Size-exclusion chromatography of procyanidins: Comparison between apple and grape procyanidins and application to the characterization of fractions of high degrees of polymerization

Reactivity of flavanols: Their fate in physical food processing and recent advances in their analysis by depolymerization

Flavanols, a subgroup of polyphenols, are secondary metabolites with antioxidant properties naturally produced in various plants (e.g., green tea, cocoa, grapes, and apples); they are a major polyphenol class in human foods and beverages, and have recognized effect on maintaining human health. Therefore, it is necessary to evaluate their changes (i.e., oxidation, polymerization, degradation, and epimerization) during various physical processing (i.e., heating, drying, mechanical shearing, high-pressure, ultrasound, and radiation) to improve the nutritional value of food products. However, the roles of flavanols, in particular for their polymerized forms, are often underestimated, for a large part because of analytical challenges: they are difficult to extract quantitatively, and their quantification demands chemical reactions. This review examines the existing data on the effects of different physical processing techniques on the content of flavanols and highlights the changes in epimerization and degree of polymerization, as well as some of the latest acidolysis methods for proanthocyanidin characterization and quantification. More and more evidence show that physical processing can affect content but also modify the structure of flavanols by promoting a series of internal reactions. The most important reactivity of flavanols in processing includes oxidative coupling and rearrangements, chain cleavage, structural rearrangements (e.g., polymerization, degradation, and epimerization), and addition to other macromolecules, that is, proteins and polysaccharides. Some acidolysis methods for the analysis of polymeric proanthocyanidins have been updated, which has contributed to complete analysis of proanthocyanidin structures in particular regarding their proportion of A-type proanthocyanidins and their degree of polymerization in various plants. However, future research is also needed to better extract and characterize high-polymer proanthocyanidins, whether in their native or modified forms.

Reversed-phase liquid chromatographic analysis of hydrophobic interaction between proanthocyanidins and a C8-alkyl compound in aqueous solution

Structural and physicochemical properties of oligomeric flavan-3-ols (proanthocyanidins) in aqueous solution were investigated by spectrometric and reversed-phase (RP) HPLC analyses. Circular dichroism and fluorescence spectra of (–)-epicatechin (EC) oligomers linked through C-4 to C-8 interflavan bonds showed that EC oligomers larger than dimers formed a stable secondary structure in water. These EC oligomers are water-soluble hydrophilic compounds, whereas the oligomers were strongly retained by a C8-alkyl stationary phase under conventional RP-HPLC conditions. In a further C8-HPLC study, the hydrophobic interaction between EC oligomers and 1-octanesulfonic acid sodium salt (OSA Na) added to the mobile phase was quantitatively evaluated based on the relationship between the logarithm of the retention factor of the solute and the OSA Na concentration in the mobile phase. The strength values of the hydrophobic interaction of EC oligomers larger than dimers were the highest of 22 tested polyphenolic standards.

Antioxidant property and storage stability of quince juice phenolic compounds

The aim of this study was to characterise, in depth, 11 quince cultivars to provide data for their industrial processing into high-quality juices. Polyphenolic composition analyses (identification and quantification), soluble fraction of procyanidins, antioxidant capacity assays and cluster analysis were measured. A total of 19 kinds of polyphenolic compounds were the following in the juices: before and after 6 month of storage time at 4 and 30 °C. Large variations in polyphenolic compounds content were found as affected by quince cultivar. The total phenolics determined by UPLC ranged from 4045 mg to 721 mg/100 mL of juices, and was high correlated with antioxidant activity. During 6 months of storage a significant change was observed in the content of polyphenols, especially in procyanidins (37% and 55%, respectively). This result may be useful for the juice industry as a starting point for the development of tasty quince juices with high levels of bioactive compounds.

Neutral sugar side chains of pectins limit interactions with procyanidins

Interactions between seven hairy regions of pectins, rhamnogalacturonans II and arabinogalactan-proteins and procyanidins with different average degrees of polymerization, low (DP9) and high (DP30), were investigated by isothermal titration calorimetry and absorption analysis to study the impact of neutral sugar side chains of pectins on these associations. Associations between pectic fractions and procyanidins involved hydrophobic interactions and hydrogen bonds. No difference in association constants between various hairy regions and procyanidins DP9 was found. Nevertheless, arabinan chains showed lower association constants, and hairy regions of pectins with only monomeric side chains showed higher association with procyanidin DP30. Only very low affinities were obtained with rhamnogalacturonans II and arabinogalactan-proteins. Aggregation could be observed only with the procyanidins of DP30 and the protein-rich arabinogalactan-protein. Associations were obtained at both degrees of polymerization of the procyanidins, but differed depending on neutral sugar composition and the structure of pectic fractions.

Profiling of phenolic compounds from different apple varieties using comprehensive two-dimensional liquid chromatography

An innovative analytical approach based on the use of comprehensive two-dimensional liquid chromatography (LC×LC) is applied to obtain the profiling of phenolic compounds in different apple varieties. The method combines the use of hydrophilic interaction liquid chromatography in the first dimension and a reversed phase separation in the second dimension, as well as the use of diode array and mass spectrometry detection. Using this methodology is possible to obtain in less than 50 min the complete profiling of phenolic compounds in a complex food matrix such as apple. In fact, different flavan-3-ols including procyanidin oligomers with degree of polymerization up to 8, as well as several dihydrochalcones, flavonols and a phenolic acid are separated and tentatively identified in these samples in a single run. Besides, the total phenols and total procyanidins amounts were determined using two in vitro assays. Reinette apples presented the highest content on total phenols (6.46 mg galic acid equiv./g dry matter) whereas Granny Smith apples were the richest on total procyanidins (0.73 mg epicatechin equiv./g dry matter). This work shows the great potential of LC×LC for phenolic compounds profiling in complex food samples.

Polyphenolic composition, antioxidant activity, and polyphenol oxidase (PPO) activity of quince (Cydonia oblonga Miller) varieties

Phytochemical profiles (phenolic compounds, L-ascorbic acid, antioxidant and PPO activities) of 13 different quince varieties and 5 genotypes were studied. Polyphenols were identified by LC-PDA-QTof/MS and quantified by UPLC-PDA and UPLC-FL. A total of 26 polyphenolic compounds found in quince tissues were identified and presented: 9 flavan-3-ols ((-)-epicatechin, procyanidin B2, 3 procyanidin dimers and trimers, and 1 tetramer); 8 hydroxycinnamates, derivatives of caffeoylquinic and coumaroylquinic acid; and 9 kaempferol and quercetin derivatives. The content of total polyphenols was between 1709.43 (genotype 'S1') and 3436.56 mg/100 g dry weight ('Leskovač'). Flavan-3-ols, which are the major class of quince polyphenols, represented between 78 and 94% of the total polyphenolic compounds. The activity of PPO enzyme ranged from 709.85 to 1284.59 ΔU/min, and that of L-ascorbic acid ranged from 5.86 to 26.42 mg/100 g. Some quince varieties and their products characterized by a higher content of phenolic compounds may be selected to promote their positive effect on health.

Chemical composition of natural and polyphenol-free apple pomace and the effect of this dietary ingredient on intestinal fermentation and serum lipid parameters in rats

Unprocessed pomace containing 61% of dietary fiber (DF) and 0.23% of polyphenols (PP) and ethanol- or ethanol/acetone-extracted pomaces containing 66% DF and 0.10% PP and 67% DF and 0.01% PP, respectively, were subjected to a 4 week study in rats. The aim of the study was assessing the advantages of dietary supplementation with the above pomaces. To measure the animal response to dietary treatments, parameters describing cecal fermentation and lipoprotein profile were assessed. The dietary use of 5% unprocessed pomace caused an increase in cecal short-chain fatty acid (SCFA) production and a decrease in blood triacylglycerols, leading to a drop in serum atherogenic index. Ethanol-extracted pomace increased the glycolytic activity of cecal microbiota and decreased cecal branched-chain fatty acid production, whereas acetone extraction led to lower cecal ammonia concentration, decreased colonic pH value, and higher HDL/total cholesterol ratio. The variations in the atherogenic index indicate flavonoids as the key pomace component in relation to blood lipid profile benefits.

Impact of noncovalent interactions between apple condensed tannins and cell walls on their transfer from fruit to juice: studies in model suspensions and application

The adsorption of procyanidins (condensed tannins) on cell-wall material was quantified by bringing into contact solutions of procyanidins and suspensions of cell-wall material. A model was developed on the basis of the Langmuir isotherm formulation and a factorial experimental design. The parameters that influenced the adsorption were the concentration and molecular weight of the procyanidins, the ionic strength of the solution, the temperature, and the apple cell-wall concentration. The model was applied to partitioning of procyanidins from apple between juice and mash. The parameters to be taken into account are the composition of the apples and, specifically, (i) the concentration and molecular weight of the procyanidins, (ii) their acidity and pH as a determinant of the ionic strength, and (iii) their cell-wall content and the temperature at pressing. To estimate the ability of the model to relate procyanidin concentrations in the juice to their concentration in the apple, apples of three varieties of widely different procyanidin compositions were pressed in conditions that prevent oxidation. In these conditions, yields in the juice were >80% for phenolic acids or catechin monomers but <50% for procyanidins, with the lowest rates obtained for the higher polymers in accordance with the model.

Concentrations and characteristics of procyanidins and other phenolics in apples during fruit growth

Apples (Malus domestica Borkh.) of two table and two cider cultivars were collected during fruit growth and maturation from the end of cell proliferation. Concentrations of flavonoids (flavan-3-ols, dihydrochalcones and flavonols) in the fruit flesh decreased sharply between circa 35 and circa 100 days after flowering. For hydroxycinnamic acids, the decrease appeared slower. In a second experiments apples of the cider cultivars Kermerrien and Avrolles were sampled every 2 weeks from 40 days after flowering to overripeness for a detailed characterisation of polyphenol accumulation kinetics in the fruit flesh. Most polyphenol synthesis had occurred at 40 days after full bloom, though it persisted at a low (Kermerrien) to very low (Avrolles) level during all the fruit growth. All qualitative characteristics of the polyphenols were remarkably stable. The degree of polymerisation of the procyanidins increased slightly in Avrolles and decreased in Kermerrien. This was accompanied by a relative increase in procyanidin B2, while size-exclusion chromatography of Kermerrien polyphenol extracts showed the disappearance of a highly polymerised fraction.

Retention behavior of oligomeric proanthocyanidins in hydrophilic interaction chromatography

A novel method was developed for the separation of proanthocyanidins (PAs; oligomeric flavan-3-ols) by hydrophilic interaction chromatography (HILIC) using an amide-silica column eluting with an aqueous acetonitrile mobile phase. The best separation was achieved with a linear gradient elution of acetonitrile-water at ratios of 9:1 to 5:5 (v/v) for 60 min at a flow rate of 1.0 ml/min. Under these HPLC conditions, a mixture of natural oligomeric PAs (from apple) was separated according to degree of polymerization (DP) up to decamers. The DP of each separated oligomer was confirmed by LC/electrospray ionization MS. In further HILIC separation studies of 15 different flavan-3-ol and oligomeric PA (up to pentamer) standards with an isocratic elution of acetonitrile-water (84:16), a high correlation was observed between the logarithm of retention factors (log k) and the number of hydroxyl groups in their structures. The coefficient of this correlation (r2=0.9501) was larger than the coefficient (r2=0.7949) obtained from the correlation between log k and log P(o/w) values. These data reveal that two effects, i.e. hydrogen bonding between the carbamoyl terminal on the column and the hydroxyl group of solute oligomer and hydrophilicity based on the high-order structure of oligomeric PAs, corporately contribute to the separation, but the hydrogen bonding effect is predominant in our HILIC separation mode.