Over the last years, polysaccharides have been linked to antioxidant effects using both in vitro chemical and biological models. The reported structures, claimed to act as antioxidants, comprise chitosan, pectic polysaccharides, glucans, mannoproteins, alginates, fucoidans, and many others of all type of biological sources. The structural features linked to the antioxidant action include the polysaccharide charge, molecular weight, and the occurrence of non-carbohydrate substituents. The establishment of structure/function relationships can be, however, biased by secondary phenomena that tailor polysaccharides behavior in antioxidant systems. In this sense, this review confronts some basic concepts of polysaccharides chemistry with the current claim of carbohydrates as antioxidants. It critically discusses how the fine structure and properties of polysaccharides can define polysaccharides as antioxidants. Polysaccharides antioxidant action is highly dependent on their solubility, sugar ring structure, molecular weight, occurrence of positive or negatively charged groups, protein moieties and covalently linked phenolic compounds. However, the occurrence of phenolic compounds and protein as contaminants leads to misleading results in methodologies often used for screening and characterization purposes, as well as in vivo models. Despite falling in the concept of antioxidants, the role of polysaccharides must be well defined according with the matrices where they are involved.

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.

Type 2 diabetes mellitus (T2DM) is one of the most prevalent chronic metabolic diseases of the 21st century. Nevertheless, its prevalence might be attenuated by taking advantage of bioactive compounds commonly found in fruits and vegetables. This work is focused on the recovery of polyphenols and polysaccharide–polyphenol conjugates from grape pomace for T2DM management and prevention. Bioactives were extracted by solid–liquid extraction and by pressurized hot water extraction (PHWE). Polyphenolic fraction recovered by PHWE showed the highest value for total phenolic content (427 μg GAE.mg⁻¹), mainly anthocyanins and proanthocyanidins, and higher antioxidant activity compared to the fraction recovered by solid–liquid extraction. Polysaccharide–polyphenol conjugates comprehended pectic polysaccharides to which approximately 108 μg GAE of phenolic compounds (per mg fraction) were estimated to be bound. Polyphenols and polysaccharide–polyphenol conjugates exhibited distinct antidiabetic effects, depending on the extraction methodologies employed. Extracts were particularly relevant in the inhibition of a-glucosidase activity, with free polyphenols showing an IC₅₀ of 0.47 μg.mL⁻¹ while conjugates showed an IC₅₀ of 2.7, 4.0 and 5.2 μg.mL⁻¹ (solid–liquid extraction, PHWE at 95 and 120 °C, respectively). Antiglycation effect was more pronounced for free polyphenols recovered by PHWE, while the attenuation of glucose uptake by Caco-2 monolayers was more efficient for conjugates obtained by PHWE. The antidiabetic effect of grape pomace bioactives opens new opportunities for the exploitation of these agri-food wastes in food nutrition, the next step towards reaching a circular economy in grape products.