Effects of boiling water treatment on the physical properties of Quercus variabilis virgin cork grown in Korea

The effects of boiling water treatment on the physical properties of Quercus variabilis virgin cork (Qv VC) were examined and compared with those of Quercus suber reproduction cork (Qs RC). The water treatment was conducted at 100 °C for 1 h. Qv VC showed a significantly higher dimensional change in the three directions and lower weight loss than Qs RC by boiling water treatment. Untreated and boiled Qv VC showed higher density, air-dried moisture content, red/green (a*) and yellow/blue (b*) chromaticity, overall color change, shrinkage in all three directions, moisture adsorption on the entire surface, and swelling per 1% moisture content than untreated and boiled Qs RC. However, the lightness (L*) and water absorption on each surface were higher for Qs RC than for Qv VC. Moisture adsorption on each surface was comparable before and after heat treatment for both species. After boiling water treatment, the air-dried moisture content, dimensions, volume shrinkage, water absorption, and moisture adsorption on each surface and the entire surface increased, whereas L*, a*, b*, and swelling per 1% moisture content decreased. The results of the present study could be useful for further utilization of Qv cork growing in Korea.


Materials
In this study, we examined previously used samples 7,15 , which was collected from three trees at breast height of Qv VC in the research forest of Kangwon National University, Chuncheon, Korea (37° 77′ N, 127° 81′ E).Two planks of Qs RC from the Castelo Branco cork forest of Amorim Group (Mozelos, Portugal) were provided by FC Korea Land Co., Ltd., Seoul, Korea.The cork samples were obtained in compliance with all institutional, national, and international guidelines and legislation.The essential information on the experimental corks is listed in Table 1.The Qv VC and Qs RC samples (Fig. 1) with dimensions of 20 mm (radial, R) × 20 mm (tangential, T) × 20 mm (longitudinal, L) were oven-dried and kept in a conditioning room at 25 ± 5 °C and 60 ± 5% relative humidity (RH).

Boiling treatment process
The boiling treatment was performed using tap water as the heating medium.Water (500 mL) was boiled in a 2000 mL glass beaker by using a hotplate (GLMS-C12; Global Lab, Siheung-si, Korea).The target temperature of the water, approximately 100 °C, was measured using an infrared thermometer (Raynger Mx Series; Raytek, Table 1.Essential information about the cork samples.Numbers within parentheses are standard deviations.

Macroscopic observation
To observe the changes in the macroscopic characteristics of the transverse, radial, and tangential surfaces before and after boiling, air-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) from each species were prepared and trimmed using a sliding microtome (MSL-H; Nippon Optical Works, Nagano, Japan) before boiling water treatment.Images of the three surfaces were obtained before and after the boiling water treatment by using a mobile phone and recorded in a digital format of 3024 × 3024 pixels with a resolution of 72 dpi and a depth of 24 bits (Samsung Note 20, 12MP with F1.8; Suwon-si, Korea).

Scanning electron microscopy
To perform scanning electron microscopy of the transverse, radial, and tangential surfaces of both untreated and boiled corks, air-dried cork samples with dimensions of 10 mm (R) × 10 mm (T) × 10 mm (L) from each species were prepared and trimmed using a sliding microtome (MSL-H model; Nippon Optical Works).The samples were coated with gold by using a sputter coater (Cressington Sputter Coater 108; Watford, UK) and observed under a scanning electron microscope (SEM; JEOL, JSM-5510, 15 kV, Tokyo, Japan).

Measurement of physical properties before and after boiling water treatment
Detailed information regarding the physical properties of both cork samples is listed in Table 2.

Color change
Color was evaluated before and after boiling water treatment by using a chromameter (CR-10 Plus; Konica Minolta, Tokyo, Japan).Ten air-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) from each species were prepared.The color of the transverse, radial, and tangential surfaces of the untreated and boiled corks was determined using the CIE L*a*b* system, which is characterized by three parameters: L* (lightness), a* (red/green chromaticity), and b* (yellow/blue chromaticity).The overall color change was calculated using Eq. ( 1).

Moisture content and density
To measure moisture content and density, twenty air-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) from each species were prepared from the untreated and boiled corks.The moisture content and density after air drying were determined according to Korean standards KS F 2199 16 and KS F 2198 17 by using Eqs.( 2) and (3), respectively. (1) Table 2. Sample information for evaluating the physical properties of heat-treated cork.Sample number = replication ( R ) × species ( Sp ) × treatment ( T ) × surface ( S ).where D a (g/cm 3 ) is the density of air-dried cork samples, M a (g) is the weight of air-dried cork samples, and V a (cm 3 ) is the volume of air-dried cork samples.

Weight loss
In this study, the weight loss of ten oven-dried samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) from each cork species was measured to understand the effects of boiling water treatment by using Eq. ( 4).
where WL (%) is the weight loss, m 1 (g) is the weight of the oven-dried samples before boiling water treatment, and m 2 (g) is the weight of the oven-dried samples after boiling water treatment.

Dimensional change after boiling water treatment
Dimensional changes after boiling water treatment in the radial (L r ), tangential (L t ), and longitudinal (L l ) directions were calculated using Eq. ( 5).Ten air-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) from each species were used to determine dimensional changes after boiling water treatment.Volumetric changes were calculated as the sum of the radial, tangential, and longitudinal changes.
where L (%) is the dimensional change in each direction of the air-dried samples before (L 1 , mm) and after (L 2 , mm) boiling water treatment.

Shrinkage
Shrinkage from green to oven-dried in the radial, tangential, and longitudinal directions of the untreated and heat-treated samples was determined according to Korean standard KS F 2203 18 by using Eq. ( 6).Untreated and heat-treated twenty oven-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) were prepared for each species.The cork samples were immersed in distilled water at 25 ± 1 °C, and length was measured in the radial (L r ), tangential (L t ), and longitudinal (L l ) directions at intervals of 3 days until the dimensions in the directions remained constant.We define these samples as green corks; subsequently, the green cork samples were oven-dried at 100 °C for 24 h.Volumetric shrinkage was obtained by adding the radial, tangential, and longitudinal shrinkages.
where S o (%) is the total shrinkage in each direction from green to oven-dry conditions, and l g (mm) and l o (mm) are the lengths in the three directions under green (more than 100% moisture content) and oven-dried conditions, respectively.

Water absorption
Water absorption on each surface of the untreated and treated samples was determined according to Korean standard KS F 2204 19 by using Eq. ( 7).Sixty oven-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) were prepared for each cork species.The oven-dried cork samples coated with paraffin, except for the measuring surface, were soaked in distilled water at 25 ± 1 °C for 24 h.
where W ab (g/cm 2 ) is water absorption, W 1 (g) is the weight before soaking the samples, W 2 (g) is the weight after soaking the samples, and A (cm 2 ) is the absorption area.

Moisture adsorption on each surface
Moisture adsorption on each surface of the untreated and treated samples was determined according to Korean standard KS F 2205 20 .Sixty oven-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 20 mm (L) were prepared for each species to determine moisture adsorption on the transverse, radial, and tangential surfaces of untreated and boiled corks.The cork samples with constant weight at 75 ± 2% RH and 40 ± 1 °C in a Thermo hygrostat (TH3-PE; JEIO TECH, Daejeon, Korea) were coated with paraffin, except for the measuring surface. (2) Vol.:(0123456789) www.nature.com/scientificreports/Subsequently, the samples were placed in a chamber at 90 ± 2% RH and 40 ± 1 °C for 24 h.Moisture adsorption was calculated using Eq. ( 8).
where M ad is moisture adsorption (g/cm 2 ), W 0h (g) is the weight stabilized at 75% RH and 40 °C, W 24h (g) is the weight after placing the samples in a desiccator at 90% RH and 40 °C for 24 h, and A (cm 2 ) is the adsorption area.
Moisture adsorption on the entire surface Moisture adsorption on the entire surface of both untreated and treated samples was conducted according to the methodology of Jo et al. 21and Korean standard KS F 2205 20 .Twenty oven-dried cork samples with dimensions of 20 mm (R) × 20 mm (T) × 5 mm (L) were prepared for each species.The constant weight of the cork samples at 75 ± 2% RH and 40 ± 1 °C and 90 ± 2% RH and 40 ± 1 °C was obtained in the Thermo hygrostat.Moisture adsorption (equilibrium moisture content [EMC]) and swelling per 1% moisture content were calculated using Eq. ( 9).
where W (g) is the oven-dried weight, and W 0h (g) and l (mm) are the weight and length stabilized at 75% RH and 40 °C, respectively.W ∞ (g) and L (mm) are the weight and length stabilized at 90% RH and 40 °C, respectively.

Statistical analysis
Statistical differences in air-dried moisture content and density, weight loss, dimensional and color changes, water absorption, and moisture adsorption between treatments and species were analyzed using one-way analysis of variance (ANOVA) followed by post-hoc Tukey's honestly significant difference (HSD) test (SPSS ver.24, IBM Corp., New York, USA).

Macroscopic characteristics
Appearances of the transverse, radial, and tangential surfaces of Qv VC and Qs RC before and after boiling water treatment are shown in Figs. 2 and 3, respectively.Qv VC changed to a darker color after boiling, whereas Qs RC changed to a pale color.In addition, the tangential surface of Qs RC exhibited a darker coloration around the lenticular channel.Some cracks were found across the cork cells, dark-brown zone (DBZ), and lenticular filling tissue (LFT) of Qv VC after the water boiling treatment.In contrast, no cracks were detected in the lenticular channels of Qs RC, with an enlarged shape and darkening after the water boiling treatment (for example, K → K' and J → J').
A few studies support our results regarding the effects of boiling water treatment on macroscopic structural changes.Song et al. 11 reported that Qv cork changed to a darker color after boiling water treatment at approximately 100 °C for 3 h.Cumbre et al. 13 reported that boiling water treatment of Qs RC caused enlargement of lenticular channels in the tangential direction.

Microscopic characteristics
SEM images of the transverse, radial, and tangential surfaces of Qv VC and Qs RC before and after boiling water treatment are shown in Figs. 4 and 5, respectively.The boiling water treatment resulted in the straightening of the corrugated cork cell wall in Qv VC and Qs RC, especially in earlycork cells.Rosa et al. 10 reported that the boiling treatment of Qs RC resulted in the straightening of the cork cell wall, particularly pronounced in the corrugated earlycork cells.Yuan et al. 14 reported that the boiling treatment of Qv VC reduced cork cell wall corrugations.
Figure 6 shows SEM images of cracks in the cork samples treated with boiling water.In Qv VC, cracks parallel to the growth rings were observed in cork cells adjacent to the LFT (Fig. 6a, a1) and within LFT (Fig. 6a, a2) on the transverse surface.Cracks perpendicular to the growth rings were observed in cork cells (Fig. 6b, b1) and DBZ (Fig. 6b, b2) on the radial surface.Cracks were also observed within compact LFT cells on the tangential surface (Fig. 6c, c1).

Color change
Color changes according to the CIE L*a*b* system on the transverse, radial, and tangential surfaces of Qv VC and Qs RC before and after the boiling water treatment are presented in Tables 3 and 4.
The L* values on the three surfaces of both corks decreased significantly after boiling water treatment.Untreated and boiled Qs RC had significantly higher L* values than untreated and boiled Qv VC.     www.nature.com/scientificreports/ The L* values on the tangential surface of untreated and boiled Qv VC were significantly higher than those on the transverse and radial surfaces, whereas no significant differences were found in the L* values between the transverse and radial surfaces of the untreated and boiled samples.In contrast, the L* values of untreated Qs RC were significantly higher on the tangential surface than on the transverse and radial surfaces, whereas those of boiled Qs RC were significantly higher on the transverse and radial surfaces than on the tangential surface.No significant differences were observed in the L* values of untreated and boiled Qs RC between the transverse and radial surfaces.
On all three surfaces of both cork species, the a* values significantly decreased after boiling water treatment.The three surfaces of untreated and boiled Qv VC exhibited significantly higher a* values than those of untreated and boiled Qs RC.In untreated Qv VC, the a* values were the highest on the transverse surface and lowest on the tangential surfaces, and significant differences were detected between the three surfaces.In boiled Qv VC, a* values were significantly higher on the transverse and radial surfaces than on the tangential surface, and the transverse and radial surfaces showed comparable a* values.In untreated Qs RC, the a* values on the transverse surface was significantly higher than those on the radial and tangential surfaces, and no significant difference was observed between the radial and tangential surfaces.For boiled Qs RC, no differences in the a* values were found among the three surfaces.
In both cork species, the b* values of the three surfaces significantly decreased after the boiling water treatment.Untreated and boiled Qv VC showed higher b* values on the three surfaces than untreated and boiled Qs RC.Significant differences were found in the b* values between untreated Qv VC and Qs RC, whereas no significant differences were observed in the b* values between boiled corks of either species.The b* values of untreated and boiled Qv VC were significantly lower on the tangential surface than on the transverse and radial surfaces.In untreated Qv VC, no significant differences were found in b* values between the transverse and radial surfaces, whereas b* values on the radial surface of boiled Qv VC were significantly higher than those on the transverse surface.For untreated Qs RC, b* values were significantly lower on the tangential surface than on the transverse and radial surfaces, whereas the transverse and radial surfaces had comparable values.In boiled Qs RC, b* values were significantly higher on the radial surface than on both transverse and tangential surfaces.No significant differences in b* values were observed between the radial and tangential surfaces.
In Qv VC, ∆E* values on the radial surface were significantly lower than those on the transverse and tangential surfaces.No significant differences in ∆E* values were observed between the transverse and tangential surfaces.In Qs RC, ∆E* values were significantly higher on the tangential surface than on the transverse and radial surfaces.No significant differences in ∆E* values were found between the transverse and radial surfaces.Qv VC showed significantly higher ∆E* values on each surface than Qs RC.
In this study, L*, a*, and b* values of both cork species significantly decreased after boiling water treatment, which is consistent with the results of Yuan et al. 14 and Song et al. 11 .Yuan et al. 14 reported that the L* values of Qv cork decreased after boiling water treatment and the color differences increased as the duration time increased.Song et al. 11   www.nature.com/scientificreports/ The decrease in L*, a*, and b* values of cork after boiling water treatment could be related to the extraction of phenolic compounds, such as tannins.As mentioned by Rosa et al. 10 and Pereira 4 , boiling water treatment could solubilize mostly phenolic compounds, such as tannins, from cork, corresponding to the brown color of the water during the boiling process.Until now, there is limited information on the color change of cork by boiling water treatment.Nevertheless, a few studies have been reported on the relationship between extractives and color change in wood, which supports our results.Moura and Brito 22 reported that the a* value of Eucalyptus grandis wood decreased after heat treatment, which could be related to the volatilization of certain chemical compounds, such as phenolic extractives.Tolvaj et al. 23 found that the woods of Pinus sylvestris and Picea abies became darker after steam treatment, and the color hue of both species turned brown.The observed color change in both species could be attributed to the alteration of the conjugated double bond systems found mainly in extractives.A high amount of extractives in a wood species resulted in significant color changes during steaming 24,25 .

Moisture content and density
The air-dried moisture content and density of boiled Qv VC and Qs RC samples are listed in Table 5.The moisture content and density of untreated and boiled Qv VC were significantly higher than those of Qs RC.The moisture content of both Qv and Qs corks increased after boiling water treatment.A significant difference in moisture content was observed between untreated and boiled Qv VC, whereas no significant differences in moisture content were found between untreated and boiled Qs RC.In addition, the densities of both Qv and Qs corks did not change before and after boiling water treatment.
The results of previous studies are consistent with those of this study.Rosa et al. 10 reported that the density of Qs RC after boiling water treatment gradually decreased as the boiling duration increased.Yuan et al. 14 reported that the density of both Qv VC and Qv RC decreased after boiling water treatment, with a ratio of 0.85-0.88for VC and 0.79-0.80 for RC.Song et al. 11 reported that the density of Qv RC after boiling water treatment decreased as the treatment duration increased, with a ratio range of 0.79-0.82.The authors also mentioned that the decrease in density was correlated with reduced cork mass and increased cork dimensions during the boiling water treatment.

Weight loss and dimensional change
Table 6 shows the weight loss and dimensional changes in boiled Qv VC and Qs RC.The weight loss after water boiling treatment for Qv VC and Qs RC was 2.20% and 2.67%, respectively, showing a significant difference between the two species.
Both corks swelled in the radial, tangential, and longitudinal directions after the water boiling treatment, and dimensional expansion was the highest in the radial direction and lowest in the tangential direction.A significant difference was observed in the dimensional expansion between each direction.Moreover, Qv VC exhibited significantly higher swelling in all three directions and volumes than Qs RC.
The results of previous studies on Qs and Qv corks are consistent with those of this study.Rosa et al. 10 reported that the volume expansion of boiled Qs RC was 10-15% in the radial direction and 5-7% in the tangential and longitudinal directions.They mentioned that the expansion in the radial direction was entirely due to the attenuation of the corrugations in the lateral cell walls, which become straightening in the radial direction.Cumbre et al. 13 found that the growth ring width of boiled Qs RC increased by 12.8% at the 1st growth ring and  www.nature.com/scientificreports/moisture content more than 200% in hot water for long periods and then subjected to oven-drying at 100 °C showed significant volume shrinkage of up to 30%.
In the present study, the shrinkage of both corks was the highest in the radial direction, showing comparable results with swelling properties.As reported in some studies 10 , cork commonly showed a noticeable change in the radial direction by drying or boiling water treatment, due to partial straightening or wrinkling of the cell lateral walls in the radial direction.

Water absorption
Water absorption on each surface before and after boiling water treatment is presented in Table 8.For both Qv VC and Qs RC, water absorption on the transverse, radial, and tangential surfaces significantly increased after boiling water treatment.For both untreated and boiled Qv VC, there was no significant difference in water absorption between the transverse, radial, and tangential surfaces.In contrast, for both untreated and boiled Qs RC, the tangential surface showed significantly greater water absorption than the transverse and radial surfaces, with no significant differences in water absorption between the transverse and radial surfaces.
Rosa and Fortes 12 reported that water absorption on the tangential surface at 20 °C and 90 °C was significantly higher and faster than that on the transverse and radial surfaces.They also mentioned that water absorption on the transverse surface was slightly higher than that on the radial surface and water absorption increased as the temperature increased.
The transverse and radial surfaces of untreated Qv VC showed significantly higher water absorption than those of untreated Qs RC, whereas the tangential surface of untreated Qs RC showed significantly higher water absorption than that of untreated Qv VC.After boiling water treatment, Qs RC showed significantly higher water absorption on all three surfaces than Qv VC.
Qv VC consists of cork cells, lenticular filling tissue, DBZ, and sclereids 7,15 .Water absorption in cork could be related to cork properties, such as composition, arrangement, and porosity of cork elements.As mentioned by Prasetia et al. 15 , the lenticular channels of Qs RC have an opening with loose lenticular filling tissue, whereas Qv VC is filled with compact lenticular filling tissue.In addition, lenticular channels in Qs RC are abundant on the tangential surface.Pereira 4 also explained that the lenticular filling tissue is more hygroscopic than cork cells because of higher polysaccharide levels and lack of suberin content, which could increase the contact area for water adsorption.

Moisture adsorption on each surface
Moisture adsorption on each surface before and after the boiling water treatment is presented in Table 9. Moisture adsorption on each surface and total moisture adsorption in Qv VC slightly increased after boiling water treatment.However, no significant differences were observed between the untreated and boiled corks.Moisture Table 8.Water absorption of Qv VC and Qs RC before and after boiling water treatment.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate non-significant outcomes at 5% significance level for comparisons between treatments and species.The mean value in the same line followed by the same capital indicates non-significant outcomes at 5% significance level for comparisons between surfaces.adsorption of untreated and boiled Qv VC was comparable for the three surfaces.No significant differences were found in either untreated or boiled Qv VC among the three surfaces.
In untreated Qs RC, the tangential surface exhibited significantly higher moisture adsorption than the transverse and radial surfaces, whereas no significant differences were observed between the transverse and radial surfaces.After boiling water treatment, moisture adsorption on the transverse and radial surfaces showed a significant increase, whereas on the tangential surface, there was no significant difference between untreated and boiled corks.Furthermore, the three surfaces of boiled Qs RC exhibited comparable moisture adsorption.The total moisture adsorption of untreated cork in Qs RC was significantly lower than that of boiled cork.
Moisture adsorption on the transverse and radial surfaces of untreated Qv VC was slightly higher than that of untreated Qs RC.In contrast, moisture adsorption on the tangential surface of untreated Qv VC was slightly lower than that of untreated Qs RC.Total moisture adsorption in untreated Qv VC was slightly higher than that in untreated Qs RC.No significant differences in moisture adsorption were found on the three surfaces between the untreated corks of both species.After boiling water treatment, Qs RC showed slightly higher moisture adsorption on the transverse, radial, and tangential surfaces than Qv VC.No significant differences in moisture adsorption were observed on the three surfaces between boiled corks of both species.In addition, the ratio of moisture adsorption in untreated and boiled Qs RC was significantly higher than that in Qv VC.

Moisture adsorption on the entire surface
EMC and swelling per 1% moisture content of the corks before and after boiling treatment are presented in Table 10.Untreated and boiled corks of both species showed a significant increase in EMC with the increase in RH.In both corks, EMC at 75% and 90% RH significantly increased after the boiling water treatment.The EMC ratios (B/U) of untreated and boiled corks of both species were higher at 75% RH than at 90% RH.No significant differences were found in the EMC ratio of untreated and boiled corks between RH and species.Untreated and boiled Qv VC showed slightly higher EMC at 75% and 90% RH than untreated and boiled Qs RC.No significant differences were observed in the EMC of both species between untreated corks and between boiled corks.
Gonzales-Adrados and Haro 27 reported that the moisture adsorption of water-boiled Qs RC at 20 °C and 40 °C constantly increased from 20 RH to 80% RH and significantly increased from 80 RH to 90% RH.The ratio (m/ m 0 : mass at 90% RH/initial mass) of moisture adsorption at 90% RH was approximately 1.12 at 20 °C and 1.10 at 40 °C, respectively.Moisture adsorption was lower at 40 °C than at 20 °C, with a decrease of approximately 2-3% between the two temperatures.Pereira 4 reported that the EMC of Qs cork at ambient temperatures of approximately 15-25 °C was 8% at 75% RH, 10% at 85% RH, and 16% at 95% RH.When compared with the EMC of other hygroscopic materials such as wood, the EMC of cork is relatively low: 15.5% to 19.5% at 20 °C and 90% RH in six Argentine wood species 28 , 20% at ambient temperatures and 80% RH in cork oak wood 4 , and 9.6% at 75% RH and 13.9% at 90% RH and 40 °C in Paulownia wood 16 .
The increase in water adsorption of cork after boiling water treatment can be attributed to the expansion of cork cell dimensions, extraction of extractives, and failures such as cracks and abnormal deformation in the cell components of cork.As mentioned by Pereira et al. 29 and Pereira 4 , boiling water treatment extracts small amounts of water-soluble extractives from cork.Rosa et al. 10 and Yuan et al. 14 reported that boiling water treatment of Qs RC and Qv VC reduced cork cell wall corrugations.
The swelling per 1% moisture content and anisotropy coefficient of both boiled cork species were significantly lower than those of the untreated samples.No significant differences in swelling per 1% moisture content and the anisotropy coefficients were observed before and after boiling treatment in both species.Moreover, in both species, swelling per 1% moisture content and the ratio (B/U) before and after boiling treatment were significantly higher in the radial direction than in the tangential direction.The reduced swelling of boiled cork may be due to the straightening of cell walls during the boiling water treatment.Several studies 10,11,13,14 have reported the positive effects of boiling water treatment on volume expansion of corks.Pereira 4 reported that a second boiling water treatment of boiled cork did not cause additional swelling.Table 10.EMC and swelling per 1% moisture content of Qv VC and Qs RC before and after boiling water treatment.B and U indicate boiled and untreated cork, respectively, and T/R is the anisotropy coefficient.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate non-significant outcomes at 5% significance level for comparisons between treatments and species.The mean value in the same line followed by the same capital indicates nonsignificant outcomes at 5% significance level for comparison between RH level and direction.

Conclusions
• The boiling water treatment of Qv VC and Qs RC resulted in straightening of the corrugated cork cell wall and cracks in the LFT, cork cells, and DBZ of Qv VC as well as enlargement of lenticular channels in the tangential direction of Qs RC. • The L*, a*, and b* values on all three surfaces of both Qv VC and Qs RC decreased significantly after boiling water treatment.The ∆E* values on the three surfaces of Qv VC were significantly higher than those on the three surfaces of Qs RC. • The air-dried moisture content, dimensional change, shrinkage in the three directions, and volume of Qv VC and Qs RC increased with the boiling water treatment, whereas the density was constant.Qv VC showed a significantly greater dimensional change after boiling water treatment than Qs RC, whereas Qs RC showed a significantly greater weight loss than Qv VC.Untreated and boiled Qv VC showed significantly higher shrinkage in the three directions and volume than untreated and boiled Qs RC. • After boiling water treatment, water absorption on the three surfaces of Qv VC and Qs RC significantly increased, and Qs RC showed significantly higher water absorption on all three surfaces than Qv VC. • Moisture adsorption on the transverse, radial, and tangential surfaces of Qv VC and Qs RC increased after the boiling water treatment.After treatment, Qs RC showed slightly higher moisture adsorption on the three surfaces than Qv VC. • EMC at 75% RH and 90% RH in both Qv VC and Qs RC significantly increased after boiling water treatment, whereas the dimensional change per 1% moisture content significantly decreased.Untreated and boiled Qv VC showed a higher EMC at 75% and 90% RH and swelling per 1% moisture content than untreated and boiled Qs RC.
In conclusion, the effects of boiling water treatment on the physical properties of Qv VC grown in Korea, such as cork cell wall straightening, increasing shrinkage, water absorption, moisture adsorption, and decreasing L*, a*, and b* values, were revealed.The results can be used to understand Qv VC quality and provide valuable information for the further utilization of the cork.Owing to the anatomical structure and physical properties, the Qv VC could be potentially use as cork granules and agglomeration for composite products.

Figure 2 .
Figure 2. Transverse (a,d), radial (b,e), and tangential (c,f) surfaces before (a-c) and after (d-f) boiling water treatment in Qv VC.Black and white arrows indicate the cracks in LFT (d,f) and DBZ (e,f) after boiling water treatment, respectively.

Figure 3 .
Figure 3. Transverse (a,d), radial (b,e), and tangential (c,f) surfaces before (a-c) and after (d-f) boiling water treatment in Qs RC.A-M in (a-c) and A'-M' in (d-f) indicate the changes in the lenticular channel of Qs RC after boiling water treatment, respectively.
reported that boiling water treatment decreased the L*, a*, and b* values of Qv cork and the ∆E* of Qv cork gradually increased with as the boiling time increased.The L*, a*, and b* values of untreated Qs RC reported in this study were lower than those of Qs cork reported by Pereira 4 .

Table 3 .
L*, a*, and b* changes of Qv VC and Qs RC before and after boiling water treatment.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate non-significant outcomes at 5% significance level for comparisons between treatments and species.The mean value in the same line followed by the same capital indicates non-significant outcomes at 5% significance level for comparisons between surfaces.

Table 4 .
∆E* of Qv VC and Qs RC after boiling water treatment.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate nonsignificant outcomes at 5% significance level for comparisons between treatments and species.The mean value in the same line followed by the same capital indicates non-significant outcomes at 5% significance level for comparisons between surfaces.

Table 5 .
Moisture content and density of Qv VC and Qs RC before and after boiling water treatment.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate non-significant outcomes at 5% significance level for comparisons between treatments and species.

Table 6 .
Weight loss and dimensional change of Qv VC and Qs RC after boiling water treatment.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate non-significant outcomes at 5% significance level for comparisons between species.The mean value in the same line followed by the same capital indicates non-significant outcomes at 5% significance level for comparisons between directions.a (0.13) Vol.:(0123456789) Scientific Reports | (2024) 14:5457 | https://doi.org/10.1038/s41598-024-56110-5

Table 9 .
Moisture adsorption on the three surfaces of Qv VC and Qs RC before and after boiling water treatment.B and U indicate boiled and untreated corks, respectively.Numbers within parentheses represent standard deviations.Numbers in the same column with the same superscript lowercase letters indicate nonsignificant outcomes at 5% significance level for comparisons between treatments and species.The mean value in the same line followed by the same capital indicates non-significant outcomes at 5% significance level for comparisons between surfaces.