Evaluation of the Process of Steaming Beech Sapwood and False Heartwood with Saturated Water Steam in Terms of Acidity Changes and Color Wood

Abstract

The paper presents changes in the color and acidity of beech wood with false heartwood in the process of pressure steaming at the temperature interval t = 105 °C and 125 °C during τ = 6 to 24 h. The light white-gray color of sapwood with a yellow tint changes to pale pink and red-brown to brown-red color during the steaming process. The color of beech wood with false heartwood changed to brown-gray color shades during 24 h of steaming with saturated water steam. From the measured data, as well as the visual evaluation of the color of the wood, I can conclude that, in the process of steaming beech wood with false heartwood, we can achieve color unification between false heartwood and sapwood in mode at temperature t = 105 °C for time τ = 18 h and in mode at temperature t = 125 °C for time τ = 12 h. Due to the influence of hemicellulose hydrolysis, the acidity of beech wood changes in the process of steaming. The decrease in acidity of beech wood in the temperature interval t = 105–125 °C and time τ = 6–24 h is in the range of values pH_sapwood = 5.2 to 3.6 and pH_{false heartwood} = 5.0 to 3.9. The relationship between the total color difference ∆E and the acidity change in beech sapwood and false heartwood is expressed by a second-degree polynomial function. The above mathematical relations represent a useful tool for evaluating the achieved color shade before further technological processing.

1. Introduction

Beech wood (Fagus sylvatica L.) is a scattered-pore, heartless wood species, which can, however, form false heartwood. In older trees, brown-red wood often appears in the middle of the trunk, which is called false heartwood [1,2]. The false heartwood of beech is a specific part of the wood that is located in the central area of the trunk but is not formed in the same way as true heartwood. Unlike true heartwood, which is rich in lignin and other substances, false heartwood is often characterized by a lower content of these compounds and has a different texture and color [3,4].

The development of false heartwood is influenced by various ecological factors and tree growth conditions, with the primary cause being the infiltration of air into the trunk through injuries on the bark or branches [5]. This exposure leads to the oxidation of carbohydrates and starch present in living or partially dead cells. Consequently, polyphenolic compounds are produced and gradually spread into the surrounding tissues, giving the wood a brown-red hue [6,7,8]. The distinction between sapwood and false heartwood impacts not only the mechanical and chemical properties of the wood but also its reaction to different treatments, such as steaming [9,10].

Wood steaming is a technological process that has been increasingly used in recent decades to modify the properties of wood, such as its mechanical, physical, and aesthetic characteristics. This process is particularly important for beech wood (Fagus sylvatica L.), which is one of the most widely used wood species in Europe. Beech wood is known and often used for its strength, flexibility, and ability to respond to various treatments. The most commonly used beech wood treatment techniques include steaming, which achieves improved physical properties and targeted color changes by using saturated water steam at higher temperatures [11,12,13].

Wood steaming is a process in which wood is exposed to the temperature of saturated water steam in closed containers, such as autoclaves. The water present in the lumens of wet wood cells is heated, forming a dilute aqueous solution that contains sugars; organic acids; and various salts such as those of calcium, magnesium, potassium, and sodium. In addition, it also contains inorganic acids that are transported to the living tree via the root system. This process not only provides nutrition to the tree but also affects the chemical composition of the wood [14,15,16]. The aim of the wood steaming process is to improve its properties, such as flexibility, color homogeneity, and reduction of internal stresses. Steaming wood with saturated water steam at high temperatures (95–130 °C) causes hydrolysis of some wood components, such as lignin and hemicelluloses, which leads to a change in the wood structure. The first chemical processes that occur during the thermal treatment of wet wood include partial hydrolysis of hemicellulose and the release of water-soluble substances to form acetic acid (CH₃COOH), formic acid (HCOOH), and others [17]. This process is influenced by temperature and exposure time, and polysaccharides gradually decompose. Under the influence of acetic acid and formic acid, saccharides and pectin are oxidized, leading to the dehydration of pentoses and the formation of 2-furaldehyde [18,19,20]. At the same time, changes also occur in the structure of lignin, and free radicals and phenolic hydroxyl groups are formed, causing chemical changes that lead to the formation of new chromophoric structures. This process is manifested by a visible change in the color of the wood, with the material turning darker. These chemical reactions are an integral part of wood steaming, affecting not only the aesthetic properties of the wood but also its physical and mechanical characteristics [18,19,21,22,23,24].

The acidity of wood, often expressed in terms of pH, is an important indicator of the chemical properties of wood. The acidity of wood depends on the number of organic acids present in its composition, which affect its processing behavior, as well as its durability and resistance to decay. The acidity of aqueous solutions in hardwood species ranges from pH 5.5 to 4.8 [21,22,23,24,25]. Steaming wood causes chemical changes that lead to a decrease in pH and thus an increase in acidity, especially in the case of sapwood and false heartwood, which contain different concentrations of acids and other compounds that can affect its further processing and use [26,27].

The color of wood is one of the most important physical and aesthetic properties, influencing its use in furniture, flooring, and other decorative applications. Wood color is determined by various chemical compounds, such as lignin, tannins, and other organic substances present within the wood. Steaming wood triggers chemical reactions that alter these compounds, leading to a change in wood color. In the case of beech wood, this transformation can be very pronounced, with light-colored sapwood typically fading and darker false heartwood either darkening or lightening, depending on the steaming process parameters, such as temperature and duration [11,28].

In the past, wood darkening during the technological steaming process was primarily used to eliminate undesirable color defects, such as stains caused by steaming, browning, or mold. However, the current research focuses on targeted color modification, particularly to unify color differences between light sapwood and dark heartwood, achieving either subtle or more pronounced color homogenization in beech wood [27,28,29].

The purpose of this study is to investigate the changes in color and acidity of beech sapwood and false heartwood during the technological process of steaming with saturated water steam at temperatures ranging from 105 °C to 125 °C over a 24-h period. The study also aims to evaluate the relationship between the total color difference (∆E*) in the color space CIE L*a*b* and the pH values of beech wood recorded during the steaming process and to demonstrate the effectiveness of steaming in achieving color uniformity between sapwood and false heartwood.

2. Materials and Methods

2.1. Material and Technology for Steaming Wood

For the research, 25 beech trunks with a length of 6.2 m with a false heartwood from the Štiavnické vrchy locality (GPS. 48.4161247N, 18.8593111E, Central Slovakia region) were selected. The center lumber with a thickness of h = 50 mm was produced by cutting along the edge, which was subsequently divided into two asymmetrical parts by longitudinal sawing. By transverse shortening of these parts, 2-m-long blanks were produced and randomly sorted into 9 groups. The blanks of the 1st group, in the number of 15 pieces, were not steamed. The other blanks were steamed in mode I: t ≈ 105 °C and pressure p = 0.12 MPa and in mode II: t = 125 °C and pressure p = 0.23 MPa. Individual groups of steamed blanks were taken from the steaming autoclave after τ = 6 h, τ = 12 h, τ = 18 h, and τ = 24 h of steaming. The technological process of wood steaming was carried out in a pressure autoclave APDZ 240 (Himmasch AD, Haskovo, Bulgaria) in the company Sundermann s.r.o. in Banská Štiavnica (Slovakia). The conditions of steaming beech wood with a false heartwood in a saturated water steam environment, including the time intervals of taking individual groups of steamed blanks, are shown in the diagram in Figure 1.

The process of steaming beech wood with a false heartwood takes place in a pressure autoclave APDZ 240 at a saturated water steam pressure that is higher than atmospheric pressure. The steam temperatures in the color treatment modes are given in

Table 1. Technological conditions for steaming wood.

Steaming Modes	tmin	tmax	t4	Time of the Technological Process of Steaming Wood
Mode I	102.5	107.5	100	τ1 = 6 h	τ2 = 12 (+0.5 a) h	τ 3 = 18 (+1.0 a) h	τ 4 = 24 (+1.5 a) h
Mode II	122.5	127.5	100

Note: ^a After the specified time periods of wood treatment, exactly according to the specified diagram (course), 0.5 h must be added to the basic steaming time of beech wood. This time between individual treatment periods serves to reduce the pressure for safe opening of the autoclave and manipulation for selecting one group of treated blanks from the autoclave and closing and pressurizing the autoclave with a saturated water steam.

. The values t_max and t_min represent the temperature range in which steam is supplied to the autoclave to ensure the technological process. The temperature t₄ indicates the temperature of saturated water steam in the autoclave after reducing the pressure to the atmospheric pressure level, which allows for safe opening of the device and taking samples after 6, 12, 18, and 24 h of wood steaming.

2.2. Determination of the Wood Moisture Content and Technological Process of Wood Drying

The moisture content of beech wood with false heartwood was determined before entering the technological steaming process by random selection at 60 sapwood points and at 60 false heartwood points.

The moisture content of steamed wood was also determined in individual modes and times, always by random selection at 30 steamed sapwood points and at 30 steamed false heartwood points after cooling to ambient temperature.

A portable moisture meter of the FMD6 type (Brookhuis Micro-Electronic, Enschede, the Netherlands) with wood penetration probes was used to measure moisture.

The moisture content of all the already-dried blanks was also monitored, where test specimens were manipulated from individual groups of steamed sapwood and false heartwood to determine the wood moisture content by the gravimetric method after the drying process according to the standard [30].

The drying technology of untreated and treated beech blanks was implemented by low-temperature drying with an emphasis on preserving the acquired color in individual modes and times of wood steaming. Drying took place in a conventional hot-air dryer: KAD 1x6 (KATRES s.r.o., Jihlava, Czech Republic), according to the two-stage drying mode:

• At drying medium temperatures of t_s = 35–40 °C and relative humidity of φ = 70%–60%, free water evaporates from wet wood without causing chemical alterations in the lignin–saccharide complex of beech wood. As a result, no decomposition of chromophoric structures or formation of new functional groups occurs, and the color of the wood remains unchanged.
• The evaporation of bound water from beech wood, which occurs below the fiber saturation point, takes place at higher temperatures of t_s = 70–80 °C [31].

The evaporation of bound water from beech wood, which occurs below the fiber saturation point, takes place at higher temperatures of t_s = 70–80 °C [31]. Subsequently, the surface of all dried blanks was processed on a JET JPT-410HH milling machine (STROJE Slovensko, Banská Bystrica, Slovakia).

2.3. Color Measurement of Native and Steamed Wood

The color measurement of steamed and unsteamed beech wood with false heartwood in the color space CIE L*a*b* was conducted using a Color Reader CR-10 colorimeter (Konica Minolta, Tokyo, Japan). Measurements were taken on the bearing and side surfaces at a distance of 300 mm from the front of the blanks. A D65 light source with an 8-mm illuminated area was used. The color assessment was based on variations in the CIE Lab* color space, considering the lightness coordinate (L*), as well as the color coordinates for red (a*) and yellow (b*) [32].

The total color difference was calculated using the following equation:

$$\Delta {\mathrm{E}}^{*}=\sqrt{{\left({\mathrm{L}}_2^{*}-{\mathrm{L}}_1^{*}\right)}^2+{\left({\mathrm{a}}_2^{*}-{\mathrm{a}}_1^{*}\right)}^2+{\left({\mathrm{b}}_2^{*}-{\mathrm{b}}_1^{*}\right)}^2}$$

(1)

where ${\mathrm{L}}_1^{*}$, ${\mathrm{a}}_1^{*}$, and ${\mathrm{b}}_1^{*}$ are the values in the color space coordinates of the surface of dried milled native wood, and ${\mathrm{L}}_2^{*}$, ${\mathrm{a}}_2^{*}$, and ${\mathrm{b}}_2^{*}$ are the values in the color space coordinates of the surface of dried milled steamed wood.

2.4. Measuring the Acidity of Native and Steamed Beech Wood

Measurement of the acidity of dried native, steamed beech sapwood and false heartwood was performed on a plane surface using a pH meter pH7110 with a SenTix Sur surface electrode from the company XYLEM (Weilheim, Germany).

Measuring the acidity of beech wood consists of applying the surface electrode of the SenTix Sur pH meter type pH7110 to the surface of the wood. The acidity value was read after the pH value stabilized on the pH meter display. At the point of measuring the pH of dry wood with a SenTix Sur surface electrode, first, one drop of distilled water was dripped onto the surface of the wood using a dropper, and then, the surface contact electrode was pressed against the surface of the wood in the place of the dropped drop, thereby creating contact between the electrode and the surface of the wood. The pH value was read after approx. 180 to 240 s of stabilization of the electrode on the pH meter pH7110.

2.5. Statistical Processing of the Measured Data

From the measured data of wood colors in individual zones and acidity in the steaming process, mathematical and graphical dependencies were determined using the Statistica 12 program (V12.0 SP2, USA). Using statistical evaluation methods such as t-test and analysis of variance (ANOVA), it was assessed whether there was a relationship between individual groups of samples, and the significance level was determined using p-values.

3. Results and Discussion

3.1. Moisture Content of Native and Steamed Beech Wood

The moisture content values of beech sapwood and false heartwood measured before the steaming process, after the steaming process in a pressure autoclave, and after drying in a hot air dryer are shown in

Table 2. Average moisture values of wood before the steaming process, after the steaming process, and after drying (mean ± SD).

Beech Wood with False Heartwood	Number of Measurements (-)	Before Steaming (%)	After Steaming (%)	Decrease in Wood Moisture Content After Steaming (%)	After Drying (%)	Decrease in Wood Moisture Content After Drying (%)
Native sapwood beech	60	64.9 ± 4.3	--	--	11.9 ± 0.4	53.0
Steamed sapwood beech (Mode I.)	30	--	57.3 ± 1.1 *	7.6	12.3 ± 0.3 *	45.0
Steamed sapwood beech (Mode II.)	30	--	54.5 ± 0.9 *	10.4	12.0 ± 0.1 *	42.5
Native false heartwood beech	60	56.7 ± 3.9	--	--	12.1 ± 0.4	44.6
Steamed false heartwood beech (Mode I.)	30	--	52.1 ± 0.7 *	4.6	12.0 ± 0.3 *	40.1
Steamed false heartwood beech (Mode II.)	30	--	49.5 ± 0.9 *	7.2	12.1 ± 0.2 *	37.4

Note: * The stated average wood moisture value is calculated from all measured data from the taken blanks at individual time intervals during the wood steaming process in a pressure autoclave in a given mode.

.

Based on the measured moisture values of beech sapwood and false heartwood, it can be confirmed that the wood was sufficiently wet and met the requirement that the wood must contain a moisture content higher than w ≥ 30% before entering the pressure steaming process in order to create conditions for the chemical reactions to take place and achieve the desired effect of wood treatment in changing and unifying its color.

Table 2 also shows that false heartwood has a lower moisture content of Δw = 8.2% than beech sapwood for several reasons, which are associated with the different physical and biological properties of these two types of wood:

• Different cell structure: False heartwood, which is the older wood in the middle of the trunk, has a denser and less permeable cell structure. This means that water has limited access to these cells, which reduces the ability of the wood to absorb moisture.
• Wood maturity: Sapwood is younger and still shows high activity in water transport, because it contains more living cells. In contrast, the false heartwood is older and often less alive, which also reduces its ability to absorb water.
• Fiber properties: The false heartwood often has a different concentration of lignin, which gives the wood greater strength and a lower ability to absorb moisture compared to sapwood. Sapwood has a higher content of holocellulose and looser cells, which can retain more water.
• Physiological factors: The false heartwood is made up of wood that is no longer directly involved in the transport of nutrients and water in the tree; therefore, its ability to absorb or retain water is lower than that of the sapwood, which participates in these processes [33].

The moisture values of beech sapwood and false heartwood after cooling to ambient temperature were lower than before steaming. The moisture reduction in the range ∆w = 12.7%–16.0% is caused by the evaporation of water from the wood into the saturated water steam environment in the autoclave during cooling to a temperature of t = 100 °C before sampling and the subsequent evaporation of water into the atmosphere during cooling of the wood to the ambient temperature. The heat, which is the source of energy for the evaporation of water, is accumulated during heating of the wood to the required technological temperature [29].

3.2. Native and Steamed Beech Wood Color

The color of dry native sapwood beech is white-gray with a yellow tinge in the color space CIE L*a*b* identified by the values in the coordinates: L* = 80.5 ± 2.6, a* = 8.4 ± 1.5, and b* = 19.5 ± 1.7. The color of the false heartwood is brown-red, which is described in the color space coordinates by the values L* = 63.8 ± 3.4, a* = 11.5 ± 1.9, and b* = 19.9 ± 1.5. These values coincide with those reported by the authors of [28,34]. During the process of steaming beech wood, changes occur in the physicochemical properties of the wood, and the intensity of the coloration of steamed beech wood depends on the temperature and duration of the process. Details of the color changes of beech wood during steaming on the lightness coordinate L*, red color a*, and yellow color b* are given in

Table 3. Average values of the color of beech sapwood in the color space CIE L*a*b* (mean ± SD).

Beech Sapwood	Number of Measurements (-)	Steaming Time (h)	Values in the Color Space CIE L*a*b*
			L*	a*	b*
Unsteamed	30	0	80.5 ± 2.6	8.4 ± 1.5	19.5 ± 1.7
Steamed Mode I. tI = 105 ± 2.5 °C	30	6	69.7 ± 1.9	11.3 ± 1.3	19.6 ± 1.5
	30	12	63.5 ± 1.4	12.2 ± 1.5	19.6 ± 1.2
	30	18	61.4 ± 1.2	12.4 ± 1.2	19.5 ± 1.4
	30	24	60.0 ± 1.1	12.6 ± 1.1	19.4 ± 1.4
Steamed Mode II. tII = 125 ± 2.5 °C	30	6	61.5 ± 2.0	11.6 ± 1.4	19.4 ± 1.5
	30	12	51.5 ± 1.4	12.6 ± 1.1	19.1 ± 1.3
	30	18	49.0 ± 1.8	12.5 ± 1.3	18.9 ± 1.2
	30	24	47.5 ± 1.2	12.7 ± 1.1	18.7 ± 1.1

for sapwood beech and

Table 4. Average values of the color of beech false heartwood in the color space CIE L*a*b* (mean ± SD).

Beech False Heartwood	Number of Measurements (-)	Steaming Time (h)	Values in the Color Space CIE L*a*b*
			L*	a*	b*
Unsteamed	30	0	63.8 ± 3.4	11.5 ± 1.9	19.9 ± 1.5
Steamed Mode I. tI = 105 ± 2.5 °C	30	6	65.5 ± 2.2	12.4 ± 1.5	19.8 ± 1.2
	30	12	61.5 ± 2.0	12.4 ± 1.2	19.8 ± 1.3
	30	18	61.3 ± 1.7	12.3 ± 1.5	19.6 ± 1.1
	30	24	60.9 ± 1.7	12.3 ± 1.3	19.6 ± 1.3
Steamed Mode II. tII = 125 ± 2.5 °C	30	6	57.7 ± 2.1	12.3 ± 1.5	19.7 ± 1.4
	30	12	51.6 ± 2.0	12.4 ± 1.3	19.3 ± 1.3
	30	18	50.0 ± 1.8	12.4 ± 1.1	19.2 ± 1.3
	30	24	49.1 ± 1.5	12.5 ± 1.3	18.9 ± 1.2

for false heartwood beech.

Based on the measured data of beech wood with false heartwood in the color space CIE L*a*b* coordinates, it is clear that the most significant changes occur on the lightness coordinate L*. During the steaming process in individual modes, the lightness of sapwood beech wood decreased from the value L₀* = 80.5 at the steaming temperature t_I ≈ 105 °C to the value L_I-24* ≈ 60.0; at the temperature t_II ≈ 105 °C, it decreased to L_II-24* = 47.5. The darkening of wood is caused by chemical changes, namely hydrolysis, partial degradation of polysaccharides, and an increase in the lignin content in the wood after steaming, which is confirmed by studies [18,34,35,36].

The lightness values L* of the false heartwood during the steaming process show the opposite trend at a saturated water steam temperature t_I ≈ 105 °C. After 6 h of steaming, the wood lightened from the value L₀* = 63.8 to the value L_I-6* = 65.5 and then gradually darkened to the value L_I-24* = 60.9. At a saturated water steam temperature t_II ≈ 125 °C, the lightness decreased to the value L_II-24* = 49.1, which means that the false heartwood darkened. The lightening of the heartwood during the first 6 h of steaming at a temperature of t_I ≈ 105 °C and the subsequent gradual darkening indicate the instability of the chromophore system of the wood, which is formed by the enzymatic processes of peroxidase and polyphenol oxidase, which are responsible for the oxidation of phenolic compounds, thus characterizing the coloration of heartwood [1,2,37,38,39].

Changes in the red color coordinate a* during 24 h of steaming indicate certain differences between sapwood and false heartwood. Sapwood during steaming in both modes recorded an increase in the a* value in the first 12 h from a₀* = 8.4 to a₁₂* ≈ 12.4, and the red color value increased on average by ∆a* ≈ 4.0, which was manifested by the visual reddening of the wood, but subsequently, the value did not change significantly in the steaming process. On the other hand, in the red color coordinate a* of the false heartwood, the value increased from a₀* = 11.5 to a₆* ≈ 12.4 during the first 6 h of steaming, while, similarly to sapwood, after this period, the value did not change further but only oscillated.

The values of the yellow color coordinate b* during the steaming of the wood did not undergo any significant changes. In sapwood, the values of the yellow color coordinate ranged around b* ≈ 19.5 and, in false heartwood, around b* ≈ 19.9. This finding is in accordance with the research of [1]. In both steaming modes, a small decrease in values was recorded, but only at the end of steaming after 24 h, of ∆b* ≈ 1.0 in both types of wood, i.e., in steamed sapwood and false heartwood.

Based on the measured values of the color of sapwood beech wood and false heartwood during the process of steaming wood ranging from t = 105 °C to 125 °C and time τ = 6–24 h, the total color difference ΔE* was calculated according to the mathematical Equation (1), and the dependence of the change in the total color difference on temperature and time was processed in the form of a 3D diagram (Figure 2).

The dependence of the total color difference ΔE* of beech sapwood and false heartwood on the temperature and steaming time is also described using the mathematical equations:

ΔE*_sapwood = 76.9276 − 1.6585·t + 0.7568·τ + 0.0089·t² + 0.011·t·τ − 0.0463·τ²

(2)

ΔE*_{false heartwood} = 1.0735 − 0.1843·t − 1.4859·τ + 0.0015·t² + 0.02·t·τ − 0.0185·τ²

(3)

where t—temperature of the steam °C, and τ—steaming time in hours.

From the multiple regression analysis of the measured data, it follows that the p-value indicates the results are statistically significant (p < 0.05), meaning the model is not random. The coefficient of determination R² = 0.945 for sapwood and R² = 0.941 for false heartwood represents the strength of the relationship between the dependent variable and the independent variable.

From the variations in the lightness coordinate (L*) and the chromatic coordinates for red (a*) and yellow (b*) presented in Table 3 and Table 4, it is evident that the unified color of steamed sapwood and false heartwood differs both in terms of steaming duration and the degree of color unification achieved. As the steaming temperature increases, the time required for color homogenization decreases. Simultaneously, the resulting darkness of the brown or brown-gray shade of steamed beech wood intensifies, even with a reduced steaming duration.

From both the measured data and the visual evaluation of the color of the wood according to Figure 3, it can be concluded that, in the process of steaming beech wood, we can achieve color unification between false heartwood and sapwood in mode I at a saturated water steam temperature t = 105 °C for time τ = 18 h and in mode II at a saturated water seam temperature of t = 125 °C for time τ = 12 h.

The achieved color unification of beech wood has a high practical use in furniture production, since native false heartwood, due to its high contrast to sapwood, is usually considered a wood defect, which places it in a lower category of usability.

3.3. Acidity of Native and Steamed Beech Wood

Due to the temperature of saturated water steam, which acts on beech wood with false heartwood during the steaming process in a pressure autoclave, chemical changes occur through the process of hydrolysis of hemicelluloses, which subsequently manifests itself in a change in the acidity of the wood (acidity of the wood decreases). The measured acidity (pH) values of beech wood before the steaming process, as well as during the steaming process in individual modes, are shown in

Table 5. Average pH values of beech sapwood and false heartwood during the steaming process (mean ± SD).

Steaming Mode	Number of Measurements (-)	Steaming Time Beech Wood (h)
		0	6	12	18	24
Steamed sapwood beech (Mode I.)	30	5.5 ± 0.1	5.2 ± 0.1	5.0 ± 0.2	4.7 ± 0.3	4.5 ± 0.1
Steamed sapwood beech (Mode II.)	30		4.6 ± 0.2	4.3 ± 0.1	3.7 ± 0.2	3.6 ± 0.1
Steamed false heartwood beech (Mode I.)	30	5.2 ± 0.2	5.0 ± 0.2	4.9 ± 0.3	4.8 ± 0.1	4.7 ± 0.1
Steamed false heartwood beech (Mode II.)	30		4.8 ± 0.1	4.4 ± 0.2	4.0 ± 0.2	3.9 ± 0.1

Note: Based on the statistical evaluation of the measured acidity data, it was found that all tested differences between the acidity of sapwood and false heartwood at all steaming times were statistically significant (p < 0.05).

.

Due to the influence of chemical reactions taking place in beech wood with false heartwood during the steaming process, the color of the wood changes, and its uniformity occurs, while the pH value of the wood decreases. The measured acidity values of beech wood with false heartwood show that the factor of saturated water steam temperature is more significant than the factor of time for the change in the pH decrease of the treated wood. With the increasing temperature of saturated water steam, the change in the pH of the treated wood is more significant, and the wood acquires acidity and a more acidic odor. Changes in the pH, whether in sapwood beech wood or false heartwood, represent a uniform decrease throughout the entire steaming process. Changes in the acidity of steamed beech wood affect its further processing, such as surface treatment or the type of glue used for joining such wood, etc. [17,20,23].

Based on the measured pH values of beech wood during the wood steaming process at saturated water steam temperatures ranging from t = 105 °C to 125 °C and times of τ = 6–24 h, the dependence of the change in acidity on temperature and time was processed in the form of a 3D diagram (Figure 4).

The dependence of the acidity (pH) of beech sapwood and false heartwood on the temperature and steaming time is also described using the mathematical equations:

pH_sapwood = 5.5 + 0.035·t − 0.0292·τ − 0.0003·t² − 0.0005·t·τ + 0.0014·τ²

(4)

pH_{false heartwood} = 5.2 + 0.0162·t + 0.658·τ − 0.0001·t² − 0.0011·t·τ + 0.001·τ²

(5)

where t—temperature of the steam °C, and τ—steaming time in hours.

The multiple regression analysis of the measured acidity data shows that the results are statistically significant (p < 0.05), indicating that the model is not random. The coefficient of determination R² = 0.969 for sapwood acidity and R² = 0.955 for false heartwood acidity reflect the strength of the relationship between the dependent variables and the independent variable.

From mathematical analyses of the magnitude of changes in the color differences ∆E* and pH values of beech wood during the steaming process with saturated water steam, the dependence ΔE*= f(pH) in Figure 5 was derived for beech sapwood and false heartwood. This dependence allows to identify the degree of beech wood discoloration through the acidity of steamed sapwood and steamed false heartwood of beech wood and can be described by the above mathematical relations.

The graph (left) shows the relationship between the pH of beech sapwood and the total color difference ΔE*. The results show that, as the pH of the wood increases, the total color difference decreases. This means that lower pH values, i.e., higher acidity of the wood, cause a more pronounced color change [1,11,18,40].

The data were fitted with a quadratic regression model with the equation ΔE* = 0.2811.pH² − 14.716.pH + 81.964. The high value of the coefficient of determination (R² = 0.9557) indicates that this model describes the relationship between pH and the total color difference very well, with up to 95.57% of the variability in color change being explained by pH values.

The practical significance of this relationship lies in the fact that more acidic wood (with lower pH) undergoes more pronounced color changes, while wood with higher pH retains its color more stably. This knowledge can be important in technological wood processing processes such as steaming, as pH control can affect the final appearance of the material.

The other graph (right) shows the relationship between the pH of beech false heartwood and the total color difference ΔE*. The results show that, with the increasing pH, there is a significant decrease in the color change of the wood. Lower pH values (higher acidity) are associated with greater color change, while higher pH signals a more stable color.

The data are fitted with a quadratic regression model with the equation ΔE = −0.6512.pH² − 6.8872.pH + 51.801. The coefficient of determination R² = 0.954 indicates a very good agreement of the model with the real data, with 95.4% of the variability in color changes being explained by pH values.

In practice, this means that false heartwood with a lower pH undergoes greater color changes, which is an important factor in technological processes such as wood steaming. Regulatory mechanisms for adjusting the pH can help stabilize the color of wood, thereby optimizing its visual quality for further use.

The determined dependences of color change expressed by the total color difference ∆E* on the acidity of beech sapwood and false heartwood are a suitable tool for evaluating the achieved color shade before its further technological processing. Based on the determined mathematical equations, it is possible to control the technological process of steaming beech wood with the aim of achieving the desired color shade or homogenizing the color of beech wood.

4. Conclusions

The results of the analysis of the influence of saturated water steam temperature during the 24-h steaming process of beech sapwood and false heartwood at a temperature of t = 105–125 °C indicate a certain change in the color of the wood and its unification, as well as a change in the acidity of the wood.

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The most significant color change occurred in the lightness coordinate (L*). In sapwood, the lightness decreased from L₀ = 80.5* at 105 °C to L_II-24 = 47.5 at 125 °C. Conversely, false heartwood initially lightened at 105 °C (from L₀ = 63.8* to L_I-6 = 65.5*) but then darkened to L_II-24 = 49.1 at 125 °C.

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Other color parameters changed minimally. The red color coordinate (a*) increased by Δa ≈ 4.0 in sapwood within the first 12 h, leading to visible reddening, before stabilizing. In false heartwood, a* rose slightly (a₀ = 11.5 to a₆ ≈ 12.4) in the first 6 h but then fluctuated. The yellow coordinate (b*) remained stable (b ≈ 19.5 for sapwood, and b ≈ 19.9 for false heartwood).

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Visual analysis (Figure 3) confirms that color unification between sapwood and false heartwood is achievable. Uniformity occurs at 105 °C after 18 h and at 125 °C after 12 h.

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Steaming induces chemical reactions that lower the wood’s pH, making it more acidic and altering its odor. Higher temperatures accelerate this effect. Changes in the pH occur consistently throughout the process.

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Mathematical analysis yielded predictive equations for the total color difference (∆E*) based on pH:

• Sapwood: ∆E* = 0.2811pH² − 14.716pH + 81.964;
• False heartwood: ∆E* = −0.6512pH² − 6.8872pH + 51.801.

These equations enable control over the steaming process to achieve specific color shades or uniformity in beech wood.