The Effect of Sweetener Type on the Quality of Liqueurs from Vaccinium myrtillus L. and Vaccinium corymbosum L. Fruits

Abstract

This study aimed to investigate the effect of the type of sweetener used (xylitol, stevia, cane sugar) on the quality of liqueurs made from Vaccinium myrtillus L. and Vaccinium corymbosum L. fruits. The quality assessment was performed based on selected organoleptic and physicochemical features, with particular emphasis on the health-promoting potential of the produced beverages. The liqueurs were assessed in terms of their physicochemical parameters: pH, total acidity, density, total soluble solids, color, ethanol and polyphenol contents, and redox potential. Antioxidant capacities were determined by a 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity assay and ferric reducing antioxidant power (FRAP). The Qualitative Descriptive Analysis method was employed for their sensory assessment. The sensory profiling method was used to determine the intensity of the flavor sensations. The study results showed that the type of sweetener did not affect the antioxidative properties of the liqueur. The ABTS test yielded values from 1081.88 to 1238.13 μmol Tx/100 mL, the DPPH test from 348.8 to 367.88 μmol Tx/100 mL, and the FRAP test from 594.20 to 653.20 μmol FeSO₄/100 mL. However, the sweetening substrate affected the content of polyphenolic compounds in the resulting products, but by no more than 15%. The liqueur sweetened with xylitol had a comparable extract content to that sweetened with cane sugar. All three variants of liqueurs were accepted by the evaluation panel, and their overall qualities were comparable in the sensory assessment. It is, therefore, possible to produce a high-quality liqueur with a reduced caloric value, which will potentially increase its attractiveness for consumers.

1. Introduction

Since 2020, global liqueur sales have been observed to increase by several per cent every year [1]. Fruit alcoholic beverages, such as tinctures and liqueurs, are becoming increasingly popular among persons who search for traditional products offering exceptional organoleptic qualities and health-promoting properties. According to the Polish food law, “liqueurs” are spirit drinks produced by macerating agricultural products and foodstuffs for at least 15 days [2]. However, there is no category of alcoholic beverages corresponding to Polish “liqueurs” in the European (EU) and American (USA) legislation, which classify such beverages as fruit liqueurs [3,4]. The range of fruit liqueurs is very wide. Many companies offer a variety of brands, and individual countries manufacture local products depending on the availability of raw materials [5].

Treating fruits with alcohol results in the release and extraction of active components from the raw material. These include, among others, phenolic compounds (e.g., anthocyanins, flavonols, phenolic acids) exhibiting antioxidative activity [6,7], which mitigates the pro-oxidative effect of alcohol [8]. Dark blue or red berries are most often used to produce liqueurs due to their high antioxidative capacity [9]. Bilberries (Vaccinium myrtillus L.) are used to produce a traditional liqueur in Slovenia. They have a high content of phenolic compounds, which are accumulated both in the skin and in the red pulp. However, the harvesting of bilberries is time-consuming because they are wild-growing small fruits. Given the above, cheaper and more accessible alternatives are sought, such as the fruits of the northern highbush blueberry (Vaccinium corymbosum L.), a cultivated shrub with larger and easier-to-harvest fruits. In their case, however, phenolic compounds are accumulated only in the skin. Scientific research has shown that liqueurs produced from highbush blueberries differ from those made of bilberries, not only in terms of content but also in terms of the profile of phenolic compounds. In bilberry liqueur, 36 phenolic compounds have been identified: 11 anthocyanins, 15 hydroxycinnamic acids, 1 flavanol, 2 flavones, and 7 flavonols. But in blueberry liqueur, only 21 phenolic compounds have been identified: 10 anthocyanins, 10 hydroxycinnamic acids and 1 flavonol [10]. For these reasons, the simultaneous use of both types of fruits may enable the production of liqueurs with a broad variety of bioactive compounds.

There has been an increase in the development of liqueur using sugar from sugar beets, which is much cheaper than traditional sweeteners [11]. Sugars are important ingredients in liqueurs and can be obtained from many sources depending on the price, availability, legality, and required properties of the finished product. Apart from sugar cane or sugar beet sucrose, other sweet substances can be used to this end, including, e.g., glucose or high-fructose syrups, honey, rectified concentrated must or other natural carbohydrate substances with effects equivalent to the above-mentioned products [2,3]. It is, therefore, possible to influence the caloric value of alcoholic fruit beverages by using an appropriate sweetening substrate, which additionally affects their biological (e.g., antioxidative and anti-inflammatory) and technological (e.g., rheological and textural) properties.

Natural sweeteners considered to be health-promoting alternatives to sucrose include xylitol and stevia. Since they do not affect blood levels of insulin, they can be recommended to diabetic patients as well as to anyone who pursues a healthy lifestyle. Xylitol is one of the most thoroughly studied sugar alcohols. It is characterized by a pleasant, sweet taste, accompanied by a delicate cooling sensation. It offers many valuable technological properties to the food industry, such as anti-caking, filling, emulsifying, stabilizing, thickening, and flavor-enhancing effects. Its conventional applications include coating hard candy, using it as a powder for dusting chewing gum, and the production of frozen desserts, baked goods, confectionery, chocolate, ice cream and beverages. In turn, steviol glycosides are considered natural, calorie-free sweeteners with a sweetening power significantly higher than that of sucrose. Stevia and its derivatives can be used as sweeteners in many food products, such as dairy products, bakery products, and beverages, including functional beverages rich in polyphenols [12].

Bearing the above in mind, this study aimed to investigate the effect of the type of sweetener used on the quality of liqueurs made from bilberry and highbush blueberry fruits, assessed based on selected organoleptic features and physicochemical parameters, with particular emphasis on their health-promoting potential. Three different sweeteners were used to produce the liqueurs: xylitol (X), stevia (S), and cane sugar (CS). To the best of the authors’ knowledge, no studies have been conducted so far on the feasibility of using xylitol and stevia in the production of alcoholic beverages, nor on the effects of the sweetener used on the quality of the fruit liqueurs.

2. Materials and Methods

2.1. Production of Liqueurs

This study investigated three liqueurs produced using fruits of bilberry (Vaccinium myrtillus L.) and highbush blueberry (Vaccinium corymbosum L.) of domestic origin, available in retail stores. Each liqueur was produced from 0.5 kg of bilberries and 0.5 kg of highbush blueberries. Prior to maceration, the berries were sorted, washed, and frozen at a temperature of −18 °C. The frozen fruits were macerated in previously sterilized glass jars by filling them with 0.5 L of 95% cereal ethanol and 0.5 L of 40% cereal ethanol, and adding a sweetener. The jars were tightly closed and left at room temperature with access to sunlight for 4 weeks. Their contents were shaken every day.

Although the liqueurs differed in terms of sweetener type, all sweeteners tested were applied in doses ensuring a sweetness level corresponding to that induced by 300 g of saccharose. The first liqueur (LX) was produced with the addition of 300 g of 100% xylitol (Danisco Sweeteners, Kotka, Finland), the second liqueur (LS) with 2.4 g of 97% stevia (NatuSweet, Perchtoldsdorf, Austria), and the third one (LCS) using 300 g of cane sugar (Bio Planet, Leszno, Poland).

After maceration, the liqueurs were filtered through 3 W paper filters and poured into bottles. The maceration produced 1400 mL, 1478 mL, and 1464 mL of liqueurs sweetened with cane sugar, xylitol, and stevia, respectively. Water was added to each liqueur in an amount of 20% of the volume of the liquid obtained after filtration. Then the liqueurs were subjected to 8-month maturation at a temperature of 1820 °C.

2.2. Physicochemical Analysis

The liqueurs were assessed in terms of the physicochemical parameters pH, total acidity, density, total soluble solids (TSS), color, ethanol content, polyphenol content, and redox potential by means of a 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity assay and ferric reducing antioxidant power (FRAP). The content of polyphenols and the redox potential of the liqueurs were determined according to the procedures described in detail by Bhajan et al. (2023) [13]. Their pH was measured using a CP-505 pH meter (ELMETRON, Zabrze, Poland). Their total acidity was determined with the titration method, and the results are expressed in g of citric acid. The liqueurs’ density was measured at a temperature of 20 °C using a Balling aerometer, whereas their ethanol content was determined using a Hach Lange LCK 300 alcohol cuvette test (HACH LANGE GMBH). The color of the liqueur samples was measured using a trichromatic CR-310 colorimeter (Konica Minolta, Tokyo, Japan) with C65 illuminant and presented in the CIE L*a*b* color space based on three color components: luminance (L*) in the range from 0 (black color) to 100 (white color), parameter a* indicating the contribution of colors from green (<0) to red (>0), and parameter b* indicating the contribution of colors from blue (<0) to yellow (>0) in the color profile. The values of parameters a* and b* were then used to compute the chroma (C*—a measure of color intensity) and hue (h°—hue tone angle) [14].

$$C^{\ast }=\sqrt{a^{\ast 2}+b^{\ast 2}}$$

$$h^{°}={tan}^{-1}\left({\displaystyle \frac{b^{\ast }}{a^{\ast }}}\right)$$

The total color difference (ΔE) was calculated from Equation (1):

$$∆E=\sqrt{({{∆L}^{\ast })}^2+{\left({∆a}^{\ast }\right)}^2+{\left({∆b}^{\ast }\right)}^2}$$

(1)

where ΔL*, Δa*, and Δb* mean the differences in L*, a*, and b* values between the compared liqueurs.

2.3. Sensory Analysis

The liqueurs were subjected to sensory assessment by means of a Qualitative Descriptive Analysis (QDA), which consisted of a quantitative evaluation of individual quality attributes, i.e., color, texture, aroma, perception of alcohol content, and palatability, with the latter understood as the general flavor, aroma, and sensory sensations, which were components of the overall quality of the analyzed products. They were evaluated on a 5-point scale. Weighting factors were established for the individual quality attributes arbitrarily and reached 0.16, 0.16, 0.2, 0.18, and 0.3, respectively. They were used to compute the overall quality of the liqueurs.

Next, the sensory profiling method was deployed to determine the intensity of the perception of the following flavors: fruity, sweet, metallic, and sour. The intensity was described on a 5-point scale ranging from “non-perceptible” to “very intense” [15].

The sensory assessment of individual liqueurs was performed by a panel of 12 evaluators with the required sensory sensitivity set in the Polish Standard PN-ISO 8586:2014 [16]. Samples of liqueurs (20 mL) were prepared in plastic, transparent, randomly numbered glasses (40 mL) and served on white sheets of paper to the panelists at the sensory evaluation stations. The panelists were also provided with water. The stations were lit with daylight. The testing room was ventilated and free from extrinsic odors. The temperature in the room was 23 °C. The panelists answered questions in an online survey.

2.4. Statistical Analysis

All physicochemical analyses were conducted with three replications. The statistical analysis of results was carried out using Statistica 13.3 software. The significance of differences between groups was established based on one-way analysis of variance (ANOVA) and Tukey’s post hoc test (p ≤ 0.05). In addition, Pearson’s correlation analysis was conducted between selected results of the physicochemical analyses and the overall quality of the liqueurs determined in the organoleptic assessment.

3. Results

3.1. Physicochemical Parameters

As assumed in the experiment, there were no statistically significant differences in ethanol content between the liqueurs. The sweetening substrates used to produce the liqueurs did not affect their pHs. In contrast, significant differences were found in the total acidity, the highest value of which was determined in the liqueur produced with stevia as a sweetener (2.76 g citric acid/L). In the two remaining variants of liqueurs, the total contents of organic acids were statistically significantly lower and reached the same values (

Table 1. Physicochemical parameters of liqueurs made of bilberry and highbush blueberry with xylitol (LX), stevia (LS), and cane sugar (LCS) as sweeteners.

Parameter		Type of Liqueur
		LX	LS	LCS
pH [-]		3.71 ± 0.02 *	3.72 ± 0.02 *	3.72 ± 0.02 *
Total acidity [g citric acid/L]		2.46 ± 0.07 *	2.76 ± 0.06 **	2.43 ± 0.02 *
Density [°Blg]		6.17 ± 0.03 *	<5 **	8.6 ± 0.00 ***
TSS [°Bx]		21.97 ± 0.06 *	13.67 ± 0.15 **	23.03 ± 0.25 ***
Ethanol [%]		36.45 ± 3.54 *	38.79 ± 2.81 *	40.83 ± 2.81 *
DPPH [µmol Tx/100 mL]		367.88 ± 5.17 *	360.26 ± 15.09 *	348.8 ± 10.93 *
ABTS [µmol Tx/100 mL]		1081.88 ± 196.62 *	1188.13 ± 186.23 *	1238.13 ± 181.57 *
FRAP [µmol FeSO4/100 mL]		653.20 ± 42.75 *	594.20 ± 38.31 *	598.20 ± 51.00 *
Polyphenols [mg gallic acid/100 mL]		95.36 ± 1.99 *	92.93 ± 2.10 *	109.16 ± 1.96 **
Color	L*	23.82 ± 0.12 *	23.41 ± 0.01 **	24.00 ± 0.03 ***
	a*	7.86 ± 0.08 *	7.09 ± 0.3 **	7.18 ± 0.12 **
	b*	−9.24 ± 0.02 *	−9.44 ± 0.02 **	−9.20 ± 0.01 ***
	C*	12.13 ± 0.06 *	11.80 ± 0.03 **	11.67 ± 0.07 **
	h°	310.33 ± 0.31 *	306.80 ± 0.10 **	307.93 ± 0.50 ***

*, **, ***—differences between results denoted with different symbols in the same row are statistically significant (p ≤ 0.05).

).

Both the density of liqueurs and their TSS were strongly dependent on the sweetening substrate used. The highest values of these parameters were determined in the liqueur with cane sugar, whereas the lowest ones were found in the liqueur with stevia used as a sugar replacer. The liqueur with the highest density also had the highest content of TSS, whereas the least dense liqueur had the lowest TSS content. The extract content of the liqueur sweetened with xylitol was comparable to that determined in the liqueur produced with cane sugar (Table 1).

The values of individual parameters indicating the redox potential of the produced liqueurs (DPPH, ABTS, FRAP) did not differ significantly. It may, therefore, be concluded that the type of the sugar substrate used (xylitol, stevia, cane sugar) had no effect on the antioxidative properties of the alcoholic beverages expressed by the aforementioned parameters. However, the lack of correlations between these parameters is noteworthy. For instance, the highest DPPH and FRAP values determined in the liqueur with xylitol were not tantamount to its highest ABTS value. The content of polyphenols, i.e., biologically active compounds exhibiting high antiradical activity, was the highest in the liqueur sweetened with cane sugar and the lowest in that sweetened with stevia (Table 1).

The total color difference determined based on measurements of individual color parameters of the liqueurs was so small that it was imperceptible to an ordinary observer (in the CIE L*a*b* space ΔE < 1). The chroma of the liqueur with xylitol was statistically higher compared to the other liqueur variants, in which it was similar. The darkest color was reported for the liqueur with stevia (L* = 23.41), whereas the lightest one was reported for that with cane sugar (L* = 24). The hue angle (h°) indicated that the color of liqueurs shifted towards hues of purple; however, it differed among the liqueur samples. The greatest contribution of red color in the color profile was shown in the liqueur sweetened with xylitol, and the difference observed compared to the two other liqueurs was statistically significant. The contribution of blue color was the highest in the liqueur produced with stevia. It was similar in the two remaining liqueur variants; however, the differences noted between individual variants were statistically significant (Table 1).

3.2. Sensory Assessment

Figure 1 presents the results of the sensory assessment of the analyzed liqueurs. The sweetening substrate used during their manufacture affected scores given by the panelists for their color, perception of alcohol content, and palatability. In turn, aroma and texture attributes achieved similar scores in the case of all three liqueur variants. Irrespective of sweetener type (xylitol, stevia, or cane sugar), the overall quality of the three liqueurs determined based on scores of their individual attributes was similar and ranged from 3.62 to 3.82 points on a five-point scale.

The lowest color score was given by the panelists to the liqueur sweetened with xylitol, and was 1 and 0.9 points lower than the scores given to the liqueurs produced with stevia and cane sugar, respectively. It was described as intense, dark, and similar to the fruits used or as moderately intense, dark, and slightly diverging from the color of the fruits used in liqueur production. The difference in the color scores of the liqueurs sweetened with stevia and cane sugar was statistically insignificant. Most panelists described their color as very intense, dark, and similar to the fruits used.

The evaluators reported the most intense perception of alcohol content when tasting the liqueur sweetened with xylitol, and concluded that there was a slight positive effect of this component on product palatability. Compared to the other two liqueur variants, the difference in alcohol content perception reached 1 point in the case of stevia and 0.57 points in the case of cane sugar used as a sweetener. The difference of 0.25 points noted between the last two products was statistically insignificant. The perception of alcohol content in the liqueurs sweetened with stevia or cane sugar was satisfactory to the panelists and did not affect their palatability.

The liqueurs produced with xylitol or cane sugar as sweeteners were characterized by similar palatability (3.85 and 3.75, respectively), described as intense with a fruity note and a good sensation of sweetness. A considerably lower score was given by the panelists for this attribute to the liqueur sweetened with stevia. In their opinion, it was only sufficient, with a delicate fruity note and an intense perception of undesirable extrinsic aromas and a moderate perception of sweetness.

The aroma of the three analyzed liqueurs was not scored lower than 3.75, meaning it was a perceptible aroma typical of the fruits used. The texture of the liqueurs was described as thin and homogenous, without observed sediments. In the panelists’ opinion, the thinnest was the liqueur sweetened with xylitol; however, the differences determined between beverage variants were statistically insignificant.

An intense fruity flavor was reported by the panelists for the liqueur with xylitol. In the two other product variants, this parameter was assessed as moderate. The perception of sweetness was similar in the case of all three liqueurs, with that sweetened using stevia considered to be the least sweet one. Most panelists sensed that it had a metallic flavor, which differentiated it from the liqueurs sweetened with xylitol and cane sugar. The perception of a sour flavor was scored by panelists as weaker than moderate in all product variants. In turn, they indicated the liqueur sweetened with stevia as the sourest one and that with cane sugar as the least sour one (Figure 2).

Table 2. Pearson correlations between physicochemical parameters and overall sensory quality.

Analysis	Overall Quality
Total acidity	−0.90
Density	0.98
TSS	0.91
L*	0.98
a*	0.11
b*	0.93
C	−0.27
h°	0.31

presents the results of the Pearson correlation analysis conducted between the physicochemical parameters and overall quality of the analyzed liqueurs. The correlation analysis included only those parameters which differed statistically significantly between the product variants. In most cases, a strong correlation was demonstrated between the overall quality and these parameters; namely, density, TSS, and luminance and blueness were positively correlated with the overall quality, whereas total acidity was negatively correlated with the overall quality.

4. Discussion

The positive strong correlations found between the results of the physicochemical analyses and overall quality (Table 2) indicate the preference of the panelists for a liqueur with a higher density and, thus, a higher content of TSS. Positive strong (0.86) and moderate (0.64) correlations between density and TSS, respectively, and overall acceptability were also found in the liqueur made of wild passion fruit (Passiflora cincinnata Mast.) [17]. However, it should be noted that the TSS content of the liqueurs preferred by the assessors in the cited study was more than twice as high (ca. 46° Bx) as the value of this parameter recorded in the berry liqueur sweetened with cane sugar in the present study (Table 1). In turn, the TSS of the stevia-sweetened liqueur (Table 1) was comparable to that of the liqueur produced from Anacardium humile fruits, which ranged from 14.33 to 16.38 °Bx [18].

The pH values of the analyzed liqueurs (Table 1) were similar to those reported for the liqueurs made of Anacardium humile fruits (from 3.53 to 3.71) [18] and Passiflora cincinnata fruits (from 3.5 to 3.6) [17]. The analyses conducted in the cited works showed that both the pH and acidity of the liqueurs were clearly dependent not only on the types of fruits but also on their amount used in the liqueur production process. The lowest pH was measured in the liqueur with the highest content of Anacardium humile fruits [18]. In turn, a slightly higher pH was determined in the liqueur made of grape stems (pH = 4.04, after 180 days of maturation) [19].

The berry liqueurs showed low total acidity (from 2.43 to 2.76 g/L). A similar value of this parameter was determined for the liqueurs produced from A. humile and acai (Euterpe oleracea), i.e., from 0.04 to 0.12% in the first case, depending on the extraction time and the amount of fruit used [18], and 0.04 g/100 mL in the second case [20]. A significantly higher total acidity was reported for the liqueurs produced using wild passion fruits. It ranged from 10.3 to 14.4 g/100 mL and depended on the amount of pulp used [17]. In turn, the titratable acidity of the liqueurs produced using grape stems increased with maturation time, from 0.98 g/L in the original liqueur to 2.35 g tartaric acid/L after 180 days of maturation [19].

There was a strong negative correlation between the total acidity and the sensory scores of the overall quality of the liqueurs analyzed in this study. The quality scores of the products decreased with increasing acidity (Table 2). In turn, the acceptability of the wild passion fruit liqueur was weakly correlated with its pH [17]. In contrast to the berry liqueurs, the highest acceptability scores were achieved by the wild passion fruit liqueur, which had the highest acidity and the highest content of TSS [17]. In the case of the banana liqueur, higher acceptability scores were achieved in the variant with a medium level of sweetness [21]. Such differences may be due to the type of fruit used to produce the liqueur, which affects its sensory characteristics.

The color of a beverage is the first thing noticed by a consumer; hence, it is crucial for purchasing decisions. The color of an alcoholic fruit drink is determined by the color of the raw material used for its production, which is strictly dependent on the type of pigment contained in the fruit, as well as the amount of pulp or syrup used [17]. It is also affected by storage temperature, pH, oxygen, and the presence of ascorbic acid, sugars and their degradation products [7]. If the beverage production process involves heat treatment, the caramelization products of sugar compounds also influence the ultimate product’s color [17]. A previous study has shown that the extraction of red rose petals with 50% ethanol in a first variant and with a sucrose solution in a second resulted in the release of colored compounds into both solutions. However, the alcohol solution showed a clearly more intense color [22]. Another study demonstrated that liqueurs made of fresh hawthorn fruits subjected to a sweetening process with a 65% sucrose solution at 65 °C for 35 min before extraction showed higher values of all color parameters (L*, a*, b*, C*, h°) than liqueurs produced from unsweetened fruits. In addition, it showed that changes in the a* parameter correlated positively with the content of anthocyanins in the samples [14].

The liqueurs analyzed in the present study were characterized by a dark purple, intense color, which may be attractive to potential consumers. The measurements showed statistically significant differences between the values of all of the determined color parameters (L*, a*, b*, C*, h°). However, the total color difference calculated on their basis was so small that it could not be perceived by an ordinary observer; however, it was perceptible to the trained panelists conducting the organoleptic assessment.

The lowest color score was given by the panelists to the berry liqueur, whose measured color intensity (chroma) was the highest. It additionally showed the highest contribution of red color in the color profile (Figure 1, Table 2). However, it is noteworthy that the measured chroma was weakly correlated with the overall organoleptic quality of the tested beverages (correlation coefficient value of −0.27). In the case of wild passion fruit liqueur, similarly to the berry liqueurs, the panelists preferred beverages with a higher proportion of blue, but with a more saturated, dark color. In this case, all color parameters were strongly correlated with consumer preferences [17]. The analysis of consumer preferences for A. humile liqueurs revealed greater acceptability of the light-colored products [18]. According to Spence et al. (2015) [23], the color of a food product is a feature of great importance to consumer expectations. A change in either the intensity or saturation of the color of beverages can positively or negatively affect their final acceptability.

The phenolic compounds found in fruit alcoholic beverages contribute to their astringent but pleasant flavor and aroma [24]. They can also mitigate the harmful (pro-oxidative) effects of alcohol [8], provided that it is consumed in moderate amounts. Therefore, as high as possible a content of phenolic compounds is desirable in liqueurs. In the present study, the highest content of phenolic compounds was determined in the berry liqueur sweetened with cane sugar (109.16 mg gallic acid/100 mL), whereas in those sweetened with xylitol and stevia, it reached 95.36 and 92.93 mg gallic acid/100 mL, respectively. It is noteworthy that the decrease was no greater than 17.5%. This may have been caused by the interaction of xylitol and stevia with polyphenolic compounds, or by the positive effect of sucrose on the process of extraction of these compounds from fruit. In the case of the A. humile liqueur, the total content of phenolic compounds depended on the maturation time, i.e., the longer the maturation time, the higher the content of phenolic compounds in the liqueur. The content of phenolic compounds in the liqueur macerated for 17 days was 10.59 mg gallic acid/100 mL. It was also found that the maturation time had a greater effect on the content of phenolic compounds in the final product compared to the amount of fruit pulp used [18]. On the other hand, the maceration time clearly affected the content of polyphenolic compounds in the liqueur produced from grape stems. The highest content of polyphenols (164.5 mg gallic acid/100 mL) was determined in the liqueur after 90 days of maceration and was higher by 66 and 16% than the values recorded in the liqueurs macerated for 0 and 180 days, respectively [19]. However, the authors of the cited work emphasized that different phenolic profiles might have triggered various responses to the Folin–Ciocalteu reagent, which could have affected the values reported in their study. This could also be the reason for the differences in phenolic compounds observed in berry liqueurs, despite them having same antioxidant properties (Table 1).

The available literature on the subject has shown a significant effect of sucrose addition to liqueurs on their content of phenolic compounds. Liqueurs made of hawthorn fruits saturated with sucrose contained about 17% more polyphenolic compounds compared to those produced from unsweetened fruits [14]. A similar observation was made by Sokół-Łętowska et al. (2014) [7], who investigated the antioxidative activity of liqueurs from red fruits. They found that the liqueurs made with sugar contained more compounds that reacted with the Folin–Ciocalteu reagent than those without added sugar. Such differences in antioxidative activity were also reported by Kallithraka, Salacha, and Tzourou (2009) [25] in wine. They believed that this might have been due to reactions between oxidized polyphenols and to the formation of new antioxidants, which significantly hampered the prediction of the antioxidative capacity of processed fruits during storage.

Antioxidative activity against DPPH radicals and the ferric ion reducing capacity (FRAP) were statistically significantly higher in the case of liqueurs produced from hawthorn fruits saturated with sucrose than from the unsaturated ones. Saturation with a sucrose solution had no statistically significant effect on antioxidative activity against ABTS radicals [14]. In contrast, the present study results showed no effect of the sweetener type on the antioxidative activity of the produced liqueurs determined in the DPPH, ABTS, and FRAP assays. The differences in antioxidative activity between the liqueurs were statistically insignificant regardless of the assay. Its highest values were obtained in the ABTS assay (from 1081.88 to 1238.13 μmol Tx/100 mL), while the lowest ones were obtained in the DPPH assay (from 348.8 to 367.88 μmol Tx/100 mL). In turn, the FRAP values ranged from 594.20 to 653.20 μmol FeSO₄/100 mL. It can, therefore, be concluded that each of the liqueurs contained antioxidative compounds that act through free radicals as well as those that reduce the capacity of metals. According to de Oliveira et al. (2023) [18], the differences between the DPPH and ABTS methods, which have the same mechanism of action, can be attributed to their specific properties, i.e., the stereochemistry of radicals, the reaction temperature, or the solvent used. The FRAP method covers the most antioxidant components in the sample, while the DPPH method covers only a portion of the most reactive, and the ABTS method covers intermediate values [26].

The antioxidative capacity of the A. humile liqueur determined in the DPPH and FRAP assays was significantly lower than that of the analyzed berry liqueurs. It ranged from 0.66 to 2.36 and from 0.06 to 0.14 μmol Tx/g of sample, respectively. The values obtained depended on the liqueur preparation procedure used. The ABTS analysis was impossible due to too low values of antioxidative activity [18]. On the other hand, the antioxidative activity of an ethanolic extract from fresh Myrtus communis L. berries reached 0.915 mg/mL at the time of preparation, then increased, reaching a maximum after 3 months of storage, and subsequently decreased and stabilized at 0.930 mg/mL after 8 months of storage. Changes in this parameter were due to the hydrolysis of flavonoid glycosides and subsequent degradation of anthocyanins [27]. The maturation time also strongly affected the antioxidative properties of the grape stem liqueur. Its highest antioxidative activity determined in the DPPH assays was recorded after 90 days of maturation, and was 57% higher than the initial value and 13% higher than the value determined after 180 days [19].

5. Conclusions

The analyses conducted in this study showed that the type of sweetener (xylitol, stevia, cane sugar) had no effect on the antioxidative properties of the produced liqueurs, expressed by the DPPH, ABTS, and FRAP values. However, the sweetening substrate affected their content of polyphenolic compounds, which was lower by no more than 15% in the liqueurs sweetened using xylitol or stevia than in that sweetened with cane sugar. It should also be noted that the liqueur sweetened with xylitol had a comparable extract content to the liqueur sweetened with cane sugar.

According to the sensory assessment results, the analyzed liqueurs also showed comparable overall quality. All three variants were accepted by the evaluation panel. However, the panelists preferred the liqueur with higher density and lower acidity. Although the results of the CIE L*a*b* measurements indicated that the average observer should not notice any difference in the color of the liqueurs, the liqueur sweetened with xylitol received a significantly lower score in the sensory assessment compared to the liqueurs sweetened with stevia and cane sugar.

Overall, the use of sweeteners such as xylitol or stevia as replacers of traditional sugar during liqueur production from bilberry and highbush blueberry fruits did not significantly affect their sensory assessment and their potential health properties. Therefore, it is possible to produce alcoholic fruit beverages with a reduced caloric value, which may contribute to increasing their attractiveness among consumers. However, to fully confirm this thesis, it is necessary to conduct further research with a broader scope, such as consumer preference studies.