive oxygen exposure due to oxygen present in the headspace of the bottle should be avoided. Moreover, iron and copper concentration also seems to make significant impact on oxygen and SO2 consumption rates and on sensory perception of wines. These findings suggest that iron and copper concentrations should also be taken into account, when oxygen management strategy is defined. Effect of Headspace Volume, Ascorbic Acid and Sulphur Dioxide on Composition and Sensory Profile of Riesling Wine In the last part of the present work the effects of different oxygen and free SO2 levels, and ascorbic acid addition on the development of white wine were investigated. Riesling wine was bottled in 500 mL bottles with four different headspace volumes (0 mL, 10 mL, 20 mL, 30 mL), two levels of free SO2 (50 mg/L and 70 mg/L), and with and without ascorbic acid (250 mg/L) addition. Dissolved oxygen and the oxygen in headspace were measured in the resulting 17 wines. Free and total SO2 concentrations, ascorbic acid concentration, colour, redox potential, and antioxidative capacity were measured regularly in wine samples. After six months of storage, the wines were evaluated using sensory descriptive analysis. It was again proved that wine exposure to oxygen at bottling plays a key role in white wine development during storage. High oxygen ingress may greatly influence the redox state of wine and affect the important quality parameters including colour, free and total SO2, and the overall sensory quality of wine. Free and total sulphur dioxide loss and the decline in ascorbic acid could be linked to the total consumed oxygen content. It was observed that in the presence of ascorbic acid less sulphite was consumed. Ascorbic acid addition also contributed to the fruity and fresh character of the bottled wines, which indicates its strong antioxidant. However, when combined with high oxygen concentration, ascorbic acid addition promoted enhanced white wine browning. This was scavenged in the wines with higher free SO2 concentrations. Based on the data shown, careful control of the oxygen ingress during bottling is crucial for white wine quality. Ascorbic acid addition seems to have positive sensory effects on the development of wines during the post-bottling period. However, the possible wine browning, associated with ascorbic acid, should be taken into account. On the contrary, sulphur dioxide, in case of the Riesling wine studied, seems to be less effective to prevent negative sensory effects in wines due to excessive exposure to oxygen, but may simultaneously decrease oxidative browning in wines with ascorbic acid addition. In summary, low oxygen ingress at bottling combined with low iron and copper concentrations, moderate additions of ascorbic acid and sulphur dioxide seem to be a good oxygen management strategy and offer a good potential to improve quality and extent the shelf-life of white wines.
Investigations into heat- and light-induced terpene modifications in essential oils
(2023) Bitterling, Hannes; Vetter, Walter
Essential oils belong to secondary plant metabolites, with terpenoids and phenylpropanoids being among the main constituents in terms of quantity. Due to their lipophilic character and high volatility, they are mainly obtained by steam distillation. Citrus essential oils (agrumen oils) are an exception , since they are usually extracted from the peels by means of pressing, whereby less volatile components such as coumarins and furocoumarins are also introduced. Due to their odor and taste-giving properties, essential oils are used in the food, beverage, and cosmetics industries. In addition, due to a wide range of pharmacological properties, they are used in phytotherapy as well as in aromatherapy. However, most essential oils are highly susceptible to oxidation, polymerization, dehydrogenation, and isomerization reactions in the presence of atmospheric oxygen, light, and at high temperatures. The resulting organoleptic changes usually lead to a significant quality reduction. The formation of terpene hydroperoxides is another problem, as these are suspected of causing intolerances such as redness and itching in 1-3% of the European population upon contact with the skin. The detection of these chemical changes forms an integral part of quality control and can be prevented as far as possible by suitable production, transport, and storage strategies. Due to their volatility, essential oils are mainly analyzed by gas chromatography. However, due to their instability, the detection of hydroperoxides places considerable demands on common analytical methods. For this reason, a novel spectrophotometric method for the detection of peroxides and hydroperoxides in terpenes and essential oils was developed (paper 1). The oxidation of N-N-dimethyl-p-phenylenediamine by peroxides yielding an intensely red-colored cation (Wursters red) allowed colorimetric detection and quantitation of even smallest amounts (LOD: 0.5 ppm). The minimal sample amount of only a few milligrams, as well as simple and fast performance predestine this method for daily laboratory routine (paper 1). Among plant terpenoids, the monoterpene R-(+)-limonene is very widespread. Thus, it is not only found in citrus oils but also of in caraway oil, where its proportion amounts to almost 50%. To investigate the storage stability, R-(+)-limonene, S-(+)-carvone, different caraway oils, and the corresponding caraway seeds were stored in desiccators at 25 °C and 40 °C for eighteen months (paper 2). The samples were analyzed monthly by GC/MS and GC/FID, as well as HPLC/DAD-MS/MS. This showed that the comparison of seed, isolated essential oil, and pure substance, whichhad not been considered in storage studies so far, was of extraordinary importance. Here, both the plant matrix and the essential oil had a protective effect on individual terpenes and delayed their degradation (paper 2). Further, a clear difference between photo-oxidation and autoxidation was observed. Light-induced oxidation of terpenes primarily resulted in the formation of hydroperoxides, whereas autoxidation led to a variety of compounds such as alcohols, ketones, and epoxides. Thus, the secondary products can serve as specific markers for conclusions about the preload and quality of essential oils. In the study presented in paper 3, further photo-oxidation experiments were conducted with beta-pinene, R-(+)-limonene, and gamma-terpinene, with added furocoumarins. Furocoumarins can absorb UV-A light in the range of 320 – 380 nm and enter an energetically excited state. This energy difference between the ground state and excited state can be dissipated again by the emission of fluorescent and phosphorescent light. In this process, short-wave energy-rich UV light is converted into lower-energy visible light (bathochromic shift). For this reason, the UV light-induced degradation of the terpenes beta-pinene, R-(+)-limonene, and gamma-terpinene could be significantly reduced by adding 5% each of xanthotoxin, bergapten, bergaptol, and bergamottin. The effect of adding bergaptol was most pronounced in the photooxidation of gamma-terpinene (paper 3). Consequently, in citrus essential oils from which the natural furocoumarins had been previously removed, irradiation with UV light resulted in a strong degradation of the terpenes. This process could be markedly reduced by the re-addition of 5% of the previously removed plant-specific furocoumarins (paper 4). In summary, it can be concluded that not only the plant matrix and the essential oil as a multicomponent mixture but also potential interactions with other substances forming part of the essential oil such as furocoumarins may significantly slow down the oxidation of terpenoids.
Selektive und effiziente Laccase-katalysierte oxidative Phenolkupplungen
(2012) Constantin, Mihaela-Anca; Beifuss, Uwe
The oxidative phenolic coupling is one of the fundamental reactions of organic chemistry. In contrast to its major role in the biosynthesis of numerous natural compounds the oxidative phenolic coupling is only of little importance in organic synthesis so far. This is due to its frequent lack of regio- and stereoselectivity. Laccases are oxidases which can be employed, amongst others, for the catalysis of oxidative phenolic couplings using O2 as the oxidant. This study highlights three examples which clearly demonstrate that laccases can be used as catalysts for regio- and stereoselective oxidative couplings of phenolic compounds. The first example deals with the laccase-catalyzed oxidative dimerization of (E)-2-propenylsesamol to carpanone (a). The oxidative cyclization starts with a phenolic oxidation, which is followed by a radical coupling and an intramolecular hetero-Diels-Alder reaction. Experiments with laccases and a number of other catalysts indicate that the diastereoselectivity of the carpanone formation doesn´t depend on the nature of the catalyst but on the double bond geometry of the substrate. With (E)-2-propenylsesamol as the substrate, a 9:1-mixture of carpanone (a) and its diastereoisomer c was formed, irrespective of the catalyst used. When (Z)-2-propenylsesamol was used as the substrate, the formation of a 5:1:4-mixture of three diastereoisomers, i.e. a, c and d, was observed. When the oxidation of (E)-2-propenylsesamol with O2 as the oxidant was run in the absence of any catalyst the diastereoisomeric benzopyrans a and b were obtained in a 3:2-ratio. From a mechanistic point of view, this reaction proceeds as a Domino oxidation/intermolecular hetero-Diels-Alder reaction. The second example selected was the laccase-catalyzed oxidative coupling of sesamol, a naturally occurring phenolic antioxidant. Here, a so far unknown trimer was formed as the main product in good yield. Experiments with different catalysts indicated that the course of the oxidative coupling of sesamol depends strongly on the catalyst chosen. Finally, the laccase-catalyzed phenolic coupling of di- and trisubstituted vanillidene derivatives with O2 as the oxidant was studied. The dimerization of (E)-ferulic acid proceeded as a 8,8?-coupling with formation of a dilactone. When the disubstituted vanillidene derivatives were reacted, the diastereoselective formation of the racemic dihydrobenzo[b]furans which can be understood as the products of a 5,8?-coupling mode were formed. In contrast to the disubstituted vanillidene derivatives, the laccase-catalyzed reaction of the trisubstituted vanillidene derivatives exclusively yielded biphenyls as the result of a 5,5?-coupling.