**1. Introduction**

Rhubarb (*Rheum rhabarbarum* L.) is a perennial plant from the family Polygonacea with valuable nutritional and medicinal properties [1] and high potential for cultivation as it provides early yields when the vegetable supply to market is deficient. Rhubarb is grown for its large, thick leaf stalks or petioles that are consumed as food [2]. The leaf stalks are of various widths, with a range of 30–50 mm in diameter. The stems range in length from 300 to 500 mm and can be green to anthocyanin in color. The position of the stems can be upright or semi-upright to horizontal [3]. The enlarged petioles develop from a central crown of the rhubarb plant. Petiole color is associated with rhubarb quality and the order of preference is red, pink, and green [2]. Although rhubarb is a well-known vegetable, scientific interest in this plant is a relatively new issue; most of the evidence of its biological activities and therapeutic potential derives from the last 15 years [4]. It shows high levels of both polyphenol content and antioxidant capacity in edible parts [5], which are petioles characterized by very high antioxidant properties and rich in many compounds that have a pro-health effect on the human body [6]. There is a wide variety of rhubarb cultivars that contain bioactive compounds such as flavonoids, anthraquinone, glycosides, tannins, volatile oils, and saponins [1]. The chemical composition of rhubarb includes anthraquinones, anthrones, stilbenes, tannins, polysaccharides, etc., which show extensive pharmacological activities including gastrointestinal regulation, anticancer and antimicrobial properties, hepatoprotection, cardiovascular and cerebrovascular anti-inflammatory protection, etc. [7,8]. The main characteristics of rhubarb quality are taste and aroma, which depend on the chemical composition [9]. Studies on the phytochemical composition of different species of rhubarb have provided information on the presence of a variety of inorganic and organic acids (including tartaric, oxalic, citric, malic, and ascorbic acids) [4].

**Citation:** Mezeyová, I.; Mezey, J.; Andrejiová, A. The Effect of the Cultivar and Harvest Term on the Yield and Nutritional Value of Rhubarb Juice. *Plants* **2021**, *10*, 1244. https://doi.org/10.3390/ plants10061244

Academic Editor: Ivo Vaz de Oliveira

Received: 21 May 2021 Accepted: 15 June 2021 Published: 18 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

´Poncho´

´Victoria´

´Valentine´

´Red Champagne´

´Canadian Red´

Increased interest in the application of natural biologically active substances in human nutrition has led scientists to develop functional foods. The production of functional drinks is an ever-changing aspect of the beverage industry, as there are still new and prospective trends in this area [10]. Some examples of these kinds of foods include juices that are not obtained from concentrate (NFC) and freshly squeezed non-pasteurized juices (FS). These juices are obtained from the fruit tissue by pressing and centrifugation of the pulp [11]. Nutritionists recommend the consumption of fruit or vegetable juices instead of replacing natural products with synthetic vitamin and mineral supplements [12]. Fruit juices as functional drinks offer promoting good health by reducing the risk of serious illness. These beverages contain ingredients that provide specific benefits and are enriched with vitamins, minerals, amino acids, fiber, or antioxidants [13]. Products made from rhubarb have a favorable taste due to a high content of organic acids and rhubarb stalks taste best in early spring when they are ripe. Because rhubarb leaves are toxic due to their content of oxalic acid [14], the petioles are used for processing juices.

It should be noted that the scientific literature lacks studies on this plant and its application in the food industry. Therefore, for constant development and consumers' search for new solutions, it is necessary to focus attention on this plant [6]. It has been utilized for thousands of years for medicinal purposes, but only recently identified for its culinary use [15]; it was not until the 18th century that the culinary use of petioles was first reported [16]. In culinary rhubarb, the high oxalate content is a major drawback. A renewed interest in rhubarb production is now directed towards the use of stalks from low-oxalate cultivars as a cheap filler for industrial production of marmalade, jam, and syrup [17]. It has potential importance in the food and pharmaceutical industries to expand the range of products in the future by adding a new product with outstanding antioxidant properties [6]. Therefore, the aim of this study was to test selected rhubarb cultivars from a specific culinary perspective, i.e., processing into juice. From qualitative parameters, the sugars and acids were established in rhubarb juices which are important from a sensory analysis point of view. In addition, the juice yield of the plant was tested as an important parameter in the case of practical uses.

#### **2. Materials and Methods**

This 2-year vegetation field experiment was carried out at the Botanical Garden (BG), Slovak University of Agriculture (SUA), in 2018 and 2019 (Nitra, Slovak Republic, 48◦180 N, 18◦050 E, 144 masl).

## *2.1. Rhubarb Cultivars' Characterization*

Morphological characterization and quantitative parameters of petioles of selected cultivars of *Rheum rhabarbarum* L. are summarized in Tables 1 and 2. *Plants* **2021**, *10*, x FOR PEER REVIEW 3 of 16

> **Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18]. **Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18].


Erect attitude, type of cross-section 2, green ground color of skin, entire distribution of skin superimposed color at base, speckled distribution of skin superimposed color at middle, absent distribution of skin superimposed

Erect attitude, type of cross-section 3, red ground color of skin, entire distribution of skin superimposed color at base, speckled distribution of skin superimposed color at middle, speckled distribution of skin superimposed color just below leaf blade, absent hairiness just below leaf blade, medium ribbing of dorsal side, pink color of flesh

Erect attitude, type of cross-section 2, red ground color of skin, entire distribution of skin superimposed color at base, speckled distribution of skin superimposed color at middle, speckled distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, weak ribbing of dorsal side, green color of flesh

Semi-erect attitude, type of cross-section **7**, red ground color of skin, entire distribution of skin superimposed color at base, entire distribution of skin superimposed color at middle, speckled distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, strong ribbing of dorsal side, green color of flesh

\* Photo by Andrejiová.

**Table 2.** Quantitative parameters of *Rheum rhabarbarum* L. petioles (*n* = 100).

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

**Cultivar Length (cm) Width (cm) Thickness (cm) Weight (g)**  ´Poncho´ 29.42 ± 6.21 a 1.79 ± 0.33 a 1.16 ± 0.29 cd 49.35 ± 21.88 a ´Victoria´ 33.99 ± 6.39 b 1.78 ± 0.28 a 0.97 ± 0.19 a 54.38 ± 15.80 a ´Valentine´ 33.38 ± 6.64 b 2.31 ± 0.34 c 1.29 ± 0.30 d 77.61 ± 33.60 c ´Red Champagne´ 37.03 ± 7.14 c 1.96 ± 0.48 b 1.02 ± 0.22 ab 63.04 ± 25.58 b ´Canadian Red´ 29.55 ± 4.09 a 2.27 ± 0.30 c 1.07 ± 0.28 bc 62.47 ± 20.43 b ´Victoria´

´Victoria´

´Victoria´

´Victoria´

´Valentine´

´Valentine´

´Valentine´

´Valentine´

´Red Champagne´

´Red Champagne´

´Red Champagne´

´Red Champagne´

´Canadian Red´

´Canadian Red´

´Canadian Red´

´Canadian Red´


*Plants* **2021**, *10*, x FOR PEER REVIEW 3 of 16

*Plants* **2021**, *10*, x FOR PEER REVIEW 3 of 16

*Plants* **2021**, *10*, x FOR PEER REVIEW 3 of 16

*Plants* **2021**, *10*, x FOR PEER REVIEW 3 of 16

**Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18].

**Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18].

**Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18].

**Table 1.** Rhubarb cultivars' characterization (morphological description according to UPOV (1999)) [18].

Semi-erect attitude, type of cross-section 1, green ground color of skin, entire distribution of skin superimposed color at base, absent distribution of skin superimposed color at middle, absent distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, absent or

Semi-erect attitude, type of cross-section 1, green ground color of skin, entire distribution of skin superimposed color at base, absent distribution of skin superimposed color at middle, absent distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, absent or

Semi-erect attitude, type of cross-section 1, green ground color of skin, entire distribution of skin superimposed color at base, absent distribution of skin superimposed color at middle, absent distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, absent or very weak ribbing of dorsal side, green color of flesh

Semi-erect attitude, type of cross-section 1, green ground color of skin, entire distribution of skin superimposed color at base, absent distribution of skin superimposed color at middle, absent distribution of skin superimposed color just below leaf blade, present hairiness just below leaf blade, absent or very weak ribbing of dorsal side, green color of flesh

 **Morphological Description of Petiole Picture of the Petioles \*** 

 **Morphological Description of Petiole Picture of the Petioles \*** 

 **Morphological Description of Petiole Picture of the Petioles \*** 

 **Morphological Description of Petiole Picture of the Petioles \*** 

**Table 1.** *Cont.* very weak ribbing of dorsal side, green color of flesh very weak ribbing of dorsal side, green color of flesh

\* Photo by Andrejiová.


\* Photo by Andrejiová.


´Valentine´ 33.38 ± 6.64 b 2.31 ± 0.34 c 1.29 ± 0.30 d 77.61 ± 33.60 c ´Red Champagne´ 37.03 ± 7.14 c 1.96 ± 0.48 b 1.02 ± 0.22 ab 63.04 ± 25.58 b ´Canadian Red´ 29.55 ± 4.09 a 2.27 ± 0.30 c 1.07 ± 0.28 bc 62.47 ± 20.43 b Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII).

#### ´Canadian Red´ 29.55 ± 4.09 a 2.27 ± 0.30 c 1.07 ± 0.28 bc 62.47 ± 20.43 b ´Canadian Red´ 29.55 ± 4.09 a 2.27 ± 0.30 c 1.07 ± 0.28 bc 62.47 ± 20.43 b Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII) ´Canadian Red´ 29.55 4.09 a 2.27 ± 0.30 c 1.07 ± 0.28 bc 62.47 20.43 Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII) *2.2. Soil and Climate Characteristics*

Values with different letters are significantly different at *p* < 0.05 by LSD test in ANOVA (Statgraphic XVII) The soil type of the experimental area is brown soil to chernozem on loess and loess loams and the part along the river Nitra belongs to the area of fluvial soils, where the original soil type was fluvial and fluvial glue. The plants were grown in growing substrate suitable for *Rheum rhabarbarum* L. In terms of climate classification of the region, Nitra is situated in a warm and dry area of Slovakia. The evaluations of experimental years according to climate normals are given in Tables 3 and 4.

´Red Champagne´ 37.03 ± 7.14 c 1.96 ± 0.48 b 1.02 ± 0.22 ab 63.04 ± 25.58 b

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


**Table 3.** Evaluation of months according to air temperature climate normals 1961–1990.

**Table 4.** Evaluation of months according to annual precipitation climate normals 1961–1990.

