*3.2. Juice Yield*

As shown in Table 6, the juice yield significantly (*p* < 0.05) differed among the tested rhubarb cultivars. The average juice yield of the rhubarb cultivars ranged from 75% ('Poncho', HTA) to 85% ('Red Champagne', HTB). Generally, 'Red Champagne' was characterized by the highest juice yield potential for both harvests, as it significantly reached the highest values as compared with other tested cultivars. The statistical analysis of all tested results (Figure 3) showed significant differences (*p* < 0.001) in the case of morphological variability and influence of tested years on rhubarb juice yield. The term of harvests has not been shown to be significantly important to juice yield. The yield and composition of petioles are clearly influenced by the technology applied to a crop, especially the cultivar, the planting distance, and the nutritional regimen [14]. There were no data about juice yield of rhubarb found in the available literature. A comparison to similar celery petioles showed that high yield of celery juice was easy with a household juicer according to Donaldson, 2020 [24]. The juice yield and enzyme activity were tested in carrot, apples, spinach, and celery. The petiole juice yield of celery varied between 67.3% and 87.5% depending on juicer type. The yield of pressing ranged from 61.9% to 71.6% in the case of three apple cultivars according to Wilczy ´nski et al. [11]. In addition, the dry matter of roots, in the case of carrot, were estimated to be from 41.1% to 65.2% depending on the juicer type [24].


**Table 6.** Influence of cultivar and term of harvest on juice yield of rhubarb petioles.

Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'.

**Table 6.** Influence of cultivar and term of harvest on juice yield of rhubarb petioles.

 **2018 (HTB) 2019 (HTB) 2018–2019 (HTB)** 

Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations:

PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´.

**PON** 84 ± 2 bc 76 ± 2 abc 80 ± 3 abc **CRE** 83 ± 3 abc 70 ± 2 a 77 ± 2 ab **VAL** 84 ± 3 bc 80 ± 2 cd 82 ± 2 bc **RCH** 87 ± 1 c 84 ± 3 d 85 ± 1 c

**JY (%) JY (%) JY (%)** 

**Cultivar 2018 (HTA) 2019 (HTA) 2018–2019 (HTA)** 

**PON** 77 ± 2 a 74 ± 3 abc 75 ± 1 a **CRE** 79 ± 2 ab 80 ± 3 cd 79 ± 2 abc **VAL** 88 ± 2 c 72 ± 2 ab 80 ± 3 abc **RCH** 90 ± 3 c 79 ± 3 bcd 84 ± 2 c **VIC** 87 ± 3 c 78 ± 2 bcd 82 ± 2 bc

> **Figure 3.** Effect of the experimental year, cultivar, and term of the harvest on the mean juice yield of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.001 (\*\*\*). **Figure 3.** Effect of the experimental year, cultivar, and term of the harvest on the mean juice yield of all rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: *p* ≤ 0.001 (\*\*\*).

showed that high yield of celery juice was easy with a household juicer according to Donaldson, 2020 [24]. The juice yield and enzyme activity were tested in carrot, apples, spinach, and celery. The petiole juice yield of celery varied between 67.3% and 87.5% depending on juicer type. The yield of pressing ranged from 61.9% to 71.6% in the case of three apple cultivars according to Wilczyński et al. [11]. In addition, the dry matter of roots, in the case of carrot, were estimated to be from 41.1% to 65.2% depending on the juicer type

#### *3.3. Sugar Content 3.3. Sugar Content*

[24].

The average fructose content ranged from 33.93 (´Valentine´, HTA) to 37.93 g/L (´Valentine´, HTB), but cultivars significantly differed (*p* < 0.05) in fructose content only in the later harvest term (HTB), as shown in Table 7. When comparing all data from the cultivar variability point of view, the differences had not been proved (Figure 4). Fructose is highly represented in rhubarb. Its percentage representation is from 67% to 78% (as compared with total sugar content) according to the average from all results (Figure 8); therefore, it has high sensorial value in juice production. On the one hand, a statistically significant The average fructose content ranged from 33.93 ('Valentine', HTA) to 37.93 g/L ('Valentine', HTB), but cultivars significantly differed (*p* < 0.05) in fructose content only in the later harvest term (HTB), as shown in Table 7. When comparing all data from the cultivar variability point of view, the differences had not been proved (Figure 4). Fructose is highly represented in rhubarb. Its percentage representation is from 67% to 78% (as compared with total sugar content) according to the average from all results (Figure 5); therefore, it has high sensorial value in juice production. On the one hand, a statistically significant effect (*p* < 0.05) of the experimental year on fructose content was not found among rhubarb cultivars.


**Table 7.** Effect of cultivar on fructose and glucose content in rhubarb juice.

Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'.

**Figure 4.** Effect of the experimental year, cultivar, and term of the harvest on the mean fructose content of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance *p* ≤ 0.001 (\*\*\*). **Figure 4.** Effect of the experimental year, cultivar, and term of the harvest on the mean fructose content of all rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance *p* ≤ 0.001 (\*\*\*). RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: non-significant (NS) or significant at *p* ≤ 0.05 (\*), *p* ≤ 0.01 (\*\*), or *p* ≤ 0.001 (\*\*\*).

effect (*p* < 0.05) of the experimental year on fructose content was not found among rhubarb

On the other hand, the term of harvest had a significant effect (*p* < 0.001) on the fruc-

According to Table 7, the glucose content in the tested cultivars varied from 6.26 (´Victoria´, HTA) to 9.97 g/L (´Red Champagne´, HTB), which was 14–16% of the total sugar content of the evaluated rhubarb cultivars (Figure 8). The obtained results confirmed the statistically significant impact of cultivars on the glucose content of rhubarb (Table 7). The higher values were found in the HTB and, like the case of fructose, those differences were proven at the *p* < 0.001 level (Figure 5). The impact of the year on glucose content was significantly important when following all data (for both harvests and all cul-

**Table 7.** Effect of cultivar on fructose and glucose content in rhubarb juice.

**PON** 35.40 ± 0.34 cd 7.88 ± 0.19 d 35.03 ± 0.27 c 7.77 ± 0.18 e 35.22 ± 0.26 abc 7.83 ± 0.08 cde **CRE** 34.58 ± 0.20 b 7.47 ± 0.16 c 34.40 ± 0.36 bc 5.69 ± 0.21 c 34.49 ± 0.13 ab 6.58 ± 1.26 ab **VAL** 33.35 ± 0.45 a 6.34 ± 0.28 a 34.50 ± 0.21 bc 6.65 ± 0.12 d 33.93 ± 0.82 ab 6.50 ± 0.22 ab **RCH** 35.50 ± 0.80 cd 9.48 ± 0.08 f 34.05 ± 0.71 b 5.33 ± 0.14 b 34.78 ± 1.03 ab 7.40 ± 2.93 bcd **VIC** 36.03 ± 0.43 d 7.78 ± 0.07 d 32.80 ± 0.53 a 4.74 ± 0.07 a 34.41 ± 2.28 a 6.26 ± 2.15 a  **2018 (HTB) 2019 (HTB) 2018–2019 (HTB)** 

**PON** 33.58 ± 0.49 a 6.86 ± 0.10 b 38.18 ± 0.58 e 7.81 ± 0.15 e 35.88 ± 3.25 bc 7.34 ± 0.67 abcd **CRE** 38.20 ± 0.41 ef 6.90 ± 0.21 b 36.63 ± 0.50 d 6.82 ± 0.15 d 37.42 ± 1.11 de 6.86 ± 0.05 abc **VAL** 38.46 ± 0.30 f 8.71 ± 0.12 e 37.40 ± 0.27 de 8.89 ± 0.18 g 37.93 ± 0.75 e 8.80 ± 0.12 e **RCH** 37.56 ± 0.09 e 11.74 ± 0.13 g 34.87 ± 1.00 bc 8.20 ± 0.21 f 36.21 ± 1.90 cd 9.97 ± 2.51 f **VIC** 34.89 ± 0.44 bc 8.48 ± 0.13 e 33.99 ± 0.19 b 8.19 ± 0.10 f 34.44 ± 0.64 a 8.33 ± 0.21 de Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´.

tose content (Figure 4), which was higher in the second harvest.

**Fructose (g/L) Glucose (g/L) Fructose (g/L) Glucose (g/L) Fructose (g/L) Glucose (g/L)** 

**Cultivar 2018 (term HTA) 2019 (HTA) 2018–2019 (HTA)** 

cultivars.

tivars).

**Figure 8.** Share of fructose, glucose, and malic acid in total sugar and total acid content in the evaluated rhubarb cultivars. Table 2018. **Figure 5.** Share of fructose, glucose, and malic acid in total sugar and total acid content in the evaluated rhubarb cultivars. TS, total sugars and TA, total acid. Values are average from both harvests and years 2018–2019.

*3.4. Organic Acids and pH*  Malic acid is the primary acid in rhubarb. Its value ranged from 14.06 (´Red Cham-Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'.

pagne´, HTA) to 21.03 g/L (´Valentine´, HTB). ´Valentine´ reached the highest values for both harvests and the morphological variability was statistically improved at *p* < 0.05 (Ta-On the other hand, the term of harvest had a significant effect (*p* < 0.001) on the fructose content (Figure 4), which was higher in the second harvest.

ble 9) in the frame of each harvest as well as in the statistical analyses of all data at *p* < 0.001 (Figure 9). Any significant differences were found in terms of harvest and the year influence on malic acid content (Figure 9). A similar situation was observed in the case of total acid content. In Table 9, cultivar variability on total acid content was significantly proven, where total acid content values were the highest in the case of the cultivar ´Valentine´ for both harvests (21.88 g/L for HTA, and 22.95 g/L for HTB). The lowest value was reached by ´Canadian Red´ at the second harvest (15.29 g/L) followed by ´Red Cham-According to Table 7, the glucose content in the tested cultivars varied from 6.26 ('Victoria', HTA) to 9.97 g/L ('Red Champagne', HTB), which was 14–16% of the total sugar content of the evaluated rhubarb cultivars (Figure 5). The obtained results confirmed the statistically significant impact of cultivars on the glucose content of rhubarb (Table 7). The higher values were found in the HTB and, like the case of fructose, those differences were proven at the *p* < 0.001 level (Figure 6). The impact of the year on glucose content was significantly important when following all data (for both harvests and all cultivars).

(*p* < 0.01). The term of harvest was not significantly important (*p* = 0.8747).

in organic acids highly depends on the cultivation and harvesting period [6].

pagne´ with a value of 15.45 g/L at the first harvest. As shown in Figure 10, cultivar vari-

Organic acids play a biochemical role in maintaining the nutritional value and the quality of a vegetable species, and therefore they are among the frequently quantified compounds [27]. According to Welbaum [2], rhubarb leaves contain oxalic acid and should not be eaten. The petiole juice has a sharp acidic flavor. The oxalic acid in the petioles is much lower, the proportion of oxalic acid is about 10% of the total 2–2.5% acidity, which is dominated by nontoxic malic acid [7]. This means that the stalks are not hazardous to eat but have a very tart taste [2]. Malic acid is the predominant one, and citric and oxalic acids are also present in a smaller quantity in rhubarb plants according to Will and Dietrich [28]. The share of malic acid in total acid content varied between 89% and 92% in the evaluated rhubarb cultivars, as shown in Figure 8. Among the organic acids, the highest content was found in the case of malic acid, with average values of 679 ± 2.88 mg.100 g−1 fresh weight, according to Stoleru et al. [9]. Our results showed higher values because, in early petiole harvest, malic acid content is higher and decreases with vegetation period advancement in which the malic acid is naturally decomposed. According to the same study, the total acid content was 2239 mg.100 g−1 fresh weight. The chemical composition

**Cultivar** 

**Figure 5.** Effect of the experimental year, cultivar, and term of the harvest on the mean glucose content of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.01 (\*\*), or *p* ≤ 0.001 (\*\*\*). **Figure 6.** Effect of the experimental year, cultivar, and term of the harvest on the mean glucose content of all rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: *p* ≤ 0.01 (\*\*), or *p* ≤ 0.001 (\*\*\*).

The total soluble solids, defined by Hegedűsová et al. [25] as additive quantity that expresses the content of dissolved substances, mainly sugars, in vegetable or fruit extracts, were also tested and are summarized in Table 8. The values for total soluble solids ranged from 3.54 (´Valentine´, HTA) to 4.37 °BRIX (´Red Champagne´, HTB). Significant differences (*p* < 0.05) were found between the tested cultivars and the values were higher in the second harvest (Table 8). When comparing all data in Figure 6, the significant effect (*p* < 0.001) of the cultivar, term of the harvest, and experimental year on the mean total soluble solids values of all rhubarb cultivars were found. The higher values in the late term of harvest could relate to the total soluble solid content increasing during ripening. This parameter is a very practical index of internal fruit quality and an accurate criterion for the decision to harvest in the field [26]. The mean value 3.8 °Brix was observed, with a range from 2.2 to 6.1° according to Pantoja and Kuhl [15], where they evaluated fifteen morphological characteristics to differentiate rhubarb cultivars. The total soluble solids, defined by Heged˝usová et al. [25] as additive quantity that expresses the content of dissolved substances, mainly sugars, in vegetable or fruit extracts, were also tested and are summarized in Table 8. The values for total soluble solids ranged from 3.54 ('Valentine', HTA) to 4.37 ◦BRIX ('Red Champagne', HTB). Significant differences (*p* < 0.05) were found between the tested cultivars and the values were higher in the second harvest (Table 8). When comparing all data in Figure 7, the significant effect (*p* < 0.001) of the cultivar, term of the harvest, and experimental year on the mean total soluble solids values of all rhubarb cultivars were found. The higher values in the late term of harvest could relate to the total soluble solid content increasing during ripening. This parameter is a very practical index of internal fruit quality and an accurate criterion for the decision to harvest in the field [26]. The mean value 3.8 ◦Brix was observed, with a range from 2.2 to 6.1◦ according to Pantoja and Kuhl [15], where they evaluated fifteen morphological characteristics to differentiate rhubarb cultivars. *Plants* **2021**, *10*, x FOR PEER REVIEW 10 of 16

**Figure 6.** Effect of the experimental year, cultivar, and term of the harvest on the mean total soluble solid content of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: significant at *p* ≤ 0.05 (\*) or *p* ≤ 0.001 (\*\*\*). **Figure 7.** Effect of the experimental year, cultivar, and term of the harvest on the mean total soluble solid content of all rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: significant at *p* ≤ 0.05 (\*) or *p* ≤ 0.001 (\*\*\*).

**TSS Total sugar TSS Total sugar TSS Total sugar °BRIX (g/L) °BRIX (g/L) °BRIX (g/L)** 

> **Figure 7.** Effect of the experimental year, cultivar, and term of the harvest on the total sugar content of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´;

Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´.

**Table 8.** Effect of cultivar on total soluble solid content and total sugar content in rhubarb juice.

**PON** 3.98 ± 0.03 d 44.89 ± 0.40 c 4.22 ± 0.02 h 52.82 ± 0.80 f 4.10 ± 0.17 cd 48.86 ± 5.61 bc **CRE** 3.74 ± 0.02 b 45.35 ± 0.14 c 3.44 ± 0.04 b 43.22 ± 0.11 a 3.59 ± 0.21 ab 44.29 ± 1.50 a **VAL** 3.48 ± 0.03 a 43.32 ± 0.12 a 3.60 ± 0.02 c 45.06 ± 0.15 b 3.54 ± 0.08 a 44.19 ± 1.23 a **RCH** 4.15 ± 0.02 f 52.59 ± 0.10 ef 3.25 ± 0.01 a 50.14 ± 0.37 e 3.70 ± 0.63 ab 51.36 ± 1.73 c **VIC** 3.87 ± 0.04 c 50.69 ± 0.22 d 3.23 ± 0.01 a 47.90 ± 0.48 d 3.55 ± 0.45 a 49.30 ± 1.91 c  **2018 (HTB) 2019 (HTB) 2018–2019 (HTB) PON** 3.76 ± 0.03 b 43.99 ± 0.16 b 3.95 ± 0.02 f 46.82 ± 0.11 c 3.86 ± 0.13 abc 45.40 ± 2.00 ab **CRE** 4.04 ± 0.05 e 52.12 ± 0.57 e 3.60 ± 0.03 c 45.25 ± 0.19 b 3.82 ± 0.32 abc 48.69 ± 4.86 bc **VAL** 4.50 ± 0.08 g 54.43 ± 0.59 h 4.12 ± 0.02 g 48.20 ± 0.23 d 4.31 ± 0.27 d 51.31 ± 4.41 c **RCH** 4.86 ± 0.06 h 60.14 ± 0.43 i 3.88 ± 0.03 e 49.78 ± 0.26 e 4.37 ± 0.70 d 54.96 ± 7.33 d **VIC** 4.03 ± 0.12 e 52.85 ± 0.33 g 3.76 ± 0.03 d 48.51 ± 0.43 d 3.90 ± 0.20 bc 50.68 ± 3.07 c Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII).


**Table 8.** Effect of cultivar on total soluble solid content and total sugar content in rhubarb juice.

*Plants* **2021**, *10*, x FOR PEER REVIEW 10 of 16

Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. **VAL** 3.48 ± 0.03 a 43.32 ± 0.12 a 3.60 ± 0.02 c 45.06 ± 0.15 b 3.54 ± 0.08 a 44.19 ± 1.23 a **RCH** 4.15 ± 0.02 f 52.59 ± 0.10 ef 3.25 ± 0.01 a 50.14 ± 0.37 e 3.70 ± 0.63 ab 51.36 ± 1.73 c

The total sugar content varied between 44.19 ('Valentine', HTA) and 54.96 g/L ('Red Champagne', HTB) and according to Table 8 the differences between the tested cultivars were significant (*p* < 0.05). The values were higher at the later harvest (HTB), in which differences between the two harvests were also significantly proven at *p* < 0.001 (Figure 8). The year impact also significantly (*p* < 0.01) influenced the total sugar content parameter when comparing all cultivars and both harvests. The significant differences (*p* < 0.001) in terms of harvest influence on tested sugars as well as in the case of agrotechnical and soil/climate conditions through the tested years are in accordance with the study by Kalisz et al. [6] where the chemical composition of organic acids, minerals, carbohydrates, proteins, and vitamins highly depended on the cultivation and harvesting period. **VIC** 3.87 ± 0.04 c 50.69 ± 0.22 d 3.23 ± 0.01 a 47.90 ± 0.48 d 3.55 ± 0.45 a 49.30 ± 1.91 c  **2018 (HTB) 2019 (HTB) 2018–2019 (HTB) PON** 3.76 ± 0.03 b 43.99 ± 0.16 b 3.95 ± 0.02 f 46.82 ± 0.11 c 3.86 ± 0.13 abc 45.40 ± 2.00 ab **CRE** 4.04 ± 0.05 e 52.12 ± 0.57 e 3.60 ± 0.03 c 45.25 ± 0.19 b 3.82 ± 0.32 abc 48.69 ± 4.86 bc **VAL** 4.50 ± 0.08 g 54.43 ± 0.59 h 4.12 ± 0.02 g 48.20 ± 0.23 d 4.31 ± 0.27 d 51.31 ± 4.41 c **RCH** 4.86 ± 0.06 h 60.14 ± 0.43 i 3.88 ± 0.03 e 49.78 ± 0.26 e 4.37 ± 0.70 d 54.96 ± 7.33 d **VIC** 4.03 ± 0.12 e 52.85 ± 0.33 g 3.76 ± 0.03 d 48.51 ± 0.43 d 3.90 ± 0.20 bc 50.68 ± 3.07 c Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´.

**Figure 7.** Effect of the experimental year, cultivar, and term of the harvest on the total sugar content of all rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; **Figure 8.** Effect of the experimental year, cultivar, and term of the harvest on the total sugar content of all rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: non-significant (NS) or significant at *p* ≤ 0.05 (\*), *p* ≤ 0.01 (\*\*), or *p* ≤ 0.001 (\*\*\*).

#### *3.4. Organic Acids and pH* **CRE** 16.47 ± 1.06 e 18.49 ± 0.10 e 16.99 ± 1.04 g 18.42 ± 1.04 f 16.73 ± 0.37 b 18.46 ± 0.25 b **CRE** 16.47 ± 1.06 e 18.49 ± 16.99 ± 1.04 g 18.42 ± 1.04 f 16.73 ± 0.37 b 18.46 ± b

Malic acid is the primary acid in rhubarb. Its value ranged from 14.06 ('Red Champagne', HTA) to 21.03 g/L ('Valentine', HTB). 'Valentine' reached the highest values for both harvests and the morphological variability was statistically improved at *p* < 0.05 (Table 9) in the frame of each harvest as well as in the statistical analyses of all data at *p* < 0.001 (Figure 9). Any significant differences were found in terms of harvest and the year influence on malic acid content (Figure 9). A similar situation was observed in the case of total acid content. In Table 9, cultivar variability on total acid content was significantly proven, where total acid content values were the highest in the case of the cultivar 'Valentine' for both harvests (21.88 g/L for HTA, and 22.95 g/L for HTB). The lowest value was reached by 'Canadian Red' at the second harvest (15.29 g/L) followed by 'Red Champagne' with a value of 15.45 g/L at the first harvest. As shown in Figure 10, cultivar variability had a significant impact (*p* < 0.001) on total acids content, as well as the year impact (*p* < 0.01). The term of harvest was not significantly important (*p* = 0.8747). **VAL** 19.40 ± 2.03 f 21.92 ± 1.05 f 20.19 ± 2.06 j 21.84 ± 0.88 h 19.80 ± 0.56 cd 21.88 ± 0.46 cd **RCH** 14.43 ± 1.07 a 15.17 ± 0.03 a 13.69 ± 0.16 c 15.72 ± 1.06 b 14.06 ± 0.52 a 15.45 ± 0.39 a **VIC** 15.49 ± 0.94 c 16.14 ± 0.65 c 12.13 ± 0.17 a 15.89 ± 2.14 c 13.81 ± 2.38 a 16.01 ± 0.17 a  **2018 (HTB) 2019 (HTB) 2018–2019 (HTB) PON** 19.79 ± 1.03 g 22.39 ± 1.06 g 18.03 ± 1.05 h 19.44 ± 2.07 g 18.91 ± 1.25 c 20.92 ± 2.08 c **CRE** 15.08 ± 0.25 b 16.16 ± 1.03 c 13.44 ± 1.09 b 14.42 ± 0.22 a 14.26 ± 1.16 a 15.29 ± 1.23 a **VAL** 22.63 ± 2.08 i 24.16 ± 2.13 h 19.43 ± 1.06 i 21.74 ± 0.83 h 21.03 ± 2.27 d 22.95 ± 1.71 d **RCH** 14.97 ± 1.06 b 15.90 ± 1.09 b 15.81 ± 0.25 e 17.10 ± 1.05 d 15.39 ± 0.60 ab 16.50 ± 0.85 a **VIC** 15.71 ± 1.12 d 16.75 ± 0.14 d 16.21 ± 0.39 f 17.06 ± 2.08 d 15.96 ± 0.35 b 16.90 ± 0.22 ab Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. **VAL** 19.40 ± 2.03 f 21.92 ± 1.05 f 20.19 ± 2.06 j 21.84 ± 0.88 h 19.80 ± 0.56 cd 21.88 ± 0.46 cd **RCH** 14.43 ± 1.07 a 15.17 ± 0.03 a 13.69 ± 0.16 c 15.72 ± 1.06 b 14.06 ± 0.52 a 15.45 ± 0.39 a **VIC** 15.49 ± 0.94 c 16.14 ± 0.65 c 12.13 ± 0.17 a 15.89 ± c 13.81 ± a 16.01 ± 0.17 a  **2018 (HTB) 2019 (HTB) 2018–2019 (HTB) PON** 19.79 ± 1.03 g 22.39 ± 1.06 g 18.03 ± 1.05 h 19.44 ± 2.07 g 18.91 ± 1.25 c 20.92 ± 2.08 c **CRE** 15.08 ± 0.25 b 16.16 ± 1.03 c 13.44 ± 1.09 b 14.42 ± 0.22 a 14.26 ± a 15.29 ± 1.23 a **VAL** 22.63 ± 2.08 i 24.16 ± 2.13 h 19.43 ± i 21.74 ± 0.83 h 21.03 ± 2.27 d 22.95 ± 1.71 d **RCH** 14.97 ± 1.06 b 15.90 ± 1.09 b 15.81 ± 0.25 e 17.10 ± 1.05 d 15.39 ± ab 16.50 ± 0.85 a **VIC** 15.71 ± d 16.75 ± 0.14 d 16.21 ± 0.39 f 17.06 ± 2.08 d 15.96 ± 0.35 b 16.90 ± 0.22 ab Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´.

*Plants* **2021**, *10*, x FOR PEER REVIEW 12 of 16

*Plants* **2021**, *10*, x FOR PEER REVIEW 12 of 16

**Table 9.** Effect of cultivar on malic acid and total acid content in rhubarb juice.

**Table 9.** Effect of cultivar on malic acid and total acid content in rhubarb juice.

**PON** 21.62 ± 0.16 h 24.73 ± 2.23 i 15.34 ± 0.59 d 17.43 ± 0.55 e 18.48 ± 4.45 c 21.08 ± 5.16 c

**(g/L) (g/L) (g/L) (g/L) (g/L) (g/L) PON** 21.62 ± 0.16 h 24.73 ± 2.23 15.34 ± 0.59 d 17.43 ± 0.55 e 18.48 ± 4.45 c 21.08 ± 5.16 c

**2018 (HTA) 2019 (HTA) 2018–2019 (HTA)** 

**2018 (HTA) 2019 (HTA) 2018–2019 (HTA)** 

**(g/L) (g/L) (g/L) (g/L) (g/L) (g/L)** 

**Figure 9.** Effect of the experimental year, cultivar, and term of the harvest on the malic acid content in rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.001 (\*\*\*). **Figure 9.** Effect of the experimental year, cultivar, and term of the harvest on the malic acid content in rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: *p* ≤ 0.001 (\*\*\*). **Figure 9.** Effect of the experimental year, cultivar, and term of the harvest on the malic acid content in rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.001 (\*\*\*).


**Table 9.** Effect of cultivar on malic acid and total acid content in rhubarb juice.

Values with different letters in columns are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'.

> Organic acids play a biochemical role in maintaining the nutritional value and the quality of a vegetable species, and therefore they are among the frequently quantified compounds [27]. According to Welbaum [2], rhubarb leaves contain oxalic acid and should not be eaten. The petiole juice has a sharp acidic flavor. The oxalic acid in the petioles is much lower, the proportion of oxalic acid is about 10% of the total 2–2.5% acidity, which is dominated by nontoxic malic acid [7]. This means that the stalks are not hazardous to eat but have a very tart taste [2]. Malic acid is the predominant one, and citric and oxalic acids are also present in a smaller quantity in rhubarb plants according to Will and Dietrich [28]. The share of malic acid in total acid content varied between 89% and 92% in the evaluated rhubarb cultivars, as shown in Figure 5. Among the organic acids, the highest content was found in the case of malic acid, with average values of 679 <sup>±</sup> 2.88 mg.100 g−<sup>1</sup> fresh weight, according to Stoleru et al. [9]. Our results showed higher values because, in early petiole harvest, malic acid content is higher and decreases with vegetation period advancement in which the malic acid is naturally decomposed. According to the same study, the total acid content was 2239 mg.100 g−<sup>1</sup> fresh weight. The chemical composition in organic acids highly depends on the cultivation and harvesting period [6].

> The flavor of fruits and vegetable can also be determined by the sugar/acid (S/A) ratio that, in our case, expresses the ratio between total sugars and total acids; when the ratio is higher, the juice is subjectively less acidic. As shown in Table 10, the lowest S/A ratio of 2.02 was detected in the 'Valentine' cultivar, which means that there are 2.02 parts of sugars per one part of acid and the 'Valentine' is sensorially sweeter as compared with the 'Red Champagne' with the highest value of S/A ratio (3.35). The year and term of harvest did not have a significant effect on the S/A ratio according to the statistical analyses (Figure 11). Although the SSC/TA ratio is currently used as a maturity index for some types of fruit, it has been recognized that this measurement does not always correlate well with the perception of sweetness or tartness in other fruits [29].


**Table 10.** Effect of cultivar on pH level and the S/A ratio in rhubarb juice. **CRE** 2.86 ± 0.22 bc 2.45 ± 0.01 d 2.93 ± 0.12 d 2.35 ± 0.00 c 2.90 ± 0.24 b 2.40 ± 0.07 b

the perception of sweetness or tartness in other fruits [29].

**Table 10.** Effect of cultivar on pH level and the S/A ratio in rhubarb juice.

**PON** 2.60 ± 0.02 a 1.82 ± 0.03 a 2.95 ± 0.04 d 3.03 ± 0.05 g 2.77 ± 0.08 b 2.42 ± 0.86 b

**pH S/A Ratio pH S/A Ratio pH S/A Ratio** 

*Plants* **2021**, *10*, x FOR PEER REVIEW 13 of 16

The flavor of fruits and vegetable can also be determined by the sugar/acid (S/A) ratio that, in our case, expresses the ratio between total sugars and total acids; when the ratio is higher, the juice is subjectively less acidic. As shown in Table 10, the lowest S/A ratio of 2.02 was detected in the ´Valentine´ cultivar, which means that there are 2.02 parts of sugars per one part of acid and the ´Valentine´ is sensorially sweeter as compared with the ´Red Champagne´ with the highest value of S/A ratio (3.35). The year and term of harvest did not have a significant effect on the S/A ratio according to the statistical analyses (Figure 11). Although the SSC/TA ratio is currently used as a maturity index for some types of fruit, it has been recognized that this measurement does not always correlate well with

Values with different letters in columns are significantly different at P < 0.05 by LSD test in ANOVA (Statgraphic XVII). Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. to prepare rhubarb dishes. The cultivar had a significant influence (*p* < 0.001) on pH, as shown in Figure 12, while the year and term of harvest did not have a significant influence.

**Figure 11.** Effect of the experimental year, cultivar, and term of the harvest on the sugar/acid ratio in the rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.001 (\*\*\*). **Figure 11.** Effect of the experimental year, cultivar, and term of the harvest on the sugar/acid ratio in the rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: *p* ≤ 0.001 (\*\*\*).

The pH of rhubarb juice varied between 2.77 ('Poncho', THA) and 3.25 ('Canadian Red', HTB), as shown in Table 10, which corresponded with Welbaum [2], where the pH of rhubarb juice was measured to be 3.2; therefore, large quantities of sugar are often used to prepare rhubarb dishes. The cultivar had a significant influence (*p* < 0.001) on pH, as shown in Figure 12, while the year and term of harvest did not have a significant influence.

**Figure 12.** Effect of the experimental year, cultivar, and term of the harvest on the pH in the rhubarb cultivars. Abbreviations: PON, ´Poncho´; CRE, ´Canadian Red´; VAL, ´Valentine´; RCH, ´Red Champagne´; VIC, ´Victoria´. LSD test with significance: *p* ≤ 0.001 (\*\*\*). **Figure 12.** Effect of the experimental year, cultivar, and term of the harvest on the pH in the rhubarb cultivars. Abbreviations: PON, 'Poncho'; CRE, 'Canadian Red'; VAL, 'Valentine'; RCH, 'Red Champagne'; VIC, 'Victoria'. LSD test with significance: *p* ≤ 0.001 (\*\*\*).

#### **4. Conclusions 4. Conclusions**

Since fresh rhubarb is considered to be not tasty, i.e., it is very sour and slightly sweet, the ideal solution for using its concentrated nutrients and antioxidants is to use it as a juice, or as an additive to mixed juices. Among the given cultivars, the highest yield was achieved by the cultivar ´Red Champagne´ (45.58 t/ha), with the highest juice yield potential (85%), which is also interesting from the tested parameters point of view. It reached the highest values of glucose (9.97 g/L), total soluble solids (4.37 g/L), and total sugars (54.96 g/L). In contrast, ´Valentine´ had glucose (6.50 g/L) and total sugar content (44.19 g/L) at the lowest level with the highest total acids (22.95 g/L). The juice yield of the plant was tested as an important parameter in the case of practical uses and its value varied in the following order: ´Poncho´ < ´Canadian Red´ < ´Valentine´ < ´Victoria´ < ´Red Champagne´. The quality of the juice was significantly affected by the harvest term, where in the later term (HTB) the values were higher in the case of sugars (total sugars, TSS, fructose, and glucose). The acids were not changed by the term of harvest. In the case of some parameters, the significant influence of the year was also amplified, as there were significant differences in the agroclimatic characteristics tested for the two years. Since studying the sensory analysis of rhubarb juice is a new scientific issue, the results can serve as a basis for widespread use of this health-promoting crop, especially in the context of gastronomic use. We recommend adding rhubarb juice to vitalize, for example, apples harvested too late, where the share of malic acid is too low and achieves a balanced ratio between sugars and acids. The results show that rhubarb juice is a very suitable fortifier for such fruits, especially apples, which are unsaleable after long-term storage and juices Since fresh rhubarb is considered to be not tasty, i.e., it is very sour and slightly sweet, the ideal solution for using its concentrated nutrients and antioxidants is to use it as a juice, or as an additive to mixed juices. Among the given cultivars, the highest yield was achieved by the cultivar 'Red Champagne' (45.58 t/ha), with the highest juice yield potential (85%), which is also interesting from the tested parameters point of view. It reached the highest values of glucose (9.97 g/L), total soluble solids (4.37 g/L), and total sugars (54.96 g/L). In contrast, 'Valentine' had glucose (6.50 g/L) and total sugar content (44.19 g/L) at the lowest level with the highest total acids (22.95 g/L). The juice yield of the plant was tested as an important parameter in the case of practical uses and its value varied in the following order: 'Poncho' < 'Canadian Red' < 'Valentine' < 'Victoria' < 'Red Champagne'. The quality of the juice was significantly affected by the harvest term, where in the later term (HTB) the values were higher in the case of sugars (total sugars, TSS, fructose, and glucose). The acids were not changed by the term of harvest. In the case of some parameters, the significant influence of the year was also amplified, as there were significant differences in the agroclimatic characteristics tested for the two years. Since studying the sensory analysis of rhubarb juice is a new scientific issue, the results can serve as a basis for widespread use of this health-promoting crop, especially in the context of gastronomic use. We recommend adding rhubarb juice to vitalize, for example, apples harvested too late, where the share of malic acid is too low and achieves a balanced ratio between sugars and acids. The results show that rhubarb juice is a very suitable fortifier for such fruits, especially apples, which are unsaleable after long-term storage and juices made from such apples are often faint because malic acid has been degraded during the storage process. By enriching such juices with a rhubarb component, we achieve an optimal level of acidity in relation to the content of total sugars, which creates a very refreshing drink.

made from such apples are often faint because malic acid has been degraded during the storage process. By enriching such juices with a rhubarb component, we achieve an optimal level of acidity in relation to the content of total sugars, which creates a very refresh-**Author Contributions:** Conceptualization—I.M.; methodology—I.M., J.M., and A.A.; resources— J.M. and A.A.; writing—original draft preparation—I.M.; writing—review and editing—I.M., J.M., and A.A. All authors have read and agreed to the published version of the manuscript.

ing drink. **Author Contributions:** Conceptualization—I.M.; methodology—I.M, J.M., and A.A.; resources— **Funding:** This research was funded by the project, Development of Theoretical Knowledge and Practical Skills of Students for Teaching of Subject Vegetable Production, KEGA 018SPU-4/2020.

and A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the project, Development of Theoretical Knowledge and Practical Skills of Students for Teaching of Subject Vegetable Production, KEGA 018SPU-4/2020.

J.M and A.A.; writing—original draft preparation—I.M.; writing—review and editing—I.M., J.M., **Conflicts of Interest:** The authors declare no conflict of interest.

**Conflicts of Interest:** The authors declare no conflict of interest.
