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Article

Towards Wine Waste Reduction: Up-Cycling Wine Pomace into Functional Fruit Bars

by
Maja Benković
1,
Filip Cigić
1,
Davor Valinger
1,
Tea Sokač Cvetnić
2,
Ana Jurinjak Tušek
1,*,
Tamara Jurina
1,
Jasenka Gajdoš Kljusurić
1 and
Ivana Radojčić Redovniković
1
1
Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia
2
Zagreb County Institute of Public Health, Mokrička ulica 54, 10290 Zaprešić, Croatia
*
Author to whom correspondence should be addressed.
Processes 2024, 12(12), 2941; https://doi.org/10.3390/pr12122941
Submission received: 19 November 2024 / Revised: 12 December 2024 / Accepted: 13 December 2024 / Published: 23 December 2024
(This article belongs to the Special Issue Feature Papers in the "Food Process Engineering" Section)
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Versions Notes

Abstract

:
Due to the beneficial composition of wine pomace, it has found several applications in the food industry, mostly in the form of flour or extracts. This study suggests the use of grape skin separated from the pomace as a functional ingredient for fruit bars based on the hypothesis that grape skin can contribute to fruit bar antioxidant potential. Fruit bars were produced with dried figs/dates, grape skin, and cocoa/hazelnut mix in different proportions (48–70%, 30–50%, and 0–2%, respectively). The addition of grape skin proved beneficial for the total polyphenolic content (TPC) and antioxidant capacity. Furthermore, consumers appeared to like the newly developed functional product, and the addition of up to 30% grape skin did not have an adverse effect of sensory properties. The bars were graded A based on the NutriScore value and were microbiologically compliant to food safety regulations. These results demonstrate the possibility of grape skin use in the development of a functional fruit bar product, which can be beneficial not only from chemical and sensory point of view, but also economically feasible and environmentally friendly.

1. Introduction

During the winemaking process, large quantities of waste are generated, which mostly include solid waste (grape stalks, grape pomace, wine lees, vine shoots, and leaves) and wastewater. This waste, if not properly managed, can cause substantial ecological damage which includes soil pollution, damage to vegetation, surface and ground water pollution, pest and insect infestation, and emission of unpleasant odors [1,2]. Recognizing the urgency of this issue, researchers have intensified efforts to find sustainable and innovative ways to valorize grape pomace, a major byproduct of winemaking. By transforming this waste into valuable products, the wine industry can minimize its environmental footprint and contribute to a more circular economy.
Grape pomace usually consists of stems, skin, and seeds remaining after grape processing [3], and is considered a rich source of dietary fiber (43–75%), protein (6–15%), and polyphenols (phenolic acids, flavonoids, and stilbenes), with the specific composition varying depending on the grape variety and processing methods [4,5,6]. Its abundance in phenolic compounds also brings multiple health benefits. Studies have shown that grape pomace has neuroprotective, anti-tumor, antioxidant, anti-inflammatory, and innate-adaptive immunity effects [7,8,9,10,11]. However, when developing a food product with the addition of pomace, care has to be taken to avoid possible toxicological effects, which are mostly connected to the presence of aflatoxins (B1, B2, G1, G2), ochratoxin, citrinin, heavy metals, and pesticides [12,13,14,15], so each batch of pomace prior to its use in food products has to be tested for health safety. Some examples of wine pomace use in the food industry include the utilization of grape seed extract in meat products [16], cheese production [17], and the addition of grape pomace flour to bread, muffins, cookies, and pasta [18,19,20,21,22]. By effectively utilizing grape pomace in diverse food products, the food industry can enhance nutritional value, meet consumer demand for functional foods, and contribute to sustainability by reducing waste [23,24]. Beyond food applications, grape pomace has also shown potential in producing biofuels, biodegradable packaging materials, and cosmetics, aligning with the principles of a circular economy [2,25]. Such innovative approaches not only reduce environmental pollution but also create economic opportunities by transforming waste into high-value products.
Due to the growing demand for healthy, natural, and convenient food, fruit bars are becoming increasingly popular among consumers [26,27,28]. Fruit bars are concentrated fruit products with high nutritional and energy value compared to fresh fruit. Additionally, they boast a longer shelf life, portability, and convenience, making them a popular choice for consumers seeking healthy and convenient on-the-go snacks. Furthermore, fruit bars can serve as an excellent instant food that can provide the necessary amount of dietary fiber and other bioactive compounds needed to meet daily needs [29,30,31]. The versatility of fruit bars lies in their adaptability to incorporate a wide range of ingredients, including dried fruits, nuts, seeds, and functional additives, which enhance their nutritional profile and cater to diverse consumer preferences [27,32].
Dried fruits which are often utilized as fruit bar ingredients include dates, figs, apricots, cranberries, raisins, mangos, apples, durian, guava, and many more [28]. Dates, in their dried form, are readily available and are considered a relatively low-cost ingredient of fruit bars. Dates are considered sources of carbohydrates (mainly fructose and glucose), proteins, dietary fibers, minerals (calcium, iron, magnesium, selenium, copper, phosphorus, potassium, zinc, sulfur, cobalt, fluorine, and manganese), and vitamins (mainly vitamin B complexes) [33]. Furthermore, according to literature data, dates possess a low glycemic index and anti-inflammatory, anti-oxidant, and anti-tumor activity [34] making them ideal candidates for creating nutrient-dense products. On the other hand, figs, as fruits often grown in the Mediterranean region, are excellent sources of vitamins, minerals, amino acids, dietary fibers, and other bioactive components, including carotenoids and polyphenolic compounds. Furthermore, they benefit gastrointestinal, respiratory, inflammatory, metabolic, and cardiovascular health [35]. Due to the above-mentioned, dates and figs were chosen as the base ingredients in this study for the development of fruit bars. Besides the basic dried fruit ingredients, bars are often supplemented with different nuts and spices to enhance their flavor, color, and antioxidant properties. Cocoa powder is widely known for its antioxidant effects and health benefits [36], while nut (walnut, almond, hazelnut, pistachios, and peanuts) ingredients are sources of unsaturated fats known for their numerous health-promoting properties [37]. Beyond nutrition, the adaptability of fruit bars extends to their flavor profiles, which can range from sweet and tangy to savory and spiced [38]. Ingredients like cocoa powder, cinnamon, vanilla, or ginger not only enhance the taste but also provide additional health benefits, such as anti-inflammatory properties [39,40]. This adaptability enables producers to experiment with unique combinations to meet regional taste preferences or seasonal trends. By accommodating such a wide variety of ingredients, fruit bars have become a dynamic product category that addresses diverse consumer demands for health, convenience, and indulgence [26]. Their versatility also makes them a promising vehicle for incorporating underutilized or upcycled ingredients, such as grape skin or other agricultural byproducts, thereby promoting sustainability while meeting the market’s evolving needs [41].
Based on the above-mentioned, this research explores the potential of utilizing grape skin separated from pomace as a functional ingredient in date and fig-based fruit bars, contributing to their antioxidant potential and overall nutritional profile. The primary novelty lies in upcycling grape skin waste, a byproduct of winemaking, into a valuable food ingredient. This addresses a significant environmental and economic concern by preventing waste and creating a new market for a previously discarded material. The study focuses on optimizing the bar composition to maximize the incorporation of grape skin while maintaining sensory appeal, microbiological safety, and a high nutritional score (high in fibers, proteins, and bioactives). This demonstrates a commitment to both product quality and sustainable practices. The research aligns with the principles of sustainability by addressing food waste, promoting the use of natural and underutilized resources, and developing a product with enhanced nutritional value.

2. Materials and Methods

2.1. Materials and Reagents

Raw materials used in this study were dried figs, dried dates, coconut flour, hazelnut, cocoa powder with 10–12% fat, all by Nutrigold (Zagreb, Croatia), citrus peel powder, orange peel powder, vanilla sugar, all by Arcadie (Méjannes-lés-Alés, France), and Graševina variety grape pomace from the Požega—Slavonija County, harvest year 2021, supplied by Kutjevo d.o.o. (Kutjevo, Croatia). The basic characteristics of pomace were 46.55% dry matter, 7.73% proteins, 4.13% fats, 4.53% sugars, 29.13% fibers, and pH of 4.6. Furthermore, the complete toxicological profile of pomace was analyzed (238 pesticide residues, total aflatoxins (B1 + B2 + G1 + G2), aflatoxin B1, ochratoxin, citrinin, and heavy metals) and they were all below the limits prescribed by the EU legislation, making the pomace safe for use in food for humans. All other ingredients, including fig, date, cocoa, hazelnuts, and spices, were bought at a local store and must conform to the health safety standards to be put on the market.
Chemicals and reagents used in this study were as follows: ethanol (Kefo d.o.o., Ljubljana, Slovenia), methanol (J.T. Baker, Hampton, NH, USA, SAD), Folin-Ciocalteu reagent (Kemika, Zagreb, Croatia), 1,1-diphenyl-1-picrylhydrazyl (DPPH) (Sigma Aldrich, Darmstadt, Germany), gallic acid 98% (AcrosOrganics, Belgium, WI, USA), sodium acetate trihydrate (CH3COONa ∙ 3H2O) (J.T. Baker, Deventer, the Netherlands), 2,4,6-Triphenyl-1,3,5-triazine (TPTZ) (Sigma-Aldrich, Darmstadt, Germany), sodium carbonate (Na2CO3) (p.a.) (Gram Mol, Zagreb, Croatia), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) (Fluka, Buchs, Switzerland), iron (III) chloride hexahydrate (FeCl3∙6H2O) (GRAM-MOL d.o.o., Zagreb, Croatia), iron (II) sulphate heptahydrate (FeSO4·7H2O) (Sigma-Aldrich, Darmstadt, Germany), and acetic acid 99.5% (CH3COOH) (T.T.T. d.o.o., Sveta Nedjelja, Croatia).

2.2. Methods

2.2.1. Raw Material Preparation

Initial separation of seeds and stalks from the pomace was achieved using a custom-designed sieve with a 5 mm pore diameter. The resulting grape skin was then homogenized into a slurry using a Multifresh blender (Delimano, Cadempino, Switzerland). Similarly, dried dates and dried figs were processed into a homogeneous state. Hazelnuts were ground to a fine powder using the same blender and subsequently combined with cocoa powder in a 1:1 ratio. After the preparation of the raw materials, the preparation of the fruit bars began.

2.2.2. Fruit Bar Preparation

Fruit bars were prepared based on a Constrained Mixture design of experiments, which included three raw materials at three levels (dried fruit—figs or dates at levels ranging from 48 to 70%; grape skin in levels from 30 to 50%, and cocoa/hazelnut in levels from 0 to 2%). The experimental design is shown in Table 1.
The preparation process began by weighing the raw materials needed for the preparation of fruit bars in the proportions defined by the experiment design for each type of bar separately (date + grape skin + hazelnut/cocoa, fig + grape skin + hazelnut/cocoa). Homogenization of raw materials was carried out manually. A total of 1 g of vanilla powder, 0.5 g of orange powder, and 0.5 g of lemon powder were added to each bar. After mixing, the mixtures were weighed (20 g each), placed in a mold and put to a convective oven (InkoLab ST60T, Zagreb, Croatia) to dry for 8 min at 180°C. After the bars were cooled, they were packed in PA/PE vacuum bags, sealed, and placed in a refrigerator (6 °C) for storage until further analysis.

2.2.3. Analysis of Physical and Chemical Properties

Moisture Content

Moisture content of the fruit bars was determined by the gravimetric drying method at a temperature of 105 °C to a constant mass [42]. Measurements were performed in duplicate and the results were expressed as mean value ± standard deviation (SD).

Extract Preparation

In order to analyze the chemical properties of the bars, extracts were made using 70% ethanol (v/v) as a solvent. A total of 3 g of the crushed sample was weighed and added to 90 mL of the extraction solvent previously heated to 50°C in an oil bath (IKA HBR4 digital, IKA-Werke, Staufen, Germany). The extraction was carried out for 30 min at a mixer speed of 500 rpm in covered glass beakers in order to prevent the evaporation of the solvent. The obtained extract was filtered on a vacuum filtration set (Rocker 300—LF 30, New Taipei City, Taiwan) and stored in 50 mL Falcon cuvettes. The samples were kept in a freezer until further analyses.

Total Dissolved Solids, Conductivity, and pH of the Extracts

Conductivity and the total dissolved solids (TDS) were determined using a conductometer (SevenCompact, MettlerToledo, Greifensee, Switzerland) by immersing the probe in the liquid extract. Three parallel measurements were carried out. The pH value of the extracts was determined using a pH meter (Jenco 601A, Schaumburg, IL, USA) by immersing the pH probe in the liquid extract. Three parallel measurements were also carried out and the results were expressed as mean value ± SD.

Determination of Total Polyphenolic Content (TPC)

The total polyphenolic content (TPC) of the prepared liquid extracts was determined spectrophotometrically by the Folin-Ciocalteu reagent, as previously described by Singleton and Rossi [43]. The measurements were performed in triplicate and the results were expressed as mean values ± standard deviation of mg gallic acid equivalents (GAE) per gram of dry matter (mg GAE/g dm).

Determination of Antioxidant Activity Using the DPPH Method

The antioxidant activity was determined spectrophotometrically based on the reaction of the tested sample and DPPH (2,2-diphenyl-1-picrylhydrazyl) methyl solution, as reported previously by Brand-Williams et al. [44]. The measurements were performed in triplicate and the results were expressed as mean values ± standard deviation of the molar fraction (mmol) of Trolox equivalents per gram of dry matter (mmol TE/g dm).

Determination of Antioxidant Activity Using the FRAP Method

Antioxidant activity was also determined by the FRAP (Ferric Reducing Antioxidant Power) method, according to Benzie and Strain [45]. The measurements were performed in triplicate and the results were expressed as mean values ± standard deviation of the molar equivalents of FeSO4 · 7H2O per gram of dry matter (mmol FeSO4/g dm).

2.2.4. Sensory Analysis of Fruit Bars

The sensory properties of the produced fruit bars were analyzed based on the hedonistic scale. Samples were presented to the analysts in a closed, marked package, in a random order. Each analyst was given a grading sheet and water to rinse the palate between samples. In order to avoid too high a number of samples which would oversaturate the sensory palate, the fig and the date samples were tested on two separate days. Sensory properties were determined according to a hedonic scale with grades from 1 to 5, with 1 indicating the least acceptability and 5 the highest acceptability. The properties that were analyzed were as follows: appearance (which was mainly connected to the homogeneity of the bar and the visual presence/absence of large chunks of grape skin, date, or fig), color (the bars should have an appealing brown color, similar to that of milk/dark chocolate), odor (the pomace smell should not be present, but masked by cocoa/hazelnuts or dried fruit component), sweetness (not excessively sweet), bitterness (not too bitter), and aftertaste (aftertaste should not contain bitter or acidic notes originating from pomace). Results were presented as mean value ± SD.

2.2.5. Optimization, Definition of Optimal Composition, and Validation Experiments

The Statistica v. 14 software (Tibco Software, Palo Alto, CA, USA) was used to create the experimental design, as well as to perform basic statistical analyses (mean values and SDs) and multivariate analysis (bi-plots). The same software was used to analyze the results of the experiment in the form of ANOVA at a probability level of p < 0.05, to get an insight into the significant influences of input variables on the outputs. Furthermore, optimization of mixture composition was carried out by defining desirability profiles in the same software package, where the proportions of fig/date, grape skin, and cocoa/hazelnut were used as input parameters, and all sensory and chemical properties were used as output parameters. After optimization, an independent validation experiment was performed in which the optimum composition mixtures were produced, and their chemical and sensory properties were determined and compared to model-obtained values. Model adequacy was estimated based on relative percentage error (RPE) values.

2.2.6. Analysis of Nutritional Composition, Nutri-Score, and Microbiological Acceptability

The optimum composition mixtures were subjected to nutritional composition analysis in a certified analysis laboratory (outsourced analyses) according to the following procedures and norms: fats according to an in-lab modified Soxhlet method, proteins according to an in-lab modified Kjeldahl method, salts according to the in-lab modified titration method, fibers according to an AOAC 991.43 method [46], carbohydrate content was determined by calculation (% carbohydrates = 100 − % moisture − % protein − % lipid − % mineral) after all the other components have been measured, while the total energy value of the bars was calculated based on energy conversion factors of the determined macronutrients (proteins, carbohydrates, and fats).
Nutri-Score was calculated based on the 2023 updated algorithm publicly available on the French Ministry of Health webpage [47].
Microbiological analysis included aerobic mesophilic bacteria determination according to the HRN EN ISO 4833-2:2013 norm [48], Staphylococcus aureus according to the HRN EN ISO 6888-1:2021 norm [49], Enterobacteriaceae according to the HRN ISO 21528-2:2017 norm [50], yeasts and molds according to the HRN ISO 21527-2:2012 [51], and Salmonella spp. according to the HRN EN ISO 6579-1:2017 norm [52].

3. Results and Discussion

3.1. Physical Properties of Newly Developed Fruit Bars

Grape pomace, as a substantial by-product of the wine-making process, is still underutilized in functional food production, which is quite detrimental given the fact that it is a valuable source of bioactives, proteins, and fibers. This research aimed to determine whether grape skin separated from the pomace can add additional benefits to functional fruit bars and to determine in which way the addition of grape skin influences the physical, chemical, and sensory properties of such bars. Two types of fruit bars were produced—the first containing dates as an additional fruit ingredient, and the second containing figs. Results describing the physical properties of the fruit bars and their extracts are presented in Table 2, and the correlations among tested variables are shown in the form of a bi-plot in Figure 1.
The dry matter content of date-based fruit bars ranged from 46.72 ± 0.22 to 54.73 ± 0.61%, while the dry matter content for the fig-based bars ranged from 48.78 ± 0.42 to 57.57 ± 0.04% (Table 2). The presented results show that the fig-based bars had a slightly higher dry matter content compared to the date-based ones, which is a result of the dry matter difference in the raw materials used to make the bars. In the dependence on the mixture composition, samples with a higher percentage of dry matter, F1 (56.57%) and F6 (56.22%), contain the lowest proportion of grape skin (30%), while samples with a higher percentage of grape skin (50%) also contain a lower percentage of dry matter, e.g., D2 (46.93%) and F2 (48.78%). It is visible that the addition of grape skin in higher concentrations increases the moisture content of the fruit bars. On the other hand, the addition of hazelnuts/cocoa increases the percentage of dry matter in fruit bars, which is a result of the low moisture content of the cocoa powder (<10%). The same relationship between moisture and grape skin content is also visible in bi-plots (Figure 1a,b), where the two variables are situated in opposite quadrants, showing inverse proportionality between the two.
Considering the variability of pH values in grape pomace [53,54], ranging from 3.4 to 5.4, which may consequently alter the pH of food products and thus impact taste, the pH of the ethanolic extract of the produced bars was also determined. pH values for fig-based bars ranged from 4.98 ± 0.01 (samples F1 and F2) to 5.21 ± 0.03 (sample F6) and for the date-based bars from 5.05 ± 0.02 (sample D2) to 5.40 ± 0.04 (sample D9). According to literature data, dates at the Tamar stage have slightly acidic pH values in the range of 5.2 to 6.3, originating from the natural organic acids present in the fruit. Furthermore, the pH value of 5 is often considered to be the limit value associated with an acidic taste, evaluated as a bad character by the consumer [55,56]. In this case, the pH levels of all date-based bars were in accordance with literature data and above the acceptable acidity level. Also, when a higher concentration of grape skin was added to the fruit bar, the pH levels dropped. Based on a study by Kalantari et al. [57], fig extract made using water as a solvent had a pH value of 4.88 ± 0.12, which is slightly lower than all the pH values for fig-based bars determined in this study. Similar to the date bars, the pH of the bars with larger grape skin content was lower, showing an inverse proportionality among the two variables (Figure 1a,b).
TDS values ranged from 34.35 ± 1.77 (D7) to 47.85 ± 0.64 mg/L (D4) for date-based bars and from 33.35 ± 0.21 (F2) to 38.10 ± 0.42 mg/L (F3) for fig-based bars. Conductivity values ranged from 71.15 ± 0.64 (D7) to 96.50 ± 0.71 mS/cm (D4) for date-based bars and from 66.75 ± 0.07 (F2) to 76.65 ± 0.64 mS/cm (F3) for fig-based bars. Literature data confirms the correlation between TDS and conductivity [58], and it is visible that conductivity values follow the same trend as TDS values in this study. The effect of grape skin addition on the TDS and conductivity values is not evident for the date-based bars. In fact, it seems that the most pronounced effect on TDS and conductivity is derived from the cocoa/hazelnut—mixture containing the highest amounts of cocoa/hazelnuts (e.g., D3 and D4), which also had the highest values of TDS and conductivity. In fig mixtures, the effect of grape skin and cocoa/hazelnut addition also does not show an evident trend, nor was a linear proportional or inverse proportional trend seen in bi-plots (Figure 1a,b).

3.2. Chemical Properties of Fruit Bar Extracts

To determine the effect of grape skin addition on the chemical properties of the bars, TPC and antioxidant activities (DPPH, FRAP) of the bar extracts were determined and the results are shown in Figure 2.
TPC content of the date-based bars ranged from 2.09 (D1) to 4.88 mg GAE/g dm (D5) (Figure 2a). It is visible that the TPC content varies depending on the addition of grape skin and cocoa/hazelnut. For example, the TPC values of the samples with low grape skin content were also among the lowest for all samples, while the samples with low cocoa content also exhibited lower TPC values compared to others. It is known from literature data that grape pomace contains a remarkable amount of polyphenols, e.g., a study by Radulescu et al. analyzes the polyphenol content and antioxidant activity of grape pomace originating from different grape varieties, and the TPC content ranged from 17.161 ± 1.346 mg GAE/g dm to 26.654 ± 3.356 mg GAE/g dm, emphasizing the higher values for red varieties relative to white ones [59]. Monteiro et al. reported a range of TPC from 23.2 to 37.9 g/kg which was dependent on the cultivar, and they successfully identified 14 different bioactive molecules [60]. Therefore, the addition of pomace-based grape or grape skin to functional products should lead to the enhancement of their bioactive profile, which was, indeed, confirmed in this study. However, it is important to emphasize that, in this case, the date content also played an important role, since it is known from literature data that dates are rich sources of polyphenols. According to a meta-analysis by AlFaris et al. [61], total polyphenolic contents of dates varied significantly depending on the place of origin, dry matter content, and solvent used for extraction, and ranged from 4.36 mg GAE/100 g dm to 753.30 mg GAE/100 g dm, based on 11 studies selected for the meta-analysis. This confirms that the date also contributed to the TPC values of the bars. Furthermore, the effect of cocoa also cannot be neglected, since cocoa powder can contain up to 611 mg GAE per serving (7.3 g) [62].
The antioxidant capacity determined by the FRAP method ranged from 0.097 (D1) to 0.227 (D5) mmol FeSO4 · 7H2O/g dm (Figure 2c), and trends are in accordance with the TPC contents, which usually correlates with the antioxidant capacity [63]. The same is valid for the antioxidant capacity determined by the DPPH method, where the values ranged from 0.0053 (D1) to 0.0213 (D9) mmol Trolox/g dm, respectively (Figure 2b). In comparison, potential functional date fruit bars were developed in several studies as follows: Maisto et al. [64] developed a date bar containing Lactobacillus spp. and concluded that the newly developed bar had a TPC content of 47.27–77.56 mg GAE/g bar and DPPH values of 111.05–150.13 mg Trolox/g bar, depending on the Lactobacillus strain used for fermentation. Date bars developed by Safdar et al. [65] contained different herbal extracts and had a TPC content of 251.99 ± 10.24 mg GAE per date bar for the 3% moringa leaf extract-fortified bar, whereas the unfortified date had the lowest polyphenol content (24.13 ± 3.61 mg GAE per date bar). However, the extraction of polyphenols from the developed bar was performed by ultrasound, and thus the much higher TPC values compared to this study were obtained.
Fig-based bars exhibited TPC values in the range from 1.499 (F5, sample with 40% grape skin and no cocoa/hazelnut) to 4.381 (F4, sample with 50% grape skin and 2% cocoa/hazelnut) mg GAE/g dm, and an evident change in TPC values with higher proportions of grape skin and cocoa was observed (Figure 2d). The lowest value of FRAP-determined antioxidant capacity was measured for sample F1 (sample with 30% grape skin and no cocoa/hazelnut) (0.072 mmol FeSO4·7H2O/g dm), and the highest for sample F9 (sample with 40% grape skin and 1% cocoa/hazelnut) (0.114 mmol FeSO4·7H2O/g dm) (Figure 2f). As for the DPPH method, values ranged from 0.0027 mmol Trolox/g dm (F1) to 0.0186 mmol Trolox/g dm (F4) (Figure 2e). In this case, it is also clear that the addition of grape skin and cocoa contributes to a rise in TPC and antioxidant capacity. According to a study by Solomon et al. [66], fig extracts produced by maceration contained 48.6 to 281.1 mg GAE/100 g of fruit polyphenols, which is highly dependent on the cultivar. On the other hand, functional bars produced using figs and supplemented with roselle can contain up to 284.22 mg GAE/100 g freshly prepared bar and an antioxidant capacity of 5454.61 μmol TE/g of freshly prepared bar [67].
When comparing date and fig bars, it was evident that date bars exhibited higher TPC and antioxidant capacity values. These results are in accordance with literature data, which states that dates contain more polyphenols and a higher antioxidant capacity compared to figs [61,66]. Furthermore, the use of dates in fruit bar development is mentioned in many literature sources [68,69], while studies utilizing figs are present to a lesser extent [67,70], mostly due to their higher price and lower bioactive contents.

3.3. Sensory Properties of Newly Developed Fruit Bars

The sensory properties of newly developed fruit bars were evaluated using a hedonistic scale, where each parameter was assigned a rating from 1 to 5 (1—the lowest acceptability, 5—the highest acceptability). For each evaluated property, the mean rating was taken, and the results are shown in Figure 3 in the form of a “spider web” for each group of fruit bars, whereby the estimated intensity of each of the evaluated sensory properties increases with the increased distance from the central point [71].
Looking at the sensory scores for each group separately, in the date group (Figure 3a), sample D6 received the highest appearance score (4.17), while sample D2 received the lowest (2.83). The date samples appeared to be more homogenous, without visible large chunks of dried fruit or grape skin. For color, which was considered acceptable if similar to that of milk or dark chocolate, sample D6 received the highest score (4.17), while sample D2 (3.33) received the lowest score. For odor, sample D3 received the highest score (4.33), while sample D9 received the lowest score (3.00). Considering the fact that the lowest score for odor was 3.0, it can be said that the date, cocoa, and hazelnut as components of the bars managed to mask the odor of grape skin present in the samples. The sample with the highest graded sweetness was D1 (4.00), while several samples, D2, D4, and D5, received the same lowest score (3.17). For the property of bitterness, samples D1, D3, D6, and D9 have the same highest score (3.67), while samples D2 and D4 have the same lowest score (3.17). Sample D3 (4.00) has the best aftertaste, while samples D4 and D5 share the lowest score. The highest score for texture was given to sample D6 (4.17), while the lowest score (3.17) was shared by samples D1, D2, D4, and D7. According to different studies of consumer acceptance of fruit bars, most consumers prefer good flavor and aroma characteristics of the bar, followed by texture, while the “healthy living” concept was often not considered significant [28,72,73]. In this study, properties were connected to texture, and appearance and color of date-based bars received higher scores in comparison to those connected to aroma, which is an indication that there is space for further improvement on the aroma scale, which would then, consequently lead to higher acceptability of the date-based bars.
In the fig group (Figure 3b), sample F6 received the highest appearance score (4.67), while sample F3 received the lowest score (3.00). For color, samples F1 and F6 share the same highest score (4.33), while sample F3 received the lowest score (3.17). For odor, samples F8 and F9 share the highest score (4.67), while samples F4 and F7 have an identical lowest score (4.17). The sample with the highest sweetness score was F6 (4.50), while the lowest score was given to sample F3 (2.83). For bitterness, sample F1 received the highest score (4.67), and sample F5 received the lowest score (2.83). Samples F6, F8, and F9 have the best aftertaste (4.17), while samples F2, F3, and F5 share the lowest score (3.33). The highest score for texture was given to sample F6 (4.33), while the lowest score was given to sample F4 (3.33). According to a review by Orrego et al. [28], in an overall analysis of sensory properties, aromas, and flavors have the most influence on consumer preferences, with color and appearance also proving to be an important factor. For fig-based bars, characteristics connected to the appearance, color, and texture received lower scores in comparison to those connected to the aroma components (sweetness, bitterness, aftertaste, and odor). The lower texture and appearance grades were due to the fact that fig contains seeds which are grainy in nature and visible in the fruit bar mixtures. Also, fig has a lighter color than date, which is less similar to chocolate, and thus the lower grades. This indicates that further improvements in the areas of texture and appearance of fig-based bars are possible (e.g., the addition of more cocoa or hazelnuts). From an aroma point of view, the fig-based bars outscored the date-based ones, indicating that consumer preference is shifted toward figs as components of the bars.

3.4. Modeling and Estimation of Significant Effects of Functional Ingredients on the Newly Developed Fruit Bar Properties

To get a detailed insight into how new functional ingredients affect the properties of fruit bars, experiment design analysis was performed which included ANOVA with date/fig content (A), grape skin content (B), and cocoa/hazelnut content (C) as independent variables and chemical and sensory properties as dependent variables. Significant effects estimated by a quadratic model with pooled AB, AC, and BC effects are shown in the form of Pareto diagrams in Figure 4 (date bars) and Figure 5 (fig bars).
For date-based fruit bars chemical properties (TPC, DPPH, and FRAP), significant (p < 0.05) influences of date and grape skin contents are visible (Figure 4a–c): higher date and grape skin contents lead to higher TPC and antioxidant capacity values. In a comparison of the two, grape skin contributed to the chemical composition to a higher degree than date. Based on the presented results, grape skin can be considered as a promising ingredient for the development of novel functional food products, since it contributes significantly to TPC and antioxidant capacity. Another aspect of the study was its influence on the sensory properties, which usually presents a bigger problem due to the specific taste of grape skin, as well as the consumers’ perception towards eating a winemaking by-product. Significant influence of date and grape skin content was detected on all analyzed sensory properties (Figure 4d–j), with all of them being the most influenced by date content. This proved to be beneficial for the product since the consumers apparently did not mind the grape skin in the product because its taste was masked by date. Also, higher content of grape skin did not lead to deteriorated sensory scores. Cocoa and hazelnut content only had a significant influence on the sweetness, bitterness, and texture of the product.
In the case of fig-based bars, TPC was significantly affected by all the mixture components (Figure 5a), while the DPPH- and FRAP-measured antioxidant capacity was highly dependent on fig and grape skin content (Figure 5b,c), with the primary effect being that of grape skin. In the case of sensory properties, all of the analyzed sensorial parameters were dependent on both, grape skin and fig content, while odor was also dependent on the cocoa and hazelnut content (Figure 5d–j). The addition of grape skin did not have a negative effect on either of the sensory properties, indicating that, in addition to contributing to a better TPC and antioxidant capacity, it does not have adverse effects on taste, and can, therefore, be successfully used as a functional ingredient. However, it is important to emphasize that grape skin use and the further development of grape skin-containing products should be in combination with other dried fruit. In that aspect, researchers have to bear in mind that different ingredients and the combinations thereof differently affect chemical properties, texture, sensory properties, and consumer acceptance [28].

3.5. Optimization and Validation

Based on the collected experimental data, the response desirability profiling method was used to define the optimal mixture composition, and the validation experiments were performed to test the adequacy of the developed model. Results are shown in Table 3.
As seen in Table 3, the optimal ratio of mixture components was the same for both bars: 64.5% date/fig, 33.58% grape skin, and 1.92% cocoa/hazelnut. The model-predicted values differed significantly for the antioxidant capacity determined by the FRAP method (RPE = −86.96% for date and RPE = −82.09% for fig), meaning that the model did not prove to be appropriate for the prediction of this property. Smaller RPE values were determined for the TPC and DPPH. In both cases, the error was smaller for the date-based bar than the fig-based one. Sensory properties of the date-based bars were adequately predicted by the model, with the exception of appearance and color, where the RPE error values were −19.34% and −28.35%. For the sensory properties of the fig-based bars, the RPE values were below the acceptable 10% level for appearance, odor, sweetness, aftertaste, and texture, while the model did not prove to be applicable for the prediction of color and bitterness.

3.6. Nutritional Composition and Microbiological Safety of Newly Developed Fruit Bars

After the optimal composition of the bars was defined, the nutritional composition and the microbiological compliance were tested and the results are shown in Table 4.
Based on the values shown in Table 4, it can be seen that date-based bars and fig-based bars have a similar nutritional composition, with the energy values ranging from 949 kJ for date-based and 967 for fig-based bars. A slightly higher content of carbohydrates was exhibited by fig-based bars, while the values for proteins, salt, and fiber were also very similar. The NutriScore values of the bars were −6 (date) and −5 (fig), which puts them in the A NutriScore group, which is considered to be the group with the highest nutritional quality. In a similar study, the nutritional composition of fruit bars based on fig and roselle was studied by Aslam et al., who reported that the protein content ranged from 8.48 to 9.93%, fiber content ranged from 2.12 to 4.10%, and fat content ranged from 0.42 to 0.60%, all depending on the ratio of fig to roselle added to the bar [67]. In comparison, the fig- and date-based bars developed in this study have markedly higher fiber contents and higher fat content, but lower protein content, which is a consequence of differences in raw materials used in bar production. Furthermore, Ghazal et al. reported a carbohydrate content of date-based fruit bars in the range of 56.65 to 59.81%, which was higher in comparison to the bars developed in this study [69].
Furthermore, microbiological analysis revealed that both types of produced bars are in accordance with microbiological requirements for foods. The microbiological analysis of the yeasts and mold content also showed the samples conform to legislation which is important since molds from the Aspergillus genera can produce ochratoxins, which are classified as possible human carcinogens [74].
The results from this study confirm that the newly developed bars, in addition to the beneficial content of bioactives and good sensory scores, also conform with the highest nutritional and microbiological standards.

4. Conclusions

This study aimed to evaluate the possibility of grape pomace use in the development of functional fruit bars and to determine what effects the grape skin addition had on the final product. The fruit bars with grape skin addition exhibited high TPC content and antioxidant capacity while scoring good sensory points at the same time. The addition of pomace proved beneficial for the TPC content and antioxidant capacity. Furthermore, consumers appeared to like the newly developed functional product, and the addition of grape skin did not have an adverse effect on sensory properties. The optimal ratio of grape pomace in both fig- and date-based bars is 33.58%. The bars were graded A based on the NutriScore value and were microbiologically compliant with food safety regulations. These results confirm the initial hypothesis that grape skin can be used in the development of a functional fruit bar product, which can be beneficial not only from a chemical and sensory point of view but also economically feasible and environmentally friendly.

Author Contributions

M.B. conceptualization, methodology, writing—original draft; F.C. data curation, formal analysis; D.V. data curation, writing—review and editing; T.S.C. formal analysis, writing—review and editing; A.J.T. formal analysis, writing—review and editing, T.J. writing—review and editing, J.G.K. writing—review and editing; I.R.R. funding acquisition, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research has been funded by the European Union through the European Regional Development Fund, Competitiveness, and Cohesion 2014–2020, project code KK.01.1.1.01.0007.

Data Availability Statement

Research data is available upon request. To request the data, contact the corresponding author of the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 12 02941 g001
Figure 1. Bi-plot of variables and date-based samples (a) and fig-based samples (b). Variables marked red are considered supplementary, while the responses (dry matter, TDS, conductivity, and pH) are the analyzed variables—marked blue.
Figure 1. Bi-plot of variables and date-based samples (a) and fig-based samples (b). Variables marked red are considered supplementary, while the responses (dry matter, TDS, conductivity, and pH) are the analyzed variables—marked blue.
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Figure 2. TPC content (a), antioxidant capacity determined by the DPPH method (b) and the FRAP method (c) for the date-based fruit bars. TPC content (d), antioxidant capacity determined by the DPPH method (e) and the FRAP method (f) for the fig-based fruit bars. Different letters/symbols above bars of the same measured chemical property represent significant differences at p < 0.05.
Figure 2. TPC content (a), antioxidant capacity determined by the DPPH method (b) and the FRAP method (c) for the date-based fruit bars. TPC content (d), antioxidant capacity determined by the DPPH method (e) and the FRAP method (f) for the fig-based fruit bars. Different letters/symbols above bars of the same measured chemical property represent significant differences at p < 0.05.
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Figure 3. Sensory properties of fruit bars: (a) date, (b) fig.
Figure 3. Sensory properties of fruit bars: (a) date, (b) fig.
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Processes 12 02941 g004aProcesses 12 02941 g004b
Figure 4. Pareto charts for the estimation of the effects of date content, grape skin content, and cocoa/hazelnut content on (a) TPC, (b) FRAP, (c) DPPH, (d) appearance, (e) color, (f) odor, (g) sweetness, (h) bitterness, (i) aftertaste, and (j) texture of the produced date-based fruit bars. Values next to the bars represent correlation coefficients, while the red line represents the significance level (p < 0.05).
Figure 4. Pareto charts for the estimation of the effects of date content, grape skin content, and cocoa/hazelnut content on (a) TPC, (b) FRAP, (c) DPPH, (d) appearance, (e) color, (f) odor, (g) sweetness, (h) bitterness, (i) aftertaste, and (j) texture of the produced date-based fruit bars. Values next to the bars represent correlation coefficients, while the red line represents the significance level (p < 0.05).
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Processes 12 02941 g005aProcesses 12 02941 g005bProcesses 12 02941 g005c
Figure 5. Pareto charts for the estimation of the effects of date content, grape skin content, and cocoa/hazelnut content on (a) TPC, (b) FRAP, (c) DPPH, (d) appearance, (e) color, (f) odor, (g) sweetness, (h) bitterness, (i) aftertaste, and (j) texture of the produced fig-based fruit bars. Values next to the bars represent correlation coefficients, while the red line represents the significance level (p < 0.05).
Figure 5. Pareto charts for the estimation of the effects of date content, grape skin content, and cocoa/hazelnut content on (a) TPC, (b) FRAP, (c) DPPH, (d) appearance, (e) color, (f) odor, (g) sweetness, (h) bitterness, (i) aftertaste, and (j) texture of the produced fig-based fruit bars. Values next to the bars represent correlation coefficients, while the red line represents the significance level (p < 0.05).
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Table 1. Design for Constrained Mixtures for the production of fruit bars containing grape skin. Samples were marked with numbers 1 to 9, with the letter in front of the number representing the dried fruit type used in the mixture (D—date, F—fig).
Table 1. Design for Constrained Mixtures for the production of fruit bars containing grape skin. Samples were marked with numbers 1 to 9, with the letter in front of the number representing the dried fruit type used in the mixture (D—date, F—fig).
SampleDate/Fig Content
(g/100 g)		Grape Skin Content
(g/100 g)		Cocoa/Hazelnut Mix Content
(g/100 g)
D1/F170300
D2/F250500
D3/F368302
D4/F448502
D5/F560400
D6/F669301
D7/F749501
D8/F858402
D9/F959401
Table 2. Physical properties of fruit bars and their extracts (* fruit bar, ** extract).
Table 2. Physical properties of fruit bars and their extracts (* fruit bar, ** extract).
SampleDry Matter * (%)pH **TDS ** (mg/L)Conductivity ** (mS/cm)
Date—based bars
D153.77 ± 1.68 a5.20 ± 0.01 a41.05 ± 0.21 a81.85 ± 0.64 a
D246.93 ± 0.86 b5.05 ± 0.02 b37.65 ± 0.07 b75.55 ± 0.07 b
D354.73 ± 0.61 a5.19 ± 0.03 c42.50 ± 0.42 c84.60 ± 0.57 c
D449.86 ± 1.06 c5.16 ± 0.04 c47.85 ± 0.64 d96.50 ± 0.71 d
D548.66 ± 0.05 d5.12 ± 0.06 d35.65 ± 0.07 e71.25 ± 0.21 e
D651.68 ± 0.00 e5.21 ± 0.08 a38.45 ± 0.35 f76.40 ± 1.13 f
D746.72 ± 0.22 b5.16 ± 0.05 c34.35 ± 1.77 e71.15 ± 0.64 e
D850.05 ± 0.30 c5.25 ± 0.03 d36.20 ± 0.71 b73.75 ± 0.78 g
D949.69 ± 1.31 c5.40 ± 0.04 e36.10 ± 1.27 b72.75 ± 0.92 g
Fig—based bars
F156.57 ± 0.28 A4.98 ± 0.01 A34.50 ± 0.00 A69.80 ± 1.13 A
F248.78 ± 0.42 B4.98 ± 0.01 A33.35 ± 0.21 B66.75 ± 0.07 B
F357.57 ± 0.04 A5.19 ± 0.12 B38.10 ± 0.42 C76.65 ± 0.64 C
F453.07 ± 1.27 C5.05 ± 0.05 C34.05 ± 0.07 A67.95 ± 0.35 A
F551.67 ± 2.87 D5.03 ± 0.01 C36.20 ± 1.13 D73.90 ± 0.14 D
F656.22 ± 0.21 A5.21 ± 0.03 B34.05 ± 0.07 A67.45 ± 0.92 A
F751.90 ± 0.96 D5.09 ± 0.05 D34.30 ± 0.28 A68.05 ± 0.92 A
F854.81 ± 0.14 E5.13 ± 0.02 E35.40 ± 0.85 E71.95 ± 0.64 E
F956.18 ± 0.02 A5.13 ± 0.02 E35.15 ± 2.05 E72.50 ± 0.57 E
Different letters in the columns for the same type of bar (fig or date) represent significant differences at p < 0.05.
Table 3. Optimal mixture composition and a comparison between model-obtained values and the validation experiment.
Table 3. Optimal mixture composition and a comparison between model-obtained values and the validation experiment.
Date Optimal Mixture Composition: 64.5% Date, 33.58% Grape Skin, 1.92% Cocoa/HazelnutFig Optimal Mixture Composition: 64.5% Fig, 33.58% Grape Skin, 1.92% Cocoa/Hazelnut
ParameterModel Predicted Values *Validation ExperimentRPEModel Predicted Values *Validation ExperimentRPE
TPC3.31003.8335 ± 0.003815.812.59003.3772 ± 0.003430.39
FRAP0.14040.0183 ± 0.0005−86.960.09660.0173 ± 0.0008−82.09
DPPH0.01230.0123 ± 0.00100.000.00840.0124 ± 0.001047.62
Appearance3.933.17 ± 1.17−19.343.553.20 ± 0.84−9.86
Color3.952.83 ± 0.98−28.353.753.20 ± 0.45−14.67
Odor3.843.83 ± 0.41−0.264.494.60 ± 0.552.45
Sweetness3.783.83 ± 0.411.323.823.60 ± 1.14−5.76
Bitterness3.593.67 ± 0.512.234.033.60 ± 1.14−10.67
Aftertaste3.813.83 ± 0.410.523.853.60 ± 1.14−6.49
Texture3.743.50 ± 0.55−6.423.763.40 ± 1.34−9.57
* Model-obtained values were all calculated within the values of a 95% confidence interval. RPE = relative percentage error (%).
Table 4. Nutritional composition and microbiological analysis of the optimized date- and fig-based fruit bars with the addition of grape skin.
Table 4. Nutritional composition and microbiological analysis of the optimized date- and fig-based fruit bars with the addition of grape skin.
Date-Based BarFig-Based BarReference Values *
Nutritional analysis
Energy (kJ/kcal)949/225967/230/
Fats (g/100 g)
Of which saturated fats (g/100 g)		2.3
0.4		3.2
0.2		/
Carbohydrates (g/100 g)
Of which sugars (g/100 g)		43.4
<0.5		42.2
<0.5		/
Proteins (g/100 g)2.93.2/
Salt (g/100 g)0.20.3/
Fiber (%)9.39.6/
Nutri-Score (/)−6 (A)−5 (A)/
Microbiological analysis
Aerobic mesophilic bacteria (CFU/g)<100<100100–1000
Staphylococcus aureus (CFU/g)<10<1010–100
Enterobacteriaceae (CFU/g)<10<1010–100
Yeasts and molds (per 25 g)<100<10010–100
Salmonella spp. (CFU/g)n.d per 25 gn.d per 25 gn.d. per 25 g
* Reference values are based on the Croatian Law on food hygiene and microbiological criteria for food. n.d stands for not detected.
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MDPI and ACS Style

Benković, M.; Cigić, F.; Valinger, D.; Sokač Cvetnić, T.; Jurinjak Tušek, A.; Jurina, T.; Gajdoš Kljusurić, J.; Radojčić Redovniković, I. Towards Wine Waste Reduction: Up-Cycling Wine Pomace into Functional Fruit Bars. Processes 2024, 12, 2941. https://doi.org/10.3390/pr12122941

AMA Style

Benković M, Cigić F, Valinger D, Sokač Cvetnić T, Jurinjak Tušek A, Jurina T, Gajdoš Kljusurić J, Radojčić Redovniković I. Towards Wine Waste Reduction: Up-Cycling Wine Pomace into Functional Fruit Bars. Processes. 2024; 12(12):2941. https://doi.org/10.3390/pr12122941

Chicago/Turabian Style

Benković, Maja, Filip Cigić, Davor Valinger, Tea Sokač Cvetnić, Ana Jurinjak Tušek, Tamara Jurina, Jasenka Gajdoš Kljusurić, and Ivana Radojčić Redovniković. 2024. "Towards Wine Waste Reduction: Up-Cycling Wine Pomace into Functional Fruit Bars" Processes 12, no. 12: 2941. https://doi.org/10.3390/pr12122941

APA Style

Benković, M., Cigić, F., Valinger, D., Sokač Cvetnić, T., Jurinjak Tušek, A., Jurina, T., Gajdoš Kljusurić, J., & Radojčić Redovniković, I. (2024). Towards Wine Waste Reduction: Up-Cycling Wine Pomace into Functional Fruit Bars. Processes, 12(12), 2941. https://doi.org/10.3390/pr12122941

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