## 1 Introduction  
The development of high-performance natural fibre composites cannot neglect the optimization of the fibre/matrix interfacial adhesion [1]. The importance of the fibre/matrix interface for the macroscopic response of fibre-reinforced composites is well consolidated, and it is not an easy task to find an ideal interface able to optimize simultaneously all the macroscopic properties that are directly or indirectly affected by its characteristics, i.e., tensile strength, fracture toughness, resistance to creep, fatigue, dimensional stability and environmental degradation. This consideration leads to the full awareness that interfacial properties need to be optimized rather than maximized [2]. For these reasons, the improvement of the adhesion properties between natural fibres and polymer matrices can be based on the optimization of the chemistry of the sizing agents or on the surface modification of the fibres to increase the bonding and wettability of the matrix. In this regard, numerous strategies have been devised to address the limitations of natural fibres, mainly ascribed to their hydrophilic character that precludes a suitable interfacial adhesion with hydrophobic polymer matrices, but at the same time increases moisture absorption and related microbiological degradation [1]. In this field, the engineering of fibres' surfaces for their effective incorporation in composite materials with reduced costs and environmental impact represents a noticeable challenge. With this in mind, many strategies that have been investigated over the years suffer from the need to handle large amounts of chemicals [3; 4; 5; 6] or complex equipment [7; 8; 9; 10; 11], which can limit their full industrial exploitation. Another recent approach consists of decorating the fibres' surface with nanostructures to increase the surface area and the stress transfer of the polymer matrix [12; 13; 14], but this strategy usually involves several processing steps that make it less attractive for industrial applications. In [15], the authors reported the grafting of TiO\({}_{2}\) onto flax fibres with a view to increasing the mechanical properties of poly lactic acid-based composites. The fibre treatment needed two steps, namely dewaxing and oxidation. In this regard, fibres were first treated with boiling acetone and then NaOH that was later neutralized by using a diluted citric acid solution. In the second step, the hydroxyl groups of the flax fibres were oxidized to carboxylic groups by using TEMPO (2,2,6,6-Tetramethylpiperidine 1-oxyl) and then finally immersed in the sol for the synthesis of TiO\({}_{2}\) based on titanium isopropoxide as a precursor. A slight increase in the interlaminar shear strength (ILSS) and in the thermal and moisture resistance of the resulting composites was achieved, but with a remarkable complexity. An environmentally friendly substitute for chemical and physical strategies is based on biological agents such as fungi [16; 17; 18] and enzymes [19; 20; 21] or supercritical fluids [22; 23]. These modifications give advantages over chemical and physical methods, including their selectivity, the lower energy input required and their recyclability and reusability. In particular, the use of enzymes for textile and natural fibre modification is swiftly growing. The enzymatic treatment represents a green way to modify natural fibres, and in literature enzymatic treatments have been used with two final aims: (i) the extraction of past fibres from non-fibrous tissues in stems as an alternative to dew retting [24; 25], and (ii) the surface modification of fibres already extracted in order to reduce hydrophilicity and increase thermal stability [21; 26]. In these latter studies, the enzymatic treatment was reported to be able to remove undesirable substances like pectin, hemicelluloses, lignin and waxes from the surface of the fibres, improving fibre/matrix adhesion, moisture resistance and thermal stability.