Research progress on the extraction and application of apple polyphenols

Apple (Malus domestica Borkh.) represents a globally significant fruit crop whose health benefits extend far beyond basic nutrition, largely attributable to its rich and diverse polyphenolic profile (APs). These secondary metabolites are mainly concentrated in the peel but are also present in the flesh, exhibiting potent biological activities, particularly remarkable antioxidant capacity. Considerable heterogeneity exists in the polyphenolic composition among apple cultivars, primarily determined by genetic factors. Heritage cultivars, such as Antonovka and Bramley’s Seedling, consistently exhibit higher total polyphenol content (TPC)-often two to five times higher than widely cultivated commercial dessert apples such as Golden Delicious, Gala, and Fuji. Polyphenols are disproportionately concentrated in the peel, accounting for up to 70%-90% of total fruit phenolics. Red-skinned cultivars, such as Red Delicious and Idared, contain particularly high peel concentrations of flavonols, mainly quercetin glycosides, whereas cultivars like Granny Smith, Reinette, and Cripps Pink (Pink Lady®) often show higher flavan-3-ol levels in the flesh.The major classes of apple polyphenols include flavan-3-ols, flavonols, dihydrochalcones, phenolic acids, and anthocyanins. Flavan- 3- ols, comprising monomers such as catechin and epicatechin as well as oligomers and polymers (procyanidins), are the most abundant class in many apples, especially Dabinett and Kingston Black. Flavonols are dominated by quercetin glycosides, predominantly located in the peel, with red- skinned cultivars generally exhibiting higher levels than yellow- or green- skinned types. Dihydrochalcones, unique to the Malus genus, are represented mainly by phloridzin (phloretin-2'-O-glucoside), often constituting a significant portion of the phenolic profile, particularly in the flesh and seeds. Chlorogenic acid (5-O-caffeoylquinic acid) is the principal phenolic acid, present in both peel and flesh, with high concentrations in cultivars such as Honeycrisp and Braeburn. Anthocyanins are present only in red-fleshed or red-skinned varieties, including the Redlove series and Pink Pearl, and are primarily composed of cyanidin-3-O- galactoside. Efficient, selective, and sustainable recovery of APs while preserving their structural integrity and bioactivity is crucial for both research and industrial applications. Conventional solvent extraction methods, using methanol, ethanol, acetone, or their aqueous mixtures, remain foundational, with optimization typically targeting solvent type and concentration, temperature, extraction time, solid- to- liquid ratio, and pH. Although maceration and Soxhlet extraction are widely employed, they are time consuming and solvent-intensive. Advanced non-conventional techniques, such as ultrasound-assisted extraction, microwave-assisted extraction, pressurized liquid or accelerated solvent extraction, supercritical fluid extraction, and enzymeassisted extraction have been developed to improve the extraction efficiency, reduce solvent consumption, and minimize thermal degradation. Emerging green solvents, including natural deep eutectic solvents and ionic liquids, offer biodegradable, low- toxicity alternatives. Multi- parameter optimization strategies, such as response surface methodology, are widely applied to maximize yield, enhance recovery of target compounds, reduce co-extraction of undesired substances, and ensure scalability and stability of the extracts.The potent and multifaceted bioactivities of APs have promoted their integration into diverse sectors. In the nutraceutical and functional food industry, APs are incorporated into dietary supplements, fortified beverages, and health foods for their antioxidant, anti-inflammatory, cardiovascularprotective, anti-diabetic, anti-proliferative, and neuroprotective effects. In pharmaceuticals, they are investigated as lead compounds? or adjunct therapies for chronic diseases, leveraging multi-target mechanisms and favorable safety profiles, with purified isolates such as phloridzin receiving particular attention. Polyphenol profiles also serve as biochemical markers in horticulture and breeding for germplasm characterization and marker-assisted selection. Their antimicrobial properties enhance food safety and shelf-life, while in cosmetics, APs provide skin protection against photoaging and oxidative stress, inhibit melanogenesis, promote collagen synthesis, and support wound healing. Additionally, APs are explored as natural feed additives in animal husbandry, improving gut health, immunity, growth performance, oxidative stability of products, and potentially reducing methane emissions in ruminants. Other emerging applications include biomaterials, edible coatings, plant biostimulants, and green synthesis of nanoparticles. Despite these advances, there are still some challenges , including limited bioavailability, incomplete mechanistic understanding, potential synergistic effects, and the need for advanced delivery systems such as encapsulation and nanoparticles. Sustainable and integrated processing, clinical validation, regulatory harmonization, and the development of novel nutrient-dense cultivars are also crucial. Circular economy approaches, including valorization of pomace and peels, would further enhance economic and environmental benefits. Apple polyphenols represent a treasure trove of bioactive compounds with profound significance for human health and diverse industrial applications. Understanding the intricate interplay between genetic diversity, structural complexity, optimized recovery technologies, and targeted applications is fundamental. Interdisciplinary research bridging basic science, applied technology, and industrial needs is essential to overcome current challenges, unlock new opportunities, and fully harness the health-promoting and economic potential of this ubiquitous and valuable fruit.