**4. Bioactivity of bast fibers**

#### **4.1 Antioxidant activity**

Flax and hemp fibers apart from the main components as cellulose, hemicellulose, pectin, lignin, fats, and waxes contain in their chemical composition phenolic acids, which are natural antioxidants [24]. Based on an effectively scavenging chain reaction and deleterious radicals as well as suppressing radiation-induced oxidative reactions, phenolic acids serve for preserving the physiological integrity of plant cells exposed to both air and impinging ultraviolet (UV) radiation [25].

The study described in [19] confirmed the diversified presence of ferulic, *p*-coumaric, syringic acids and small amounts of sinapinic acid in the chemical composition of flax and hemp fibers. The differences resulted from various fibrous plant varieties and the method of fiber extraction, which have effect on their chemical composition. Syringic acid naturally occurring in O-methylated phenolic acid shows high antioxidant and antibacterial activity and can be enzymatically degraded by some bacteria as a source of methane or methanol [26]. As is visible in **Table 2**, the decorticated fibers of hemp Bialobrzeskie as well as Modran and B14 flax varieties show the highest content of syringic acid. The lower share in retted fibers results from the fact that syringic acid can be degraded during the biological retting process.

Sinapinic acid (3,5-dimethoxy-4-hydroxycinnamic acid) exhibits antioxidant, anti-inflammatory, anticancer, antimutagenic, antiglycemic, neuroprotective, and antibacterial activities [27]. Sinapinic acid occurs only in hemp fibers; the highest amount was detected in the decorticated Bialobrzeskie hemp, as well as in the waterretted Wojko and Tygra hemp.

Coumaric and ferulic acids are the main hydroxycinnamic acids in flax [28]. *p*-coumaric acid is mainly a plant metabolite, which exhibits antioxidant and antiinflammatory properties. It also shows bactericidal activity by damaging bacterial cell membrane and by interacting with bacterial DNA.

Ferulic acid, together with dihydroferulic acid, is a component of lignocellulose, serving to crosslink the lignin and polysaccharides, thereby conferring rigidity to the cell walls [29]. It is an intermediate in the synthesis of monolignols, the monomers of lignin, and is also used for the synthesis of lignans. Ferulic acid shows antioxidant and anti-inflammatory properties and is able to suppress UV-radiation-induced oxidative reductions, which has a negative effect on the skin.

Ferulic acid is easily soluble in water and can be easily removed from the fibers during the retting process; however, coumaric acid is poorly soluble in water, and its removal could be only partial. Among all types of tested flax and hemp fibers, almost all the decorticated fibers contained the highest amount of *p*-coumaric and ferulic acids because decortication is an entirely dry mechanical process.


**Table 2.** *Acid content in the flax and hemp fiber. Results are expressed as mean ± standard deviation (SD), n = 4. Lowercase letters indicate significant differences at p ≤ 0.05 according to the Tukay's HSD test [19].*
