Effects of Berry Thinning on the Physicochemical, Aromatic, and Sensory Properties of Shine Muscat Grapes

Abstract

The effects of the level of berry thinning (30% and 50% berry removal) on the quality and sensory properties of Shine Muscat grapes were investigated. As berry thinning increased, the total soluble solids content increased and titratable acidity decreased. Berry thinning increased berry size and cluster weight but caused no change in individual berry weight. Phenolic concentrations as measured by total phenolic, proanthocyanidin, and polymeric tannin concentrations tended to increase with an increase in berry thinning. Gas chromatographic analysis indicated that C6-compounds were the significant constituents of volatile alcohols and aldehydes; linalool was the most abundant monoterpene. Odor activity analysis indicated that (E)-2-hexen-1-ol, (E)-2-hexenal, 1-hexanal, (Z)-3-hexenal, (E)-β-damascenone, linalool, and (E)-linalool oxide were active odorants. Berry thinning increased the accumulation of linalool contributing to high sensory flavor scores in thinned berries. Furthermore, its oxidized derivative-linalool oxide-contributed to enhancing the Muscat flavor. In conclusion, berry thinning induced compositional changes in Shine Muscat grape berries by accelerating the ripening rate, contribution to improved sensory properties.

1. Introduction

Shine Muscat, a diploid table grape variety, was obtained in 1988 in Japan from a cross of Akitsu-21 (a hybrid of Steuben—V. labruscana × Muscat of Alexandria—V. vinifera) and Hakunan (V. vinifera) [1]. Shine Muscat has recently gained public attention in Asian countries including Korea and China owing to its excellent sweetness, muscat flavor, crispness, and low sourness [2]. The quality of table grapes can be judged by the size of clusters, uniformity and symmetry of berries, and the characteristic color, flavor, and texture of the variety. Variation in grape quality depends primarily on soil management, irrigation, fertilization, pruning, and climate [3]. In addition, several other treatments, including thinning, defoliation, spraying growth regulators, girdling, and canopy management, affect the quality of grape berries [4,5,6,7].

Excessive fruit load often reduces grapes’ quality, such as weakening vine, poor berry size and color, low sweetness, and delayed ripening. Thinning is a common and simple vineyard practice used to improve berry quality during grape cultivation. Researchers have evaluated the effects of various thinning techniques, including shoot thinning, cluster tipping and thinning, berry thinning, shoot topping, and defoliation on different grape varieties. Dardeniz [6] investigated the effects of cluster tipping on the yield and quality of Uslu (V. vinifera L.) and Cardinal (V. vinifera L.) grapes and found that cluster tipping improved the quality of both grape varieties, although yields were not affected. Sivilotti et al. [8] investigated the compositional changes of anthocyanins and proanthocyanins in Refosco dal peduncolo rosso (Vitis vinifera L.) grapes treated with selective berry thinning and classical cluster thinning. They reported that the thinning treatments increased the total anthocyaninins concentration while decreasing the high molecular weight proanthocyanidins, mean degree of polymerization, and percentage of galloylation in grape skin. Palliotti and Cartechini [9] investigated the effects of cluster thinning on the yield and grape composition of three different grapevine varieties: Sangiovese, Merlot, and Cabernet Sauvignon. Their study indicated that cluster thinning by 40% was significant and reduced yields in all varieties. As results, the quality of the grapes was improved by increasing the concentrations of total soluble solids, anthocyanins, polyphenols, and total nitrogen and decreasing titratable acidity.

However, to the best of our knowledge, the effects of berry thinning on the compositional changes of Shine Muscat grapes have hardly been studied. Shin et al. [7] evaluated the effects of the applications of growth regulators and floral cluster thinning on the fruit quality and aroma properties of Shine Muscat grapes, but more in-depth investigations are required. Moreover, there have been no studies on the effects of thinning treatment on the sensory properties of Shine Muscat grapes. The present study assessed the effects of berry thinning on the compositional changes by evaluating total soluble solids content, titratable acidity, berry and cluster size and weight, skin color, phenolic composition and concentrations, volatile aroma compounds, and sensory properties of Shine Muscat grapes.

2. Materials and Methods

2.1. Grape Samples

Shine Muscat grapes were cultivated in an irrigated vineyard (Ansung, Korea). The experiment was a randomized block design, with three treatments in three replications. Each plot consisted of five vines. The flower cluster length was adjusted to 3 cm by removing the apical part of each cluster five days before full bloom. Vines were sprayed with a combination of gibberellic acid and thidiazuron (25 and 2 ppm, respectively) twice at 2 and 14 days after full bloom. After the fruit set, each cluster usually 80~90 fruitlets left. Berry thinning was performed by removing 0%, 30%, and 50% of berries from each cluster nine days after full bloom. Grapes were harvested at 20 weeks after full bloom and classified into three groups based on the level of berry thinning: grape without berry thinning (BT0; control), grape with 30% berry thinning (BT30), and grape with 50% berry thinning (BT50). The harvested grapes were stored at 15 °C prior to analysis.

2.2. General Properties

Free-flowing juice was collected by squeezing ten berries randomly selected from each group. The total soluble solids content (TSS) in the juice was measured using a digital refractometer (PR-32 Alpha; ATAGO Co. Ltd., Tokyo, Japan). Titratable acidity (TA) was measured using an automatic titrator (TitroLine Easy; SI Analytics GmbH, Mainz, Germany). The collected juice (5 mL) was diluted in 20 mL deionized water and a 0.1 M NaOH standard solution was titrated into the sample solution until the pH of the sample reached 8.2. Titratable acidity was expressed as the tartaric acid equivalent [2].

The Brix/acid ratio was obtained by dividing the TSS by the TA of the grape juice samples. The average horizontal and vertical diameters of ten berries randomly selected from each group were measured using digital Vernier calipers (CD-15CP, Mitutoyo, Japan). An electronic scale (SW-02, CAS Corporation, Seoul, Korea) was used to measure the average berry weight. Berry skin color was determined on 20 berries randomly selected from each group and measured using a colorimeter (DP-400, Konica Minolta, Tokyo, Japan). The skin color was expressed with L, a, and b color coordinates, and standard illuminant C was used as a reference. The color coordinates L, a, and b indicate the perceptual lightness, greenness, and yellowness, respectively.

2.3. Phenolic Composition and Concentrations

2.3.1. Extraction of Phenolic Compounds

The grape skin was manually separated from the pulp using a plat spatula and washed thoroughly with deionized water. The collected skin was lyophilized for 3 days under vacuum and pulverized using an electric blade grinder. The skin powder was stored in a desiccator at room temperature before extraction. For the extraction, 0.3 g of dried skin powder was immersed in 10 mL of extraction solvent (water:acetone:methanol = 0.36:0.48:0.16, v/v), and the mixture was sonicated in an ultrasonic bath (DH.WUC.D10H, Daihan Scientific, Wonju, Korea). The extraction was performed twice (30 min each), and the extracts were combined, centrifuged, and filtered. The extract was stored at 4 °C before analyses.

2.3.2. Total Phenolic Concentration

The total phenolic concentration (TPC) of the extracts was measured using the modified Folin-Ciocalteu’s reagent assay [10]. An aliquot (1 mL) of extract solution was evaporated and dissolved in dimethyl sulfoxide. A 0.1 mL of sample solution was mixed with 0.5 mL of a working solution of Folin-Ciocalteu’s reagent 10-fold diluted in deionized water. The reaction was initiated by adding 0.4 mL of a 20% Na₂CO₃ solution and the reaction solution was incubated at 40 ℃ for two hours in a water bath (Maxturdy-18, Daihan Scientific, Wonju, Korea). The absorbance of the reaction mixture was measured at 760 nm on a 96-well microplate reader (Multiskan GO, Thermo Fisher Scientific, Waltham, MA, USA). TPC was expressed as mg gallic acid equivalent/g dry skin powder (mg GAE/g DW).

2.3.3. Proanthocyanidin Concentration

The proanthocyanidin concentration (PAC) in the extracts was measured using a vanillin-acetic acid assay [10]. A 30 μL extract solution was pipetted into each well of a 96-well microplate, and 150 μL of a vanillin working solution (0.5% vanillin in 4% HCl in acetic acid) was added. The microplate was incubated at 25 °C for 4 min on a microplate reader (shaking on for 3 min, and off for 1 min, and finishing with shaking off). The absorbance of the reaction mixture was measured at a wavelength of 500 nm. PAC was expressed as mg catechin equivalent/g dry skin powder (mg CE/g DW).

2.3.4. Polymeric Tannin Concentration

The polymeric tannin concentration (PTC) in the extracts was measured using a BSA precipitation assay [10]. A 0.2 mL of the extract solution was mixed with 1 mL of BSA solution (1 mg/mL BSA in a washing buffer) in a microtube and incubated at 25 °C for 10 min. The tannin-protein complex was precipitate and separated by centrifugation at 10,000 rpm for 2 min, and washed with 1 mL of washing buffer (170 mM NaCl in 200 mM acetic acid, pH 4.9). A 875 µL of 8.3 M aqueous urea solution with 5% triethanolamine (pH 7.0) was added to the washed precipitate and incubated at 25 °C for 10 min to isolate polymeric tannin from protein-tannin complex. A 175 µL of each re-suspended tannin solution was mixed with 25 µL of FeCl₃ solution (10 mM FeCl₃ in 10 mM HCl) in a well of a 96-well microplate. After incubation at 25 °C for 10 min on a microplate reader (shaking on for 2 min, off for 8 min, and finishing with shaking off), the absorbance of the reaction mixture was measured at a wavelength of 510 nm. PTC was expressed as mg tannic acid equivalent/g dry skin powder (mg TAE/g DW).

2.4. Volatile Free Aroma Compounds

Grape berries randomly selected from each group were ground using an electric blade grinder and the grape juice was obtained by centrifugation and filtration. Grape juice (10 mL) was transferred to a 20 mL capacity headspace vial containing 10 µL of acetonitrile and 0.3 g of NaCl. Acetonitrile was used as an internal standard to quantify aroma compounds, and NaCl was used to enhance the volatility of aroma compounds. The sample vial was incubated at 50 °C with constant stirring for 1 h. SPME fiber (50/30 µm DVB/CAR/PDMS; Supelco, Bellefonte, PA, USA) was introduced into the headspace for 20 min to adsorb volatile aroma compounds. The SPME fiber was injected into the injection port (250 °C) of a gas chromatograph (6890N, Agilent Technologies, Santa Clara, CA, USA) equipped with an Agilent 5975 Series Mass Selective Detector operated in positive ion electron impact ionization mode at 70 eV with a scan range of 50–700 m/z. The volatile aroma compounds were separated in an HP-INOWAX capillary column (30 mm × 0.32 mm × 0.25 µm; Agilent Technologies) with purified helium gas at a constant flow rate of 2 mL/min. The purge flow to the split vent was set to 144.9 mL/min for 1 min. The oven temperature was programmed to initially hold at 40 °C for 5 min, increase to 250 °C at a rate of 5 °C/min, and finally held at 250 °C for 5 min. Volatile aroma compounds were identified by comparing their retention times with authentic standards and their mass spectra with those from the NIST 11 (National Institute of Standards and Technology) mass spectral library. The concentrations of volatile aroma compounds were estimated using the following equation [11]:

$$R{F}_{SC}=\frac{A_{IS}\times C_{SSC}}{C_{IS}\times A_{SC}}$$

$$C_{SC}=\frac{C_{IS}\times A_{SC}\times R{F}_{SC}}{A_{IS}}$$

where RF_SC is the response factor for a specific compound, A_IS is the peak area of an internal standard, C_SSC is the concentration of the standard of a specific compound, C_IS is the concentration of the internal standard, A_SC is the peak area of a specific compound, and C_SC is the concentration of a specific compound.

The odor activity value (OAV) of the identified volatile aroma compounds was calculated from the concentration of each compound divided by its perception threshold in water, and values higher than one were considered active odorants. Perception threshold data were obtained from the literature [12,13,14,15,16,17].

2.5. Sensory Evaluation

The effects of berry thinning on the sensory properties of Shine Muscat grapes were evaluated with the approval of the Institutional Review Board (IRB) in May 2021 (P01-202105-13-004). An untrained panel of 33 people consisting of 26 women and 7 men aged 20–60 years who had no repulsion against Muscat flavor were recruited from the National Institute of Horticultural and Herbal Science. The panelists were tested using linalool, hexanal, and hexanol standard compounds with different concentrations whether they were able to distinguish muscat flavor from herbaceous and fruity flavors and perceive different intensities of muscat flavor. The test was performed the day after harvest. All grape samples were equilibrated to room temperature prior to the evaluation. The panelists were trained using a grape sample at 21 Brix before testing. Grape berries were served to the panelists, and water was provided freely to clean their palates. They evaluated the sensory parameters, including sweetness, sourness, firmness, flavor, and overall acceptance. The intensities of all sensory parameters for the tested grape samples were scored on a seven-point scale [18].

2.6. Statistical Analysis

All experimental results were analyzed using the SPSS software (IBM, Armonk, NY, USA). Mean comparisons were performed by analysis of variance (ANOVA), followed by Tukey’s multiple comparison test at p < 0.05.

3. Results and Discussion

3.1. General Properties

Visual appearances of Shine Muscat grapes treated with different levels of berry thinning are shown in Figure 1 and their color coordinates are presented in

Table 1. Effects of berry thinning on the skin color indices of Shine Muscat grapes.

Level of Berry Thinning (BT, %)	Color Coordinates
	L	a	b
0	37.04 ± 1.91 a	−6.67 ± 0.33 a	11.67 ± 0.76 b
30	37.74 ± 1.31 a	−6.3 ± 0.20 b	11.63 ± 0.48 b
50	37.97 ± 0.73 a	−6.45 ± 0.27 a,b	12.41 ± 0.32 a

Means with different letters within the same row are significantly different at p < 0.05.

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L values were not different among the sample groups. The a values of BT30 and BT50 were lower than those of BT0. This result indicates that berry thinning decreased the greenness of the berry skin color resulting from the acceleration of berry ripening. On the other hand, the b value of BT50 was higher than those of BT30 and BT0. The increase in the b value indicates the color change of berry skin from green to yellowish green due to the accumulation of carotenoids [2].

The effects of berry thinning on the TSS, TA, and TSS/TA ratio are presented in Figure 2. Berry thinning increased the TSS and decreased TA in Shine Muscat grapes. Both BT30 and BT50 groups showed higher TSS and lowered TA than the control group (BT0). Furthermore, TSS and TA were affected by the level of berry thinning. Specifically, The BT50 group had higher TSS and lowered TA than the BT30 group. The ratio of TSS to TA (TSS/TA) tended to increase due to the increasing trend of TSS and decreasing trend of TA with increasing the level of berry thinning. A previous study by Sivilotti et al. [8] showed that selective berry thinning of Refosco dal peduncolo rosso (V. vinifera L.) grapes increased TSS. However, it showed no changes in TA or pH. Marko et al. [19] reported that berry thinning increased the TSS of Merlot grapes, but had little effect on Cabernet Sauvignon grapes. It was assumed that the berry thinning treatment accelerated the ripening of the grape berries because the changes in TSS and TA are good indicators of the ripeness of grapes. Thinning can provide thinned berries more opportunities to access enough nutrients compared to untreated ones, resulting in the acceleration of berry ripening [6].

The effects of berry thinning on the size and weight of Shine Muscat grapes are shown in Figure 3. The horizontal diameter showed no differences between BT0 and BT30, but BT50 was lower. On the other hand, the vertical diameters of BT30 and BT50 were higher than those of BT0. The cluster weight tended to decrease with increasing level of berry thinning. The average cluster weight was 1034, 825, and 524 g for BT0, BT30, and BT50, respectively. However, the weight of individual berries was not affected by thinning treatment, although Abd El-Razek et al. [20] found that berry thinning increased both berry size and weight of Crimson Seedless grape berries.

3.2. Phenolic Composition and Concentration

The effects of berry thinning on the evolution of phenolic compounds in Shine Muscat grapes were determined in terms of total phenolic concentration (TPC), proanthocyanidin concentration, and polymeric tannin concentration (PTC) as shown in Figure 4 due to their structural diversity giving rise to different biological and organoleptic properties. Berry thinning increased the accumulation of phenolic compounds in berry skin. The TPC of BT30 and BT50 were 65.6 and 66.8 mg GAE/g DW, respectively, and higher than BT0 (55.5 mg GAE/g DW). However, there were no differences in TPC between BT30 and BT50. PAC showed a trend similar to that of TPC. PAC of BT0, BT30, and BT50 were 19.9, 25.2, and 25.9 mg CE/g DW, respectively. Gil et al. [21] investigated the influence of berry thinning and cluster thinning in Vitis vinifera Syrah grapes on wine composition and quality. They found that berry thinning increased the concentration of proanthocyanidins in wine. Furthermore, berry thinning was more effective in increasing the proanthocyanidin concentration in wines than cluster thinning. However, the opposite trend was observed in the study by Sivilotti et al. [8]. It was observed that selective berry thinning of R. peduncolo rosso (Vitis vinifera L.) grapes decreased the accumulation of proanthocyanidins. PTC also showed a similar trend to that of TPC and PAC. The concentrations of polymeric tannins in BT0, BT30, and BT50 were 13.4, 17.1, and 16.6 mg TAE/g DW, respectively. Higher PTC values in BT30 and BT50 than in BT0 resulted from higher tannin maturation due to the acceleration of berry ripening. This result was in line with a previous study by Gil et al. [21] that berry thinning increased the degree of tannin polymerization in grape berries.

3.3. Volatile Free Aroma Compounds

The effects of berry thinning on the profiles of free volatile aroma compounds of Shine Muscat grapes are presented in

Table 2. Effects of berry thinning on the composition and concentration of volatile free aroma compounds in Shine Muscat grape juices.

Classification	Aroma Compounds	RT (min)	Identification	The Level of Berry Thinning (%)
				0	30	50
Alcohols	(E)-2-Hexen-1-ol	17.36	Mass spectrum	533.3 ± 133.1 a	505.4 ± 55.1 a	298.3 ± 59.0 b
	(E)-3-Hexen-1-ol	15.45	Standard	1 ± 0.3 a	0.3 ± 0.02 b	0.9 ± 0.2 a
	(Z)-2-Penten-1-ol	13.55	Standard	0.5 ± 0.1 b	0.5 ± 0.1 b	1.5 ± 0.2 a
	(Z)-3-Hexen-1-ol	16.29	Standard	39.9 ± 5.1 b	52.1 ± 4.2 a	28.9 ± 0.7 c
	1-Heptanol	19.32	Standard	0.3 ± 0.1	0.4 ± 0.1	N.D *
	1-Hexanol	15.09	Standard	323.4 ± 77.6 a	265.1 ± 34.1 a	294 ± 57.31 a
	1-Nonanol	27.42	Standard	0.2 ± 0.02 a	0.1 ± 0.03 b	0.1 ± 0.01 b
	1-Octanol	23.51	Standard	0.5 ± 0.03	N.D	0.4 ± 0.05
	1-Octen-3-ol	19.29	Standard	0.2 ± 0.02 a	0.2 ± 0.03 a	0.1 ± 0.03 a
	2-Ethyl-1-hexanol	20.85	Standard	4.1 ± 0.4 b	5 ± 0.4 a,b	5.2 ± 0.5 a
Aldehydes	(E)-2-Heptenal	13.16	Mass spectrum	0.8 ± 0.1	N.D	N.D
	(Z)-3-Hexenal	6.01	Standard	4.8 ± 0.1 b	4.9 ± 0.5 b	8.4 ± 0.1 a
	(E)-2-Hexenal	9.00	Standard	1140.3 ± 90.9 a	1149 ± 287.2 a	1419.7 ± 104.6 a
	Benzaldehyde	21.47	Standard	2.8 ± 0.2 b	4.1 ± 0.2 a	1.3 ± 0.6 c
	Hexanal	4.14	Standard	749.3 ± 91.4 b	983.2 ± 15.2 b	1538.5 ± 205.0 a
	Nonanal	16.46	Standard	0.3 ± 0.05 a	0.4 ± 0.03 a	0.2 ± 0.05 b
C13-Norisoprenoids	(E)-β-Damascenone	32.48	Standard	0.6 ± 0.1	0.2 ± 0.002	N.D
Esters	Ethyl octanoate	18.24	Mass spectrum	0.2 ± 0.05	N.D	N.D
	Methyl salicylate	30.73	Standard	1.1 ± 0.4	0.4 ± 0.1	N.D
Ketones	2,2-dimethyl-3-octanone	23.57	Mass spectrum	0.6 ± 0.1 ab	0.8 ± 0.2 a	0.4 ± 0.03 b
	2-Heptanone	7.50	Standard	1.2 ± 0.2 a	0.4 ± 0.1 b	0.2 ± 0.08 b
	2-Octanone	11.76	Standard	2.9 ± 0.6 a	0.8 ± 0.4 b	0.4 ± 0.06 b
	4-Methyl-2-Heptanone	8.61	Mass spectrum	N.D	N.D	0.2 ± 0.03
	6-Methyl-5-hepten-2-one	14.13	Standard	3.5 ± 0.7 a	3.8 ± 0.1 a	2.8 ± 0.4 a
Monoterpenes	(E)-Linalool oxide	18.44	Standard	N.D	7.9 ± 1.0	8.7 ± 1.4
	(E)-β-Ocimene	9.82	Standard	0.2 ± 0.03 a	0.3 ± 0.1 a	0.3 ± 0.07 a
	(R)-(+)-Limonene	7.82	Standard	0.1 ± 0.03 a	0.2 ± 0.05 a	0.2 ± 0.09 a
	(Z)-Citral	27.64	Standard	N.D	0.2 ± 0.003	0.2 ± 0.004
	(Z)-β-Ocimene	10.48	Standard	0.2 ± 0.04 b	0.4 ± 0.1 a	0.3 ± 0.08 ab
	Epoxylinalool	30.13	Mass spectrum	N.D	1.8 ± 0.4	2.8 ± 0.7
	Geraniol	34.15	Standard	13.4 ± 2.2 b	26.9 ± 1.5 a	8 ± 1.3 c
	Hotrienol	25.63	Mass spectrum	12.4 ± 1.0 b	16.7 ± 0.4 a	9.7 ± 1.5 c
	Linalool	23.25	Standard	38.7 ± 2.7 c	76.6 ± 10.7 b	116.3 ± 10.0 a
	Nerol	32.39	Standard	1.9 ± 0.5 b	5.9 ± 1.0 a	1.7 ± 0.2 b
	Nerol oxide	19.90	Mass spectrum	1.8 ± 0.09	1.1 ± 0.4	N.D
	Terpinen-4-ol	24.46	Standard	N.D	N.D	0.2 ± 0.08
	Terpinolene	11.47	Standard	N.D	0.1 ± 0.03	0.2 ± 0.06
	α-Terpineol	28.46	Standard	13.4 ± 1.2 a	13.2 ± 3.3 a	22.4 ± 5.4 a
	β-Citronellol	31.33	Standard	1.7 ± 0.3 a	1.9 ± 0.3 a	0.4 ± 0.1 b
	β-Myrcene	7.02	Standard	N.D	0.2 ± 0.07	0.1 ± 0.03
	β-Pinene	6.85	Standard	0.2 ± 0.04 b	0.6 ± 0.1 a	0.7 ± 0.06 a
	γ-Terpinene	7.09	Standard	N.D	0.1 ± 0.02	N.D
	Total			2895.8	3131.3	3773.7

* N.D: Not detected. The concentrations of the free aroma compounds were expressed in μg/L juice. Means with different letters within the same row are significantly different at p < 0.05.

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A total of 42 volatile compounds, including 10 alcohols, 6 aldehydes, 1 C13-norisoprenoids, 2 esters, 5 ketones, and 18 monoterpenes were identified. Berry thinning decreased the alcohol concentration while increasing the aldehyde concentration. Four C6-alcohols, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, (Z)-3-hexen-1-ol, and 1-hexanol constituted the majority of the total alcohol concentration. Among them, only (E)-2-hexen-1-ol was found to be an active odorant in all the sample groups. Three C6-aldehydes, (Z)-3-hexenal, (E)-2-hexenal, and hexanal, were the most abundant volatile aldehydes and were active odorants in all the sample groups. These C6-compounds are considered responsible for the herbaceous and grassy flavors of grapes [22]. Yuan and Qian [23] reported that the C6-alcohol concentration of Pinot noir grapes reached its highest around veraison and decreased after that, while the C6-aldehyde concentration increased with ripening. Thus, decreasing and increasing trends in C6-alcohols and aldehydes with increasing levels of berry thinning likely resulted from the different ripening rates among the sample groups. β-Damascenone and β-ionone are the major C13-norisoprenoids commonly found in grape berries. However, only trace amounts of (E)-β-damascenone were detected in all grape samples and were found to be active odorants of BT0 and BT30 due to its low odor threshold (0.002 μg/L in water). Esters and ketones were present in trace amounts.

In Muscat varieties, monoterpenes such as linalool, geraniol, nerol, citronellol, and α-terpineol are the major aroma compounds responsible for floral and fruity flavors [2]. However, in the present study, only linalool was an active odorant among them in all sample groups. Linalool was the most abundant monoterpene, and its concentration tended to increase with increasing berry thinning. Linalool oxide an oxidized derivative of linalool, was detected only in BT30 and BT50 and was an active odorant.

3.4. Sensory Properties

The effects of berry thinning on the sensory properties are shown in Figure 5. Berry thinning improved Shine Muscat grapes’ sweetness, flavor, and overall acceptance. BT50 and BT30 gained higher scores for sweetness, flavor, and overall acceptance than BT0. The sweetness score was the highest for BT50, followed by BT30 and BT0. The trend of sweetness scores was similar to that of TSS, showing an increasing trend with increasing berry thinning. The sourness score was the lowest in BT50 and was similar between BT30 and BT50, despite a decreasing trend of TA with increased berry thinning. The firmness and astringency scores were similar between the sample groups. The flavor scores showed a similar trend to that of sweetness. It was assumed that the increasing trend in flavor scores was related to the increasing trend of linalool concentration with increasing berry thinning. Furthermore, linalool oxide seemed to help enhance Muscat flavor in BT30 and BT50, since it was an active odorant in these sample groups. The overall acceptance scores showed a similar pattern to that of sweetness and flavor. The overall acceptance was highly affected by the sweetness and flavor attributes rather than the other attributes.

4. Conclusions

This study investigated the effect of berry thinning on Shine Muscat grape berries in terms of effects on sensory properties, increasing the TSS and reduced the TA of grape berries. The treatment increased the TSS and reduced the TA of grape berries. The treatment increased berry size and cluster weight but showed no effect on individual berry weight. TPC, PAC, and PTC tended to increase as the level of berry thinning increased. The compositional analysis of the volatile compounds confirmed that C6-compounds were the most abundant aroma compounds. Linalool, which provides a floral scent, was found to be the most abundant monoterpene. OAV analysis showed that (E)-2-hexen-1-ol, (E)-2-hexenal, 1-hexanal, (Z)-3-hexenal, (E)-β-damascenone, linalool, and (E)-linalool oxide were active odorants. The increasing trend of linalool concentration with increasing berry thinning was responsible for the high sensory flavor scores in the thinned berries. Moreover, linalool oxide detected only in BT30 and BT50 enhanced the Muscat flavor of thinned berries. It was concluded that berry thinning accelerated ripening and induced compositional changes in Shine Muscat grapes, resulting in improved sensory properties.