A review of vibrational frequencies reported for tannin molecules is provided to create a database with typical fingerprints for different types of tannins. This will provide researchers working on vibrational spectroscopy applications and technicians a fast, easy, and reliable method to assess the quality and authenticity of these compounds, both extracted in laboratories and provided from commercial sources. The widespread use of these compounds has highlighted the need for a systematic characterization of the vibrational frequencies and molecular fingerprints for their identification and discrimination. According to our knowledge, this is the first systematic collection of typical peak frequencies for tannins, which can be applied in several research fields and technological applications.

Pulsed electric fields (PEF) technology is an innovative food processing system and it has been introduced in relatively recent times as a pre-treatment of liquid and semi-solid food. Low cost-equipment and short processing time, coupled to the effectiveness in assisting the extraction of valuable compounds from vegetable tissues, makes PEF a challenging solution for the industrial red winemaking; a tailored PEF-assisted maceration was demonstrated to promote an increase in wine color quality and an improvement in the polyphenolic profile. Despite the application of PEF has been studied and the positive effects in selected wine varieties were demonstrated on batch and pilot-scale systems, there is a need for a more detailed characterization of the impact in different grapes, and for a better understanding of potential undesirable side-effects. This review aims to summarize the state of the art in view of a detailed feasibility study, to promote the introduction of PEF technology in the oenological industry.

The winemaking process generates a large amount of residues such as vine shots, stalks, grape pomace, and wine lees, which were only recently considered for exploitation of their valuable compounds. The purpose of this work was to investigate the performance of nanofiltration for the recovery of phenolic compounds, with bioactive capacity like antioxidant, from red grape pomace extract. Four membranes were compared in this study—three cellulose acetate (CA series: lab-prepared by phase inversion) and one commercial (NF90). All membranes were characterized for their hydraulic permeability and rejection coefficients to reference solutes like saccharose, glucose, raffinose, polyethylene glycol, sodium chloride, and sodium sulfate. Permeation flowrates and rejection coefficients towards total phenolics content, antioxidant activity, proanthocyanidins, glucose and fructose were measured in the nanofiltration of grape pomace extract using selected operating conditions. Among the investigated membranes, the CA400-22 exhibited the highest permeate flux (50.58 L/m² h at 20 bar and 25 °C), low fouling index (of about 23%), the lowest rejection coefficients towards the reference solutes and the best performance in terms of separation between sugars and phenolic compounds. Indeed, the observed rejections for glucose and fructose were 19% and 12%, respectively. On the other hand, total phenolics content and proanthocyanidins were rejected for 73% and 92%, respectively.

Valorization of agricultural waste has become increasingly important. Wastes generated by wineries are high in phenolic compounds with antioxidant and antibacterial properties, which contribute to phytotoxicity, making their immediate use for agricultural means limited. Utilizing a water-based extraction method, the phenolic compounds from winery waste were extracted and purified. The resulting extract was characterized for phenolic composition using high-pressure liquid chromatography-ultraviolet/visible and electrochemical detectors (HPLC-UV/Vis, ECD) for monomers, and spectral assessment of the tannins present using attenuated total reflectance- Fourier transform infrared (ATR-FTIR), FT-Raman, and solid-state nuclear magnetic resonance (SSNMR) spectroscopies. The extract’s antioxidant activity was assessed by the scavenging of the 2,2-diphenyl-1–picrylhydrazyl (DPPH) radical and Folin-Ciocalteu total phenolic assay, and was found to be as effective as a commercially obtained grape extract. The extract’s antimicrobial efficacy was tested for minimum bactericidal concentration using Candida albicans, Escherichia coli 25922, and Staphylococcus aureus 6538, which resulted in greater efficacy against gram-positive bacteria as shown over gram-negative bacteria, which can be linked to both monomeric and tannin polyphenols, which have multiple modes of bactericidal action.

Twenty commercially available oenological tannins (including hydrolysable and condensed) were assessed for their antiradical/reducing activity, comparing two analytical approaches: The 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radical scavenging spectrophotometric assay and the cyclic voltammetry (CV) electrochemical method. Electrochemical measurements were performed over a −200 mV–500 mV scan range, and integrated anodic currents to 500 mV were used to build a calibration graph with (+)-catechin as a reference standard (linear range: From 0.0078 to 1 mM, R² = 0.9887). The CV results were compared with the DPPH• assay (expressed as % of radical scavenged in time), showing high correlation due to the similarity of the chemical mechanisms underlying both methods involving polyphenolic compounds as reductants. Improved correlation was observed by increasing the incubation time with DPPH• to 24 h (R² = 0.925), demonstrating that the spectrophotometric method requires a long-term incubation to complete the scavenging reaction when high-molecular weight tannins are involved; this constraint has been overcome by using instant CV measurements. We concluded that the CV represents a valid alternative to the DPPH• colorimetric assay, taking advantage of fast analysis and control on the experimental conditions and, because of these properties, it can assist the quality control along the supply chain.