Plant-derived phenolic compounds have multiple positive health effects for humans attributed to their antioxidative, anti-inflammatory, and antitumor properties, etc. These effects strongly depend on their bioavailability in the organism. Bioaccessibility, and consequently bioavailability of phenolic compounds significantly depend on the structure and form in which they are introduced into the organism, e.g., through a complex food matrix or as purified isolates. Furthermore, phenolic compounds interact with other macromolecules (proteins, lipids, dietary fibers, polysaccharides) in food or during digestion, which significantly influences their bioaccessibility in the organism, but due to the complexity of the mechanisms through which phenolic compounds act in the organism this area has still not been examined sufficiently. Simulated gastrointestinal digestion is one of the commonly used in vitro test for the assessment of phenolic compounds bioaccessibility. Encapsulation is a method that can positively affect bioaccessibility and bioavailability as it ensures the coating of the active component and its targeted delivery to a specific part of the digestive tract and controlled release. This comprehensive review aims to present the role of encapsulation in bioavailability of phenolic compounds as well as recent advances in coating materials used in encapsulation processes. The review is based on 258 recent literature references.

Grape pomace is a sustainable source of bioactive phenolic compounds used in various industries. The recovery of phenolic compounds could be improved by biological pretreatment of grape pomace, as they are released from the lignocellulose structure by the activity of the enzymes produced. The influence of grape pomace pretreatment with Rhizopus oryzae under solid-state conditions (SSF) on the phenolic profile and chemical composition changes was studied. SSF was performed in laboratory jars and in a tray bioreactor for 15 days. Biological pretreatment of grape pomace resulted in an increase in the content of 11 individual phenolic compounds (from 1.1 to 2.5-fold). During SSF, changes in the chemical composition of the grape pomace were observed, including a decrease in ash, protein, and sugar content, and an increase in fat, cellulose, and lignin content. A positive correlation (r > 0.9) was observed between lignolytic enzymes and the hydrolytic enzyme’s xylanase and stilbene content. Finally, after 15 days of SSF, a weight loss of GP of 17.6% was observed. The results indicate that SSF under experimental conditions is a sustainable bioprocess for the recovery of phenolic compounds and contributes to the zero-waste concept by reducing waste.

Grape pomace is a byproduct of wineries and a rich source of phenolic compounds that can exert multiple pharmacological effects when consumed and enter the intestine where they can then be absorbed. Phenolic compounds are susceptible to degradation and interaction with other food constituents during digestion, and encapsulation may be a useful technique for protecting phenolic bioactivity and controlling its release. Therefore, the behavior of phenolic-rich grape pomace extracts encapsulated by the ionic gelation method, using a natural coating (sodium alginate, gum arabic, gelatin, and chitosan), was observed during simulated digestion in vitro. The best encapsulation efficiency (69.27%) was obtained with alginate hydrogels. The physicochemical properties of the microbeads were influenced by the coatings used. Scanning electron microscopy showed that drying had the least effect on the surface area of the chitosan-coated microbeads. A structural analysis showed that the structure of the extract changed from crystalline to amorphous after encapsulation. The phenolic compounds were released from the microbeads by Fickian diffusion, which is best described by the Korsmeyer–Peppas model among the four models tested. The obtained results can be used as a predictive tool for the preparation of microbeads containing natural bioactive compounds that could be useful for the development of food supplements.

Grape pomace is a by-product of winemaking characterized by a rich chemical composition from which phenolics stand out. Phenolics are health-promoting agents, and their beneficial effects depend on their bioaccessibility, which is influenced by gastrointestinal digestion. The effect of encapsulating phenol-rich grape pomace extract (PRE) with sodium alginate (SA), a mixture of SA with gelatin (SA-GEL), and SA with chitosan (SA-CHIT) on the bioaccessibility index (BI) of phenolics during simulated digestion in vitro was studied. A total of 27 individual phenolic compounds (IPCs) were quantified by UHPLC. The addition of a second coating to SA improved the encapsulation efficiency (EE), and the highest EE was obtained for SA-CHIT microbeads (56.25%). Encapsulation affected the physicochemical properties (size, shape and texture, morphology, crystallinity) of the produced microbeads, which influenced the delivery of phenolics to the intestine and their BI. Thus, SA-GEL microbeads had the largest size parameters, as confirmed by scanning electron microscopy (SEM), and the highest BI for total phenolic compounds and IPCs (gallic acid, 3,4-dihydroxybenzoic acid and o-coumaric acid, epicatechin, and gallocatechin gallate) ranged from 96.20 to 1011.3%. The results suggest that encapsulated PRE has great potential to be used as a functional ingredient in products for oral administration.

Wineries produce considerable amounts of grape pomace, which is a readily available natural source of bioactive phenolic compounds. In this study, grape pomace was used as a substrate for the cultivation of eleven filamentous fungi (Trametes versicolor TV6, Trametes versicolor TV8, Trametes versicolor AG613, Trametes gibbosa, Phanerochaete chrysosporium, Ceriporiopsis subvermispora, Pleurotus eryngii, Ganoderma lucidum, Ganoderma resinaceum, Humicola grisea, and Rhizopus oryzae) under solid-state conditions (SSF) for 15 days with the aim of improving the recovery of the individual phenolic compounds. Twenty-one phenolic compounds were quantified and the recovery of seventeen of them (gallic acid, ellagic acid, p-hydroxybenzoic acid, syringic acid, vanillic acid, 3,4-dihydroxybenzoic acid, ferulic acid, o-coumaric acid, p-coumaric acid, epicatechin gallate, galocatechin gallate, quercetin, kaempferol, procyanidin B1, procyanidin B2, resveratrol, and ε-viniferin) were positively affected by SSF. Ellagic acid is the most recovered compound, whose content increased 8.8-fold after 15 days of biological treatment with Ceriporiopsis subvermispora compared to the untreated initial sample. Among the microorganisms tested, the fungi Pleurotus eryngii and Rhizopus oryzae proved to be the most effective in increasing the recovery of most phenolic compounds (1.1–4.5-fold). In addition, the nutrient composition (proteins, ash, fats) of grape pomace was positively affected by the biological treatments.

The phenols from grape pomace have remarkable beneficial effects on health prevention due to their biological activity, but these are often limited by their bioaccessibility in the gastrointestinal tract. Encapsulation could protect the phenolics during digestion and influence the controlled release in such an intestine where their potential absorption occurs. The influence of freeze-drying encapsulation with sodium alginate (SA) and its combination with gum Arabic (SA-GA) and gelatin (SA-GEL) on the encapsulation efficiency (EE) of phenol-rich grape pomace extract and the bioaccessibility index (BI) of phenolics during simulated digestion in vitro was investigated. The addition of a second coating to SA improved the EE, and the highest EE was obtained with SA-GEL (97.02–98.30%). The release of phenolics followed Fick’s law of diffusion and the Korsmeyer–Peppas model best fitted the experimental data. The highest BI was found for the total phenolics (66.2–123.2%) and individual phenolics (epicatechin gallate 958.9%, gallocatechin gallate 987.3%) using the SA-GEL coating were used. This study shows that freeze-dried encapsulated extracts have the potential to be used for the preparation of various formulations containing natural phenolic compounds with the aim of increasing their bioaccessibility compared to formulations containing non-encapsulated extracts.