A-type proanthocyanidins: Sources, structure, bioactivity, processing, nutrition, and potential applications.

A-type proanthocyanidins (PAs) are a subgroup of PAs that differ from B-type PAs by the presence of an ether bond between two consecutive constitutive units. This additional C-O-C bond gives them a more stable and hydrophobic character. They are of increasing interest due to their potential multiple nutritional effects with low toxicity in food processing and supplement development. They have been identified in several plants. However, the role of A-type PAs, especially their complex polymeric form (degree of polymerization and linkage), has not been specifically discussed and explored. Therefore, recent advances in the physicochemical and structural changes of A-type PAs and their functional properties during extraction, processing, and storing are evaluated. In addition, discussions on the sources, structures, bioactivities, potential applications in the food industry, and future research trends of their derivatives are highlighted. Litchis, cranberries, avocados, and persimmons are all favorable plant sources. Α-type PAs contribute directly or indirectly to human nutrition via the regulation of different degrees of polymerization and bonding types. Thermal processing could have a negative impact on the amount and structure of A-type PAs in the food matrix. More attention should be focused on nonthermal technologies that could better preserve their architecture and structure. The diversity and complexity of these compounds, as well as the difficulty in isolating and purifying natural A-type PAs, remain obstacles to their further applications. A-type PAs have received widespread acceptance and attention in the food industry but have not yet achieved their maximum potential for the future of food. Further research and development are therefore needed.

Plant leaf proanthocyanidins: from agricultural production by-products to potential bioactive molecules.

Proanthocyanidins (PAs) are a class of polymers composed of flavan-3-ol units that have a variety of bioactivities, and could be applied as natural biologics in food, pharmaceuticals, and cosmetics. PAs are widely found in fruit and vegetables (F&Vegs) and are generally extracted from their flesh and peel. To reduce the cost of extraction and increase the number of commercially viable sources of PAs, it is possible to exploit the by-products of plants. Leaves are major by-products of agricultural production of F&Vegs, and although their share has not been accurately quantified. They make up no less than 20% of the plant and leaves might be an interesting resource at different stages during production and processing. The specific structural PAs in the leaves of various plants are easily overlooked and are notably characterized by their stable content and degree of polymerization. This review examines the existing data on the effects of various factors (e.g. processing conditions, and environment, climate, species, and maturity) on the content and structure of leaf PAs, and highlights their bioactivity (e.g. antioxidant, anti-inflammatory, antibacterial, anticancer, and anti-obesity activity), as well as their interactions with gut microbiota and other biomolecules (e.g. polysaccharides and proteins). Future research is also needed to focus on their precise extraction, bioactivity of high-polymer native or modified PAs and better application type.

The Leaf proanthocyanidins (LPAs) are mostly oligomeric procyanidins, with a small proportion of leaves containing A-type procyanidins.Foliage is a sustainable source of PAs.LPAs are a potential source of valuable bioactive compounds.The content, structure, extraction and identification and bio-activity of LPAs are discussed.Processing improvement is beneficial to enhance the production of LPA.

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.

Interactions between cell wall polysaccharides and polyphenols: Effect of molecular internal structure.

Cell wall polysaccharides (CPSs) and polyphenols are major constituents of the dietary fiber complex in plant-based foods. Their digestion (by gut microbiota) and bioefficacy depend not only on their structure and quantity, but also on their intermolecular interactions. The composition and structure of these compounds vary with their dietary source (i.e., fruit or vegetable of origin) and can be further modified by food processing. Various components and structures of CPSs and polyphenols have been observed to demonstrate common and characteristic behaviors during interactions. However, at a fundamental level, the mechanisms that ultimately drive these interactions are still not fully understood. This review summarizes the current state of knowledge on the internal factors that influence CPS-polyphenol interactions, describes the different ways in which these interactions can be mediated by molecular composition or structure, and introduces the main methods for the analysis of these interactions, as well as the mechanisms involved. Furthermore, a comprehensive overview is provided of recent key findings in the area of CPS-polyphenol interactions. It is becoming clear that these interactions are shaped by a multitude of factors, the most important of which are the physicochemical properties of the partners: their morphology (surface area and porosity/pore shape), chemical composition (sugar ratio, solubility, and non-sugar components), and molecular architecture (molecular weight, degree of esterification, functional groups, and conformation). An improved understanding of the molecular mechanisms that drive interactions between CPSs and polyphenols may allow us to better establish a bridge between food processing and the bioavailability of colonic fermentation products from CPSs and antioxidant polyphenols, which could ultimately lead to the development of new guidelines for the design of healthier and more nutritious foods.

Unraveling the pectinolytic function of Bacteroides xylanisolvens using a RNA-seq approach and mutagenesis.

BACKGROUND: Diet and particularly dietary fibres have an impact on the gut microbiome and play an important role in human health and disease. Pectin is a highly consumed dietary fibre found in fruits and vegetables and is also a widely used additive in the food industry. Yet there is no information on the effect of pectin on the human gut microbiome. Likewise, little is known on gut pectinolytic bacteria and their enzyme systems. This study was undertaken to investigate the mechanisms of pectin degradation by the prominent human gut symbiont Bacteroides xylanisolvens. RESULTS: Transcriptomic analyses of B. xylanisolvens XB1A grown on citrus and apple pectins at mid- and late-log phases highlighted six polysaccharide utilization loci (PUL) that were overexpressed on pectin relative to glucose. The PUL numbers used in this report are those given by Terrapon et al. (Bioinformatics 31(5):647-55, 2015) and found in the PUL database: http://www.cazy.org/PULDB/. Based on their CAZyme composition, we propose that PUL 49 and 50, the most overexpressed PULs on both pectins and at both growth phases, are involved in homogalacturonan (HG) and type I rhamnogalacturonan (RGI) degradation, respectively. PUL 13 and PUL 2 could be involved in the degradation of arabinose-containing side chains and of type II rhamnogalacturonan (RGII), respectively. Considering that HG is the most abundant moiety (>70%) within pectin, the importance of PUL 49 was further investigated by insertion mutagenesis into the susC-like gene. The insertion blocked transcription of the susC-like and the two downstream genes (susD-like/FnIII). The mutant showed strong growth reduction, thus confirming that PUL 49 plays a major role in pectin degradation. CONCLUSION: This study shows the existence of six PULs devoted to pectin degradation by B. xylanisolvens, one of them being particularly important in this function. Hence, this species deploys a very complex enzymatic machinery that probably reflects the structural complexity of pectin. Our findings also highlight the metabolic plasticity of B. xylanisolvens towards dietary fibres that contributes to its competitive fitness within the human gut ecosystem. Wider functional and ecological studies are needed to understand how dietary fibers and especially plant cell wall polysaccharides drive the composition and metabolism of the fibrolytic and non-fibrolytic community within the gut microbial ecosystem.

Does hydration of 'Deglet Nour' date palm fruits improve their quality and help to reduce waste?

This work investigates the quality change of date palm fruits after hydration treatment which is commonly applied to enhance the hard textured 'Deglet Nour' fruits that are unacceptable for consumption. Date palm fruits were treated at 60-62 °C with saturated steam for 4 h in three different processing units (DPU). Mid Infrared Spectroscopy (MIR) giving a global spectral evaluation discriminates samples from the three DPUs and highlights date palm fruits of the first DPU regarding hydration treatment. Treatment led to a decrease of fruit firmness, skin lightness, and of sucrose and malic acid contents whereas citric acid and procyanidins contents and procyanidins 'degree of polymerization increased. Thermal treatment had no effect on glucose and fructose contents, on cell wall content and composition and on minor phenolic groups. Significant differences existed on dates from the three DPUs, discriminating dates presenting high firmness. Hydration treatment improve dates texture as expected while nutritional parameters were quite stable, confirming that is very promising and could be highly recommended to valorise fruit that are currently not commercialized. However, optimisation is needed for the very hard-type dates.

Effect of long-term deficit irrigation on tomato and goji berry quality: from fruit composition to in vitro bioaccessibility of carotenoids.

Drought is a persistent challenge for horticulture, affecting various aspects of fruit development and ultimately fruit quality, but the effect on nutritional value has been under-investigated. Here, fruit quality was studied on six tomato genotypes and one goji cultivar under deficit irrigation (DI), from fruit composition to in vitro bioaccessibility of carotenoids. For both species, DI concentrated most health-related metabolites in fresh fruit. On a dry mass basis, DI increased total phenolic and sugar concentration, but had a negative or insignificant impact on fruit ascorbic acid, organic acid, and alcohol-insoluble matter contents. DI also reduced total carotenoids content in tomato (-18.7% on average), especially ß-carotene (-32%), but not in goji berry DW (+15.5% and +19.6%, respectively). DI reduced the overall in vitro bioaccessibility of carotenoids to varying degrees depending on the compound and plant species. Consequently, mixed micelles produced by digestion of fruits subjected to DI contained either the same or lesser quantities of carotenoids, even though fresh fruits could contain similar or higher quantities. Thus, DI effects on fruit composition were species and genotype dependent, but an increase in the metabolite concentration did not necessarily translate into greater bioaccessibility potentially due to interactions with the fruit matrix.

Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length.

The interactions between procyanidins and pectic compounds are of importance in food chemistry. Procyanidins with low (9) and high (30) average degrees of polymerization (DP9 and DP30) were extracted from two cider apple varieties. Commercial apple and citrus pectins, as well as three pectin subfractions (homogalacturonans, partially methylated homogalacturonans with degree of methylation 30 and 70) at 30 mM galacturonic acid equivalent, were titrated with the two procyanidin fractions (at 30 mM (-)-epicatechin equivalent) by isothermal titration calorimetry and UV-vis spectrophotometry. Slightly stronger affinities were recorded between commercial apple or citrus pectins and procyanidins of DP30 (Ka = 1460 and 1225 M(-1) respectively, expressed per monomer units) compared to procyanidins of DP9 (Ka = 1240 and 1085 M(-1), respectively), but stoichiometry and absorbance maxima differed between apple and citrus pectins. It was proposed that methylated homogalacturonans interacted with procyanidins DP30 mainly through hydrophobic interactions. The stronger association was obtained with the longer procyanidin molecules interacting with highly methylated pectins.

Characterization of microbial metabolism of Syrah grape products in an in vitro colon model using targeted and non-targeted analytical approaches.

PURPOSE: Syrah red grapes are used in the production of tannin-rich red wines. Tannins are high molecular weight molecules, proanthocyanidins (PAs), and poorly absorbed in the upper intestine. In this study, gut microbial metabolism of Syrah grape phenolic compounds was investigated. METHODS: Syrah grape pericarp was subjected to an enzymatic in vitro digestion model, and red wine and grape skin PA fraction were prepared. Microbial conversion was screened using an in vitro colon model with faecal microbiota, by measurement of short-chain fatty acids by gas chromatography (GC) and microbial phenolic metabolites using GC with mass detection (GC-MS). Red wine metabolites were further profiled using two-dimensional GC mass spectrometry (GCxGC-TOFMS). In addition, the effect of PA structure and dose on conversion efficiency was investigated by GC-MS. RESULTS: Red wine exhibited a higher degree of C1-C3 phenolic acid formation than PA fraction or grape pericarp powders. Hydroxyphenyl valeric acid (flavanols and PAs as precursors) and 3,5-dimethoxy-4-hydroxybenzoic acid (anthocyanin as a precursor) were identified from the red wine metabolite profile. In the absence of native grape pericarp or red wine matrix, the isolated PAs were found to be effective in the dose-dependent inhibition of microbial conversions and short-chain fatty acid formation. CONCLUSIONS: Metabolite profiling was complementary to targeted analysis. The identified metabolites had biological relevance, because the structures of the metabolites resembled fragments of their grape phenolic precursors or were in agreement with literature data.

Experimental and theoretical investigation on interactions between xylose-containing hemicelluloses and procyanidins.

During processing of plant-based foods, cell wall polysaccharides and polyphenols, such as procyanidins, interact extensively, thereby affecting their physicochemical properties along with their potential health effects. Although hemicelluloses are second only to pectins in affinity for procyanidins in cell walls, a detailed study of their interactions lacks. We investigated the interactions between representative xylose-containing water-soluble hemicelluloses and procyanidins. Turbidity, ITC and DLS were used to determine the relative affinities, and theoretical calculations further ascertained the interactions mechanisms. Xyloglucan and xylan exhibited respectively the strongest and weakest interactions with procyanidins. The different arabinoxylans interacted with procyanidins in a similar strength, intermediate between xyloglucans and xylans. Therefore, the strength of the interaction depended on the structure itself rather than on some incidental properties, e.g., viscosity and molar mass. The arabinose side-chain of arabinoxylan did not inhibit interactions. The computational investigation corroborated the experimental results in that the region of interaction between xyloglucan and procyanidins was significantly wider than that of other hemicelluloses.

Revisiting the contribution of ATR-FTIR spectroscopy to characterize plant cell wall polysaccharides.

The contribution of ATR-FTIR spectroscopy to study cell wall polysaccharides (CWPs) was carefully investigated. The region 1800-800 cm-1 was exploited using principal component analysis and hierarchical clustering on a large range of different powders of CWPs based on their precise chemical characterization. Relevant wavenumbers were highlighted for each CWP: 1035 cm-1 was attributed to xylose-containing hemicelluloses, 1065 and 807 cm-1 to mannose-containing hemicelluloses, 988 cm-1 to cellulose, 1740 and 1600 cm-1 to homogalacturonans according to the degree of methylation. Some band positions were affected by macromolecular arrangements (especially hemicellulose-cellulose interactions). However, as arabinan and galactan did not reveal distinctive absorption bands, ATR-FTIR spectroscopy did not allow the discrimination of cell walls differing by the abundance of these polysaccharides, e.g., those extracted from apple and beet. Therefore, the application of ATR-FTIR could remain sometimes limited due to the complexity of overlapping spectra bands and vibrational coupling from the large diversity of CWP chemical bonds.

Changes in cell wall neutral sugar composition related to pectinolytic enzyme activities and intra-flesh textural property during ripening of ten apricot clones.

The changes of texture and cell wall characteristics of apricot were investigated in ten clones at two maturity stages. Fruit firmness, cell wall composition and enzyme activity of three apricot flesh zones were analysed. The AIS (alcohol-insoluble solids) were characterised by high amounts of uronic acid (179-300 mg g-1 AIS) and relatively high amounts of cellulosic glucose (118-214 mg g-1 AIS). The methylesterification degree varied significantly among the different clones ranging from 58 to 97 in Ab 5 and Mans 15 respectively. Conversely to zones firmness, enzymatic activity was higher in pistil followed by equatorial and peduncle zones. The ripening effect has been observed in firmness evolution according to enzymatic activity. This correlation allowed a classification of clones depending on softening. Among studied clones, Ab 5, Marouch 16, Mans 15 and Cg 2 were less influenced by softening and have the advantage of a technological valorisation for the processing industry.

Iron-induced peroxidation of trilinolein nano-emulsions under model gastric conditions and its inhibition by dietary phenolic antioxidants.

An early mechanism for the health benefits of dietary plant phenols is their antioxidant activity in the upper digestive tract. Indeed, these non-essential micronutrients abundant in fruits and vegetables can efficiently fight the iron-induced peroxidation of dietary lipids in the gastric compartment, a recognized form of postprandial oxidative stress. In this work, this phenomenon is investigated through a simple model based on nano-emulsions of trilinoleylglycerol, which permits a direct spectroscopic monitoring and mechanistic insights sustained by extensive kinetic analysis. Polyphenols belonging to the main dietary classes are tested, in particular, flavonols, anthocyanins, flavanols and oligomeric procyanidins. Overall, the common polyphenols tested are good inhibitors of lipid peroxidation induced by metmyoglobin (heme iron) in the early stage of digestion (pH 5-6). For instance, under our peroxidation conditions (2 µM heme, 0.7 mM linoleic acid equivalent, 4.5 mM Brij®35), IC50 concentrations in the range 0.4-1.9 µM were estimated for the set of polyphenols, with oligomeric procyanidins being less inhibitory than the flavanol monomers. However, the polyphenols are ineffective at lower pH (pH 4) when the hematin cofactor is dissociated from its protein (globin). On the other hand, a moderate protection against lipid peroxidation induced by free iron (e.g., released by the oxidative degradation of hematin) persists. This protocol, which combines simplicity and nutritional relevance, could provide a basis for standard tests aimed at assessing the antioxidant capacity of foods and food additives.

Multiscale Localization of Procyanidins in Ripe and Overripe Perry Pears by Light and Transmission Electron Microscopy.

Histochemical staining with 4-dimethylaminocinnamaldehyde (DMACA), light microscopy, and transmission electron microscopy (TEM) were applied to characterize procyanidin localization at ripe and overripe stages in perry pear flesh (cv. 'De Cloche'). Pear flesh contained stone cell clusters surrounded by very large parenchyma cells. DMACA staining showed procyanidins mainly located in parenchyma cells from the fruit mesocarp. Under light microscopy and TEM, procyanidins appeared in the vacuole of parenchyma cells as uniformly stained granules, probably tannosomes. They were differently dispersed in ripe and overripe perry pears, as the granules remained free inside the vacuole in ripe pears and mostly attached to the tonoplast in overripe pears.

Interactions of arabinan-rich pectic polysaccharides with polyphenols.

Given the high prevalence of arabinan side chains in pectic polysaccharides, this work aims to unveil the impact of their structural diversity on pectic polysaccharides-polyphenol interactions. To assess the effect of arabinan branching degree, sugar beet arabinans (branched and debranched) were used and compared to the well-known structure of apple arabinan and other pectic polysaccharides. Furthermore, arabinans contribution to pectic polysaccharides/polyphenol interactions was assessed. The interactions were evaluated using chlorogenic acid, phloridzin and procyanidins (degree of polymerization of 9). Linear arabinans had 8-fold and 2-fold higher retention for chlorogenic acid and phloridzin, respectively, than branched arabinans. This trend was also observed for the interaction of arabinans with procyanidins. However, arabinans with covalently linked polyphenols showed lower interactions. The interactions involved between arabinans and polyphenols explained 1-28 % of the interactions of pectic polysaccharides, allowing us to conclude that the whole polysaccharide structure is more relevant for polyphenol interactions than each part.

Effects of the apple matrix on the postprandial bioavailability of flavan-3-ols and nutrigenomic response of apple polyphenols in minipigs challenged with a high fat meal.

Food matrix interactions with polyphenols can affect their bioavailability and as a consequence may modulate their biological effects. The aim of this study was to determine if the matrix and its processing would modulate the bioavailability and the postprandial nutrigenomic response to a dietary inflammatory stress of apple flavan-3-ol monomers. We carried out an acute randomized controlled study in minipigs challenged with a high fat meal (HFM) supplemented with raw fruit, puree, or apple phenolic extract with matched content of flavan-3-ol monomers. Fasting and postprandial blood samples were collected over 3 h to quantify flavan-3-ol monomers in sera by UPLC-Q-TOF/MS and to isolate peripheral blood mononuclear cells (PBMCs) for assessing the changes in the gene expression profile using a microarray analysis. When compared to the extract-supplemented meal, the peak of the total flavan-3-ol concentration was reduced by half with both raw apple and puree supplements. The apple matrices also affected the gene expression profile as revealed by the Principal Component Analysis of the microarray data from PBMCs which discriminated the supplementation of HFM with the polyphenol extract from those with raw apples or puree. A total of 309 genes were identified as differentially expressed by the apple-derived products compared to HFM, with 63% modulated only in the presence of the food matrix (apple and puree). The number of differentially modulated genes was higher with the puree (246) than with the unprocessed apple (182). Pathway enrichment analyses revealed that genes affected by the apple-derived products control inflammation and leukocyte transendothelial migration both involved in the onset of atherosclerotic processes. Overall, this study showed that the two apple matrices reduce the postprandial serum concentration of flavon-3-ols whereas they increase the nutrigenomic response of PBMCs. The biological processes identified as modulated by the apple products suggest an attenuation of the transient pro-inflammatory response induced by a HFM. The differences observed between the nutrigenomic responses support that the apple matrix and its processing affect the nutrigenomic response, probably by increasing the bioavailability of other apple phytochemicals. To conclude, this study raises awareness for considering the impact of the food matrix and its processing on the biological response of polyphenols in nutritional studies.

ATR-FTIR spectroscopy to determine cell wall composition: Application on a large diversity of fruits and vegetables.

Infrared spectroscopy coupled with multivariate analyses such as linear regressions was applied to assess the main cell wall components of a huge diversity of fruits and vegetables belonging to 29 plant species. The methodology was tested on the raw freeze-dried powders and on their corresponding AIS (Alcohol Insoluble Solids) dried by solvent exchanges. The most informative spectral region was 1750-1035 cm-1. Excellent predictions (determination coefficient R2 ≥ 0.9 and residual predictive deviation RPD ≥ 3.0) were obtained for AIS yields and for arabinose, total glucose, non-cellulosic glucose, total neutral sugars, methanol and starch contents in the AIS samples. The key wavenumbers were: 1740 cm-1 for total neutral sugars; 1075, 1440-1450, 1616 and 1740 cm-1 for pectins; 895, 1035-1041 and 1160-1163 cm-1 for cellulose and 1035-1041 cm-1 for lignin. Limitations of the reference methods to analyze cell wall components (biochemical assays, spectrophotometry, chromatography) affecting the prediction accuracy were also discussed.

Impact of air-drying on polyphenol extractability from apple pomace.

Little data are available on the impact of pomace pre-treatment, notably drying, on the nature and yield of polyphenols. Pomace from two apple varieties ('Avrolles' and 'Kermerrien'), pressed with and without oxidation, were air-dried to different degrees. Drying led to the loss of native molecules, notably 5-O-caffeoylquinic acid and flavan-3-ols. Total polyphenol yields, after sequential pressurized liquid extraction (water 10 MPa, 70 °C, then ethanol 48%, 10 MPa, 70 °C), varied between 5 and 15 g/kg dry weight but showed no marked trend with drying. Extracts from dried pomace contained few native polyphenols. Water extracts from 'Kermerrien' contained flavonols, flavanols and phloridzin and those from 'Avrolles' contained phloridzin. Water:ethanol extracts were rich in procyanidins, especially from 'Avrolles', where they represented >80% of analysable polyphenols. Presence of polyphenol molecules with modified structures in the extracts of dried pomaces might lead to different biological properties than those with native molecules.

Pear ripeness and tissue type impact procyanidin-cell wall interactions.

Procyanidins and cell walls were extracted from pear at ripe and overripe stages in order to investigate the impact of ripening stage on their association. Procyanidin composition and structure remained stable at the overripe stage. Mid Infrared Spectroscopy (MIR) discriminated cell wall-procyanidin complex from initial purified cell wall material (CWM). Interactions between procyanidins and CWM isolated from the whole flesh (FL), parenchyma cells (PC), stone cells (ST) and skin (SK) at ripe and overripe stage were characterized using UV-Vis spectrometry using Langmuir isotherm formulation and Isothermal Titration Microcalorimetry (ITC). The affinity between procyanidins and CWM decreased as follows: PC > FL > ST > SK. The proportion of bound procyanidins increased at the overripe stage for all CWM and the maximal saturation level was obtained for overripe FL and ST. ITC indicated that associations between pear cell walls and procyanidins involved hydrogen bonds and mainly hydrophobic interactions for overripe PC.