Study of quinones reactions with wine nucleophiles by cyclic voltammetry

A novel EGCG-Histidine complex improves gelling and physicochemical properties of porcine myofibrillar proteins: Insight into underlying mechanisms

Herein, an EGCG-Histidine complex is prepared, characterized, and further used to improve gel properties of myofibrillar proteins (MP). Results of FTIR, XRD, UV-Vis spectroscopy showed that histidine is covalently bound to EGCG by Michael addition or Schiff base reaction to form EGCG-Histidine complex, and antioxidant activity of EGCG-Histidine complex is significantly increased compared to EGCG or histidine alone (P < 0.05). The addition of EGCG-Histidine complex results in cooking loss of gel decreasing from 66.7 ± 0.23 % to 40.3 ± 2.02 %, and improves rheological properties of MP, and enhances gel strength from 0.10 ± 0.01 N to 0.22 ± 0.03 N, indicating positive effect of EGCG-Histidine complex on MP gel formation, above results is supported by results of SEM, CD spectroscopy, SDS-PAGE, and tryptophan fluorescence. These results indicated that EGCG-Histidine complex can be used as a functional ingredient to improve gel quality of meat products.

Enzymatic browning: The role of substrates in polyphenol oxidase mediated browning

Enzymatic browning is a biological process that can have significant consequences for fresh produce, such as quality reduction in fruit and vegetables. It is primarily initiated by polyphenol oxidase (PPO) (EC 1.14.18.1 and EC 1.10.3.1) which catalyses the oxidation of phenolic compounds. It is thought that subsequent non-enzymatic reactions result in these compounds polymerising into dark pigments called melanins. Most work to date has investigated the kinetics of PPO with anti-browning techniques focussed on inhibition of the enzyme. However, there is substantially less knowledge on how the subsequent non-enzymatic reactions contribute to enzymatic browning. This review considers the current knowledge and recent advances in non-enzymatic reactions occurring after phenolic oxidation, in particular the role of non-PPO substrates. Enzymatic browning reaction models are compared, and a generalised redox cycling mechanism is proposed. The review identifies future areas for mechanistic research which may inform the development of new anti-browning processes.

From polyphenol to o-quinone: Occurrence, significance, and intervention strategies in foods and health implications

Polyphenol oxidation is a chemical process impairing food freshness and other desirable qualities, which has become a serious problem in fruit and vegetable processing industry. It is crucial to understand the mechanisms involved in these detrimental alterations. o-Quinones are primarily generated by polyphenols with di/tri-phenolic groups through enzymatic oxidation and/or auto-oxidation. They are highly reactive species, which not only readily suffer the attack by nucleophiles but also powerfully oxidize other molecules presenting lower redox potentials via electron transfer reactions. These reactions and subsequent complicated reactions are capable of initiating quality losses in foods, such as browning, aroma loss, and nutritional decline. To attenuate these adverse influences, a variety of technologies have emerged to restrain polyphenol oxidation via governing different factors, especially polyphenol oxidases and oxygen. Despite tremendous efforts devoted, to date, the loss of food quality caused by quinones has remained a great challenge in the food processing industry. Furthermore, o-quinones are responsible for the chemopreventive effects and/or toxicity of the parent catechols on human health, the mechanisms by which are quite complex. Herein, this review focuses on the generation and reactivity of o-quinones, attempting to clarify mechanisms involved in the quality deterioration of foods and health implications for humans. Potential innovative inhibitors and technologies are also presented to intervene in o-quinone formation and subsequent reactions. In future, the feasibility of these inhibitory strategies should be evaluated, and further exploration on biological targets of o-quinones is of great necessity.

Novel insight into the role of sulfur dioxide in fruits and vegetables: Chemical interactions, biological activity, metabolism, applications, and safety

Sulfur dioxide (SO₂) are a category of chemical compounds widely used as additives in food industry. So far, the use of SO₂ in fruit and vegetable industry has been indispensable although its safety concerns have been controversial. This article comprehensively reviews the chemical interactions of SO₂ with the components of fruit and vegetable products, elaborates its mechanism of antimicrobial, anti-browning, and antioxidation, discusses its roles in regulation of sulfur metabolism, reactive oxygen species (ROS)/redox, resistance induction, and quality maintenance in fruits and vegetables, summarizes the application technology of SO₂ and its safety in human (absorption, metabolism, toxicity, regulation), and emphasizes the intrinsic metabolism of SO₂ and its consequences for the postharvest physiology and safety of fresh fruits and vegetables. In order to fully understand the benefits and risks of SO₂, more research is needed to evaluate the molecular mechanisms of SO₂ metabolism in the cells and tissues of fruits and vegetables, and to uncover the interaction mechanisms between SO₂ and the components of fruits and vegetables as well as the efficacy and safety of bound SO₂. This review has important guiding significance for adjusting an applicable definition of maximum residue limit of SO₂ in food.

Insight on a Competitive Nucleophilic Addition Reaction of Nε-(Carboxymethyl) Lysine or Different Amino Acids with 4-Methylbenzoquinone

ο-benzoquinone is a common intermediate which is mostly formed by the oxidation of phenolics or polyphenols containing catechol structure. ο-benzoquinone has an outstanding nucleophilic ability, while advanced glycation end products (AGEs) are nucleophilic and can undergo a nucleophilic addition reaction with ο-benzoquinone to mitigate the harmful effects of AGEs on the body. As common nucleophilic substances, amino acids existing in large quantities in food processing and in vivo may bind competitively with ο-benzoquinone, thus influencing the trapping of ο-benzoquinone with AGEs. In this study, cyclic voltammetry and coexistence experiments were used to compare the reactivities of Nε-(carboxymethyl) lysine (CML) and amino acids with 4-methylbenzoquinone (4-MBQ). The results showed that CML is more reactive with ο-benzoquinone than most amino acids, and even in complex systems, ο-benzoquinone still captured CML. Moreover, almost all adducts were identified by UPLC-QTOF-MS/MS, and their chemical formulas were deduced. Quantum chemistry accurately predicts the efficiency and site of reactions of ο-benzoquinone and nucleophiles to a certain extent, and found that a secondary amine has a greater reactivity with 4-MBQ than a primary amine in a similar molecular structure. In general, ο-benzoquinone could capture AGEs, thereby showing potential to reduce the harmfulness of AGEs.

Electrochemical Oxidation of Sodium Metabisulfite for Sensing Zinc Oxide Nanoparticles Deposited on Graphite Electrode

A novel concept was successfully evaluated for the electrochemical quantitative analysis of zinc oxide nanoparticles originally in aqueous suspension. An aliquot of the suspension was first placed on the working area of a graphite screen-printed electrode and the water was evaporated to form a dry deposit of ZnO nanoparticles. Deposition of ZnO nanoparticles on the electrode was confirmed by energy-dispersive X-ray spectroscopy. A probe solution containing KCl and sodium metabisulfite was added on top of the deposit for electrochemical analysis by cyclic voltammetry. The anodic peak current (Ipa) for metabisulfite, measured at +1.2 V vs. Ag/AgCl, afforded a lower detection limit of 3 µg and exhibited a linear dependence on the mass of deposited ZnO nanoparticles up to 15 μg. Further, the current increased nonlinearly until it reached a saturation level beyond 60 μg of ZnO nanoparticles. The diffusion coefficient of metabisulfite anions through the electrical double layer was determined to be 4.16 × 10−5 cm2/s. Apparently the surface reactivity of ZnO originated from the oxide anion rather than the superoxide anion or the hydroxyl radical. Enhancement of the metabisulfite oxidation peak current can be developed into a sensitive method for the quantitation of ZnO nanoparticles.

Oxygen and SO2 Consumption of Different Enological Tannins in Relationship to Their Chemical and Electrochemical Characteristics

The oxidative behavior of five commercial enological tannins of different sources (tea, grape marc, grape seed, untoasted oak, and toasted oak) was investigated in model wine solutions in the presence or absence of SO2. Solutions of the tannins were also analyzed for total phenolics, methyl cellulose precipitable tannins, high-performance liquid chromatography, and linear sweep voltammetry. Tea and oak-derived tannin solutions were characterized by the highest oxygen consumption rates, with oak-derived tannins exhibiting the highest oxygen consumption rates per milligram of phenolic material present. Linear sweep and derivative voltammetry parameters were well-correlated with oxygen consumption rates, whereas total phenolics or total tannins were not. All tannins were associated with consumption of SO2 upon reaction with oxygen, with the lowest rate of SO2 lost per milligram of O2 reacted being observed for oak tannins.

Impact of Phenolic Compounds in Strecker Aldehyde Formation in Wine Model Systems: Target and Untargeted Analysis

The Strecker degradation of phenylalanine has been studied in a phenolic compound/phenylalanine wine model system. Six phenolic compounds (3,4-dihydroxybenzoic acid, gallic acid, caffeic acid, ferulic acid, catechin, and epicatechin) were compared in the formation of phenylacetaldehyde when in the presence of glucose or methylglyoxal (MG). The addition of glucose reduced the formation of Strecker aldehyde, independently of the phenolic compound. The addition of MG, on the other hand, increased phenylacetaldehyde formation for hydroxybenzoic acids and decreased phenylacetaldehyde formation for flavan-3-ols, confirming their capacity to trap the dicarbonyl compound. As a target phenolic compound, catechin was chosen to perform kinetic studies to further understand the reaction intermediates involved in the mechanism of phenylacetaldehyde formation, in particular, catechin o-quinone and catechin-MG adduct. The addition of glucose and MG increased the consumption of catechin, while a reduction in the respective o-quinone was observed, suggesting that these substrates have an impact in other reactions involving catechin. In that regard, for the first time, it was demonstrated that the catechin-MG adduct was capable of oxidizing and forming a new o-quinone, contributing to wine instability promoted by oxidation reactions.

Study of reactions of Nε-(carboxymethyl) lysine with o-benzoquinones by cyclic voltammetry

The reaction of Nε-(carboxymethyl) lysine (CML) with eight kinds of non-flavonoid o-benzoquinones and five kinds of flavonoid o-benzoquinones were investigated by cyclic voltammetry at pH 5.0, 7.0 and 8.0 and scan rate of 10, 50 and 100 mV/s. The reactivity of o-benzoquinones towards CML is weakened by the electron-donating substituent and strengthened by the electron-withdrawing substituent on the o-benzoquinone rings. The steric hindrance of the substituents on o-benzoquinone rings also weakens the quinone reactivity. Reaction of 4-methylbenzoquinone with CML (38.0 ± 1.3%) was found to be faster than that with l-lysine (31.3 ± 1.5%) and Nα-acetyl-l-lysine (14.5 ± 0.1%) but slower than that with l-cysteine (≥100.0%) and Nα-acetyl-l-cysteine (≥100.0%) at pH 7.0 and scan rate of 10 mV/s. Products obtained by the reaction of CML with o-benzoquinones were found to include a CML-quinone adduct according to the cyclic voltammetry and UPLC-QTOF-MS/MS analysis.

Quantifying the efficiency of o-benzoquinones reaction with amino acids and related nucleophiles by cyclic voltammetry

The reaction efficiency of o-benzoquinones with amines (L-lysine, Nα-acetyl-L-lysine, glycine, L-methionine and L-arginine), thiols (L-cysteine and Nα-acetyl-L-cysteine) and protein (bovine serum albumin) were determined at pH 5.0, 7.0 and 8.0 and scan rate of 10, 50 and 100 mV/s by cyclic voltammetry. Nucleophiles containing multiple nucleophilic groups and nucleophilic group possessing low pKa value would enhance the reactivity of nucleophiles towards o-benzoquinones. The reactivity of different o-benzoquinones with L-lysine/L-cysteine followed the order: protocatechuic acid quinone ≈ catechol quinone > 4-methylbenzoquinone ≈ caffeic acid quinone > rosmarinic acid quinone > chlorogenic acid quinone. The reactivity of quinones would be decreased by the steric hindrance of substituents on quinone ring, and it would also be weakened by enhancing electron cloud density of quinone ring. Adducts generated by the interaction of 4-methylbenzoquinone with amines and thiols were tentatively identified as amine-quinone adduct and thiol-phenol adduct respectively by UPLC-QTOF-MS/MS and cyclic voltammetry.

Response surface methodology: A tool to minimize aldehydes formation and oxygen consumption in wine model system

A response surface methodology was applied to study the effect of precursors on o-quinone and phenylacetaldehyde formation in wine model systems stored at 40 °C during 24 h. The results confirmed that glucose plays an important role in reducing aldehyde formation by inhibiting the formation of o-quinone. The regression equations showed that oxygen consumption followed a 2nd polynomial equation whereas phenylacetaldehyde and o-quinone were best fit with a polynomial function containing quadratic terms. These behaviors indicate that different pathways are involved in the respective aldehyde formation and oxygen consumption. RSM has been shown to be a powerful tool to better understand key chemical reactions. By considering a number of factors, individually and in combinations, the derived equations predicted that the best combination to minimize phenylacetaldehyde was achieved for high glucose levels and low amounts of gallic acid and metals. This is valuable information when trying to improve wines sensorial properties during shelf-life.

Colorimetric, electroanalytical and theoretical evaluation of the antioxidant activity of Syzygium aromaticum L., Origanum vulgare L., Mentha spicata L. and Eremanthus erythropappus M. essential oils, and their major constituents

Antioxidant mechanism of essential oils and their major constituents, and the synergism between them.

Untargeted voltammetric approaches for characterization of oxidation patterns in white wines

Chemical and electrochemical changes associated with controlled oxidation were measured in thirteen commercial white wines, in order to evaluate the potential of linear sweep voltammetry to provide relevant information on the oxidative behavior of individual wines. For a given amount of oxygen consumed, substantial diversity of oxidative behaviors was observed. A good correlation (R² = 0.69) was observed between the rate of O₂ consumption of individual wines and the total charged passed during linear sweep voltammetry, but not with their Folin-Ciocalteu values. Onset potential of anodic oxidation was also related to oxygen consumption capacity of wine, indicating an important contribution of easily oxidizable substrates. Subtraction of voltammograms of oxidized wines from their corresponding non-oxidized controls generated new voltammograms representative of the global changes induced by oxidation. These new voltammograms contained several features related to oxygen consumption rates of each wine, and could be considered as a 'wine oxidation signature'.

Green Tea Polyphenols Decrease Strecker Aldehydes and Bind to Proteins in Lactose-Hydrolyzed UHT Milk

The effect of epigallocatechin gallate enriched green tea extract (GTE) on flavor, Maillard reactions and protein modifications in lactose-hydrolyzed (LH) ultrahigh temperature (UHT) processed milk was examined during storage at 40 °C for up to 42 days. Addition of GTE inhibited the formation of Strecker aldehydes by up to 95% compared to control milk, and the effect was similar when GTE was added either before or after UHT treatment. Release of free amino acids, caused by proteolysis, during storage was also decreased in GTE-added milk either before or after UHT treatment compared to control milk. Binding of polyphenols to milk proteins was observed in both fresh and stored milk samples. The inhibition of Strecker aldehyde formation by GTE may be explained by two different mechanisms; inhibition of proteolysis during storage by GTE or binding of amino acids and proteins to the GTE polyphenols.

Quinones as Strecker degradation reagents in wine oxidation processes

The Strecker aldehydes formed during the reaction between α-amino acids (phenylalanine or methionine) and either gallic acid, caffeic acid or (+)-catechin ortho-quinones were evaluated in wine-model systems. It was demonstrated that phenylacetaldehyde was formed by quinone intermediates at wine pH. The highest amounts of phenylacetaldehyde during the 10days of experiment (69±5µg/L/day; 7x>Control) were obtained from (+) catechin, followed by gallic acid (61±4µg/L/day; 6x>Control) and caffeic acid (41±4µg/L/day; 4x>Control). The intermediate structures delivered from the reaction of ortho-quinones with α-amino acids were demonstrated by MSⁿ.

Electrochemistry and surface-enhanced Raman spectroscopy of CTAB modulated interactions of magnetic nanoparticles with biomolecules

In this study, cyclic voltammetry was used to unearth the electrochemical behavior of MNPs/CTAB and their interaction with biomolecules whereas SERS provided an insight into the mode of interaction in these molecular associations.