Protein-polyphenol Interaction Research Articles

Effect of Milk Protein-Polyphenol Conjugate on the Regulation of GLP-1 Hormone.

Modern functional foods are designed to provide health benefits beyond basic nutrition. They are enriched with bioactive ingredients like probiotics, vitamins, minerals, and antioxidants. These foods support overall health, enhance immune function, and help prevent chronic diseases. Milk proteins and tea are known to influence satiety and regulate body weight. Studies have shown that green tea polyphenols, namely, (-)-epigallocatechin gallate (EGCG), and whey proteins, predominantly lactoferrin (LF) from milk, play a role in regulating satiety. This study aims to investigate the effect of conjugating EGCG with apo-lactoferrin (Apo-LF) and assessing these effects on satiety through monitoring glucagon-like peptide-1 (GLP-1) regulation in a human colon (NCI-H716) cell line. Apo-LF-EGCG conjugates were synthesized and characterized in terms of structural and functional properties. The effect on GLP-1 regulation was assessed by real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA) to monitor gene and protein expressions, respectively. The results revealed that the protein-polyphenol interaction occurs through the complex formation of hydrogen bonds at the O-H and carbonyl groups of EGCG. The conjugates also showed a significant up-regulation of gene and protein expression levels of GLP-1 while also preventing EGCG from degradation, thereby preserving its antioxidant properties. The Apo-LF-EGCG conjugates increase satiety via increasing GLP-1 secretion in human colon cells while simultaneously retaining the antioxidant properties of EGCG. Therefore, these conjugates show potential for use as dietary supplements to enhance satiety.

(-)-Epigallocatechin gallate as an inhibitor of hemoglobin-catalyzed lipid oxidation: molecular mechanism of action and nutritional application

Hemoglobin (Hb) is effective inducer for lipid oxidation and protein–polyphenol interaction is a well-known phenomenon. The effects of the interaction of (-)-epigallocatechin gallate (EGCG) with Hb on lipid oxidation were rarely elucidated. The detailed interaction between bovine Hb and EGCG was systematically explored by experimental and theoretical approaches, to illustrate the molecular mechanisms by which EGCG influenced the redox states and stability of Hb. EGCG would bind to the central pocket of protein with one binding site to form Hb-EGCG complex. The binding constant for Hb-EGCG complex was 0.34 × 104 M−1 at 277 K, and thermodynamic parameters (ΔH > 0, ΔS > 0 and ΔG < 0) revealed the participation of hydrophobic forces in the binding process. The binding of EGCG would increase the compactness of protein molecule and diminish the crevice near the heme cavity, which was responsible for the reduction of met-Hb to oxy-Hb and inhibition of hemin release from met-Hb. Moreover, EGCG efficiently suppressed Hb-caused lipid oxidation in liposomes and cod muscles, which was possibly attributed to the reduction to oxy-Hb state and declined hemin dissociation from met-Hb. Altogether, our results provide significant insights into the binding of EGCG to redox-active Hb, which represents a novel mechanism for the anti-oxidant capacity of EGCG in human health and is favorable to the applications of natural EGCG in the good quality of Hb-containing products.

Roasted yerba mate (Ilex paraguariensis) infusions in bovine milk model before and after in vitro digestion: Bioaccessibility of phenolic compounds, antioxidant activity, protein–polyphenol interactions and bioactive peptides

Yerba mate is increasingly acknowledged for its bioactive properties and is currently being incorporated into various food and pharmaceutical products. When roasted, yerba mate transforms into mate tea, consumed as a hot aqueous infusion, and has gained popularity. This study investigated the bioaccessibility of phenolic compounds, protein–polyphenol interactions, antioxidant activity, and bioactive peptides in roasted yerba mate infusions, utilizing whole, semi-skimmed, and skimmed bovine milk models. The phytochemical profile of roasted yerba mate was analyzed in infusions with water and milk (whole, semi-skimmed, and skimmed), before and after in vitro digestion, identifying 18 compounds that exhibited variations in composition and presence among the samples. Bioavailability varied across different milk matrices, with milk being four times more efficient as a solvent for extraction. Gastric digestion significantly impacted (p < 0.05) the release of phenolic compounds, such as chlorogenic acid and rutin, with only chlorogenic acid remaining 100 % bioavailable in the infusion prepared with skimmed milk. Protein-polyphenol interaction did not influence protein digestion in different infusions, as there was a similarity in the hydrolysis pattern during the digestive process. Changes in antioxidant activity during digestion phases, especially after intestinal digestion in milk infusions, were related to alterations in protein structures and digestive interactions. The evaluation of total phenolic compounds highlighted that skimmed milk infusion notably preserved these compounds during digestion. Peptidomic analysis identified 253, 221, and 191 potentially bioactive peptides for whole, semi-skimmed, and skimmed milk-digested infusions, respectively, with a focus on anti-inflammatory and anticancer activities, presenting a synergistic approach to promote health benefits. The selection of milk type is crucial for comprehending the effects of digestion and interactions in bioactive compound-rich foods, highlighting the advantages of consuming plant infusions prepared with milk.

Modification of the Structural and Functional Characteristics of Mung Bean Globin Polyphenol Complexes: Exploration under Heat Treatment Conditions.

During the storage and processing of mung beans, proteins and polyphenols are highly susceptible to interactions with each other. Using globulin extracted from mung beans as the raw material, the study combined it with ferulic acid (FA; phenolic acid) and vitexin (flavonoid). Physical and chemical indicators were combined with spectroscopy and kinetic methods, relying on SPSS and peak fit data to statistically analyze the conformational and antioxidant activity changes of mung bean globulin and two polyphenol complexes before and after heat treatment and clarify the differences and the interaction mechanism between globulin and the two polyphenols. The results showed that, with the increase in polyphenol concentration, the antioxidant activity of the two compounds increased significantly. In addition, the antioxidant activity of the mung bean globulin-FA complex was stronger. However, after heat treatment, the antioxidant activity of the two compounds decreased significantly. The interaction mechanism of the mung bean globulin-FA/vitexin complex was static quenching, and heat treatment accelerated the occurrence of the quenching phenomenon. Mung bean globulin and two polyphenols were combined through a hydrophobic interaction. However, after heat treatment, the binding mode with vitexin changed to an electrostatic interaction. The infrared characteristic absorption peaks of the two compounds shifted to different degrees, and new peaks appeared in the areas of 827 cm-1, 1332 cm-1, and 812 cm-1. Following the interaction between mung bean globulin and FA/vitexin, the particle size decreased, the absolute value of zeta potential increased, and the surface hydrophobicity decreased. After heat treatment, the particle size and zeta potential of the two composites decreased significantly, and the surface hydrophobicity and stability increased significantly. The antioxidation and thermal stability of the mung bean globulin-FA were better than those of the mung bean globulin-vitexin complex. This study aimed to provide a theoretical reference for the protein-polyphenol interaction mechanism and a theoretical basis for the research and development of mung bean functional foods.

Comparison of interaction, structure, and cell proliferation of α-lactalbumin-safflower yellow complex induced by microwave heating or conventional heating.

The protein-polyphenol interaction mechanism has always been a research hotspot, but their interaction is affected by heat treatment, which is widely applied in food processing. Moreover, the effects of microwave or water-bath heating on the protein-polyphenol interaction mechanism have been not clarified. The pasteurization condition (65 °C, 30 min) was selected to compare the effects of microwave or water bath on binding behavior, structure, and cell proliferation between α-lactalbumin (α-LA) and safflower yellow (SY), thus providing a guide for the selection of functional dairy processing conditions. Microwave heat treatment of α-LA-SY resulted in stronger fluorescence quenching than that of conventional heat treatment. Moreover, the binding constant Ka of all α-LA-SY samples was augmented significantly after microwave or water bath treatment, and microwave-heated α-LA-SY showed the maximum Ka . Fourier transform infrared spectroscopy showed that microwave heating resulted in more ordered structures of α-LA into its disordered structures than water bath heating. However, the ferric reducing antioxidant power and chroma value of α-LA-SY were more reduced by microwave heating than by water bath heating. Moreover, microwave heating facilitated the cell proliferation of α-LA-SY compared with water bath treatment. It was demonstrated that microwave heating promoted interaction between α-LA and SY more than water bath heating did. Microwave heat treatment was a safe and effective way to enhance the binding affinity of α-LA to SY, being a potential application in food industry. © 2022 Society of Chemical Industry.

Phenolic compounds in walnut pellicle improve walnut (Juglans regia L.) protein solubility under pH-shifting condition

This study focused on the interaction of walnut protein with phenolic extracts of walnut pellicle (PEWP) under alkaline condition, leading to enhancement of protein solubility under neutral condition. First, the change of PEWP under alkaline condition was determined by RP-HPLC and mass spectrometry, and the results showed that most ellagitannins in PEWP could be retained under alkaline condition within 3 h. Interaction between PEWP and walnut protein under pH-shifting condition resulted in the remarkable increase of protein solubility (above 90%) at neutral pH. The results from SDS-PAGE and SEC showed that the improved solubility lied in the formation of large and soluble protein aggregates due to the covalent interaction among walnut protein and polyphenols. A significant change in tertiary structure of protein–phenolic complex was witnessed by fluorescence spectrum and near-UV circular dichroism. Meanwhile, walnut protein–polyphenol interaction led to a slight increase in β-turn while a slight decrease in β-sheet. Combined with amino acid composition, it could be illustrated that the covalent bonding for walnut protein with polyphenol mainly occurred at Lysine residues.

Bioactive compounds in tea: Effect of imbalanced intake on digestive enzymes activity, cytochrome inhibition and drug interaction

• Chemical and biological characteristics of proteins and polyphenols result in changes in the properties of each other as well as health functionality. • Polyphenols-protein complexes cause the poor function of digestive enzymes. • tea consumption may interfere with the oral bioavailability or activity of some drugs through CYPs inhibition. Biological active compounds in tea such as phenolic compounds have become a focus of research due to their health-beneficial impacts on the human body as well as disease prevention. Moreover, these substances also have beneficial functions as they work as food preservatives. Polyphenols are known to form a complex with protein molecules leading to change in several properties. In this review, the effect of protein-polyphenol interaction on the structure, functionality, nutritional and digestive system as well as the inhibition activity of cytochrome 450 (CYP450) and drug interaction were described. Also, the chemical bonding, which discovered leading to precipitation and changes of protein structure were explained. The feasibility of proteins-polyphenols interaction cause associated with the ability on the digestive system whether enhance the enzymatic activity for digestion or a negative effect of the enzyme inhibition activity. As the bioactive compounds attracting consumer attention in recent years and they do not provide only an advantage side, therefore, it should be significantly studied what impacts to the human body these substances include which are helpful for further investigation.

Evaluation of physicochemical and functional attributes of whey powder incorporated with pomegranate peel extract

A study has been conducted to evaluate the effect of pomegranate peel extract (PPE) (1, 2 and 3%) on physico-chemical and functional attributes of whey powder such as total phenolic content (TPC), flavonoid, proanthocyanidin, alkaloid, antioxidant activity, colour profile, protein-polyphenol interaction, rheology, differential scanning calorimetry (DSC), moisture adsorption isotherm (MAI) and microstructure. The TPC (1.02–27.80 mg/g), flavonoid (1.08–10.93 mg/g), proanthocyanidin (0.62–5.82 mg/g), alkaloid (0.63–2.28%), antioxidant activity (ABTS, 16.61–88.01%; DPPH, 17.02–49.34%), redness (2.33–8.05) and yellowness (27.77–32.24) were increased significantly (P < 0.05) with the increase in PPE. Protein-polyphenol interaction was confirmed with fluorescence spectra, FTIR and SDS-PAGE assays. The protein-polyphenol interaction brought significant changes in exothermic events (103.97–109.61 ºC) and viscosity (0.02–0.11 Pa.s) of the reconstituted whey samples. The DSC (glass transition temperature 77.15–71.37 ºC) and MAI (Equilibrium moisture content 38.75–70.45%) studies revealed that hygroscopic nature of whey powder increased with increase in the PPE. The microstructure analysis revealed that particles combined to form large aggregates upon addition of PPE. The PPE significantly increased the hygroscopicity of the powder and viscosity after reconstitution. These results are useful during drying of PPE added powders as well as polyphenol rich powders.

Noncovalent Interactions of Sea Buckthorn Polyphenols with Casein and Whey Proteins: Effect on the Stability, Antioxidant Potential, and Bioaccessibility of Polyphenols

In this work, effect of casein and whey protein interaction on the stability, antioxidant potential, and bioaccessibility of sea buckthorn polyphenols was investigated. The results revealed that casein–polyphenol and whey protein–polyphenol interaction both decreased the in vitro antioxidant capacity of polyphenols. On the other hand, the protein–polyphenol complexes increased the stability of sea buckthorn polyphenols during simulated in vitro digestion. Moreover, both the casein–polyphenol and whey protein–polyphenol complexes exhibited an increased bioaccessibility of polyphenols (60.97–64.97%) in comparison to that of free polyphenolic extract (40.01%). The main bonding force between sea buckthorn polyphenols and the proteins was found to be a hydrophobic interaction, as revealed by ATR-FTIR, UV–vis, and fluorescence spectroscopy. The particle size of casein–polyphenol and whey protein–polyphenol complexes was in the nanosize region (650–983 nm). The low span factors of polyphenol–protein complexes revealed the narrow range of particle size distribution and thus more homogeneity in the particle size. This finding highlights that the interaction between casein and whey proteins with sea buckthorn polyphenols could protect these bioactive compounds against the influence of simulated digestion.

Functional Tinospora cordifolia (giloy) based pasteurized goat milk beverage: Impact of milk protein-polyphenol interaction on bioactive compounds, anti-oxidant activity and microstructure

Tinospora cordifolia (giloy), a medicinal plant widely used in Indian system of medicine, is known to cure various ailments owing to its therapeutic values. Milk is considered as an ideal/delivery carrier system for its bioactive molecules. Therefore, in view of ‘green milk’ concept, and to potentiate the therapeutic benefits of goat milk and giloy juice, pasteurized functional giloy-goat milk beverage was developed using goat milk and debittered giloy juice (90 : 10) and evaluated for its' storage stability at refrigeration temperature (4 ± 1°C). Results revealed that functional beverage prepared with herbal extract treated giloy juice had highest (p < 0.05) total solids, bioactive compounds and anti-oxidant activity as compared to goat milk alone. HMF content showed retardation of browning reaction upon herbal extract application. Viscosity of the beverages was observed to be increasing with increasing shear rate. Fluorescent microscopy indicated polyphenol-milk protein interaction which was well confirmed with FTIR analysis and SEM, thus, supporting the compatibility of giloy juice with goat milk system. Three main principal components were identified for sensory evaluation data using Principal Component Analysis and results revealed beverage acceptability even over the 5 days of storage.

Whey proteins-epicatechin interactions in a model specialized food product treated by high pressure

ABSTRACTThe addition of protein to fruit, as a protein-enriched food, can modify the nutritional intake of both components. To mimic the protein-polyphenol interaction, a model product composed of whey protein (WP) and epicatechin (epi) was evaluated with various compositions (0%–20% polyphenol/protein). The effect of high pressure (HP) processing as a preservation method was tested and compared with control samples. The formation of complexes was evaluated by nephelometry, and oxygen radical absorbance capacity (ORAC) measurement was used to examine the variation in antioxidant capacity. Interactions were observed at the proportion of epi/WP 1%–2% and 15%–20% mass ratio. A high increase in turbidity, due to WP denaturation, was observed after HP processing. ORAC assays show that the antioxidant capacity of the epicatechin solution was improved by 22% with WP addition at epi/WP 1%. Preservation by high pressure slightly reduced antioxidant capacity.

Quality improvement of a rice-substituted fried noodle by utilizing the protein-polyphenol interaction between a pea protein isolate and green tea (Camellia sinensis) extract

The quality of rice-substituted fried noodles was improved by applying interaction between pea protein isolate (PPI) and green tea extract (GTE). Radical-scavenging activities of GTE were stably maintained when exposed to acidic pH, UV light, and fluorescent light, but decreased by approximately 65% when exposed to 80°C for 168h. The RVA profiles of noodle dough showed that peak viscosity and breakdown increased significantly but that setback and final viscosity remained unchanged with 20% rice flour replacement. PPI significantly decreased the viscosity parameters of rice-supplemented dough, and the addition of GTE recovered these values significantly. The cooking loss and viscoelasticity (Rmax) of cooked rice-supplemented noodles were fully restored by combined treatment of PPI and GTE. GTE decreased the peroxide value of fried noodles by 14% after storage at 63°C for 16days. Therefore, PPI+GTE treatment has great potential for use in fried noodles owing to the reinforced network and antioxidant activity.

The β-casein-resveratrol complex: Physicochemical characteristics and implications for enhanced nutrition

Food proteins have been widely used as carrier materials for the encapsulation and protection of bioactive molecules. Clarification the mechanism of protein-bioactive molecule interaction is important for the development protein-based carrier systems. Interaction of b-casein with resveratrol, a natural polyphenol, was studied using ultraviolet-visible absorption and fluorescence spectroscopy. It was found that the interaction shifted the protein fluorophores to a more hydrophilic environment but the polyphenol to a more hydrophobic environment. Formation of the complex with b-casein did not affect trans-cis isomerization of resveratrol or the total antioxidant activity of the protein-polyphenol system, as analyzed respectively using spectrophotometry and 2,2-azinobis-3-ethyl-benzothiazoline-6-sulfonic acid assay. The protective effect of resveratrol against the photodecomposition of folic acid was not affected by binding to b-casein. The data obtained should provide insight into protein-polyphenol interaction mechanisms and aid the development of b-casein-based carrier systems for the delivery of bioactive molecules.

Development of a gluten-free rice noodle by utilizing protein-polyphenol interaction between soy protein isolate and extract of Acanthopanax sessiliflorus.

The potential of the protein-polyphenol interaction was applied to crosslinking reinforced protein networks in gluten-free rice noodles. Specifically, inter-component interaction between soy protein isolate and extract of Acanthopanax sessiliflorus fruit (ogaja) was examined with a view to improving its quality. In a components-interacting model system, a mixture of soy protein isolate (SPI) and ogaja extract (OE) induced a drastic increase in absorbance at 660 nm by haze formation, while the major anthocyanin of ogaja, cyanidin-3-O-sambubioside, sparsely interacted with SPI or gelatin. Individual or combined treatment of SPI and OE on rice dough decreased all the viscosity parameters in rapid visco analysis. However, SPI-OE treatment significantly increased all the texture parameters of rice dough derived from Mixolab(®) analysis (P < 0.05). Incorporation of SPI in rice dough significantly reduced endothermic ΔH, and SPI-OE treatment further decreased this value. SPI-OE interaction significantly increased the tensile properties of cooked noodle and decreased 53.7% of cooking loss compared to the untreated rice noodle. SPI-OE treatment caused a considerable reinforcement of the network as shown by reducing cooking loss and suggested the potential for utilizing protein-polyphenol interaction for gluten-free rice noodle production.

A new look on protein-polyphenol complexation during honey storage: is this a random or organized event with the help of dirigent-like proteins?

Honey storage initiates melanoidin formation that involves a cascade of seemingly unguided redox reactions between amino acids/proteins, reducing sugars and polyphenols. In the process, high molecular weight protein-polyphenol complexes are formed, but the mechanism involved remains unknown. The objective of this study was twofold: to determine quantitative and qualitative changes in proteins in honeys stored for prolonged times and in different temperatures and to relate these changes to the formation of protein-polyphenol complexes. Six -month storage decreased the protein content by 46.7% in all tested honeys (t-test, p<0.002) with the rapid reduction occurring during the first three month. The changes in protein levels coincided with alterations in molecular size and net charge of proteins on SDS –PAGE. Electro-blotted proteins reacted with a quinone-specific nitro blue tetrazolium (NBT) on nitrocellulose membranes indicating that quinones derived from oxidized polyphenols formed covalent bonds with proteins. Protein-polyphenol complexes isolated by size-exclusion chromatography differed in size and stoichiometry and fall into two categories: (a) high molecular weight complexes (230–180 kDa) enriched in proteins but possessing a limited reducing activity toward the NBT and (b) lower molecular size complexes (110–85 kDa) enriched in polyphenols but strongly reducing the dye. The variable stoichiometry suggest that the large, “protein-type” complexes were formed by protein cross-linking, while in the smaller, “polyphenol-type” complexes polyphenols were first polymerized prior to protein binding. Quinones preferentially bound a 31 kDa protein which, by the electrospray quadrupole time of flight mass spectrometry (ESI-Qtof-MS) analysis, showed homology to dirigent-like proteins known for assisting in radical coupling and polymerization of phenolic compounds. These findings provide a new look on protein-polyphenol interaction in honey where the reaction of quinones with proteins and polyphenols could possibly be under assumed guidance of dirigent proteins.

Protein–polyphenol interaction on silica beads for astringency tests based on eye, photography or reflectance detection modes

Astringency is an organoleptic property of beverages and food products resulting mainly from the interaction of salivary proteins with dietary polyphenols. It is of great importance to consumers, but the only effective way of measuring it involves trained sensorial panellists, providing subjective and expensive responses. Concurrent chemical evaluations try to screen food astringency, by means of polyphenol and protein precipitation procedures, but these are far from the real human astringency sensation where not all polyphenol–protein interactions lead to the occurrence of precipitate. Here, a novel chemical approach that tries to mimic protein–polyphenol interactions in the mouth is presented to evaluate astringency. A protein, acting as a salivary protein, is attached to a solid support to which the polyphenol binds (just as happens when drinking wine), with subsequent colour alteration that is fully independent from the occurrence of precipitate. Employing this simple concept, Bovine Serum Albumin (BSA) was selected as the model salivary protein and used to cover the surface of silica beads. Tannic Acid (TA), employed as the model polyphenol, was allowed to interact with the BSA on the silica support and its adsorption to the protein was detected by reaction with Fe(III) and subsequent colour development. Quantitative data of TA in the samples were extracted by colorimetric or reflectance studies over the solid materials. The analysis was done by taking a regular picture with a digital camera, opening the image file in common software and extracting the colour coordinates from HSL (Hue, Saturation, Lightness) and RGB (Red, Green, Blue) colour model systems; linear ranges were observed from 10.6 to 106.0 μmol L−1. The latter was based on the Kubelka–Munk response, showing a linear gain with concentrations from 0.3 to 10.5 μmol L−1. In either of these two approaches, semi-quantitative estimation of TA was enabled by direct eye comparison. The correlation between the levels of adsorbed TA and the astringency of beverages was tested by using the assay to check the astringency of wines and comparing these to the response of sensorial panellists. Results of the two methods correlated well. The proposed sensor has significant potential as a robust tool for the quantitative/semi-quantitative evaluation of astringency in wine.

The effects of green tea drinking on salivary polyphenol concentration and perception of acid astringency

Samples of dilute HCl intended to result in mixtures with saliva with pH levels below, near and above the level of maximum protein–polyphenol interaction were presented to panelists. Significant differences in astringency were seen, but no evidence of a decline in astringency with stronger acid. Panelists abstained from tea drinking for some time, then drank two or more cups of green tea per day for some days, and finally omitted tea drinking for a period. Salivary polyphenol levels were determined throughout the experiment. Drinking green tea resulted in a highly significant ( p < 0.01) increase in salivary polyphenol levels that persisted for some days. Very dilute HCl solutions (0, 0.005, 0.006 and 0.007 N) were presented to panelists before, during and after the period of tea drinking and rated for astringency and sourness. Astringency and sourness intensity ratings increased significantly ( p < 0.01) during the period of tea drinking. It appears that there is a metabolic pool of polyphenol that is influenced by dietary habits. It appears likely that the salivary polyphenol level influences perception of astringency caused by acids.

Polyphenol‐enriched apple extract attenuates food allergy in mice

The immune system may be modulated with nutrition to prevent the development or to treat the symptoms of allergy. Among other foods, consumption of apples has been linked to reduced incidence of atopic dermatitis and respiratory allergy. We evaluated the efficacy and mechanisms of a polyphenol-enriched apple extract in reducing symptoms of food allergy. In a model of food allergy to ovalbumin (OVA), BALB/c mice were fed with an apple extract either during sensitization or just before the challenge. After the challenge, allergic symptoms were scored, OVA-specific serum immunoglobulins were determined by ELISA, cytokine production by mesenteric lymph node (MLN) cells was measured by a multiplex assay and gene expression profiles in the intestine were addressed using quantitative real-time PCR. Consumption of the apple extract reduced symptoms of food allergy upon challenge. This was paralleled by reduced levels of intestinal mast cell protease, diminished cytokine secretion by MLN cells and reduced local intestinal mRNA expression of various T-helper type-2 associated and pro-inflammatory genes. Mechanistic studies suggested decrease of mediator release by effector cells and reduction of allergenicity by protein-polyphenol interaction as potential mechanisms responsible for protection. Polyphenol-enriched apple extract can attenuate food allergy symptoms in sensitized mice via two distinct possible mechanisms.

Haze formation in beverages

The most frequent cause of hazes in beverages is due to protein–polyphenol interaction. Proteins that bind polyphenols contain proline, and the more proline, the greater the haze-forming activity. Polyphenols that cause haze have at least two sites that can bind to proteins; this enables them to cross-link proteins together and results in insoluble particles that scatter light. Protein–polyphenol interaction is non-covalent and, at least initially, reversible. The size of haze particles and the haze intensity are both affected by the ratio of haze-active (HA) polyphenol to HA protein. Alcohol content and pH also exert strong influences. Particle size in a model system does not vary smoothly as conditions change; rather shifts in the proportions of particles of a few discrete sizes are seen. Haze development occurs over time in packaged products. In clear beverages a treatment (fining, adsorption or ultrafiltration) is normally applied to reduce the concentration of either HA protein or HA polyphenol in order to delay the onset of haze formation beyond the intended product shelf life. A nephelometer observation gives a good indication of the amount of haze people can perceive.

Characterization of protein–polyphenol interactions

Dietary polyphenols have received attention for their biologically significant functions as antioxidants, anticarcinogens or antimutagens, which have led to their recognition as potential nutraceuticals. Polyphenols also characteristically possess a significant binding affinity for proteins, which can lead to the formation of soluble and insoluble protein–polyphenol complexes. Questions remain concerning whether and to what extent the protein–polyphenol interaction influences functionality. For example, is the formation of protein–polyphenol complexes an obstacle to the nutritional bioavailability of either species? This article discusses the development of suitable methodologies to investigate the physicochemical basis of protein–polyphenol interactions and the influence of structure–activity relationships on binding affinities.