PAPERmaking! Vol11 Nr2 2025

Barrios et al. Biotechnology for Biofuels and Bioproducts

(2025) 18:48

Page 6 of 23

measurements. HexA content was calculated using the following equation:

isothermally at 90 °C for 45 min until the sample mass stabilized. Bound water (BW) TGA and differential scanning calorimetry (DSC) were employed to determine the bound water content [45]. The analyses were performed using a TGA-550 thermogravimetric analyzer and a DSC-Discover differential scanning calorimeter (TA Instruments, New Castle, DE, USA). Approximately 100 mg of the sample with ~ 10 wt.% solid content was placed in a 100 μL platinum pan. The nitrogen gas flow rates were set to 40 mL/min for the balance gas and 60 mL/min for the sample gas. Isothermal heating at 90 °C was applied and interrupted just before the falling rate zone (as described elsewhere [46]) to obtain fully saturated fibers at a moisture ratio of 1.4, corresponding to approximately 58 wt.% water content. The samples were then rapidly removed from the TGA furnace, placed into DSC T-zero pans (TA Instruments, New Castle, DE, USA), and sealed. The samples were equilibrated at room temperature (23 ± 1 °C) for 1 h before analysis using the modulated DSC mode. The DSC thermal procedure included a temperature drop to − 30 °C, isothermal cooling for 10 min, and a heating ramp at 3 °C/min until reaching 15 °C. After the analysis, the pans were perforated and returned to the TGA furnace for solid content determination, using isothermal heating at 90 °C until a constant weight was achieved. The non-reversing melting curves obtained were Gaussian deconvoluted to determine the melting enthalpy of each peak, corresponding to free and freezing-bound water [47]. Non-freezing bound water was calculated by subtracting the amounts of freezing- bound water and unbound water (detected by DSC) from the total water measured by TGA, according to Eqs. 2–4 [47, 48]:

g 

C HexA 

( A 260 − 1.2 A 290 ) • V ( mL ) w ( g )

μ mol

= 0.287 ×

(1)

with a calibration constant of 0.287 and a correction factor of 1.2 for lignin absorption [42], where V is the hydrolysis solution volume (mL), and w is the o.d. pulp sample weight (g). Water retention value (WRV) WRV tests were conducted using the TAPPI Useful Method 256 [43]. Treated pulp samples were first disintegrated for 5 min (15,000 revolutions) following TAPPI Method 205. The pulp was then thoroughly washed with excess deionized water and soaked overnight. Although some loss of fines might occur, the impact was expected to be minimal in this study due to the low refining level and the fact that the pulp had already been washed. After additional washing, the pulp was collected on a vacuum filter and dewatered to approximately 25% solids content. For the WRV test, moist pulp samples equivalent to 0.16 g of dry mass were placed into sintered centrifuge tubes (pore size 0.22 μm, volume 3 mL). The samples were centrifuged at 900 g for 30 min. Post-centrifugation, the moisture content was determined by weighing the samples immediately, followed by drying at 105 °C for 2 h. The dried samples were then cooled in a desiccator for 30 min before a final weighing to assess moisture content accurately. Hard-to-remove water (HRW) Thermogravimetric analysis (TGA) was employed to determine the HRW content [44]. The study was performed using a TGA-550 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) operating isothermally at 90 °C, after ramping the temperature from room temperature to 90 °C at a rate of 90 °C/min. The HR water content is defined as the moisture content of the fibers at the transition point between the constant rate drying zone and the falling rate zone and is calculated as the ratio of the water weight at the onset point of this transition (determined thermogravimetrically) to the dry weight of the fibers. During the analysis, nitrogen gas was used as purge gas, with flow rates set at 40 mL/min for the balance and 60 mL/min for the sample. Wet pulp samples containing approximately 10 mg of solid mass and 90 mg of water were prepared for the experiments. The temperature was increased from 25 °C to 90 °C at a rate of 90 °C/min, and the drying process was continued

 H peak H f *w dry

 g / g 

(2)

W water =

(3)

W NFBW = M ratio − W FBW − W FW

(4)

W BW = W NFBW + W FBW

where W water is the water mass fraction, g/g, H f is the specific heat of fusion (334 J/g),  H peak is the enthalpy peak of each curve, (J), and W dry is the dry weight of sample, (g):

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