Valorization of Wood-Based Waste from Grapevine

Abstract

This article deals with the possibility of valorizing wood waste from grapevine cultivation of the varieties Pesecká leánka (white graft) and Frankovka modrá (red graft), grown in Slovakia. From the point of view of chemical composition, two methods (water and ethyl alcohol) were performed for the determination of extractives, acid-insoluble (Klason) lignin, and structural carbohydrates, and FTIR spectra of the grape samples were recorded. Mechanical strength, compression test parallel to the grain, and morphological properties (fiber length, width, and shape factor using a fiber tester) were carried out. The energy potential of grapevines was evaluated by determining the calorific value. According to the results, the relatively high content of carbohydrates (54.19%–55.27%) provides a prerequisite for acid or enzymatic hydrolysis to produce monosaccharides or second-generation bioethanol. FTIR spectra confirmed the higher content of lignin and cellulose in red grapes. The compression strength of grape cuttings (37.34 MPa—red; 32.34 MPa—white) was comparable to the strength of softwood species; hence, these wastes can be used for particleboard or fiberboard production. Average fiber length is comparable to non-wood species; thus, grape cutting can be used for pulp and paper production. The calorific value of grape cuttings ranged from 18.68 MJ·kg⁻¹ (white) to 18.91 MJ·kg⁻¹ (red), with pellets having 16.96 MJ·kg⁻¹. The energetic potential of grape cuttings was comparable to that of other wooden materials; on the other hand, the ash content of pellets from grape cuttings (10.54%) greatly exceeded the limit given by the EN ISO 17225-1 standard, which is a significant disadvantage to pellets used for heating.

1. Introduction

A significant amount of waste is generated in agriculture, which is both an economic and an environmental problem. The biomass resources considered are forest residues, agricultural residues, wood from surplus forest wood, and biomass from energy crops [1]. One of the remains from agriculture is represented by grapevine wastes. Grapevines require annual intensive pruning to obtain adequate production quality and volume [2,3,4]. According to the study of Çetin et al. [5], cane pruning weight varies from 0.56 kg/vine to 2.01 kg/vine depending on the season. Worldwide grape production represents 77.8 million tons per year [6]. According to Guardia et al. [7], 25 kg of waste is produced for every 100 kg of grapes. Winemaking in Slovakia has a long tradition of almost 2500 years. Currently, there are six winegrowing regions, 40 districts, and 603 wine-growing municipalities. The most important cultivated varieties are Veltlínske zelené, Rizling vlašský, Müller-Thurgau, and Frankovka modrá [8]. In Slovakia, the total area of vineyards is approximately 22,000 hectares, and waste in the form of grape canes represents a content of approximately 18 tons per year [9]. Globally, the content of this waste is around 14.8–29.6 million tons per year [10].

Wastes from grapes (grape shoots, grape pomace, and seeds) have several potential applications. They are rich sources of bioactive compounds with beneficial effects on human health [11,12]. The content depends on several factors such as grapevine variety, age, and growth conditions [13,14]. They are a source of antioxidants [15], substances that support the immune system, have an antitumoral effect, or serve to prevent cardiovascular and nervous diseases. These compounds can be used in the pharmaceutical, cosmetic, or food industry [16,17,18,19]. Following Cruz-Lopes et al. [20], grape stalks are a massive byproduct of winemaking and could be an interesting source of solid biomass for energy needs. Grape waste in the form of wine lees is produced during fermentation [21,22]. Wine lees are usually distilled to recover ethanol [23]. Grapevine waste can be also used as fertilizers for soils [24] or hybrid particleboard production due to its high lignin content, high density, and compaction with less compressive strain [25]. Santos et al. [10] used grape cane particles, combined with a melamine formaldehyde urea binder, as raw material for particleboard production. Wong et al. [26] prepared hybrid particleboards based on 10% grapevine and 90% pine, exhibiting mechanical properties that showed great promise. Eugenio et al. [27] described the possibility of grape canes for the papermaking process. The properties of the papers depend on a variety of characteristics, fiber dimensions, and the chemical composition of this alternative raw material. One of the ways of using grape waste, especially pomace, is to produce ethanol with stalk sugars, but this is unattractive in terms of costs and benefits [21]. Egüés et al. [28] obtained fermentable sugars from grape stalks to produce bioethanol.

Wood-based grape waste is lignocellulosic material. Lignocelluloses contain organic polysaccharides, such as cellulose, hemicelluloses, aromatic lignin, and inorganic minor compounds (ash). Stalks are essentially composed of cellulose (approximately 30%), hemicelluloses (approximately 20%), lignin (approximately 18%), tannins (approximately 16%), and proteins (approximately 6%) [20]. Grapevine canes are rich in lignin, cellulose, nitrogen, and potassium; therefore, they are highly burned or composted in the field [7]. Grape canes are composed of a high content of proteins, polyphenols, stilbenes [15,29,30,31], minerals, and carbohydrates, and they are a source of natural antioxidants and food supplements [7]. They also have anti-inflammatory and antimicrobial properties [32,33].

Agro-industrial wastes contain non-negligible amounts of bioactive compounds, which are believed to be protective agents against certain diseases such as cardiovascular diseases, cancer, and diabetes [34]. There are still some challenges in waste management and valorization with regard to economic, safety, sensory, consumer acceptance, and regulatory aspects, and more research is required to overcome these challenges and improve the quality and quantity of value-added products obtained from agricultural wastes and byproducts [35]. Polyphenols possess high utilization value in many fields such as human health, energy, and environmental protection. Emerging green extraction methods such as supercritical fluid extraction, ultrasonic extraction, microwave extraction, and other methods can shorten extraction time and improve solvent extraction efficacy, resulting in the green and safe recovery of polyphenols from grape residue and other food processing waste [36].

This article aimed to analyze two types of grape cane wastes from the pruning of white and red canes. Value-added compounds (extractives, lignin, and saccharides) and the mechanical strength of grape cuttings were determined, morphological characteristics of fibers due to the potential of their use for paper production were evaluated, and the energetic potential of grape cutting and grape pellets (mixture of white and red grape canes) was determined by determining their calorific value. The benefit of this contribution is to highlight the potential of wood-based wastes from grapes to improve both environmental and economic sustainability.

2. Materials and Methods

2.1. Materials

For analysis, we used wood-based waste from grapevine and grape cuttings (canes):

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Type of grape: white graft—Pesecká léánka (Feteasca regala), red graft—Frankovka modrá (Vitis vinifera ‘Blaufränkisch’), native, graft;

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Pruning time: the second half of February 2022;

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Location: Nitra—Dolné Krškany, Nitra wine region, Slovakia;

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WGS84 (φ, λ): 48.256958°, 18.082436°

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Soil: brown earth soils with sandy-clay soils;

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Direction of the vineyard: southwest, sloping terrain (south, southwest).

Pellet preparation for the calorimetric analysis. Both red and white grape cuttings (fraction from 3 to 5 mm, moisture of 15%) were dosed into a vertical pelletizing device, Pellet Press 120 mm matrix (AELA, Bratislava, Slovakia), where the input material was spread over the matrix with the support of pressure trains. The matrix was preheated to a temperature of 100 °C.

2.2. Methods

2.2.1. Chemical Analyses

The grape cane samples were disintegrated into sawdust, and fractions of 0.5 mm to 1.0 mm (Figure 1) in size were extracted by successive extraction with water and ethyl alcohol (Figure 1) as proposed by Atatoprak et al. [21]. Both extractions were performed in a Soxhlet apparatus. Acid-insoluble (Klason) lignin and structural carbohydrates were determined according to Sluiter et al. [37]. All analyzes were performed in four replicates.

2.2.2. Fourier-Transform Infrared (ATR-FTIR) Analysis

The FTIR spectra of the grape samples were recorded on a Nicolet iS10 FT-IR spectrometer, equipped with Smart iTR using an attenuated total reflectance (ATR) sampling accessory attached to a diamond crystal (Thermo Fisher Scientific, Madison WI, USA). The spectra were acquired by accumulating 32 scans at a spectral resolution of 4 cm⁻¹ in an absorbance mode from 4000 to 650 cm⁻¹ using OMNIC 9.0 software (Thermo Fisher Scientific, Madison WI, USA). For the evaluation of differences between examined samples, the FTIR spectra were normalized on the most intense band of the C–O, C–C, and C–C–O stretching vibrations of lignin, cellulose, and hemicelluloses at 1025 cm⁻¹ [10,38]. Measurements were made in four replicates per sample.

2.2.3. Mechanical Strength

Samples for compression test parallel to the grain were prepared from canes. The internode parts between the nodes were used. The length of the sample was 10 mm. The cross-section of samples was treated as an ellipse on the maximum and minimum diameter of the cross-section. The samples were conditioned at a temperature of 20 °C and a relative humidity of 65%. The moisture content of the samples was determined using a gravimetric method. The conditioned samples were loaded in compression parallel to the grain (testing machine FPZ 100, Dresden, Germany). Compression strength was calculated from the maximum force F_max at failure according to the following equation:

$$f_c=\frac{F_{max}}{\pi r_{min}{r}_{max}}.$$

(1)

Differences in strength between red and white wine cutting were tested by one-factor analysis of variance (ANOVA). Before ANOVA, the Kolmogorov–Smirnov test was completed. The material inhomogeneity was tested by regression analysis between strength and cross-section.

2.2.4. Fiber Tester Analysis

First, 200 mL mixtures of concentrated CH₃COOH and 30% H₂O₂ (1:1, v/v) were poured into the samples (weight = 10 g and dimensions = 20 mm × 2 mm × 2 mm). Then, the samples were refluxed for 3 h, suction-filtered through a sintered glass filter (S1), and washed with distilled water. An L&W Fiber Tester (Lorentzen and Wettre, Kista, Sweden) was used to determine the fiber dimensional characteristics. This measurement is based on the principle of two-dimensional imaging technology. Measurement technology is automated, allowing for frequent and rapid analysis of fiber quality. The instrument measures various fiber properties, such as the length and width of the fibers and fine portion (from 0.1 mm to 0.2 mm). Measurements were made in a single replicate per sample, and the number of fibers within each replication population was 20,007 cells for the red grape sample and 20,024 cells for the white sample.

2.2.5. Calorimetry

Two types of samples (cuttings and pellets), with a weight of circa 0.8 g, were analyzed using the calorimeter C 200 (IKA^®-Werke GmbH & Co. KGIKA, Staufen, Germany) and evaluated by Cal Win software (IKA^®-Werke GmbH & Co. KG, Staufen, Germany according to the standard ISO 1928 [39]. The percentage of ash content was calculated as the difference between the weight of the original sample before incineration and the residue after incineration in the calorimeter. Measurements were made in ten replicates per sample.

3. Results and Discussion

3.1. Chemical Composition of the Grapevine Cuttings

Two methods were performed for the determination of extractives in red and white grape samples; however, only a small difference between the two types of grapes was found. Atatoprak et al. [21] reported much larger amounts of water extractives (75.08% for the white grape stalk and 56.29% for red grape stalk, respectively); on the other hand, our results were close to the results of Pujol et al. [40], who found water extractives ranging from 18.38% (particle size 0.250–0.425 mm) to 21.61% (particle size 1.0–1.6 mm).

It is worth noting that the chemical composition of grape samples depends on various factors, e.g., grape types, growing location, parts of the grape, and used analytical methods. Spigno et al. [41] used various analytical methods for the characterization of six grape cultivars and found 19.9%–36.5% water extractives, 6.5%–9.0% ash, 16.6%–18.8% cellulose, 2.6%–5.7% hemicelluloses, and 16.6%–24.2% lignin. Their result showed that the grape cultivars significantly influenced all the parameters investigated. This was not in agreement with González-Centeno et al. [42], who found no significant differences between stalks obtained from different red and white grape cultivars. Following González-Centeno et al. [42], we also found no significant differences in the chemical composition between white and red grape samples (

Table 1. Extractive, lignin, and saccharide content in grape cuttings (% odw; mean ± standard deviation).

Sample	White	Red
EX-W	16.65 ± 0.11	16.18 ± 0.25
EX-E	1.63 ± 0.01	1.39 ± 0.01
EX-SUM	18.28 ± 0.13	17.57 ± 0.26
LIG	20.15 ± 0.77	20.57 ± 0.02
GLC	32.81 ± 0.86	34.26 ± 0.15
XYL	13.24 ± 0.40	11.52 ±0.97
GAL	3.04 ± 0.20	2.89 ± 0.06
ARA	3.06 ± 0.13	2.73 ± 0.08
MAN	3.13 ± 0.15	2.80 ± 0.35
SACCHARIDES	55.27 ± 1.44	54.19 ± 0.74
ASH	0.94 ± 0.13	1.54 ± 0.16

EX-W—extractives in water; EX-E—extractives in ethanol; EX-SUM = EX-W + EX-E; LIG—lignin; GLC—glucose; XYL—xylose; GAL—galactose; ARA—arabinose; MAN—mannose.

). Results in Table 1 are presented as the percentages of oven-dry weight per weight of unextracted wood (odw).

The vine shoots of the Muscat of Alexandria variety obtained from pruning of grapevine (Vitis vinifera) contained 31.9% cellulose, 23.2% hemicelluloses, and 55.1% total saccharides [43]. The authors recommended the use of liquefied vine shoots to produce flexible films with suitable thermal and mechanical properties, which could be an interesting use for the samples examined in this work. The relatively high content of carbohydrates (54.19%–55.27%) also provides a prerequisite for their acid or enzymatic hydrolysis to produce monosaccharides or second-generation bioethanol, as suggested by Atatoprak et al. [21].

3.2. ATR-FTIR Analysis of Grapevine Cuttings

No differences between both spectra were observed at 3334 cm⁻¹ (stretching vibrations of the hydroxyl groups), 2918 cm⁻¹ (–CH₂ vibrations), and 2850 cm⁻¹ (CH₂) (Figure 2).

The red grape sample showed higher absorbance intensities at 1606 cm⁻¹, 1511 cm⁻¹, and 1421 cm⁻¹ assigned to aromatic skeletal vibration of lignin [44,45,46]. Similarly, the skeletal vibrations of syringyl ring breathing with C–O stretching (1320 cm⁻¹) and guaiacyl ring breathing with C–O stretching (1265 cm⁻¹) units exhibited higher absorbances in red grape in comparison to the white sample. The peak at 1236 cm⁻¹ (aromatic ring breathing with C–O and C–O stretching) was also more intense in the red grape sample (Figure 3).

The band at about 1421 cm⁻¹, belonging to crystalline cellulose, can be ascribed to the symmetric scissoring vibration of CH₂ at C6; the band at about 1371 cm⁻¹ corresponds to C–H bending of amorphous cellulose [47]. Both bands exhibited higher absorbance in the red grape sample compared to the white grape sample, which is consistent with the higher glucose content in this sample (Table 1).

3.3. Mechanical Properties

The results of the mechanical tests are listed in

Table 2. Results of compression test of grape cuttings.

	N	Mean (MPa)	Min (MPa)	Max (MPa)	Var. (%)
Red	39	37.34	27.34	42.97	10.01
White	37	32.34	22.77	38.93	10.80
All	76	34.90			12.57

. An average compression strength of 37.34 MPa was determined in red cuttings. For comparison, Rodríguez-González et al. [48] reported a compression strength of 11.3 MPa in the Tempranillo wine trunk and a bending strength of 75.3 MPa in the Tempranillo wine branches. The compression strength of red cuttings was 15% higher than the strength of white cuttings. According to the Kolmogorov–Smirnov test, the strength values were normally distributed (p > 0.20 for white cuttings and red cuttings, respectively). The coefficient of variation of individual groups was approximately 10%. The variance showed significant differences (F(1.74) = 36.106, p = 0.000) between the compression strength of the white and red grape cutting (Table 2). Strength is strongly negatively correlated with cutting cross-section area. The correlation coefficient was 0.871 (Figure 4). This correlation suggests that strength is not a material property, and it is influenced by the strong inhomogeneity of cross-sections of cuttings. A large cross-section is caused by the large pith of the branches, and soft parenchyma cells of the pith are not dominated in load-caring capacity. The strength of the wine cuttings is comparable to the strength of softwood species such as spruce or poplar [49]. If the cuttings are further disintegrated, for example, for production of the particleboards [50] or fiberboards, there is potential for better utilization of load caring capacity of outer layers of the cross-section cuttings.

3.4. Fiber Analysis

Results of the fiber tester analysis (Figure 5a–c) show that there were no significant differences between white and red grape cuttings from the point of view of fiber length, width, and shape factor distribution. On the basis of the results, the major proportion (approximately 41.5%) of fibers was smaller than 0.3 mm. The average fiber length was 0.58 mm for white and 0.60 mm for red grape samples. A big proportion of fibers, from 32.27% (red) to 32.43% (white), was identified in the length class ranging from 0.51 mm to 1.0 mm. A relatively large proportion of fibers, from 23.61% (red) to 23.84% (white), was also identified in the length class ranging from 0.31 mm to 0.5 mm.

From the point of view of fiber width distribution, a large proportion of fibers, from 45.77% (white) to 48.64% (red), was in the width class ranging from 20.1 µm to 30.0 µm. The average fiber width was 22.37 µm for white and 22.70 µm for red grape samples. Furthermore, 30.68% (red) and 32.83% (white) fibers were identified in the width class ranging from 15.1 µm to 20.0 µm. From the perspective of fiber shape factor distribution, the largest proportion (approximately 72.5%) of fibers was identified in the class ranging from 90% to 100%, and the average shape factor was 94.1% for white and 93.99% for red grape samples. This means that a major proportion of fibers were straight.

Wood fibers (softwood, hardwood) are primarily used for paper production. The effort of paper manufacturers is to develop alternative fiber sources, i.e., other lignocellulosic sources that could replace or complement the fibers currently used for paper manufacturing [51]. In this paper, we determined the morphological heterogeneity of examined wooden material (grape canes) due to the possibility of its utilization in the paper. According to Niskanen [52], fiber properties are important in two respects. They affect the formation and consolidation of paper structure in the papermaking process, and they are responsible for the paper properties. As we mentioned above, the average fiber length of grape samples was 0.52 mm for white and 0.60 mm for red grape samples. Compared to the results of wood (e.g., eucalyptus, spruce wood) and non-wood (e.g., bamboo; wheat straw) morphological properties obtained from other authors [53,54,55,56], grape canes have lower average fiber length. On the other hand, the proportion of fibers longer than 0.3 mm was ca. 59%, which is comparable to the results of spruce wood [57]. The average fiber length according to Stankovská et al. [58] ranged from 0.8 to 1.5 mm for hardwood and 2.5 to 4.0 mm for softwood. According to Ferdous et al. [59], the fiber length of non-wood species ranged from 0.58 (eggplant stalks) to 2.02 (jute fiber). The results achieved in this work are comparable to the results obtained from the fiber length analysis of non-wood species (e.g., bagasse; bamboo; chia, corn, cotton, and jute stalks). Several authors [52,60,61,62] described a significant correlation between the paper strength and fiber length. According to the research of Fišerová et al. [60], higher fiber length supports higher paper strength. For paper production, the optimal fiber length distribution is not known and may never be determined [54]. If the strength of the paper was primarily on the proportion of long fibers, it would be possible to prepare the paper from a combination of wood and grape cuttings. Short fibers with thin cell walls and low fines content can impart superior softness in tissue paper [53]. Short fibers play an important role in the drainage characteristics of pulp and in the mechanical and optical properties [56].

From the point of view of fiber width, the average value is comparable to the results of spruce wood [57]. On this basis, the fibers could be suitable for paper production.

In addition to the length and width of fibers, the shape factor affects some of the paper’s properties. Mohlin et al. [63] described that, with an increase in the number of fiber deformations (lower shape factor), the tensile strength and tensile stiffness are lower. Ferdous et al. [59] stated in their contribution that, with a decrease in the shape factor, there is a decrease in the activation of the fiber segments in the fiber network, followed by a decrease in the tensile and tensile stiffness index and an increase in the tear and fracture toughness indices of the pulp sheets.

3.5. Calorific Value Determination

Calorimetry and the evaluation of calorific value (CV) are necessary for defining the potential energy of the materials or the amount of heat that the material generates on its complete combustion. On the basis of the results, it can be stated that the determined calorific values of the grape cuttings ranged from 18.68 MJ·kg⁻¹ (white) to 18.91 MJ·kg⁻¹ (red), with pellets having 16.96 MJ·kg⁻¹. The energetic potential of grape cuttings is comparable to that of other wooden materials. Olisa and Ajoko [64] indicated a CV for wood materials in general of 16.58 MJ·kg⁻¹, while Lunguleasa et al. [65] indicated a CV of 21.20–20.70 MJ·kg⁻¹ for Guaiac and Rose species. Lieskovský et al. [66] analyzed wood chip samples, and the mean ash content was 2.64%, the mean moisture content was 38.8%, and the mean gross calorific value was 19.43 MJ·kg⁻¹. Günther et al. [67] stated a gross calorific value between 17.9 MJ·kg⁻¹ (birch) and 20.5 MJ·kg⁻¹ (Santos rosewood). According to Demirbas [68], there is a highly significant linear correlation between the heating values of biomass fuel and the lignin content. Maj et al. [3] analyzed wood waste from lignified 1 year shoots from the cultivation of grapevines of the Seyval Blanc, Solaris, Regent, and Rondo varieties. The study showed that the material has a high energy potential of 15.88–16.19 MJ·kg⁻¹. Research by Burg et al. [69] dealt with the energetic evaluation and potential of pomace—a waste product originating during the production of grapevine. According to their results, pomace is an interesting energetic resource with a gross calorific value of 16.07–18.97 MJ·kg⁻¹. They stated that, through purposeful and efficient usage of pomace, 6.4 GWh of electric energy and 28 GWh of thermal energy can be generated. Cruz-Lopes et al. [20] evaluated grape stalks as a pelletized solid fuel and determined a CV of 16.7 MJ·kg⁻¹.

Cruz-Lopes et al. [20] stated that grape stalks contain an ash content of 2.90%, which is 10 times higher than in softwoods (e.g., spruce or pine). Maj et al. [3] determined the ash content ranging from 3.68% to 4.21% of the abovementioned grape shoots. They found that the use of this waste instead of hard coal could reduce CO emissions by 26–27%, CO₂ by 24–26%, NO_x by 55–56%, SO₂ by 96–77%, and dust by 77–80%. According to our results, a higher ash content of 10.54% was determined in grape pellets, compared to red (1.54%) and white (0.94%) grape cuttings. This could be due to the partial thermal modification of the grape samples during the pellet pressing process, as well as incomplete combustion of the pellets (lower calorific value). The ash content limit is 0.7% according to EN ISO 7225-1 [70]. The ash content of grape pellets (10.54%) greatly exceeded this limit, giving a significant disadvantage to pellets used for heating. It also exceeded the 1.2% limit of ash content at 550 °C. The other potential that has been already commercially introduced is to use pellets from grape cuttings for smoking meat or vegetable products. According to elemental analysis (

Table 3. The elemental analysis and ash content of grape cuttings.

No.	Sample of Grape Cuttings/Element	White	Red	Pellets	Ash from Pellets	Used Method	Principle
1.	Carbon (g/kg)	455	462	454	75	STN ISO 10684 [72]	EA-TCD
2.	Nitrogen (g/kg)	6.61	5.98	6.52	2.75	ISO 13878 [73]
3.	Phosphorus (g/kg)	1.15	1.07	1.01	5.85	ISO 11885 [74]	AES-ICP
4.	Calcium (mg/kg)	5698	9651	6892	48,943
5.	Magnesium (mg/kg)	1592	2003	2234	12,649
6.	Potassium (mg/kg)	3277	4023	3588	7396
7.	Sodium (mg/kg)	103	94.50	87.30	2142
8.	Iron (mg/kg)	183	804	1857	5802
9.	Manganese (mg/kg)	27.60	40.80	44.10	215
10.	Zinc (mg/kg)	26.70	56.20	23.60	39.60
11.	Aluminum (mg/kg)	37	123	379	3125
12.	Boron (mg/kg)	12.60	15.60	15.40	106
13.	Copper (mg/kg)	9.39	22.30	10.80	19.70
14.	Silicon (mg/kg)	90	291	576	88.80
15.	Chrome (mg/kg)	2.15	2.76	12.80	73.10

EA-TCD—elemental analysis by thermal conductivity detection. AES-ICP—atomic emission spectrometry with inductively coupled plasma. Elements 3–15 were determined in the mineral by microwave decomposition in the presence of HNO₃ and H₂O₂.

), grape pellets contained a higher amount of Mg, Fe, Mn, Al, Si, and Cr compared to white and red grape cuttings. Differences were visible when comparing individual types of grape canes. Red grape samples contained higher amounts of all elements: Ca, Mg, K, Fe, Mn, Zn, Al, B, Cu, Si, and Cr. Chandrasekaran et al. [71] performed elemental analysis of 23 wood chips. According to the results, wood chips also contained a proportion of the elements monitored herein: Mg from 91 mg/kg to 563 mg/kg, K from 381 mg/kg to 2066 mg/kg, Na from 92 mg/kg to 106 mg/kg, Fe from 2 mg/kg to 345 mg/kg, Mn from 3 mg/kg to 272 mg/kg, Zn from 1.53 mg/kg to 17 mg/kg, Al from 0.85 mg/kg to 399 mg/kg, Cu from 0.58 mg/kg to 3.41 mg/kg, and Cr from 0.012 mg/kg to 7.36 mg/kg. Compared to our results, grape cuttings contained higher contents of Mg, K, Fe, Zn, and Cu.

On the basis of the results, it can be stated that grape cuttings are materials with great energetic potential, and the results of calorific values are comparable to the results obtained for other wooden materials. On the other hand, grape pellets had a lower calorific value and a large amount of ash, which could pose a problem for their energy use. On the basis of the results, we would not recommend pressing the pellets in this form, as stated in this contribution; instead, we recommend burning only the canes.

4. Conclusions

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The content of carbohydrates of grape cuttings provides a prerequisite for their acid or enzymatic hydrolysis to produce monosaccharides or second-generation bioethanol.

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The higher glucose content in red grape samples compared to the white ones was confirmed by FTIR, as well as by chemical analysis.

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The strength properties of both red and white cuttings showed the potential of the material to produce particleboards, fiberboards, or pulp and paper.

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The fibers’ morphological properties of grape canes are comparable to those of non-wood species, which is a predisposition to the possibility of its further use for pulp and paper production.

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Differences between cuttings from varieties Pesecká leánka (white graft) and Frankovka modrá (red graft) were obvious in terms of elements, extractive and saccharide (glucose and xylose) content, and compression strength.

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The energetic potential of grape canes is comparable to wood; the slightly higher calorific value of red grapes is due to a higher content of carbon and lignin, and a lower content of carbohydrates compared to white grapes.

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Grape pellets had a lower calorific value and very high amount of ash compared to the grape canes; thus, they are less suitable for energy use.