Microwave hydrodiffusion and gravity as a sustainable alternative approach for an efficient apple pomace drying.

Apple pomace valuation has been impaired by its high perishability and absence of fast drying approaches demanded by industry. This work aimed to assess the feasibility of Microwave Hydrodiffusion and Gravity (MHG) process applied for apple pomace drying using discrete delivery powers (300-900 W) and comparison with hot-air drying (40-100 °C). To dry 0.4 kg of apple pomace (81% moisture), hot-air drying required 3.6-9.9 h with estimated water evaporation flux of 1.0-3.5 mL/min. For MHG, which processed 1.2 kg, these corresponded to 1.0-2.6 h and 5.1-13.9 mL/min. Furthermore, MHG allowed water recovery containing part of apple pomace phenolic compounds and carbohydrates. The dried pomace was stable for 2 years, after which phenolic compounds and polysaccharides were still recoverable by hot water extractions. These results pave the way for MHG to be used for apple pomace and other by-products preservation, boosting their conversion into valuable co-product for valuation of its components.

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.

The hydrophobic polysaccharides of apple pomace.

In this work, polysaccharides extracted with hot water from apple pomace were isolated by C18 cartridge solid-phase extraction at pH 7 (Fr7). Dialysis (12-14 kDa) of this fraction allowed to obtain 17% (w/w) of polymeric material composed by 65% of polysaccharides, mainly arabinose (58 mol%), galacturonic acid (16 mol%) and glucose (10 mol%). Folin-Ciocalteu assay showed 62 g of phloridzin equiv/kg of polyphenols. Moreover, adjusting to pH 3, it was possible to retain an additional fraction (Fr3) representing a further 4% of the polymeric material. Fr3 contained 53% of polysaccharides composed mainly by galacturonic acid (66 mol%) and polyphenols accounted for 37 g of phloridzin equiv/kg. Precipitation with ethanol and subsequent methylation and NMR spectroscopic analysis of Fr7 dialysate allowed the identification of covalently-linked pectic-polyphenol-xyloglucan and arabinan-polyphenol complexes. These structures are possibly formed as a result of polyphenol oxidation reactions during the industrial processing of apples, conferring hydrophobic characteristics to apple pomace polysaccharides.

Apple Pomace Extract as a Sustainable Food Ingredient.

Apple pomace is a by-product of apple processing industries with low value and thus frequent disposal, although with valuable compounds. Acidified hot water extraction has been suggested as a clean, feasible, and easy approach for the recovery of polyphenols. This type of extraction allowed us to obtain 296 g of extract per kg of dry apple pomace, including 3.3 g of polyphenols and 281 g of carbohydrates. Ultrafiltration and solid-phase extraction using C18 cartridges of the hot water extract suggested that, in addition to the apple native polyphenols detected by ultra-high-pressure liquid chromatography coupled to a diode-array detector and mass spectrometry UHPLC-DAD-ESI-MSn, polyphenols could also be present as complexes with carbohydrates. For the water-soluble polyphenols, antioxidant and anti-inflammatory effects were observed by inhibiting chemically generated hydroxyl radicals (OH•) and nitrogen monoxide radicals (NO•) produced in lipopolysaccharide-stimulated macrophages. The water-soluble polyphenols, when incorporated into yogurt formulations, were not affected by fermentation and improved the antioxidant properties of the final product. This in vitro research paves the way for agro-food industries to achieve more diversified and sustainable solutions towards their main by-products.

Revisiting the chemistry of apple pomace polyphenols.

Hot water is an easily implementable process for polyphenols extraction. To evaluate the effect of this process on apple pomace, the overall polyphenolic composition was assessed before and after hot water extraction, followed by extractions with aqueous/organic solutions. As determined by UHPLC-DAD, flavan-3-ols were the main apple native polyphenols. Their amount decreased 50% after hot water extraction, while the other classes remained unchanged. Dihydrochalcones and hydroxycinnamic acid oxidation products, were also observed, alongside with non-extractable oxidised procyanidins that represented more than 4-fold the amount of native apple polyphenols in the pomace. Microwave superheated-water extraction of the insoluble cell wall material in water/acetone solutions and the high amounts of polyphenols that were insoluble in water/ethanol solutions suggested that oxidised procyanidins could be covalently linked to polysaccharides. These complexes represented up to 40% of the available polyphenols from apple pomace, potentially relevant for agro-food waste valuation.

Interaction of wine mannoproteins and arabinogalactans with anthocyanins.

Wine polymeric material (WPM), which includes polysaccharides, proteins, and polyphenolic compounds, interacts with anthocyanins. To determine the contribution of polysaccharides in these interactions, the diffusion performance of anthocyanins along a dialysis membrane was determined in the presence and absence of isolated mannoproteins (MP) and arabinogalactans (AG) from WPM. Furthermore, to estimate the extent of the interaction between WPM and polyphenolic compounds, the activation energy (Ea) required for their diffusion in the presence of WPM was determined. AG, generally more abundant than MP in wine, interact in a greater extent with anthocyanins, showing their relevant contribution for WPM/anthocyanins interactions. The Ea for the diffusion of polyphenolic compounds in presence of WPM indicated the occurrence of interactions with relative weak to strong intensities (2.6-50.8kJ/mol). As not all polyphenolic compounds were able to be released from WPM, stronger interactions, possibly by covalent linkages, are involved, providing new insights on WPM/polyphenolic compounds relationships.