Grape Pomace as a Functional Ingredient in Bread Production: Effects on Quality and Nutritional Properties

3.1 Determination of the effect of grape variety on the composition and antioxidant properties of pomace and wood flour

A comparative analysis of flour obtained from the pomace of white and red grape varieties was carried out. It was found that, depending on the grape variety, the content of nutrients in pomace flour varies within a wide range. Data on the composition of pomace flour for the varieties used in the study are presented in the table (Table 1).

Table 1. Chemical composition of pomace flour obtained from different grape varieties

The analysis of the chemical composition of pomace flour obtained from the Bayanshira and Madrasa grape varieties showed that the crude protein content in the pomace flour from the Madrasa variety was 2.5% higher than that of the Bayanshira variety. Examination of the data in the table indicates that pomace flour is not only a good source of protein but also rich in minerals. The mineral content of flour derived from Madrasa grape pomace was as follows: iron 170 mg/kg, iodine 4.05 mg/kg, zinc 25 mg/kg, manganese 9.5 mg/kg, phosphorus 3.62 g/kg, calcium 15.10 g/kg, and cobalt 0.22 mg/kg.

Notably, the iron content, essential for hemoglobin formation in the blood, ranged from 145 to 170 mg/kg, highlighting the functional significance of these products. Additionally, the presence of other functionally relevant elements such as cobalt, zinc, manganese, and others further supports their nutritional value. As is well known, cobalt is a component of vitamin B12, zinc is involved in numerous enzymatic processes, manganese activates oxidation reactions, and copper regulates erythrocyte maturation, among other functions. Collectively, these factors make the use of pomace flour as a functional feed ingredient not only feasible but also highly advantageous.

Pomace flour is also notable for its high iodine content. The iodine content was 11.25 mg/kg in the Bayanshira variety and 4.05 mg/kg in the Madrasa variety.

Research has shown that grape pomace is rich in essential amino acids, which are readily and efficiently assimilated by the human body. These amino acids play critical roles in several vital physiological processes, including the synthesis of skin proteins and certain hormones, the regulation of fat metabolism (methionine), the formation of urinary metabolites (arginine), and the stimulation of growth processes (lysine), among others.

Studies indicate that the amino acid profile of flour obtained from grape pomace is comparable to that of other valuable plant flours, particularly in lysine, arginine, histidine, threonine, serine, asparagine, and glutamine content [27].

A comparative analysis of the amino acid composition of pomace flour from Bayanshira and Madrasa grape varieties revealed that most amino acids were present in higher amounts in the Madrasa variety compared to Bayanshira (Table 2). Specifically, lysine was 0.18% higher, histidine 0.10% higher, serine 0.07% higher, proline 0.16% higher, glutamic acid 0.39% higher, and aspartic acid 0.8% higher. Regarding essential amino acids, the Madrasa variety also showed superior content for lysine, valine, methionine, leucine, threonine, and phenylalanine. Overall, the total amino acid content in pomace flour from the Madrasa variety was 1.15% higher than that from the Bayanshira variety.

Table 2. Amino acid composition of pomace flour obtained from different grape varieties

Research shows that viticulture and winemaking are production sectors in which almost all products are utilized. In particular, in viticulture, all parts of the vine, from green leaves to the green and dry shoots formed during pruning, are not discarded but can be subjected to recycling and returned to production.

Analyses show that the woody part of the grapevine contains a considerable amount of mono and polysaccharides depending on its age (Table 3).

Table 3. Composition of shoots obtained from grape pruning

It was found that the one-year-old dry shoots obtained from grape pruning have a composition similar to that of the perennial woody parts of the vine. Although the content of easily digestible polysaccharides in the one-year-old shoots is several times higher than in the perennial parts, other compositional characteristics are comparable.

After grinding, the pruned shoots can be incorporated into feed flour and used as animal feed. The residues obtained from green pruning can be ensiled. Moreover, unless other options are considered, grape pomace can also be processed into feed flour.

The one-year-old shoots obtained from dry pruning stand out in terms of composition and nutritional content, being rich in the components of vine wood and other essential nutrients (Table 4).

Table 4. Chemical composition of flour obtained from one-year-old dry shoots produced during grape pruning

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Figure 1. Comparison of the composition of flour obtained from residues of different grape varieties

As can be seen, the flour obtained from one-year-old dry shoots generated during grape pruning is inferior to pomace flour in terms of several compositional parameters, including crude protein, crude fat, crude fiber (Figure 1).

The flour obtained from one-year-old dry shoots generated during vine pruning also differed from pomace flour in terms of amino acid composition (Table 5).

Table 5. Amino acid composition of flour obtained from one-year-old dry shoots produced during grape pruning

As can be observed, in terms of chemical richness, pomace flour was superior to the flour obtained from one-year-old dry shoots produced during vine pruning. Specifically, the total amino acid content in pomace flour from the Bayanshira and Madrasa varieties was 8.0% and 9.15%, respectively, whereas in the corresponding woody flour from these varieties, the values were 5.39% and 5.54%, respectively (Figure 2).

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Figure 2. Comparison of total amino acid content in flour obtained from residues of different grape varieties

In other words, the total amino acid content in pomace flour from the Bayanshira variety was 2.61% higher than in the corresponding woody flour, while in the Madrasa variety, it was 3.21% higher. This difference is also reflected in the balanced amino acid profile along with the content of minerals, particularly iron, iodine, and other beneficial elements.

The systematic study of these compounds has only begun in recent years. As a result, nutrients such as tannins and related compounds, as well as color substances like anthocyanins, have been extensively investigated, and research in this area continues successfully. The attention given to phenolic compounds is not coincidental, as these compounds play a crucial role in determining the taste, color, body, and ultimately the organoleptic properties of grapes, juice, wine, and related products. Additionally, the therapeutic properties of phenolic compounds in wine and wine-based products have enhanced their significance in food product technology. Since these compounds are mainly concentrated in the solid parts of the grape clusters, particular emphasis has been placed on the study of these fractions.

3.2 Investigation of the effect of drying method on the extraction of dietary fibers from grape skins

Previous studies have shown that grape pomace varies in both chemical and mechanical composition depending on the grape variety and processing technology. The grape skin serves as the direct raw material for dietary fiber extraction (Grape Dietary Fiber, GDF), and its content varies widely depending on the grape type and processing method.

During the research, it was found that Isabella grapes cultivated in the foothill areas of the Goygol region in western Azerbaijan exhibit more intense aroma and different compositional characteristics compared to those grown in lowland areas. After processing this grape material, the physical and chemical properties of the wet pomace obtained from the press were analyzed (Table 6).

Table 6. Some physicochemical properties of wet grape pomace

It was found that the pomace obtained from Isabella grapes grown in the foothill region exhibited a titratable acidity that was 0.18% higher, a moisture content that was 1.73% lower, and a pH value that was 0.11 units lower compared to pomace from lowland areas.

The color values of the analyzed pomace samples, measured using the Hunter L, a, b system, also differed notably (Table 7).

Table 7. Hunter color values (L, a, b) of wet pomace samples

According to the analysis, the L value of wet pomace from the Goygol region was 0.40 units higher compared to Samukh; the a value was 0.11 units lower, and the b value remained nearly the same.

The mechanical composition analysis of grape pomace indicated that it contains various extraneous materials, including parts of the grapevine itself. Therefore, it is necessary to remove these impurities to obtain a homogeneous mass. To minimize the loss of biologically valuable compounds, washing the pomace with water is preferred. In this process, water is poured over the pomace, which is then manually washed, and the remaining mass is stirred in water.

During the study, it was observed that when the pomace was washed in a container with water, some of its color leached into the water, and the water contained a significant amount of suspended particles from the pomace. Microscopic examination of the wash water revealed a high microbial load. Consequently, the pomace was washed several times, and the microbiological quality of the final solution was strictly monitored (Figure 3).

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Figure 3. Purification of grape pomace and extraction of dietary fiber

Accordingly, the pomace was washed several times, and the microbiological quality of the final solution was strictly monitored.

Table 8 presents the composition of grape skins from the investigated varieties. The sugar content in the skins varies depending on the ripening stage of the berries and differs significantly from the carbohydrate content in the pulp. For instance, if the pulp of one ton of grapes contains 140–170 g of sugars, the corresponding grape skins contain 0.6–2.9 g of sugars. Approximately 20% of the skin consists of cellulose, pectin, and proteins. The skins also contain organic acids, including citric, tartaric, and malic acids.

Polyphenols are present in the skins of various grape varieties. It has been reported that the polyphenol content in red grape skins is approximately twice as high as that in white grape varieties. Additionally, wide varieties contain yellow and red pigments as well as aromatic compounds. Anthocyanins are located in several cell layers beneath the epidermis of the skin.

Table 9. Physicochemical properties of Isabella grape skin powders obtained by drying under different temperatures and durations

The skins of fully ripened berries also contain significant amounts of xanthophylls, carotenoids, and chlorophyll, which are largely degraded as the berries reach full maturity. The brighter coloration of the berries develops during ripening and reaches its maximum at full maturity.

Table 9 presents the results of drying the peel derived from the Isabella grape variety under various temperature and time conditions.

As can be seen from Table 9, the moisture content of dried and fermented grape pomace samples varied between 0.830 and 3.955%. It was found that the drying temperature and method have a significant effect on the moisture content of pomace.

The content of titratable acids in pomace samples varied between 4.36 and 5.18%. As can be seen, with increasing temperature, the amount of titratable acids first increased and then decreased. When dried in a sublimator, the amount of titratable acids was higher, reaching 5.18%.

Hunter color values were determined in skin powders obtained by drying with different methods. It was found that L values ranged between 21.994–25.446, a values between 11.020–21.960, and b values between 3.146–6.790, depending on different variants. Both L and a values were higher in the powder obtained by sublimation drying, while the b value was higher (6.790) in samples dried at a higher temperature (90 ℃) in drying chambers.

Phenolic compounds and antiradical activity of powder obtained from dried pomace of the Isabella grape variety were studied. It was found that these indicators vary depending on the drying method and temperature.

During drying in conventional drying chambers at three different temperatures, total phenolic compounds were observed to vary between 18.760 and 25.630, while in sublimation drying, the total phenolic compounds reached a maximum of 31.520 mg/g. Antiradical activity in experimental samples varied between 13.860 and 17.640 mg/dm³, and the highest value was observed in freeze (sublimation) drying.

Researchers consider the drying of grape pomace to be essential in order to prevent its spoilage and ensure its year-round utilization. Studies have evaluated the nutrient composition, in vitro digestibility, gas production, and phenolic profile of dried grape pomace processed using various micronization and air-classification techniques [28]. Additionally, research has been conducted on the development of functional products using dried grape pomace obtained through different drying methods [29], as well as on the production of value-added extracts derived from it [30, 31].

During drying in conventional dryers, with increasing temperature, a slight increase in antiradical activity was first observed, and at 90 ℃, a decrease was recorded. At 50 ℃, partial degradation of the cell wall occurs, allowing phenolic compounds to be more extracted, and bound phenolic compounds are partially released. Extraction efficiency increases, and the yield of bioactive compounds rises. At 80–90 ℃, a large number of cells are destroyed, transfer of substances into the medium increases, and thermal degradation of phenolic compounds occurs, followed by polymerization and loss of active form. At 90 ℃, degradation outweighs extraction. As can be seen, while extraction dominates at 50 ℃, degradation becomes the main process at 80–90 ℃. Thus, the decrease in antiradical activity at 80–90°C is associated with thermal oxidative degradation and structural changes of phenolic compounds due to high temperature. Although the release of bioactive compounds continues, their degradation rate predominates, resulting in reduced overall antioxidant activity.

At elevated temperatures, thermal oxidation occurs, during which phenolic compounds are oxidized in the presence of oxygen to form quinones. These reactive quinones subsequently interact with other compounds. As a result, antioxidant activity decreases, and browning intensifies.

Under conditions of high temperature and acidic medium, hydrolysis processes take place, leading to the breakdown of phenolic glycosides and the formation of free phenolic aglycones. Consequently, phenolic compounds initially increase due to their release, but are later oxidized and degraded.

In such conditions, polymerization and condensation reactions also occur, resulting in the formation of high-molecular-weight polymers from oxidized phenols. These compounds exhibit low solubility and reduced antioxidant activity, making their extraction more difficult. Additionally, high temperatures induce structural changes in the aromatic rings of phenolic compounds, including the degradation of hydroxyl groups, which further contributes to the decline in antiradical activity.

The degradation of phenolic compounds follows first-order kinetics and accelerates with increasing temperature; thus, higher temperatures lead to a faster rate of degradation.

An increase in drying temperature exerted a complex effect on both the content of phenolic compounds and their antiradical activity in grape peel powder. The obtained results indicate that the level of phenolic compounds was higher at 50 ℃, whereas it decreased within the 80–90 ℃ range. These changes can be explained by the balance between extraction and degradation processes.

At 50 ℃, partial disruption of the cell wall occurs, facilitating the release of phenolic compounds from the intracellular matrix. Under these conditions, a portion of bound phenolics is converted into free forms, resulting in more efficient extraction. The relatively low temperature helps preserve the thermal stability of phenolics and limits the rate of oxidative reactions. Consequently, extraction predominates over degradation, leading to an increased overall yield of bioactive compounds.

In contrast, increasing the temperature to 80–90 ℃ significantly alters the process mechanism. At these temperatures, cell structures are extensively disrupted, intensifying the release of phenolic compounds into the medium. However, high temperature simultaneously accelerates both thermal and oxidative degradation of phenolics. In the presence of oxygen, phenolic compounds are oxidized to reactive quinones, which further react with other components to form more complex, high-molecular-weight compounds. These polymerization and condensation products generally exhibit lower solubility and reduced antioxidant activity.

Additionally, under high temperature and acidic conditions, hydrolysis of phenolic glycosides occurs, leading to the formation of free aglycones. Although a temporary increase in phenolic content may be observed at this stage, these free forms are more susceptible to rapid oxidation and degradation. Structural modifications in the aromatic ring and the loss of hydroxyl groups reduce their radical-scavenging capacity.

Furthermore, oxidation processes at elevated temperatures also affect color parameters. The formation of quinones and their derivatives leads to product darkening, which is consistent with the changes observed in Hunter color values.

From a kinetic perspective, the degradation of phenolic compounds follows first-order reaction behavior, and the rate of these processes increases exponentially with temperature. Therefore, at 90 ℃, degradation predominates over extraction, resulting in a decrease in both total phenolic content and antiradical activity.

In summary, it was determined that extraction processes dominate at 50 ℃, whereas at 80–90 ℃, thermal degradation, oxidation, and polymerization reactions become predominant. Thus, although the release of bioactive components continues at higher temperatures, their reduced stability leads to a decline in the overall antioxidant potential.

As with total phenolic compounds, higher antiradical activity was also observed in sublimation drying. It is also known that freeze-dried grapes are used in beverage production.

The analysis of Isabella grape pomace shows that it is a product rich in biologically active substances, especially antioxidants. The use of such residues in the production of combined products is important for obtaining functional food products.

3.3 Investigation of functional bakery production using grape pomace extracts

Bread and bakery products are basic food products, and their quality does not always fully meet modern scientific nutritional requirements. One of the main directions for solving this problem is the creation of new safe functional bread varieties to regulate human nutrition.

A number of studies indicate that grape pomace is a heterogeneous yet rich source of bioactive compounds, supporting its targeted use as a functional ingredient in future food and nutraceutical applications [32, 33].

The possibility of using pomace powder obtained from the processing of white and red grape varieties, as well as seed extract, in the production of bakery products was studied. For this purpose, the obtained pomace samples are fermented separately according to a specific methodology. Using high-quality wheat flour, bread samples are prepared with the application of pomace-based additives. Bread samples prepared with different amounts of additives are evaluated in comparison with the control.

Quality indicators of pectin extracts obtained from different grape varieties are presented (Table 10).

It was observed that among the grape varieties studied, the Rkasiteli variety exhibited the highest dry matter content, which corresponded to the maximum pectin yield per 100 g of pomace. However, in terms of pectin extract purity (conventional unit), the Chardonnay variety demonstrated superior quality.

During the study, pectin extract in powder form was used. The yield of dry pectin obtained from the pomace of different grape varieties is presented (Table 11).

Table 11. Dry pectin yield from 100 g of pomace of different grape varieties

As can be seen, Rkatsiteli showed the highest extractability among the varieties; however, due to its lower pectin proportion compared to the other varieties, the pectin yield from 100 g of pomace in this cultivar was 3.33 g. Among the grape varieties, Aligote and Bayanshira demonstrated slightly higher pectin yields compared to the others.

A production technology for traditional bread products enriched with pectin extract powder has been developed. Traditional wheat-based bakery products served as the research objects. To elucidate the effect of pectin extract powder, laboratory-scale serial baking experiments were conducted.

Researchers have conducted a study aimed at producing wheat bread with desirable texture and slow starch digestibility by incorporating citrus pectin. The study demonstrates that the addition of an appropriate amount of citrus pectin has the potential to produce bread with both a lower glycemic index and improved textural properties [34].

Pectin extracts were// added to the dough at concentrations ranging from 3% to 10% (w|w, based on flour) of the flour mass. The results indicated that the addition of pectin extract promoted acid development and resulted in more intensive fermentation (Table 12).

It was found that as the amount of pectin extract powder added to the dough increases, the amount of gluten remains almost unchanged. However, as the amount added increases, there is a decrease in the extensibility. This is especially evident at a 10% addition.

Table 12. Effect of pectin extract on the quality of wheat dough

To determine the leavening power of baker's yeast, dough samples prepared with a certain composition are fermented under standard conditions. The time it takes for the dough to reach a height of 7 cm is measured. The shorter the time, the greater the leavening power.

The leavening power of the bread dough was measured as follows: with 3% pectin extract, the dough required 4.09 min; with 5%, 4.0 min; with 7%, 5.49 min; and with 10%, 8.53 min. It was determined that the dough with 5% (w|w, based on flour) pectin extract exhibited the optimal leavening capacity.

Laboratory-scale baking tests were conducted using dough enriched with pectin extract powder, and the quality of the resulting bread was evaluated (Table 13).

Laboratory-scale baking tests revealed that increasing the amount of pectin extract added to the dough enhanced the dough’s acid retention and fermentation activity. The dough’s leavening capacity, the bread’s absolute volume, and the structural-mechanical properties of the dough improved with higher extract content.