Mechanistic Understanding of Tyrosinase Inhibition by Polymeric Proanthocyanidins from Acacia confusa Stem Bark and Their Effect on the Browning Resistance of Fresh-Cut Asparagus Lettuce.

Tyrosinase inhibitors are capable of preventing unfavorable enzymatic browning of fruits and vegetables. In this study, the capacity of Acacia confusa stem bark proanthocyanidins (ASBPs) to inhibit tyrosinase activity was evaluated. ASBPs were shown to be a high-potential inhibitor of tyrosinase with IC50 values of 92.49 ± 4.70 and 61.74 ± 8.93 µg/mL when using L-tyrosine and L-DOPA as the substrate, respectively. The structural elucidation performed with UV-vis, FT-IR spectroscopy, ESI-MS and thiolysis coupled to HPLC-ESI-MS suggested that ASBPs had structural heterogeneity in monomer units and interflavan linkages and consisted mainly of procyanidins dominant with B-type linkages. To gain insights into the inhibitory mechanisms of ASBPs against tyrosinase, different spectroscopic and molecular docking methods were further conducted. Results validated that ASBPs possessed the ability to chelate copper ions and could prevent the oxidation process of substrates by tyrosinase. The hydrogen bond formed with Lys-376 residue played a key role in the binding force of ASBPs with tyrosinase that induced a certain alteration in the microenvironment and secondary structure of tyrosinase, resulting in the enzymatic activity being ultimately restricted. It was also observed that ASBPs treatment effectively inhibited the activities of PPO and POD to retard the surface browning of fresh-cut asparagus lettuce and thus extended their shelf-life. The results provided preliminary evidence supporting the exploitation of ASBPs into potential antibrowning agents for the fresh-cut food industry.

Acidic conditions induce the accumulation of orange Monascus pigments during liquid-state fermentation of Monascus ruber M7.

The influence of pH on the biosynthesis of orange Monascus pigments (OMPs) in Monascus ruber M7 was investigated. Under acidic fermentation conditions, pigment mixtures predominantly rich in OMPs were obtained. HPLC analysis revealed the presence of four orange components (O1-O4) and four yellow components (Y1-Y4) in the mixtures, and the dominant ones were O1 and O3, which accounted for 56.0% to 75.9% of the total pigments in the pH range 3-6. Subsequently, O1 and O3 were identified by LC-DAD-ESI/MS as Rubropunctatin and Monascorubrin, respectively. The yield of OMPs was observed to be inversely dependent on pH. At pH 3, large amounts of OMPs with high purity (79.1%) were accumulated. A real-time quantitative PCR analysis revealed that the expression of genes related to the biosynthesis of OMPs in M. ruber M7 was upregulated at acidic pH as compared to neutral pH, and the variation in the level of expression of these genes with pH was consistent with the production of OMPs. These results indicated that the large accumulation of OMPs under acidic condition involved the acidic pH-induced transcription of genes related to the biosynthesis of OMPs. These results would contribute towards the development of an efficient technology for large-scale production of OMPs.

Simple and Label-Free Fluorescent Detection of Melamine Based on Melamine⁻Thymine Recognition.

In the past few years, melamine has been illegally added into dairy products to increase the apparent crude protein levels. If humans or animals drink the milk adulteration of melamine, it can form insoluble melamine⁻cyanurate crystals in their kidneys which causes kidney damage or even death. In the present work, we constructed a simple and label-free fluorescent method for melamine detection based on melamine-thymine recognition. SYBR Green I was utilized as a reporter for this method as it did not require any modification or expensive equipment. In the absence of melamine, polythymine DNA was digested by Exo I, which caused a decrease in the fluorescence signal. In the presence of melamine, the polythymine DNA was able to fold into a double chain structure, however this was done with the help of T-melamine-T mismatches to prevent degradation. Then, the SYBR Green I combined with the double-stranded DNA to result in an intense fluorescence signal. The limit of detection in this method was 1.58 µM, which satisfied the FDA standards. This method also had a good linear relationship within the range of 10⁻200 µM. In addition, this new method has a good selectivity to distinguish melamine from the component of milk. As a result, we developed a simple and highly selectivity method for melamine detection.

Enhanced stability of Pickering emulsions through co-stabilization with nanoliposomes and thermally denatured ovalbumin.

Pickering emulsions were co-stabilized by nanoliposome (NL) and thermally denatured ovalbumin (DOVA) based on the induction of OVA with strong particle characteristics through thermal denaturation. DOVA-NL particles were spherical and their sizes were mainly distributed between 50 and 100 nm. The surface tension and interfacial tension of DOVA-NL were significantly reduced, and the surface hydrophobicity, amphiphilicity and free -SH content of DOVA were enhanced after complexation with NL. The content of α-helix and ß-sheet in DOVA decreased, whereas the content of ß-turn and random coil increased after complexation with NL. Hydrophobic interactions, hydrogen bonding and electrostatic forces played a vital role in the interactions between NL and DOVA, leading to conformational changes in DOVA. The number of binding sites between NL and DOVA was more than one, and the interaction between NL and DOVA was exothermic and spontaneous. The emulsification index showed that DOVA-NL-stabilized Pickering emulsions (DNPE) were significantly more stable than DOVA-stabilized emulsions. DOVA-NL particles adsorbed at the oil-water interface and the droplet size of DNPE was smaller than that of DOVA-stabilized emulsions. This study suggests that it may be an effective strategy to improve the stability of Pickering emulsions through co-stabilization with NL and DOVA.

Stability enhancement of proanthocyanidin-loaded liposomes via surface decoration with oxidized konjac glucomannan.

The stability enhancement of proanthocyanidin-loaded liposomes (PC-Lip) via surface decoration with oxidized konjac glucomannan (OKGM) was investigated. The encapsulation efficiency and drug loading capacity of OKGM-coated PC-Lip (OKGM-PC-Lip) rose significantly. The average size and PDI of OKGM-PC-Lip increased, while the zeta potential decreased compared to those of PC-Lip. PC-Lip membrane fluidity reduced after coating with OKGM. The morphology of OKGM-PC-Lip showed that OKGM "halo layer" was formed on the liposome surface. Hydrogen bonding played an indispensable role in the combination between OKGM and PC-Lip, and the phase transition temperature of PC-Lip slightly increased after coating with OKGM. The retention rate of OKGM-PC-Lip was higher than that of PC-Lip at extreme pH. In vitro release, no significant difference in cumulative release was detected between OKGM-PC-Lip and PC-Lip at gastric stage, while the cumulative release rate of OKGM-PC-Lip was remarkably lower than that of PC-Lip at intestinal stage. The antioxidant activity of OKGM-PC-Lip was notably higher than that of PC-Lip. These results suggested that the resistance of PC-Lip to external influences was fruitfully enhanced after coating with OKGM. Compared with other polysaccharides, OKGM-coated liposomes may be more promising and advantageous in functional foods due to the polysaccharide's benefits to human health.

Potential application of nanoliposomes loaded with complex tannins from the seed shell of Euryale ferox in the anti-browning of fresh-cut asparagus lettuce.

Surface browning of plant-derived fresh-cut products is mainly caused by conversion of the phenolic compounds into o-quinones under tyrosinase catalysis. In this study, the rarely reported complex tannins from Euryale ferox seed shell (ECTs) constituted by the units of 35.60% condensed tannins and 64.40% hydrolysable tannins were shown to suppress the activity of tyrosinase efficiently, supporting the exploitation of ECTs into novel anti-browning agents. However, the utilization of ECTs in food preservation is often restricted because of their chemical instability to external environment. Further fabrication of nanoliposomes loaded with ECTs (ECTs-NLs) herein was carried out to improve the stability of ECTs. DLS, TEM, FTIR, DSC and XRD confirmed that ECTs were encapsulated into nanoliposomes successfully, and ECTs-NLs appeared as vesicle-like spherical morphology with favorable encapsulation efficiency, uniform particle size distribution and negative zeta-potential. The resulting ECTs-NLs were relatively stable in the dark at 4 °C. Nanoliposomal encapsulation significantly enhanced ECTs stability, thus protecting inhibitory effect of ECTs against tyrosinase. Furthermore, anti-browning evaluation proved that ECTs-NLs had distinct advantages over free ECTs in alleviating surface browning of fresh-cut asparagus lettuces. These results suggested that nanoliposomes were effective in stabilizing ECTs and ECTs-NLs could be potentially applied to the fresh-cut food industry.

Discrimination and prediction of Qingzhuan tea storage year using quantitative chemical profile combined with multivariate analysis: Advantages of MRMHR based targeted quantification metabolomics.

The duration of storage significantly influences the quality and market value of Qingzhuan tea (QZT). Herein, a high-resolution multiple reaction monitoring (MRMHR) quantitative method for markers of QZT storage year was developed. Quantitative data alongside multivariate analysis were employed to discriminate and predict the storage year of QZT. Furthermore, the content of the main biochemical ingredients, catechins and alkaloids, and free amino acids (FAA) were assessed for this purpose. The results show that targeted marker-based models exhibited superior discrimination and prediction performance among four datasets. The R2Xcum, R2Ycum and Q2cum of orthogonal projection to latent structure-discriminant analysis discrimination model were close to 1. The correlation coefficient (R2) and the root mean square error of prediction of the QZT storage year prediction model were 0.9906 and 0.63, respectively. This study provides valuable insights into tea storage quality and highlights the potential application of targeted markers in food quality evaluation.

Insights from ß-Conglycinin and Glycinin into the Mechanism of Nanoliposome-Soybean Protein Isolate Interactions.

The interaction mechanism between nanoliposomes (NL) and a soybean protein isolate (SPI) was investigated via the complexation between NL and two major components of SPI, i.e., ß-conglycinin (7S) and glycinin (11S). The endogenous fluorescence emissions of 7S and 11S were statically quenched after complexation with NL, and the polarity of the SPI fluorophore increased. The interaction between NL and SPI was exothermic and spontaneous, 7S/11S secondary structures were altered, and more hydrophobic groups were exposed on protein surfaces. Moreover, the NL-SPI complex had a large zeta potential to attain system stability. Hydrophobic forces and hydrogen bonds played vital roles in the interaction between NL and 7S/11S, and a salt bridge was also involved in the NL-11S interaction. The binding characteristics between NL and 7S/11S were chiefly governed by the protein characteristics, such as amino acid composition, surface hydrophobicity, and advanced structure. These findings could deepen the understanding of the interaction mechanism between NL and SPI.

Revealing the chemical differences and their application in the storage year prediction of Qingzhuan tea by SWATH-MS based metabolomics analysis.

It's generally believed that the longer the storage, the better the quality of dark tea, but the chemical differences of Qingzhuan tea (QZT) with different storage years is still unclear. Herein, in this work, an untargeted metabolomic approach based on SWATH-MS was established to investigate the differential compounds of QZT with 0-9 years' storage time. These QZT samples were roughly divided into two categories by principal component analysis (PCA). After orthogonal projections to latent structures discriminant analysis (OPLS-DA), 18 differential compounds were putatively identified as chemical markers for the storage year variation of QZT. Heatmap visualization showed that the contents of catechins, fatty acids, and some phenolic acids significantly reduced, flavonoid glycosides, triterpenoids, and 8-C N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSFs) increased with the increase of storage time. Furthermore, these chemical markers were verified by the peak areas corresponding to MS2 ions from SWATH-MS. Based on the extraction chromatographic peak areas of MS and MS2 ions, a duration time prediction model was built for QZT with correlation coefficient R2 of 0.9080 and 0.9701, and RMSEP value of 0.85 and 1.24, respectively. This study reveals the chemical differences of QZT with different storage years and provides a theoretical basis for the quality evaluation of stored dark tea.

Surface characteristics and emulsifying properties of whey protein/nanoliposome complexes.

The surface characteristics and emulsifying properties of whey proteins (WP) after complexation with nanoliposomes (NL) were investigated. WP surface hydrophobicity enhanced after complexation with NL, and it indicated the exposure increase of WP hydrophobic groups. WPNL interfacial tension significantly decreased compared with that of WP. The interfacial protein content of WPNL-stabilized emulsions was slightly different from that of WP-stabilized emulsions. WP emulsifying properties were significantly improved after complexation with NL. The mean sizes and polydispersity indexes of WPNL-stabilized emulsion droplets were smaller than those of WP-stabilized emulsion droplets. The absolute zeta-potential values of WPNL-stabilized emulsions were greater than those of WP-stabilized emulsions. Electrostatic repulsion played a vital role in WPNL-stabilized emulsion stability. Moreover, surface and emulsifying properties of WPNL were changed by exterior factor-induced alteration of protein advanced structures. The emulsifying properties of WP after complexation with NL were improved due to the modification of WP surface characteristics.