Introduction
2-thiobarbituric acid is a barbiturate, the structure of which is that of barbituric acid in which the oxygen at C-2 is replaced by sulfur. It has a role as a reagent and an allergen. 2-Thiobarbituric acid has been reported in Psidium guajava with data available.
1. The 2-Thiobarbituric Acid (Tab) Methodology For The Evaluation Of Warmed-Over Flavor And Oxidative Rancidity In Meat Products
Summary
Although warmed-over flavor and rancidity are primarily organoleptic characteristics of foods, chemical methods for their quantization have been developed. Malonaldehyde (MA) is one of the several decomposition products generally estimated as a marker of lipid oxidation. An extraction/filtration or a distillation procedure for the isolation of malonaldehyde followed by color development with the 2-thiobarbituric acid reagent is often used.
Each evaluation method has its own advantage(s)/disadvantage(s). The filtration procedure often affords more realistic results and prevents overestimation of the TBA reactive substances (TBARS). However, presence of coloured additives such as the cooked cured-meat pigment, as well as turbidity of the extracts, may interfere with accurate determination of the coloured chromogen of TBARS-MA. On the other hand, the distillation method generally affords higher values of TBARS due to further breakdown of labile hydroperoxides.
Addition of antioxidants to the mixtures prior to distillation proved beneficial in some cases. Fur cured meats, the residual nitrite may react with malonaldehyde at elevated temperatures and this would result in underestimation of the TBARS. Addition of sulfanilamide to the distillation mixture prevented the nitrosation of malonaldehyde.
However, sulfanilamide itself gave rise to the formation of condensation products with malonaldehyde. Thus, the TBA determination of oxidative rancidity in meat products may give rise to different results and interpretations.
Introduction
The 2 – thiobarbituric acid (TBA) test was first used by Kohn and L iv e r s e d g e (1944). They observed that animal tissues, upon aerobic incubation, gave a pinkcoloured compound with the 2-thiobarbituric ac;d (TBA reagent). During autoxidation of polyunsaturated fatty acid (PUFA) components of lipids, malonaldehyde (MA) is produced. This secondary oxidation product is highly reactive and remains bound to other food ingredients. An acid/heat treatment of food would presumably release the bound malonaldehyde (Tarladgis ej; aj^. I960)
- Malonaldehyde is the major substance reacting with the TBA reagent aS formulated in Figure 1 (Nair and Turner, 1984). The pink-coloured chromogen so produced has an absorption maximum at 532 nm. Spectrophotometric determinati°n of this complex is the method usually employed for quantification malonaldehyde and other TBA reactive substances. In addition to MA, 2 ,1* ‘ alkadienals, and to a lesser extent, 2-alkenals also produce a pink-coloureh pigment which absorbs at 532 nm (Marcuse and Johansson, 1973).
The TBA test may be performed directly on a food product (Wills, 1965), anci this may be followed by the extraction of the coloured pigment into butanol oi butanol-pyridine mixture (Placer et a l . . 1966; Uchiyama and Mihara, 1978).
Test may also be carried out on a aliquot of an acid extract of food (usually 7.5 to 28% trichloroacetic acid, TCA, solution) (Siu and Draper, 1978) or on a portion of a steam distillate of the sample under investigation (Tarladgis et al., I960, 1964). The latter, the distillation procedure developed by Tarladgis et al. (1960) is the method frequently used. The extraction in TCA is also another procedure used by many researches.
The TBA number which expresses warmed over flavour or lipid oxidation, defined as mg of malonaldehyde per kg of sample (Sinnhuber and Yu, 1958), is calculated by multiplying the absorbance of the TBA-MA complex at 532 nm by a constant. This constant is in turn obtained by the use of standard precursors of malonaldehyde such as 1,1,3,3-tetramethoxypropane (TMA) or 1,1,3,3- tetraethoxypropane (TEP).
1. Materials and Methods
Materials
All chemicals used in these studies were reagent – grade, and were used without any further purification. The cooked cured-meat pigment was prepared as described previously (Shahidi and Pegg, 1988). The meat, loin pork, was deboned and trimmed of most of its surface fat. It was then ground twice using an Oster meat grinder. Additives, along with 20% of distilled water were added to meat samples prior to cooking.
It all cases the mixtures were thoroughly mixed to obtain homogeneous samples. The addition level of different additives are given in corresponding tables where they appear. Homogenized meat samples were cooked in a thermostated water bath for 40 minutes to reach an internal temperature of 75°C. After cooling to room temperature, they were homogenized and stored in plastic bags at 4°C until use. The TBA number of meat samples were determined by a distillation and/or an extraction procedure. The distillation procedure was essentially that of Tarladgis et al.
(1960) with minor modifications as described elsewhere (Shahidi a_l. 1986). A 10 g meat sample was placed into a 500 ml round-bottom flask containing 97.5 ml distilled water and 2.5 ml 4N HC1, along with few drops of Dow Antifoam A and several glass beads. For cured meats sulfanilamide was added to the mixture, in some cases, as described elsewhere (Shahidi, 1989).
The mixture Was then heated for approximately 20 min to collect 50 ml of distillate. A 5 ml aliquot of the distillate was pipetted into a 50 ml vial containing 5 ml 0.02 M aqueous solution of 2-thiobarbituric acid reagent.
The vial was then capped and 1009 heated in a boiling water bath for about 35 min to obtain the TBA-MA chromogen. After cooling the vial to room temperature, the absorbance of the complex was read at 532 nm on a DU-8 spectrophotometer. Using 1,1,3,3-tetramethoxypropane standard, a conversion factor of 8.1 was obtained for converting the TBA-MA absorbance readings to TBA numbers.
2. Results And Discussion
The absorption spectra of the TBA-malonaldehyde is depicted in Figure 2. The absorption maxima of the TBA-MA from a sample of meat distillate and from malonaldehyde prepared directly from its precursor 1,1,3,3-tetramethoxypropane was 532 nm. Both distillation and extraction methods showed similar absorption patterns.
Table 1 summarizes the results for the TBA numbers of meat systems prepared by addition of certain additives such as butylated hydroxyanisole (BHA), tertbutylhydroquinone (TBHQ), sodium tripolyphosphate (STPP), disodium salt of ethylenediaminetetraacetic acid (Na2EDTA), sodium nitrite (NaN02) and the performed cooked cured-meat pigment (CCMP), with or without the addition of TBHQ.
A close scrutiny of the results indicated the following trends: a) the TBA numbers of meat systems determined by the distillation method generally gave results which were numerically higher than those obtained by the extraction procedure; b) storage of the meat samples at 4°C for nearly 3 weeks resulted in a substantial increase in the TBA numbers; c) presence of sodium nitrite in the systems gave results
which varied in different direction when sulfanilamide was added to the mixture prior to distillation; d) addition of CCMP to meats gave substantially higher TBA values when extraction procedure was employed; however, this trend was reversed when meats where were stored for nearly 3 weeks; and e) addition of TBHQ to the meat system containing CCMP gave results which were always higher when the extraction procedure was employed.
In another set of experiments the effect of addition of antioxidants/ chelators during the distillation or extraction process on the TBA values of the samples was monitored. Results summarized in Table 2 indicate that the addition of antioxidants had a slight effect in lowering the TBA values by the distillation procedure. However, little effect was observed when the extraction process was employed.
Presence of Na2EDTA, alone or in combination with an antioxidant, resulted in a slight increase in the TBA numbers. This may be due to the fact that iron in the systems prior to its release would be kept by EDTA ln its more powerful pro-oxidant state of Fe(II) rather than being converted to its less potent pro-oxidant state of Fe(III).
(The pro-oxidant activity of ferrous and ferric ions, as quantified by their effect on the TBA number of meats, have been given in Table 2). Furthermore, EDTA-Fe(II) may cause the decomposition of hydroperoxides, thus giving rise to artifically high TBA values.
3. In Situ Formation Of 2-Thiobarbituric Acid Incorporated G-C3N4 For Enhanced Visible-Light-Driven Photo catalytic Performance†
Abstract
Embedding heterocycles into the skeleton of g-C3N4 has been proved to be a simple and efficient strategy for improving light response and the separation of photo-excited charges. Herein, 2-thiobarbituric acid incorporated g-C3N4 (TBA/CN) with good photocatalytic efficiency for Rh B degradation and H2 production was successfully achieved via a facile thermal copolymerization approach.
The incorporation of aromatics and S atoms into the skeleton of g-C3N4 was identified via systematic characterizations. This unique structure contributed to the narrowed band-gap, extended delocalization of lone pair electrons and changed electron transition pathway, which led to the enhanced visible light utilization, accelerated charge migration and prolonged electron lifetime, subsequently resulting in the significant boost of photocatalytic activity. The optimal TBA/CN-3 sample yielded the largest Rh B degradation rate constant k value of 0.0273 min−1 and simultaneously highest rate of H2 evolution of 0.438 mmol g−1 h−1, which were almost 3.5 and 3.8 folds as fast as that of the pristine CN, respectively.
Finally, the photocatalytic mechanism was proposed for the detailed elucidation of the process of Rh B degradation coupled with H2 production.
4. Salts of Barbituric and 2-Thiobarbituric Acids with Imidazole: Polymorphism, Supramolecular Structure, Thermal Stability And Water Solubility†
Introduction
In recent decades, emerging global energy dilemma and environmental pollution have become the two urgent threats to human existence and social development.1,2 Semiconductor photocatalysis has widely proven to be one of the most encouraging and prospective alternative techniques for effectively easing up the energy crisis and environmental issues due to its incorporated merits of high efficiency, energy-saving ability and eco-friendliness.3–5 Numerous photocatalysts have been constructed and successfully employed for H2 evolution, pollutant degradation and the conversion of CO2 to CO and CH4.
However, the practical application of traditional photocatalysts, such as TiO2 and WO3, was mostly suppressed by the limitations of toxicity, high cost, low efficiency and insufficient visible light utilization.6–8
Recently, graphitic carbon nitride (g-C3N4, CN), a metal-free semiconductor polymer, has received unprecedented attention due to its unique traits of visible-light-driven, tunable electronic structure, physical–chemical stability, good-durability, inexpensive nature and facile synthesis, which are perfect fit for the H2 production, pollutant degradation, and CO2 reduction.
9–11 Nevertheless, the practical applicability of CN is still restricted by its inherent bottlenecks of limited visible light harvesting and ultrafast recombination rate of photogenerated charge carriers.12,13 To overcome the above-mentioned defects, considerable efforts have been devoted to boost the photocatalytic property of CN, including porous morphology formation,14 heteroatom doping,15 heterostructure construction,16 aromatic ring integration17 and dye sensitization.
1. Experimental section
1. Photo catalyst preparation and characterization
Analytical pure grade urea and 2-thiobarbituric acid purchased from Aladdin Chemical Reagent Co., Ltd (Shanghai, China) were used as precursors. The modified CN composites were synthesized via a direct annealing route. In detail, the mixture of 20 g urea and desired amounts (0.05, 0.1, 0.2 and 0.3 g) of 2-thiobarbituric acid was completely dissolved in 5 mL ultrapure water via magnetic stirring. After drying at 60 °C overnight, the obtained residues were placed into a covered crucible and heated at 520 °C for 120 min at an increasing rate of 5 °C min−1.
When restoring to room temperature, the resulting composites were collected and labeled as TBA/CN-1, TBA/CN-2, TBA/CN-3 and TBA/CN-4 correspondingly.
For comparison, pristine g-C3N4 was also obtained via the direct calcination of urea under the same procedure, and the final product was named as CN.
The crystal and molecular structures, the surface morphologies and elemental composition and the light response behaviors of the as-synthesized composites were systematically characterized via X-ray diffraction (XRD), Fourier transformed infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy.
The photoelectrochemical behaviors of these samples, including photocurrent (PC) response and electrochemical impedance spectra (EIS), were recorded on a CHI 760E electrochemical workstation.
2. Photo catalytic performance measurements
The visible-light driven photocatalytic properties of the as-synthesized samples were assessed by Rh B degradation coupled with H2 production. A 300 W Xe lamp (λ > 420 nm) was chosen as the radiation source and ultrapure water was employed throughout the experiments.
In a typical degradation experiment, 50 mg of the photocatalyst was fully mixed with 100 mL of Rh B (10 mg L−1) by ultrasonic treatment for 5 min.
Before exposure to light, the reaction system achieved adsorption–desorption equilibrium by magnetic stirring for 30 min in darkness. At designated intervals, 3.0 mL suspension was collected and centrifuged at 12 000 rpm for 10 min. Finally, the concentration of Rh B in the supernatant was estimated using a Unico UV-2800A spectrophotometer at the characteristic wavelength of 552 nm.
In the photocatalytic H2 generation test, 30 mg of sample deposited with 1.0 wt% Pt was fully suspended into 50 mL ultrapure water containing 10% (vol/vol) triethanolamine (TEOA), where TEOA was used as the hole scavenger and Pt was used as the co-catalyst. H2 evolution during the photocatalytic reaction was calculated using an online gas chromatograph (Agilent, GC-8890) equipped with a thermal conductivity detector and high-purity nitrogen was used as the carrier gas.
3. Results and discussion
1. Material characterization
The possible reaction route for the TBA/CN framework is shown in Fig. 1. Here, the crystal and molecular structures of the as-synthesized composites were first identified by the XRD and FT-IR analyses. As depicted in Fig. 2a, all the samples possessed two typical diffraction peaks of graphitic materials at around 12.9° and 27.5°, corresponding to the (100) and (002) crystal facets (JCPDS 87-1526),26 which suggested that the original crystal structure of CN was largely intact after the introduction of small amounts of TBA.
Specifically, the front peak was related to the in-plane repeating motifs of tri-s-triazine, while the subsequent peak was attributed to the repeated interfacial stacking of the conjugated aromatic system in the CN nanosheets, respectively.27,28
3. Photo catalytic property
The photodecomposition of Rh B was first carried out to evaluate the photocatalytic activity of the as-synthesized samples. As presented in Fig. 9a, a negligible change of Rh B after 90 min visible light exposure demonstrated that it was very stable under the experimental conditions.
After adding the photocatalyst, the degradation efficiency significantly enhanced, and the TBA/CN samples showed better degradation effect than the pristine CN. As expected, TBA/CN-3 showed the optimal photocatalytic efficiency and about 92.0% of Rh B could be removed after 90 min of visible light illumination. However, the TBA/CN-4 sample with the narrowest band gap of 2.09 eV exhibited reduced degradation activity, which can be possibly due to the formation of recombination centers and the reduction of charge separation efficiency caused by excessive TBA incorporation.52 The corresponding degradation kinetics was calculated by the following pseudo-first-order equation: ,53 and the results are shown in Fig. 9b.
The largest rate constant k value of 0.0273 min−1 for TBA/CN-3 was obtained, which was almost 3.5-fold as that of the pristine CN. Fig. 9c depicts the UV-Vis absorption change of Rh B in a given reaction time over TBA/CN-3. With prolonged reaction time, the UV-Vis absorption intensity of Rh B markedly declined, and the characteristic absorption wavelength at 552 nm shifted to 518 nm, indicating the complete disruption of the conjugated aromatic system and chromogenic groups.
Conclusion
Overall, the TBA modified g-C3N4 was synthesized by copolymerizing the mixture of urea and TBA. The systematic characterization results proved that the structural units of TBA were well incorporated into the skeleton of g-C3N4 nanosheets. The PL, TRPL and photoelectrochemical experimental results suggested that the TBA/CN samples possessed prolonged lifetime of the photo-excited carriers, enhanced carrier transfer efficiency and reduced photocurrent resistance, which were in favor of the Rh B degradation and H2 evolution.
The optimized TBA/CN exhibited 3.5 and 3.8-folds improved visible-light-driven Rh B degradation and H2 evolution activities compared to the pristine CN, respectively. This research may provide a new insight into the rational design of g-C3N4-based photocatalysts for efficiently visible light capture and conversion.
4. Salts of Barbituric and 2-thiobarbituric acids with imidazole: polymorphism, supramolecular structure, thermal stability and water solubility
Abstract
Barbituric acids are a useful tool for the construction of various supramolecular compounds with intriguing physicochemical properties. Also, barbiturates play a significant role in biology and medicine. The same is true for imidazole derivatives.
Herein, three novel salts of barbituric (H2BA) and 2-thiobarbituric (H2TBA) acids with imidazole (Im), namely imidazolium barbiturate in two polymorphic modifications, HIm(HBA) (1) (pale yellow) and (2) (pale orange), and imidazolium 2-thiobarbiturate, HIm(HTBA) (3), were synthesized and characterized by CHNS elemental analysis, single-crystal and powder X-ray diffraction, FT-IR and UV-Vis spectroscopy techniques.
Crystal structure analysis revealed that the compounds feature three different supramolecular architectures with the formula unit count (Z′) varying from 1 for 3 to 4 for 1, which is governed by an unusual geometry of hydrogen bonding motifs. Intermolecular interactions in crystals 1–3 were analyzed using 2D-fingerprint plots derived from the Hirshfeld surfaces. According to the TG-DSC analysis, compounds 1–3 were thermally stable up to ∼100 °C in air.
The solubility studies (solubility product constants and solubility at a fixed pH value were determined) showed that salts 2 and 3 were more soluble in water than H2(T)BA. To the best of our knowledge, this is one of the first reports on the phenomenon of polymorphism among organic salts with barbiturate moieties. Thus, the present findings broaden our understanding of the supramolecular organization of barbiturates, complement the systematic studies of the correlation between their crystal structure and physicochemical properties and lay the foundation for the further development of novel materials based on imidazolium (2-thio)barbiturates.
Question
1. What is Thiobarbituric acid also known as?
The thiobarbituric acid (TBA) test measures malonaldehyde (MDA) produced due to the oxidation of fatty acids with three or more double bonds, and it measures other TBA reactive substances such as 2-alkenals and 2,4-alkadienals. Therefore, TBA is also referred to as TBARs (TBA reactive substances).
2. What is the TBA test used for?
The TBA test is a rapid and simple method for determining the extent that a fat has degraded to non-metabolizable aldehydes, such as malondialdehyde. A fat that has a high peroxide number or high value in the TBA test would not be suitable for use in feeds because it would contain a high peroxide or aldehyde content.
3. What is the thiobarbituric acid index?
Principle. The thiobarbituric acid index is a measure of the cumulative thermal stress brought about by exposure to heat (intensity) in malt and wort.
4. What is thiobarbituric acid value?
8.3 Thiobarbituric acid (TBA) value
TBA is one of the most widely used methods of assessing the extent of lipid oxidation in foods. The TBA value is typically expresses in milligrams of malondialdehyde (MDA) equivalents per kilogram of sample, as determined by the methods described.
5. What are the uses of TBA?
TBA is used as a high-purity, high-quality intermediate in the production of organic peroxides and antioxidants, and as a reaction component and solvent in the manufacture of fine chemicals and pharmaceuticals.