Properties and anti-fungal activity of liquid by-products from softwood bark carbonization

Abstract

This study investigated the potential use of pyrolysis liquid from bark as an anti-fungal substance against food decaying fungi. Four different fractions of pyrolysis liquid were collected during variable temperature phases of the pyrolysis process: F1 (25–260 °C), F2 (260–512 °C), F3 (512–800 °C), and F4 (800–25 °C). The thermal degradation of bark material was assessed using TGA analysis. The concentration, pH, total phenolic content, and functional groups of the liquid samples were determined. Additionally, the molecular composition was examined using UHPLC and QToF mass spectrometry methods. Fungal species were isolated from bell pepper and animal fat and identified through microscopic observation and DNA sequencing. The anti-fungal activity of the liquid fractions was evaluated using the disk diffusion test. The obtained degradation thermograms had a typical shape characteristic of lignocellulosic materials, revealing different thermal degradation phases of the bark. These phases served as a basis for the collection of the pyrolysis liquid in fractions, which were expected to differ in properties and molecular composition. In the fractions collected above 260 ºC (F2, F3, F4), the pyrolysis liquid presented an acidic character, resulting from the complex thermochemical reactions that occur during the slow pyrolysis of bark. F2 had the highest concentration of total phenolic compounds (6.46 mg GAE/g extract) while F1 and F4 contained only negligible amounts. The FTIR spectra of F2 displayed additional peaks compared to other samples which provided information on the occurrence of various compounds. The reversed phase UHPLC-UV analysis revealed that furfural, 5-hydroxymethyl furfural, and 5-methyl furfural were the most abundant compounds, and F2 had the highest concentration of summed furans (570 µg/mL) among all samples. The morphological assessment and DNA sequence analysis of the fungal strains revealed that Penicillium crustosum and Cladosporium sp were isolated from fat and bell pepper, respectively. The antifungal activity of the liquid fractions was limited due to their low concentration (ranging from 0.24% to 0.01% (v/v)), with only minor inhibition observed for F2, indicated by a small inhibition zone of approximately 1 mm around the 10 mm filter paper. However, concentrating the fraction F2 up to 1% (v/v) demonstrated a stronger inhibitory zone against Cladosporium pseudocladosporioides and Penicillium sp., indicating its antifungal potential at higher concentrations. Overall, the pyrolysis liquid demonstrated promising antifungal activity, particularly after concentration, with F2 exhibiting the highest bioactivity and strongest inhibition effect. These findings highlight its potential for controlling food-decaying fungi while emphasizing the need for further purification, toxicity assessments, and application studies to ensure its feasibility for agro-industrial applications.

Graphical Abstract

Keywords

Pyrolysis liquid / Valorization / Bark / Antifungal compounds

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Funding

Javna Agencija za Raziskovalno Dejavnost RS(N2-0225)

Slovenian ministry of higher education, research, and innovation (C3330-21-252003)

ForestValue program(C3330-21-252003)

University of the Basque country UPV/EHU(RYC2022-035782-I)

ESF+(RYC2022-035782-I)

HORIZON EUROPE Marie Sklodowska-Curie Actions(#101067636)

RIGHTS & PERMISSIONS

The Author(s)