Timber and wood industry generate a large amount of residues that are available for further processing and valorization. Manufacturing value-added products from underutilized forest by-products could improve the whole forest-wood supply chain, not only by reducing the environmental and economic cost of waste management, but also providing an additional income.
Among several compounds available in forest residues, tannins belong an important class of phenolic structures, which are abundant in the bark, wood and in a smaller amount in leaves and fruits of a variety of higher plants. They are produced by plants mainly as a defense against insects, fungi, or bacteria attack, due to their ability to complex proteins permanently. Based on their structural characteristics, tannins are conventionally divided into hydrolysable tannin (gallotannins, ellagitannins), and condensed tannins (or proanthocyanidins, consisting in flavonoid oligomers) [1].
Tannins have been traditionally used in leather manufacturing and in the production of bio-based adhesives [2], but in last few decades their application range expanded remarkably, from oenology to materials engineering. In the latter field, due to their metal chelating ability, tannins were applied as low-cost and effective adsorbent materials to remove toxic metals from wastewater [3, 4]. Several researches tested the possibility of using tannins in the manufacturing of biobased foams, wood preservatives, corrosion inhibitors, polyurethane surface coatings, etc. [1, 5]. More recently, food packaging applications of tannin gained attention, showing a possible contribution to the development of polymeric films with improved mechanical and physicochemical properties for food preservation [6].
Due to their biological activities, i.e., antioxidant, anti-inflammatory, anti-diabetic, cardioprotective, and antimicrobial, recently tannins gained attention for their applications in the fields of medicine and pharmacology [7–10]. Studies in veterinary medicine focused on the use of tannins as feed supplements aimed at decreasing the antibiotic treatments, improving animal and human health, and lowering antimicrobial resistance [11–13].
Applications of tannins in the food sector are increasing, driven by the evidence of their biological value and the market demand for functional and healthier products [14–17]. In addition to the long-practiced use of tannins in wine, beer and spirits manufacturing for the improvement of taste, clarification and antioxidant properties, their use as ingredients or additives in the food industry is still limited, however with growth potential especially with regards to the antioxidant properties.
The global tannin market size was evaluated at USD 2.23 billion in 2019, expanding at an estimated compound annual growth rate (CAGR) of 6.7% from 2020 to 2025, with leather tanning represent the largest application segment (62.3%) of the total market revenue in 2016 [18].
Besides mimosa, pine and oak, chestnut is one of the most important tannin sources [19]. In particular, Castanea sativa Mill. (European or sweet chestnut), mainly containing hydrolysable tannins [20, 21], showed higher antioxidant activity compared with quebracho [22], and mimosa [23].
C. sativa is an economically important resource in Europe, mainly on the northern shores of the Mediterranean basin, having a key role in different production chains (timber, flour, honey, and tannins) [24]. Based on current and emerging uses of tannin and the strong market demand, process improvements and the availability of methods enabling also limited-scale productions by local stakeholders in rural and mountainous areas could be environmentally, economically and socially beneficial [25].
This study presents the first evidence of the possibility of using controlled hydrodynamic cavitation (HC), an emerging green, efficient and straightforwardly scalable method for the extraction of natural products [26], as a single-unit operation performing the aqueous extraction of tannins from chestnut wood waste. Performance data, including extraction yields, process time and specific energy consumption, are provided in order to allow both replication and comparison with other methods.