## 2 Materials and Methods
### Extraction of Volatiles, GC-MS Analysis and Data Processing  
The samples of un/treated hemp shives and spruce strands were placed separately in desiccators and analyzed for their volatile content after days 1, 3, 7 and 14 using GC-MS. To avoid instrumental sensitivity changes, samples were measured in one sequence. Before analysis, samples were stored airtight closed in vials in deepfreeze (-80 \({}^{\circ}\)C). For volatile organic compound collection, solid-phase microextraction, fibers with a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS 50/30 \(\upmu\)m) coating from Supelco (Bellefonte, PA, USA) was employed. Vials were incubated for 10 min to increase volatiles emission from the sample and then, volatiles were collected onto a fibers' stationary phase for the next 10 min, both at 100 \({}^{\circ}\)C.  
Gas chromatography coupled to mass spectrometry (GC-MS) was applied for VOCs separation and identification. Basic measurements were performed using Quadrupole Shimadzu GC-MS QP2010 SE--Ultra, applying SLB-SMS capillary column (30 m, 0.25 mm i.d., 0.25 \(\upmu\)m film thickness) from Supelco. The injection was performed at 250 \({}^{\circ}\)C, while the transfer line was kept at 280 \({}^{\circ}\)C. The temperature program was as follows: 40 \({}^{\circ}\)C for 1 min and then with grad 5 \({}^{\circ}\)C min\({}^{-1}\) to 250 \({}^{\circ}\)C and held for 2 min. Total run time was 45 min. Helium was used as a carrier gas at a flow rate of 1 mL-min\({}^{-1}\).  
A group of target compounds was based on the literature. Nevertheless, in order not to focus only on few selected compounds, a mass analyzer was operated in SCAN mode (scan speed 2000 ns, range 30-400 \(m\)/z). The identification of chemical compounds was based on mass spectral similarity with the in-built NIST MS library (NIST, Gaithersburg, MD, USA; 2017 released version). For confirmation of target compounds identity, retention times of respective standards (Sigma-Aldrich; Germany) were used.  
Consequently, GC-MS with time of flight (TOF) mass analyzer Pegasus 4D (LECO, St. Joseph, MI, USA) was used to analyze an identical sample set. The aim was to obtain full spectral data for future chemometric evaluation. The sampling procedure remained the same, while for GC separation faster ramping was used. The GC oven temperature program was as follows: 40 \({}^{\circ}\)C for 1 min; then ramped at a rate of 10 \({}^{\circ}\)C min\({}^{-1}\) to 70 \({}^{\circ}\)C; then at 5 \({}^{\circ}\)C min\({}^{-1}\) to 200 \({}^{\circ}\)C and at 20 \({}^{\circ}\)C min\({}^{-1}\) to 28 \({}^{\circ}\)C and held for 1 min. The total GC run time was 21 min.  
Automated spectral deconvolution and peak finding algorithms were carried out using ChromaTOF software (LECO, St. Joseph, MI, USA). A build in peak alignment tool, Statistical Compare, was used to align all chromatographic signals with a signal to noise ratio (S/N) higher  
Figure 1: The samples closed in vials ready for GC-MS analysis—visual color change: (S—spruce strands, H—untreated hemp shive, H\({}_{\rm A}\)—hemp shive after tartaric acid treatment, Hg—hemp shive after citric acid treatment, HC—hemp shive after sodium bicarbonate treatment).  
than 50 in all samples. The data measured using the TOF mass spectrometer were normalized (constant raw sum) and then evaluated using principal component analysis (PCA) and partial least square-discriminant analysis (PLS-DA) in SIMCA 15 software (Sartorius Stedim Data Analytics AB, Malmo, Sweden).  
The reported intensities are areas of a unique mass--the specific mass of the compounds' mass spectrum--that does not co-elute with another compounds signal at a signal's retention time. For reported compounds RSD (relative standard deviation, expressed as %) from 7 individual measurements was checked and was below 20%.