Exploring the Potential of Indigenous Grape Varieties for Sparkling Wine Production in the Hrvatska Istra Subregion (Croatia)

Abstract

Indigenous grape varieties from the Hrvatska Istra subregion (Croatia) represent a significant proportion of regional wine production. In this study, the potential of six indigenous varieties—Malvazija istarska, Garganja, Duranija, Surina, Hrvatica, and Teran—for the traditional method for the production of sparkling wines was evaluated. Several of these varieties are currently underutilized or neglected in contemporary viticulture. A total of 85 volatile aroma compounds, including acids, alcohols, esters, C13-norisoprenoids, and terpenes, were identified and quantified using SPME-Arrow-GC/MS. Sensory evaluation was conducted using a structured nine-point hedonic scale. Among the compounds identified, C13-norisoprenoids (notably β-damascenone, TPB, and TDN) and esters (including ethyl 3-methylbutanoate, ethyl butanoate, and ethyl hexanoate) were found to contribute most significantly to the overall aromatic profile of the sparkling wines. Sensory profiles varied distinctly among the varieties. Some varieties demonstrated pronounced aromatic and structural characteristics, making them suitable for monovarietal sparkling wine production, while others exhibited complementary sensory properties more appropriate for cuvées. This study represents the first comprehensive chemical and descriptive sensory profiling of sparkling wines produced from these Istrian indigenous grape varieties. These findings aim to support their valorization and integration into the broader spectrum of sparkling wine production, thereby enhancing their recognition and market potential.

1. Introduction

According to the definition of the International Organization of Vine and Wine (OIV) [1], sparkling wines belong to the category of special wines, which means that they are obtained from fresh grapes, from musts or wines that have undergone certain technological processes and whose properties do not come only from the grapes but also from the techniques used during production. Quality sparkling wine is a product that has an excess pressure, due to carbon dioxide originating exclusively from alcoholic fermentation in solution, of at least 3.5 bar when kept at a temperature of 20 °C in closed containers and for which the total alcoholic strength of the cuvée intended for their preparation must not be less than 9% vol [2]. The production of sparkling wines involves two fermentations, the second of which is usually carried out in a tank (Charmat or Martinotti method) or in the bottle (traditional or Champenoise method).

The quality of sparkling wine depends on several parameters such as the characteristics of the foam, color, acidity, and aroma [3]. The overall sensory properties of sparkling wine, especially its aroma, are influenced by a multitude of factors: the production method, grape variety, composition of the base wine, yeast strain, yeast autolysis, and the duration of maturation in contact with the lees [4], as well as the ingredients used for liqueur d’expedition, the oxygen level during the disgorging process, the type of closure used, and the levels of SO₂ and CO₂ [5]. The aroma of wine is defined by a mixture of volatile components that can be categorized into three groups: varietal, fermentative, and post-fermentative, along with their corresponding aromatic components. Aging on the lees in sparkling wines produced by the traditional method can significantly affect changes in the composition of volatile components [6], as well as the characteristics of foaming, mouthfeel, and taste [7]. Several authors [3,5,6] report an increase in the concentrations of ethyl esters, some alcohols, and varietal aromas, while acetate esters and fatty acids show a decrease in concentration due to adsorption to the yeast cell walls. 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) and vitispirane have been identified as the main markers of autolysis in sparkling wines [8,9,10].

In Croatia, sparkling wines are produced in all wine-growing regions, and as stated by Jagatić et al. [5], most of them are produced using the traditional method with a noticeable upward trend in production. There is a great diversity of grape varieties from conventional ones (Chardonnay, Pinot Blanc, Pinot Noir, Riesling), to indigenous ones (Graševina, Kraljevina, Pošip, Grk, Žlahtina, Malvazija istarska, Teran) that are used for the production of sparkling wines in Croatia. According to data from the Croatian Agency for Agriculture and Food in 2017, the total production of sparkling wines in Croatia amounted to 3440.33 hL, of which 860.47 hL was produced in the wine-growing subregion Hrvatska Istra. The possibility of producing sparkling wines from indigenous and/or neglected grape varieties is increasingly the subject of scientific research around the world [11,12,13,14,15,16,17].

Hrvatska Istra wine region has a long tradition of viticulture and wine production, with still wines historically dominating the market. Sparkling wines have not traditionally been produced in this region; however, following the broader trends in global wine production, during the last two decades the production of sparkling wines in Istria has largely increased, and now most of the producers have also a sparkling wine in their offer.

The primary varieties currently used for sparkling wine production in Istria are Chardonnay and Pinot Blanc, along with the indigenous varieties Malvazija istarska as a white skin variety and Teran as a dark violet skin variety. Although all four varieties are technically suitable for sparkling wine production, the first two lack regional authenticity, as they are not native to Istria. Malvazija istarska typically has lower titratable acidity [18,19], making it more suitable for the production of still wines, while Teran is rich in anthocyanins and other phenolic compounds [20,21,22], which makes it more appropriate for aged red still wines.

Despite the current dominance of Malvazija istarska and Teran (currently planted on 56% and 9% of total vineyard area, respectively), Hrvatska Istra is home to a wide array of indigenous grapevine varieties [23,24]. As stated in Vitolović [25], historical records from the detailed register of Istrian vineyards in 1951 list Duranija, Garganja, Surina, and Hrvatica among the ten most widely cultivated varieties in the region. Currently, their cultivation is limited, and they are maintained mostly by old generations of winegrowers. Their decline in cultivation during the previous 70 years was primarily driven by their lower sugar content and the consequently lower wine quality according to the common standards prevailing before the onset of climate changes, when the higher sugar content in grapes was considered a positive trait in winemaking [26,27,28]. Duranija and Garganja are white-skinned varieties; Surina has a pink skin, while Hrvatica has a dark blue skin.

In recent years, several factors have emerged that warrant a reevaluation of the production of these neglected indigenous Istrian varieties, which include: the increasing sugar content in grapes (and alcohol content in wine) as a consequence of climate change, making varieties with lower sugar content more advantageous; a general consumer trend toward lighter, lower-alcohol wines; the growing popularity of sparkling wines, which are produced from grapes with lower sugar levels; market demand for distinctive products, especially wines made from local grape varieties that reflect regional heritage and are not cultivated elsewhere; the need to diversify the wine portfolio and offer a wider range of wine styles at a local level.

Therefore, the objective of this study was to assess the suitability of the Istrian indigenous grape varieties for sparkling wine production.

2. Materials and Methods

2.1. Grapevine Varieties

Six grapevine varieties traditionally grown in the wine-growing subregion Hrvatska Istra were used in this study—three white varieties (Malvazija istarska, Duranija, and Garganja), two red varieties (Teran and Hrvatica), and one rosé variety (Surina). All varieties were grafted on Vitis berlandieri × Vitis riparia SO4 rootstock and planted in 2006 in the experimental vineyard of the Institute of Agriculture and Tourism in Poreč, Croatia, which exhibits the characteristics demonstrated in our previous study [29]. The training system used was a vertically shoot-positioned single-cane Guyot. Regular cultural practices for the region were performed during the season, including the positioning of the shoots between two pairs of catching wires, mechanical leaf removal in the fruit zone at the berry set, mechanical shoot trimming to 130 cm canopy height, and a protection against major diseases and pests. During the season of 2022, grapes from a total of 30 vines for each variety were used for this study. Harvest was performed manually and sequentially for each variety, according to the ripening stage and the capacity of the variety to obtain the desired sugar level; Malvazija istarska and Hrvatica were harvested on 31 August 2022, Duranija, Garganja, and Surina were harvested on 7 September 2022, while Teran was harvested on 12 September 2022.

2.2. Sparkling Wines Production

Grapes (100 kg of each variety) intended for sparkling wine production via the traditional method were pressed without crushing and destemming using a hydraulic press (Letina inox d.o.o., Čakovec, Croatia) at 1.5 bar. After pressing, approximately 60–72 L of cloudy must was obtained, depending on the variety. The resulting grape juice was subjected to gravity settling for 24 h at 12 °C, with the addition of potassium metabisulfite (K₂S₂O₅) (Esseco Srl, Trecate, Italy) used for sulfiting at 6 g hL⁻¹ to provide 30 mg L⁻¹ and pectolytic enzyme (1 g hL⁻¹, Lallzyme C-MAX™, Lallemand, Montreal, QC, Canada). After gravity settling, 45 L of clear must of each variety was used for alcoholic fermentation. The clarified must of each variety (Malvazija istarska, Duranija, Garganja and Surina white must, and Hrvatica and Teran rosé must) was divided into three replicates (15 L glass bottles), and alcoholic fermentation was initiated through the inoculation of rehydrated Saccharomyces cerevisiae GAL (ex var. bayanus) yeast (0.3 g L⁻¹, Lalvin EC 1118™, Lallemand, Montreal, QC, Canada) and yeast nutrient (0.3 g L⁻¹, Fermaid E, Lallemand, Montreal, QC, Canada), conducted at 17 °C. Protein stability was assessed by treating the wine with a bentonite suspension. In this experiment, we used MASTERVIN^® COMPACT bentonite (Enologica Vason S.p.A, Nassar, Italy), a product consisting of silica gel adsorbed onto specifically activated sodium bentonite and activated silica. Samples were considered protein-stable when NTU < 5. The bentonite doses required to achieve protein stability varied significantly among the wines of autochthonous varieties as follows: Malvazija istarska—1.8 g L⁻¹, Duranija—2.5 g L⁻¹, Garganja—2.5 g L⁻¹, and Surina—1.5 g L⁻¹, Hrvatica—0.5 g L⁻¹, and Teran—0.3 g L⁻¹. For secondary fermentation, rehydration of Saccharomyces cerevisiae GAL (ex var. bayanus) yeast (Lalvin EC 1118™, Lallemand, Montreal, QC, Canada) was carried out in 50 mL of chlorine-free water (Meteor grupa-Labud d.o.o., Zagreb, Croatia) for 20 min at 36 °C. Then, 50 mL of base wine and 50 mL of liqueur (a mixture of sugar and wine at a concentration of 25 g per 50 mL) were added to the rehydrated yeast, homogenized, and left in a water bath at 22 °C for 16 h. Afterward, the specific gravity was measured and found to be 1018 ± 3. To the 150 mL of this mother culture, 850 mL of base wine, 250 mL of water, and 250 mL of liqueur (containing 125 g of sugar) were added. The mixture was homogenized and maintained at 20 °C for two days with stirring and aeration and added to the remaining base wine, which had been previously filtered and mixed with dissolved sugar (24 g L⁻¹ sucrose, to achieve a pressure of 6 bar) and 0.3 g L⁻¹ yeast nutrients (Fermaid E, Lallemand, Montreal, QC, Canada). The wines were then bottled (0.75 L) for sparkling wine, sealed with a bidule and a crown stopper, and stored horizontally at 16 °C for secondary fermentation. After the completion of secondary fermentation (residual sugar < 4.0 g L⁻¹) and eighteen months of aging on the lees at a temperature of 15 °C with the bottles rotation (remuage), the bottles were placed on pupitres and subjected to riddling to facilitate sediment accumulation in the bottle neck. The wines (four white; Malvazija istarska, Garganja, Duranija, and Surina, and two rosé Hrvatica, and Teran) were then disgorged, sulphited with 50 mg L⁻¹ SO₂ (Agrolit d.o.o., Litija, Slovenia), without the addition of expedition liqueur, corked, caged, and stored at 15 °C until further analysis.

2.3. Chemical Analysis of Must, Base, and Sparkling Wines

Soluble solid content (Brix), titratable acidity, and pH in must prior to alcoholic fermentation, as well as alcohol content, reducing sugars, titratable acidity, volatile acidity, and pH in base as well as pressure in sparkling wines, were analyzed using the methods outlined by the OIV (2019) [30]. The concentrations of individual organic acids (citric, tartaric, lactic, succinic, and malic acid; expressed in g/L) were determined using High-Performance Liquid Chromatography (HPLC). Samples were filtered through a 0.22 µm pore size membrane filter (Merck KGaA, Darmstadt, Germany) and 5 µL was injected directly into the system. Chromatographic separation was performed under isocratic conditions with a flow rate of 0.6 mL/min, using a column temperature of 65 °C and UV detection at 210 nm. Separation was achieved on a cation-exchange column (Aminex HPX-87H, 300 × 7.8 mm i.d.; Bio-Rad Laboratories, Hercules, CA, USA) with a 0.0065% (v/v) aqueous phosphoric acid solution as the mobile phase. Quantification was conducted using the external standard method.

2.4. Identification and Quantification of Volatile Compounds in Sparkling Wines

Volatile compounds in sparkling wine were analyzed using SPME-Arrow-GC/MS with a RSH Triplus autosampler (Thermo Fisher Scientific, Waltham, MA, USA) according to method described by Tomaz et al. [31]. For each analysis, 5 mL of wine and 2 g of NaCl were placed in 20 mL headspace vials. Samples were incubated at 60 °C for 20 min, followed by fiber exposure (DVB/CWR/PDMS, 120 μm × 20 mm) for 49 min. Desorption occurred at 250 °C for 10 min in splitless mode. Analyses were performed on a TRACE 1300 GC coupled with an ISQ 7000 MS (Thermo Fisher Scientific, Waltham, MA, USA), using a TG-WAXMS A column (60 m × 0.25 mm × 0.25 μm). Helium was the carrier gas (1 mL/min). The oven temperature started at 40 °C (5 min), increased to 210 °C at 2 °C/min, and was held for 10 min. Mass spectra were acquired in EI mode (70 eV) over m/z 30–300. Data were processed with Chromeleon 7.0 software, and quantification was performed using the external standard method.

2.5. Calculation of Odor Activity Value (OAV) and Relative Odor Contribution (ROC)

The evaluation of wine’s overall aroma is commonly conducted through the assessment of the odor activity value (OAV) and relative odor contribution (ROC). The OAV is derived by dividing the concentration of volatile compounds by their respective odor detection threshold (ODT). Volatile compounds exhibiting an OAV > 1 are considered to be aromatically significant and contribute substantially to the development of the wine’s aromatic profile. These compounds are deemed to possess sufficient potency to influence the sensory characteristics of the wine, thereby playing a critical role in its overall olfactory composition. The ROC of each aroma compound is calculated as the ratio of the OAV of the respective compound to the total OAV of each wine.

2.6. Sensory Evaluation

A panel of eight oenologists conducted a descriptive analysis of sparkling wines, evaluating sensory attributes across three main categories: (1) visual attributes, including foamability, bubble rate, bubble size, and foam stability; (2) olfactory attributes, encompassing aromatic intensity, fruity notes, red/black fruit, floral, toast/yeast/brioche, herbal, and overall complexity; and (3) gustatory attributes, acidity, sweetness, bitterness, and persistence. The intensity of each attribute was assessed using a 9-point scale, where 9 represented the highest intensity and 0 indicated the absence of the attribute. Each sample was analyzed in triplicate, and the results for each attribute are reported as mean values.

2.7. Statistical Analysis

Analysis of variance (ANOVA) was employed for the statistical evaluation of the data, while Fisher’s least significant difference (LSD) test was used to determine significant differences among means at a significance level of p < 0.05. In order to gain deeper insight into the data set (volatile components OAV > 1, and sensory attributes with significant differences), a principal component analysis (PCA) was performed. Statistical analyses were conducted using Statistica v. 13.2 software (TIBCO Inc., Palo Alto, CA, USA).

3. Results and Discussion

3.1. Chemical Composition of Grape Must, Basic, and Sparkling Wines

The most important chemical parameters for harvesting grapes intended for the production of sparkling wines include titratable acidity, pH, and sugar content in the grape must [32]. Must parameters (

Table 1. Chemical composition of must of indigenous varieties.

Parameters	Malvazija Istarska	Garganja	Duranija	Surina	Hrvatica	Teran
	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Brix	18.30 ± 0.03 b	17.00 ± 0.25 d	17.40 ± 0.09 c	17.00 ± 0.12 d	18.90 ± 0.12 a	17.20 ± 0.09 cd
Titratable acidity * (g L−1)	6.20 ± 0.05 e	7.10 ± 0.09 c	6.80 ± 0.07 d	7.80 ± 0.08 b	6.30 ± 0.05 e	7.95 ± 0.12 a
Citric acid (g L−1)	0.12 ± 0.01 d	0.23 ± 0.01 a	0.17 ± 0.01 b	0.13 ± 0.01 d	0.15 ± 0.01 c	0.15 ± 0.01 c
Tartaric acid (g L−1)	3.40 ± 0.04 c	3.80 ± 0.06 b	3.50 ± 0.06 c	3.90 ± 0.04 b	3.50 ± 0.04 c	4.20 ± 0.11 a
Malic acid (g L−1)	2.00 ± 0.07 f	2.70 ± 0.05 c	2.40 ± 0.06 d	3.05 ± 0.02 b	2.10 ± 0.04 e	3.22 ± 0.03 a
pH	3.18 ± 0.01 a	3.09 ± 0.03 b	3.05 ± 0.01 bc	3.04 ± 0.01 cd	3.15 ± 0.01 a	3.02 ± 0.01 d

*—as tartaric acid, SD—standard deviation, different letters in the same row indicate significant differences (p ≤ 0.05).

) for the six autochthonous varieties investigated in this study were for sugar content from 17.00 to 18.90 Brix, titratable acidity from 6.20 to 7.95 g L⁻¹, and pH value in the range from 3.02 to 3.18.

The content of titratable acidity as well as tartaric and malic acids significantly depended on the varieties, which is in accordance with the statements of Preiner et al. [33]. The profile of individual organic acids is important due to the influence on the pH value as well as on the titratable acidity content [34]. Tartaric and malic acids are the main organic acids in all musts, and the highest concentrations were recorded in the varieties Teran and Surina.

Table 2. Chemical composition of base wines of indigenous varieties.

Parameters	Malvazija Istarska	Garganja	Duranija	Surina	Hrvatica	Teran
	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Alcohol (vol%)	10.90 ± 0.14 a	10.27 ± 0.33 b	10.34 ± 0.09 b	10.02 ± 0.02 b	11.18 ± 0.03 a	10.00 ± 0.03 b
Titratable acidity * (g L−1)	6.38 ± 0.04 d	7.75 ± 0.19 b	7.15 ± 0.08 c	8.20 ± 0.01 a	6.58 ± 0.04 d	8.27 ± 0.08 a
Citric acid (g L−1)	0.22 ± 0.02 ab	0.25 ± 0.01 ab	0.16 ± 0.01 b	0.15 ± 0.01 b	0.24 ± 0.02 a	0.17 ± 0.03 ab
Tartaric acid (g L−1)	3.38 ± 0.04 d	3.65 ± 0.05 c	3.43 ± 0.01 d	3.84 ± 0.05 b	3.38 ± 0.04 d	4.05 ± 0.07 a
Malic acid (g L−1)	2.03 ± 0.04 d	2.74 ± 0.06 b	2.45 ± 0.09 c	2.99 ± 0.01 a	2.06 ± 0.03 d	3.13 ± 0.11 a
Succinic acid (g L−1)	0.49 ± 0.04 b	0.88 ± 0.10 a	0.73 ± 0.07 a	0.82 ± 0.02 a	0.50 ± 0.10 b	0.73 ± 0.07 a
Lactic acid (g L−1)	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
Volatile acidity ** (g L−1)	0.25 ± 0.03	0.31 ± 0.02	0.31 ± 0.05	0.33 ± 0.09	0.32 ± 0.09	0.27 ± 0.05
Residual sugar (g L−1)	1.50 ± 0.2	0.85 ± 0.4	1.30 ± 0.1	1.05 ± 0.02	1.00 ± 0.00	1.50 ± 0.05
pH	3.20 ± 0.02 a	3.10 ± 0.01 b	3.03 ± 0.09 b	3.03 ± 0.02 c	3.14 ± 0.02 b	3.03 ± 0.02 c

*—as tartaric acid, **—as acetic acid, SD—standard deviation, n.d.—not detected; different letters in the same row indicate significant differences (p ≤ 0.05).

shows the physicochemical composition of the base wines after protein stabilization on thermolabile proteins. No stuck/sluggish alcoholic fermentation was recorded in the production of the base wines (residual sugar < 4 g L⁻¹). Furthermore, malolactic fermentation was not performed, and consequently there was no decomposition of malic and citric acids, which resulted in a low content of volatile acidity (0.25–0.36 g L⁻¹). The concentrations of succinic acid (the main acid produced by yeasts) differed significantly among the base wines. The differences in the concentrations of succinic acid in our case probably depend on the nitrogen source originating from the grapes [35] and not on the yeast (as the same yeast was used in all treatments). Given that the pH values ranged from the lowest 3.03 in the base wines Teran and Surina to the highest 3.20 in the base wine Malvazija istarska, additional acidification was not performed.

The alcoholic strength of the base wines is in line with expectations given the characteristics of the varieties and the harvest date. The obtained base wines of all varieties provided a good basis in terms of chemical composition and sensory characteristics for the production of sparkling wines by the traditional method. Almost complete decomposition of sugar was recorded in all sparkling wines (values from 0.67 to 2.07 g L⁻¹), which makes them fall into the “Brut nature” category. The alcohol content (11.28–12.29 vol. %) followed the alcohol values of the base wines increased by the addition of sucrose through expedition liqueur. Furthermore, the volatile acidity was below the olfactory detection threshold in all samples. The data for the chemical composition of sparkling wines (

Table 3. Chemical composition of sparkling wines of indigenous varieties.

Parameters	Malvazija Istarska	Garganja	Duranija	Surina	Hrvatica	Teran
	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Alcohol (vol%)	12.21 ± 0.07 a	11.46 ± 0.22 c	11.78 ± 0.20 b	11.29 ± 0.04 c	12.28 ± 0.21 a	11.50 ± 0.12 c
Titratable acidity * (g L−1)	7.23 ± 0.04 cd	7.85 ± 0.26 b	7.42 ± 0.35 c	8.41 ± 0.17 a	6.83 ± 0.17 d	8.56 ± 0.20 a
Residual sugar (g L−1)	2.07 ± 0.06 a	1.07 ± 0.06 b	0.67 ± 0.15 d	0.67 ± 0.17 d	0.79 ± 0.12 cd	0.97 ± 0.25 bc
Volatile acidity ** (g L−1)	0.36 ± 0.04	0.41 ± 0.03	0.32 ± 0.04	0.37 ± 0.07	0.37 ± 0.08	0.35 ± 0.07
pH	3.13 ± 0.04 ab	3.06 ± 0.01 bc	3.06 ± 0.05 bc	3.03 ± 0.06 c	3.18 ± 0.04 a	3.01 ± 0.03 c
Pressure (bar)	4.9 ± 0.10 ab	5.2 ± 0.06 a	5.0 ± 0.25 ab	4.90 ± 0.20 ab	4.50 ± 0.31 bc	4.30 ± 0.56 c

*—as tartaric acid, **—as acetic acid, SD—standard deviation; different letters in the same row indicate significant differences (p ≤ 0.05).

) produced by the traditional method with eighteen months of aging are in line with the data of Caliari et al. [36].

3.2. Volatile Compounds

Table 4. Individual volatile compound concentrations (µg L⁻¹) of sparkling wines of indigenous varieties.

Parameters	Malvazija Istarska	Garganja	Duranija	Surina	Hrvatica	Teran
	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Fatty acids
9-Decenoic acid	0.01 ± 0.00 d	5.63 ± 0.12 b	0.01 ± 0.00 d	7.47 ± 0.08 a	3.34 ± 0.06 c	0.01 ± 0.00 d
Decanoic acid	801.96 ± 58.33 a	559.29 ± 25.22 c	543.71 ± 22.12 c	683.15 ± 67.11 b	441.59 ± 31.05 d	503.51 ± 19.05 cd
Dodecanoic acid	0.36 ± 0.08 c	8.39 ± 1.14 b	0.23 ± 0.05 c	10.30 ± 2.18 a	0.01 ± 0.00 c	8.22 ± 0.91 b
3-Hydroxy-dodecanoic acid	4.02 ± 0.05 d	5.09 ± 0.56 b	4.00 ± 0.01 d	4.09 ± 0.80 cd	4.79 ± 0.19 bc	6.05 ± 0.08 a
Hexanoic acid	3212.12 ± 87.55 a	2148.08 ± 159.35 c	1816.66 ± 92.33 d	2752.34 ± 187.21 b	2397.26 c ± 156.33 c	3369.03 ± 153.05 a
2-Ethyl-hexanoic acid	50.77 ± 9.21 a	10.09 ± 0.58 bc	16.22 ± 1.69 b	11.77 ± 1.09 bc	7.32 ± 1.21 d	8.17 ± 1.11 d
Nonanoic acid	2.88 ± 0.67 b	3.80 ± 0.05 a	2.52 ± 0.07 b	3.84 ± 0.50 a	2.51 ± 0.80 b	0.05 ± 0.01 c
Octanoic acid	5109.40 ± 102.36 a	3690.80 ± 232.22 d	4204.41 ± 192.33 c	4718.29 ± 215.11 b	3619.45 ± 122.48 d	4643.54 ± 256.33 b
Propanoic acid	0.76 ± 0.01 c	1.31 ± 0.04 b	0.48 ± 0.03 d	0.52 ± 0.05 d	0.40 ± 0.01 e	1.65 ± 0.04 a
2-Methyl-propanoic acid	0.42 ± 0.05 b	0.64 ± 1.31 b	0.50 ± 0.01 b	0.27 ± 0.08 b	1.00 ± 0.05 b	35.40 ± 2.28 a
Σ fatty acids	9182.71 ± 258.31 a	6433.11 ± 419.34 c	6588.75 ± 308.64 c	8192.03 ± 474.21 b	6477.65 ± 312.18 c	8575.62 ± 432.86 ab
Alcohols
1-Butanol	219.15 ± 12.25 a	94.41 ± 2.25 c	123.57 ± 11.54 b	17.51 ± 1.04 e	76.48 ± 8.04 d	131.87 ± 10.08 b
1-Hexanol	1940.23 ± 94.55 a	590.14 ± 36.57 c	404.75 ± 29.64 d	634.94 ± 57.24 c	1484.22 ± 59.07 b	1892.94 ± 61.05 a
2-Ethyl-1-hexanol	2.49 ± 0.23 c	2.19 ± 0.45 c	2.35 ± 0.15 c	3.77 ± 0.88 b	2.89 ± 0.40 c	5.71 ± 0.22 a
1-Nonanol	2.02 ± 0.10 a	1.15 ± 0.02 b	n.d.	0.97 ± 0.17 c	n.d.	n.d.
1-Octanol	10.57 ± 1.22 b	12.91 ± 0.57 a	8.57 ± 0.98 c	11.41 ± 0.08 b	3.35 ± 0.56 d	13.57 ± 0.84 a
1-Pentanol	0.23 ± 0.05 b	0.09 ± 0.02 cd	0.06 ± 0.01 d	0.22 ± 0.02 b	0.16 ± 0.07 bc	0.54 ± 0.05 a
3-Methyl-1-pentanol	54.27 ± 3.57 b	50.19 ± 3.56 b	0.05 ± 0.00 d	115.53 ± 10.24 a	0.04 ± 0.01 d	28.82 ± 1.33 c
4-Methyl-1-pentanol	20.37 ± 2.18 c	27.45 ± 4.22 b	16.31 ± 1.24 cd	41.57 ± 3.23 a	0.11 ± 0.02 e	12.36 ± 1.12 d
1-Propanol	5.40 ± 0.48 d	108.19 ± 11.25 a	91.85 ± 6.25 b	0.34 ± 0.02 d	69.57 ± 5.55 c	9.71 ± 0.48 d
3-Ethoxy-1-propanol	22.31 ± 1.29 d	10.33 ± 1.08 d	42.16 ± 3.24 c	52.89 ± 11.12 c	281.98 ± 15.96 a	148.37 ± 12.11 b
2,3-Butanediol, isomer 2	706.32 ± 24.33 b	616.15 ± 38.99 c	224.28 ± 14.66 f	446.98 ± 29.57 e	536.25 ± 37.01 d	906.36 ± 56.22 a
trans- 2-Heksen-1-ol	7.03 ± 0.84 a	0.03 ± 0.00 d	3.68 ± 0.23 b	0.04 ± 0.00 d	1.40 ± 0.03 c	0.06 ± 0.01 d
2-Nonanol	0.46 ± 0.03 b	0.01 ± 0.00 e	0.20 ± 0.01 d	0.99 ± 0.02 a	0.35 ± 0.02 c	0.39 ± 0.06 c
2-Octanol	0.65 ± 0.07 a	0.26 ± 0.04 b	0.06 ± 0.00 c	n.d.	0.01 ± 0.00 c	0.01 ± 0.00 c
2-Octen-1-ol	4.03 ± 0.51 a	0.02 d ± 0.00	0.01 ± 0.00 d	3.17 ± 0.07 b	3.13 ± 0.51 b	2.13 ± 0.08 c
3-Ethyl-4-methylpentan-1-ol	0.95 ± 0.47 c	2.91 ± 0.07 b	1.38 ± 0.06 c	0.21 ± 0.01 d	4.07 ± 0.65 a	0.01 ± 0.00 d
trans-3-Hexen-1-ol	77.05 ± 3.66 a	18.88 ± 0.58 cd	17.15 ± 1.25 d	44.11 ± 2.56 b	19.44 ± 1.02 cd	21.02 ± 0.94 c
cis-3-Hexen-1-ol	119.42 ± 9.54 a	40.22 ± 1.25 c	5.22 ± 0.45 e	14.20 ± 1.21 d	47.60 ± 2.33 c	93.66 ± 3.95 b
Benzyl alcohol	197.69 ± 53.33 a	41.95 ± 3.24 b	0.15 ± 0.02 c	65.47 ± 3.66 b	35.79 ± 1.05 bc	61.63 ± 4.05 b
2,3-Butanediol, isomer 1	4040.53 ± 158.15 a	3541.54 ± 125.49 bc	1355.56 ± 95.55 e	3308.09 ± 145.21 c	2034.93 ± 70.09 d	3806.97 ± 255.22 ab
Isoamyl alcohol	107,735.08 ± 567.22 c	107,085.81 ± 689.25 c	68,884.08 ± 224.10 e	145,280.99 ± 986.67 a	70,947.74 ± 453.31 d	109,200.59 ± 593.34 b
Isobutyl alcohol	5895.28 ± 211.27 b	3397.80 ± 154.22 d	4549.45 ± 84.60 c	6638.78 ± 266.25 a	4516.05 ± 166.62 c	5815.28 ± 210.07 b
2-phenylethanol	27,463.93 ± 1125.37 a	27,293.99 ± 985.34 a	20,949.34 ± 335.27 b	28,167.08 ± 426.05 a	16,857.61 ± 299.31 d	19,276.83 ± 305.22 c
Σ alcohols	148,525.46 ± 2270.71 b	142,936.59 ± 2058.46 c	96,680.22 ± 809.25 d	184,849.26 ± 1945.32 a	96,923.15 ± 1121.63 d	141,428.81 ± 1516.44 c
C13-norisoprenoides
Dehydro-β-ionone	11.07 ± 0.98 a	2.07 ± 0.07 d	3.51 ± 0.07 c	5.17 ± 0.51 b	0.59 ± 0.02 e	4.74 ± 0.31 b
4-Hydroxy-ß-ionone	0.81 ± 0.05 a	0.11 ± 0.01 c	0.12 ± 0.02 c	0.10 ± 0.00 c	0.13 ± 0.01 c	0.73 ± 0.09 b
α-Ionon	0.01 ± 0.00 c	0.55 ± 0.05 a	0.01 ± 0.00 c	0.02 ± 0.00 c	0.25 ± 0.03 b	0.00 ± 0.00 c
β-Damascenone	10.42 ± 0.59 a	3.25 ± 0.11 c	4.62 ± 0.54 b	3.22 ± 0.08 c	2.40 ± 0.05 d	4.94 ± 0.57 b
β-Ionone	16.48 ± 1.10 a	4.94 ± 0.06 b	5.48 ± 0.68 b	4.88 ± 0.12 b	2.58 ± 0.11 c	5.69 ± 0.66 b
TDN	6.55 ± 0.51 d	2.67 ± 0.07 e	8.78 ± 0.87 c	3.52 ± 0.22 e	10.95 ± 0.69 b	13.79 ± 1.13 a
TPB	13.02 ± 0.91 a	6.67 ± 0.09 b	6.64 ± 0.22 b	5.39 ± 0.19 c	2.58 ± 0.05 d	4.95 ± 0.06 c
Vitispirane A	7.33 ± 0.20 b	5.63 ± 0.04 c	4.72 ± 0.02 c	8.34 ± 1.23 b	8.01 ± 0.41 b	13.50 ± 0.59 a
Vitispirane B	6.80 ± 0.63 c	9.34 ± 0.15 b	6.34 ± 0.34 c	15.50 ± 2.15 a	7.08 ± 0.06 c	14.81 ± 1.05 a
Σ C13-norisoprenoides	65.68 ± 4.97 a	25.88 ± 0.65 c	33.88 ± 2.76 bc	46.16 ± 4.50 b	34.56 ± 1.43 c	63.16 ± 4.46 a
Esters
Isoamyl acetate	54.92 ± 3.55 c	48.11 ± 0.94 d	45.45 ± 0.74 d	70.91 ± 4.22 b	46.08 ± 2.22 d	82.76 ± 4.08 a
Ethyl-2-butenoate	0.92 ± 0.07 b	0.58 ± 0.06 d	0.80 ± 0.10 c	0.43 ± 0.05 e	0.65 ± 0.05 d	1.26 ± 0.05 a
2-Methylbutyl octanoate	0.93 ± 0.06 cd	0.85 ± 0.07 de	0.77 ± 0.05 e	1.08 ± 0.09 b	1.00 ± 0.12 bc	1.42 ± 0.01 a
3-Hexen-1-ol acetate	0.02 ± 0.00 d	0.06 ± 0.01 b	0.01 ± 0.00 d	0.33 ± 0.01 a	0.02 ± 0.00 d	0.29 ± 0.02 b
Ethyl-3-hexenoate	0.35 ± 0.04 c	0.61 ± 0.01 b	n.d.	0.01 ± 0.00 d	0.62 ± 0.06 b	0.80 ± 0.06 a
Ethyl-4-hexenoate	3.73 ± 0.22 a	1.62 ± 0.02 b	1.38 ± 0.33 bc	1.23 ± 0.02 d	0.01 ± 0.00 e	0.12 ± 0.01 e
Hexyl acetate	0.20 ± 0.00 d	0.58 ± 0.01 b	0.27 ± 0.01 c	0.08 ± 0.03 e	0.73 ± 0.04 a	0.04 ± 0.01 f
Isopropyl salicylate	5.58 ± 0.94 a	0.34 ± 0.00 c	0.01 ± 0.00 c	0.28 ± 0.04 c	2.55 ± 0.64 b	1.82 ± 0.24 b
Ethyl-2-methylbutanoate	68.88 ± 8.66 bc	76.90 ± 2.25 b	62.54 ± 5.47 bcd	109.83 ± 15.22 a	59.16 ± 7.12 cd	53.11 ± 2.06 d
Ethyl-3-methylbutanoate	116.19 ± 9.57 bc	124.01 ± 8.64 b	101.28 ± 9.09 d	157.70 ± 5.68 a	93.30 ± 9.22 d	104.66 ± 5.99 cd
Ethyl butanoate	286.88 ± 15.36 d	274.14 ± 11.68 d	378.54 ± 21.33 c	248.15 ± 21.33 d	453.16 ± 37.05 b	604.05 ± 54.49 a
Ethyl decanoate	68.25 ± 8.47 ab	58.14 ± 8.57 b	58.56 ± 5.22 b	80.42 ± 14.04 a	38.39 ± 1.11 c	58.55 ± 2.24 b
Diethyl malate	389.50 ± 24.62 c	487.71 ± 15.59 b	273.47 ± 21.07 d	373.09 ± 32.64 c	550.24 ± 26.04 b	1106.49 ± 90.05 a
Diethyl succinate	10,621.01 ± 843.28 b	10,078.74 ± 633.51 bc	8635.88 ± 369.56 d	12,509.33 ± 572.33 a	8605.25 ± 298.55 d	9194.60 ± 249.33 cd
Ethyl dodecanoate	2.71 ± 0.08 b	2.99 ± 0.21 b	3.57 ± 0.60 a	3.07 ± 0.09 b	1.50 ± 0.02 c	2.77 ± 0.05 b
Ethyl-2-hydroxy-4-methylpentanoate	8.79 ± 0.57 c	13.32 ± 0.86 a	8.46 ± 0.98 cd	7.18 ± 1.05 d	3.37 ± 0.79 e	10.82 ± 0.58 b
Ethyl lactate	4663.93 ± 342.25 d	5211.99 ± 235.66 d	3636.35 ± 156.99 e	10,244.98 ± 561.05 a	7045.62 ± 211.09 c	7671.15 ± 113.99 b
Ethyl-3-furoate	0.59 ± 0.08 b	0.49 ± 0.03 c	0.60 ± 0.05 b	0.60 ± 0.01 b	0.64 ± 0.05 b	0.86 ± 0.02 a
Ethyl-3-hydroxybutanoate	227.86 ± 15.23 a	177.71 ± 11.37 b	93.16 ± 5.46 e	191.73 ± 11.16 b	102.08 ± 5.66 d	128.62 ± 1.26 c
Ethyl-9-hexadecenoate	11.75 ± 0.80 c	10.86 ± 0.68 cd	6.25 ± 0.11 e	14.17 ± 1.22 a	13.51 ± 1.02 b	9.48 ± 0.50 d
Ethyl hydrogenglutarate	34.18 ± 0.95 d	43.36 ± 1.59 c	30.68 ± 1.02 d	41.19 ± 3.25 c	57.77 ± 5.63 b	77.07 ± 2.36 a
Ethyl hydrogensuccinate	738.92 ± 24.55 a	667.44 ± 33.25 b	572.23 ± 27.88 c	598.81 ± 31.33 c	601.04 ± 33.34 c	701.45 ± 11.50 ab
Ethyl hexadecanoate	16.51 ± 1.29 a	13.41 ± 2.15 b	10.86 ± 1.21 c	6.57 ± 0.56 d	11.08 ± 0.98 c	2.98 ± 0.04 e
Ethyl-3-hydroxyhexanoate	1.34 ± 0.05 b	1.06 ± 0.05 b	1.19 ± 0.60 b	1.30 ± 0.08 b	1.39 ± 0.05 b	2.98 ± 0.63 a
Ethyl hexanoate	256.68 ± 2.59 a	172.79 ± 4.25 c	151.39 ± 3.49 d	260.38 ± 19.34 a	192.27 ± 4.26 b	272.16 ± 9.05 a
Isobutyl decanoate	0.93 ± 0.08 a	0.05 ± 0.01 bc	0.01 ± 0.00 c	0.02 ± 0.00 c	0.01 ± 0.00 c	0.11 ± 0.05 b
Isopentyl hexanoate	2.04 ± 0.02 cd	2.28 ± 0.06 b	1.93 ± 0.04 e	1.99 ± 0.07 de	2.11 ± 0.06 c	4.72 ± 0.06 a
Methyl octanoate	1.58 ± 0.04 b	1.30 ± 0.00 d	1.07 ± 0.01 f	1.82 ± 0.03 a	1.16 ± 0.04 e	1.50 ± 0.01 c
Methyl stearate	0.71 ± 0.01 b	0.06 ± 0.00 e	0.70 ± 0.02 b	0.37 ± 0.02 c	0.89 ± 0.03 a	0.12 ± 0.01 d
Ethyl octanoate	316.78 ± 11.24 b	259.61 ± 11.35 c	223.10 ± 9.66 c	393.09 ± 35.08 a	219.81 ± 33.11 c	303.88 ± 21.22 b
Ethyl pentadecanoate	0.01 ± 0.00 b	n.d.	1.29 ± 0.06 a	n.d.	1.30 ± 0.02 a	n.d.
Ethyl pentanoate	0.65 ± 0.02 b	0.31 ± 0.02 d	0.90 ± 0.04 a	0.47 ± 0.05 c	0.04 ± 0.01 e	0.02 ± 0.01 e
Phenyl acetate	2.05 ± 0.05 a	1.62 ± 0.05 b	1.25 d ± 0.03	1.58 ± 0.06 b	1.36 ± 0.03 c	2.08 ± 0.06 a
Σ esters	17,905.32 ± 1314.74 c	17,733.03 ± 982.95 c	14,303.95 ± 641.22 d	25,322.18 ± 1330.15 a	18,108.09 ± 678.41 c	20,402.73 ± 570.04 b
Terpenes
ß-Fanesene	0.50 ± 0.00 d	0.75 ± 0.06 b	0.56 ± 0.04 cd	0.92 ± 0.07 a	0.61 ± 0.05 c	0.91 ± 0.08 a
3,7-dimethyl-3,6-octadien-1-ol	0.01 ± 0.00 b	0.02 ± 0.00 b	0.03 ± 0.01 b	0.03 ± 0.01 b	0.03 ± 0.01 b	0.83 ± 0.04 a
α-Farnesen	1.60 ± 0.02 a	1.20 ± 0.00 b	0.35 ± 0.07 d	0.84 ± 0.11 c	0.82 ± 0.06 c	0.27 ± 0.01 d
Citronelol	1.42 ± 0.06 b	1.67 ± 0.02 b	1.97 ± 0.09 b	3.36 ± 0.05 a	3.95 ± 0.91 a	0.18 ± 0.01 c
Limonene	0.22 ± 0.01 c	0.12 ± 0.01 d	0.01 ± 0.00 e	0.05 ± 0.02 e	1.42 ± 0.04 a	0.85 ± 0.05 b
trans-Linalool oxide, furan	8.14 ± 0.58 a	2.29 ± 0.03 d	3.29 ± 0.08 b	3.46 ± 0.08 b	2.83 ± 0.06 c	1.09 ± 0.08 e
Terpinene-4-ol	2.55 ± 0.66 d	3.67 ± 0.25 bc	3.30 ± 0.15 c	2.53 ± 0.21 d	4.04 ± 0.57 b	6.19 ± 0.33 a
α-Terpinene	0.04 ± 0.01 bc	0.20 ± 0.00 bc	0.01 ± 0.00 c	0.01 ± 0.00 c	1.43 ± 0.33 a	0.28 ± 0.05 b
β-Myrcene	n.d.	n.d.	0.06 ± 0.00 c	0.17 ± 0.02 b	n.d.	0.50 ± 0.10 a
β-Ocimene	0.02 ± 0.00 b	n.d.	0.04 ± 0.00 a	0.01 ± 0.00 c	n.d.	n.d.
Σ terpenes	14.50 ± 1.34 a	9.93 ± 0.37 b	9.60 ± 0.44 b	11.37 ± 0.57 b	15.14 ± 2.03 a	11.10 ± 0.75 b

SD—standard deviation, n.d.—not detected; different letters in the same row indicate significant differences (p ≤ 0.05).

presents the mean concentrations with standard deviation of volatile compounds identified in sparkling wines produced from indigenous grape varieties in the Istrian region of Croatia, using SPME-Arrow-GC/MS analysis. A total of 85 volatile components were identified, among which esters (33) were the most abundant, followed by alcohols (23), fatty acids (10), terpenes (10), and C13-norisoprenoids (9).

Alcohols, which are important secondary aromatic metabolites in wine and are formed by yeasts through amino acid metabolism during alcoholic fermentation [37], represent the most abundant class of volatiles. When present at concentrations below 300 mg L⁻¹, as is the case in this study, these compounds contribute positively to the complexity of wine aroma [38]. Montevecchi et al. [39] identified isoamyl alcohol and 2-phenylethanol as the primary fermentative alcohols in Malvasia di Candia sparkling wine. Isoamyl alcohol, characterized by solvent and chemical-like aromas, was the most abundant higher alcohol across all samples, consistent with findings by De Souza Nascimento et al. [40]. The highest isoamyl alcohol concentration was observed in Surina (145,280.99 µg L⁻¹), and the lowest in Duranija (68,884.08 µg L⁻¹)., The second most abundant higher alcohol in all samples was 2-phenylethanol, known for its rose-like aroma, enhancing the aromatic complexity of the wine [41], which has been confirmed by several authors [5,40,42]. The highest concentrations of 2-phenylethanol were found in Surina, Garganja, and Malvazija istarska (28,167.08; 27,293.99; 27,463.93 µg L⁻¹, respectively), and the lowest in Hrvatica (16,857.61 µg L⁻¹). Previous studies also report varietal differences in the concentrations of these alcohols [6,12,32]. In Malvazija istarska sparkling wines, significantly higher concentrations of “leaf alcohols” (C6-compounds) were recorded, including hexanol, cis-3-hexenol, trans-3-hexenol, and trans-2-hexenol. Although these concentrations remained below their respective odor thresholds, their elevated presence aligns with findings by Jagatić et al. [5]. For instance, Malvazija istarska exhibited concentrations of 1-hexanol (1940.23 µg L⁻¹), trans-3-hexen-1-ol (77.05 µg L⁻¹), and cis-3-hexen-1-ol (119.42 µg L⁻¹), which surpass values reported by Carlin et al. [43] for Trentodoc and Franciacorta sparkling wines.

Esters, which contribute fruity and floral aromas, represent the most significant class of fermentation-derived volatiles in sparkling wines, formed during both alcoholic and malolactic fermentation, as well as during aging [37,44]. Significant varietal differences were observed in total ester concentrations, consistent with the literature [6,32]. The highest total ester levels were detected in Surina, followed by Teran, Hrvatica, Malvazija istarska, Garganja, and Duranija. The most abundant fermentative esters across all samples included ethyl octanoate, ethyl butanoate, ethyl hexanoate, ethyl-2-methylbutanoate, ethyl-3-methylbutanoate, and isoamyl acetate as findings that align with reports on Ribolla Gialla [37], Glera [45], Chenin Blanc, and Syrah [40]. Teran exhibited the highest concentrations of ethyl butanoate (604.05 µg L⁻¹), ethyl hexanoate (272.16 µg L⁻¹), and isoamyl acetate (82.76 µg L⁻¹), while Surina showed the highest concentrations of ethyl octanoate (393.09 µg L⁻¹), ethyl-2-methylbutanoate (109.83 µg L⁻¹), and ethyl-3-methylbutanoate (187.70 µg L⁻¹). These results are consistent with those of Caliari et al. [36], who also reported varietal influences on ester composition. Ethyl lactate and diethyl succinate, known as “aging esters,” tend to increase with wine maturation and following malolactic fermentation [36]. In this study, they were the most abundant esters detected. Diethyl succinate concentrations ranged from 8605.25 µg L⁻¹ in Hrvatica to 12,509.33 µg L⁻¹ in Surina and this range of concentrations is consistent with Martínez-García et al. [46]. Five esters, isoamyl acetate, ethyl-2-methylbutanoate, ethyl-3-methylbutanoate, ethyl butanoate, and ethyl hexanoate, exceeded their odor thresholds (OAV > 1), indicating a significant contribution to the wine’s aroma profiles.

Total fatty acid concentrations also varied significantly across the sparkling wines. The lowest concentration was observed in Garganja (6433.11 µg L⁻¹), and the highest in Malvazija istarska (9182.71 µg L⁻¹). Within the 4–10 mg L⁻¹ range, fatty acids are generally considered to enhance aroma, whereas concentrations above 20 mg L⁻¹ may exert a negative sensory effect [47]. Octanoic, hexanoic, and decanoic acids were the predominant fatty acids identified.

C13-norisoprenoids, formed via oxidative degradation of carotenoids through acid-catalyzed reactions, are generally present in low concentrations but have a substantial sensory impact due to their low odor thresholds [5]. Voce et al. [37] state that C13-norisoprenoids are generally not associated with specific aromatic grape varieties and generally have a low odor threshold. On the other hand, Li et al. [48] in their study observed a significant role of grape variety in the accumulation of C13-norisoprenoids and the associated gene expression. They are formed by the oxidative degradation of carotenoids, as a result of acid-catalyzed reactions, and some of these compounds are of great importance for the aroma of sparkling wine, especially produced by the traditional method [7,49]. In addition to TDN and vitispiranes [5,50], β-damascenones and (E)-1-(2,3,6-trimethylphenyl)-buta 1,3-diene (TPB) [51] were also identified as the main indicators of the autolysis process on the yeast surface. In this study, a significant difference was found in the concentration of both total and individual C13-norisoprenoids. The sparkling wines Malvazija istarska and Teran stood out with the highest total concentration of these volatile compounds. Of all detected C13-norisoprenoids, three of them, β-damascenone, TDN and TPB were found in concentrations above the detection threshold in all analyzed sparkling wines of autochthonous varieties. For β-damascenone with sweet, fruity, floral, honey olfactory impressions, the highest concentrations were detected in Malvazija istarska wine. On the other hand, for TDN (gasoline and kerosene aromas), the highest concentrations were detected in Teran wine. For the first time, TPB was detected in sparkling wines of autochthonous varieties from Croatia. Concentrations for TPB vary from 2.58 to 13.02 µg L⁻¹. Higher concentrations were found in sparkling wines produced from white grape varieties (Malvazija istarska 13.02 µg L⁻¹, Garganja 6.67 µg L⁻¹, and Duranija 6.64 µg L⁻¹), which was higher than reported for Durello [52] and Prosecco [45] sparkling wines.

Terpenes, primarily responsible for floral aromas, are transferred from grapes to must during pressing, either as free volatiles or glycosidically bound precursors [5]. Their concentrations are influenced by grape variety, geographic origin, and winemaking practices [53]. Among all volatile classes, terpenes exhibited the lowest total concentrations. Based on total terpene content, the wines were grouped into those with higher levels (Hrvatica and Malvazija istarska) and those with lower levels (Teran, Surina, Garganja, Duranija). The predominant individual terpenes across all samples were trans-linalool oxide (furan), terpinen-4-ol, α-farnesene, and citronellol, though none exceeded their respective odor thresholds.

3.3. Odor Activity Values (OAV) and Relative Odor Contribution (ROC)

Among the eighty-five identified volatile compounds, twelve (consisting of five esters, three C13-norisoprenoids, two alcohols, and two fatty acids) showed odor activity values (OAV) greater than 1, indicating that their concentrations exceeded the olfactory detection threshold.

Table 5. Odor activity values (OAV) and relative odor contributions (ROC) of aroma compounds with OAV > 1 in sparkling wines of indigenous varieties.

Parameters	ODT (µg L−1)	Odor Descriptor	Malvazija Istarska		Garganja		Duranija		Surina		Hrvatica		Teran
			OAV	ROC (%)	OAV	ROC (%)	OAV	ROC (%)	OAV	ROC (%)	OAV	ROC (%)	OAV	ROC (%)
Fatty acids
Hexanoic acid	420 [45]	cheese, oily [5]	7.65	1.20	5.11	1.58	4.33	1.24	5.50	1.79	5.71	2.76	1.05	0.42
Octanoic acid	500 [45]	cheese, oily [5]	10.22	1.60	7.38	2.28	8.41	2.42	9.44	3.06	7.24	3.50	9.29	3.75
Σ Fatty acids				2.80		3.85		3.66		4.85		6.26		4.17
Alcohols
Isoamylalcohol	30,000 [4]	alcohol, nail polish [5]	3.59	0.56	3.57	1.10	2.30	0.66	4.84	1.57	2.36	1.14	1.01	0.41
Phenylethyl Alcohol	14,000 [4]	floral, rose, honey [5]	1.96	0.31	1.95	0.60	1.50	0.43	2.01	0.65	1.20	0.58	0.70	0.28
Σ Acohols				0.87		1.70		1.09		2.22		1.73		0.69
C13-norisoprenoides
β-Damascenone	0.05 [45]	sweet, fruity, floral, honey [5]	208.36	32.68	64.90	20.01	92.44	26.57	64.48	20.93	47.96	23.20	98.88	39.89
TDN	2 [45]	petrol, kerosene [5]	3.28	0.51	1.34	0.41	4.39	1.26	1.76	0.57	5.47	2.65	6.89	2.78
TPB	0.04 [45]	tobacco [45]	325.38	51.04	166.75	51.42	166.10	47.74	134.80	43.75	64.45	31.18	123.73	49.91
Σ C13-norisoprenoides				84.24		71.85		75.57		65.25		57.03		92.58
Esters
Isoamyl acetate	30 [45]	banana [45]	1.83	0.29	1.60	0.49	1.51	0.44	2.36	0.77	1.54	0.74	1.51	0.61
Ethyl-2-methylbutanoate	18 [5]	apple, strawberry [5]	3.83	0.60	4.27	1.32	3.47	1.00	6.10	1.98	3.29	1.59	0.77	0.31
Ethyl-3-methylbutanoate	3 [5]	fruity, pineapple [5]	38.73	6.08	41.34	12.75	33.76	9.70	52.57	17.06	31.10	15.05	0.90	0.36
Ethyl butanoate	20 [45]	pineapple, apple, peach [5]	14.34	2.25	13.71	4.23	18.93	5.44	12.41	4.03	22.66	10.96	2.11	0.85
Ethyl hexanoate	14 [45]	fruity, green apple, banana [5]	18.33	2.88	12.34	3.81	10.81	3.11	11.84	3.84	13.73	6.64	1.06	0.43
Σ esters				12.09		22.59		19.68		27.68		34.98		2.56

presents the OAV values and relative odor contribution (ROC) index for individual volatile components, with the aim of assessing their influence on the overall aroma profile of sparkling wines produced from autochthonous grape varieties. The highest total OAV was recorded for the sparkling wine Malvazija istarska, with the most pronounced individual OAVs recorded for TPB (associated with tobacco aromas) and β-damascenone (contributing to sweet, fruity, floral and honey notes). These two volatile compounds also constitute the main aromas in other sparkling wine samples. In contrast, esters such as ethyl 3-methylbutanoate, ethyl butanoate, and ethyl hexanoate, characterized by their fruity aromatic qualities, form the core of the ester aroma profile.

The ROC index indicates variations among different sparkling wine samples, however, it is evident that C13-norisoprenoids and esters have the most significant influence on the overall aromatic composition of all sparkling wines.

3.4. Sensory Analysis

Figure 1 shows data on (a) visual, (b) olfactory, and (c) gustatory profiles of sparkling wines.

Significant differences between sparkling wines were observed in three of the four visual attributes examined. The most pronounced variation was recorded in the Garganja and Duranija wines, which showed significantly larger bubbles compared to the other samples, which may be a consequence of higher doses of bentonite required to achieve protein stability. Sensory analysis of the olfactory profile revealed that the rosé wines (Hrvatica and Teran) were characterized by aromas of red and black fruits, with medium to very pronounced aroma intensity. In contrast, the olfactory profile of the white sparkling wines was strongly dependent on the varieties and was predominantly defined by fruity and herbal aromatic sensations. The data on aroma complexity closely matched those on aroma intensity, with the Malvazija istarska, Surina, and Hrvatica wines showing the most pronounced properties in both attributes. Regarding the gustatory profile, the highest perception of acidity was recorded in the Surina and Teran wines. In contrast, the most pronounced feelings of sweetness and creaminess were observed in the Malvazija istarska and Hrvatica wines.

3.5. Multivariate Analysis

Principal component analysis (PCA) was applied to the data set in which sparkling wines served as cases (a), and aromatic components with OAV > 1 (Table 5) and sensory properties showing significant differences (Figure 1) were used as variables (b). The first four principal components (PCs) had eigenvalues greater than 1. Figure 2 presents the results for the first two principal components, which together explain 64.03% of the total variability in the dataset (PC1: 37.87%, PC2: 26.16%). The first principal component (PC1) revealed a separation into two groups: Group 1 (Malvazija istarska, Surina, and Garganja) and Group 2 (Teran, Hrvatica, and Duranija). Most sparkling wines in Group 1 (Malvazija istarska and Surina) are located in quadrant IV, influenced on the negative side of PC1 by higher concentrations of isoamyl alcohol, phenylethyl alcohol, ethyl 2-methylbutanoate, and ethyl 3-methylbutanoate. On the positive side of PC2, they are associated with hexanoic acid, octanoic acid, and ethyl hexanoate. Additionally, Malvazija istarska and Surina are characterized by specific sensory attributes, including enhanced foamability and an aromatic profile reminiscent of toast, yeast, and brioche, with pronounced sweetness and flavor persistence.

4. Conclusions

In this study, it was established that the examined autochthonous grape varieties are suitable for the production of sparkling wines via the traditional method. Analysis of the aromatic profile revealed that the sparkling wines exhibited a high concentration of C13-norisoprenoids, followed by esters. Notably, β-damascenone, TPB, TDN, as well as isoamyl acetate, ethyl-2-methylbutanoate, ethyl-3-methylbutanoate, ethyl butanoate, and ethyl hexanoate, were present at concentrations exceeding their respective odor activity thresholds (OAV > 1), indicating their substantial contribution to the aroma profiles of wines. Furthermore, sensory analysis demonstrated significant differences in the evaluated attributes among the samples. Multivariate analysis (PCA) of both volatile and sensory data revealed that the majority of volatile compounds and sensory attributes were associated with the sparkling wines produced from Malvazija istarska, Surina, Hrvatica, and Teran, suggesting their potential for the production of high-quality monovarietal sparkling wines. Conversely, the varieties Garganja and Duranija appeared more suitable for use in blended sparkling wines (cuvées). It is important to note that these findings are based on sparkling wines aged for eighteen months. Future research should explore the impact of different aging durations on both monovarietal and blended sparkling wines derived from these indigenous varieties.