Leveraging Horseradish's Bioactive Substances for Sustainable Agricultural Development

The problem of ensuring access to healthy food has been increasingly in the scope of attention since the Second International Conference on Nutrition (ICN2) in 2014, becoming particularly up-to-date within the framework of the UN Decade of Action on Nutrition (2016-2025). After all, healthy diet helps avoid the problem of poor nutrition in all its forms, as well as prevent a number of non-communicable diseases (NCDs), including diabetes, heart diseases, stroke and cancer. If the current patterns of food consumption are maintained, diet-related annual expenses in the field of health care, triggered by NCDs and the cause-specific mortality, are projected to reach USD 1.3 trillion by 2030. On the other hand, it is projected that the transition to a healthier diet will reduce direct and indirect health care expenses by 97%, freeing up significant funds that could be invested in reducing the cost of nutritious foods.

The United Nations action in the framework of sustainable development of agriculture and nutrition over the past 10 years has not significantly addressed the issue of horseradish. Research on horseradish and its biological aspects can contribute to food security, and in a secure environment, sustainable development is also possible.

Dietary quality is determined by four aspects: assortment and variety (within food groups and among them), adequacy (sufficient amounts of nutrients or food groups in accordance with guidelines), moderation (foods and nutrients consumed in limited amounts), as well as overall balance (ratio of nutritional macronutrients in the consumed food) [1]. If the exact composition of a healthy diet depends on individual characteristics, cultural environment, locally available food products and formed food traditions, then the basic principles of healthy nutrition are universal. A healthy diet should contain wholegrain products, legumes, as well as a wide range of fruit and vegetables. Expanding the range of fruit and vegetable products in the diet provides for necessary vitamins, micronutrients, amino acids and other phytonutrients necessary for normal functioning of the human body. It is believed that today we consume for food barely a quarter of about 600 types of vegetable crops [2].

The other the vegetable crops may sometimes be referred to as secondary, minor, rare or wild vegetables. Regular consumption of forgotten or underexploited vegetables is an effective way to maintain a varied and healthy diet and to combat micronutrient deficiencies or hidden hunger, as well as some other nutrient deficiencies, especially among rural populations and more vulnerable social groups in developing countries in the context of ensuring sustainable development of the country's agricultural sector [3]. The probable cause for the low consumption of underexploited vegetables, despite their recognized importance, is insufficiency of quality seeds, lack of data regarding their productivity and input requirements, as well as the lack of information on how they can fit into sustainable production systems. At the same time, underutilized vegetables easily adapt to poor and degraded soils, withstand drought, flooding or salinity and can be resistant to extreme climatic events [4]. Besides, they can be used as medical plants, for fodder production and as industrial raw materials. Moreover, the consumption of underutilized vegetables can become the instrument of improvement of food security to address the nutritional needs of the constantly growing population of the world [5] and expanding the range of food products. After all, the underexploited vegetables may often contain specific spicy-aromatic and coloring components which provides for their use as functional additives to food products and as enrichment of foods with necessary phytonutrients [6]. In addition, underexploited vegetables are highly profitable, which makes them a good source of income for smallholder farmers and the right marketing strategies will help increase their consumption in the context of ensuring sustainable development of the country's agricultural sector.

Horseradish (Armoracia rusticana) remains a well-known yet underappreciated vegetable. Horseradish has been cultivated for its root for more than 2,000 years. Its exact origin is unknown. Most likely it is Eastern Europe and Western Asia. Its commercial production is currently taking place mainly in North America and Europe with exclusively vegetative propagation. It is a perennial culture, a well-known traditional medicinal plant, a natural preservative and seasoning for dishes, a base for hot sauces [7].

Both roots and leaves of the plant are used for food. The fresh root is mainly used grated or processed into a sauce for fried and boiled meat and fish. The root is also preserved in white vinegar, beetroot juice, being used in mayonnaise-based sauces and sauces for fish [8]. After harvesting, the roots are usually stored under the conditions of cooling to prevent spoilage and loss of turgor [9]. In Albania, horseradish roots are used for pickling pears or added to fermented vegetable products such as cabbage, tomatoes and peppers. Horseradish leaves are widely used in Eastern European countries for preparation of fermented cucumbers [10]. Furthermore, it is believed that horseradish leaves prevent food spoilage [11], whereas oil and water extracts of horseradish root are used to store tomatoes and peppers [12]. Horseradish root is also used as a cheaper alternative to wasabi.

In recent years, scientists have been paying more and more attention to horseradish due to its high content of biologically active compounds. The specificity of these compounds provides horseradish with antioxidant [13], antimicrobial, antifungal, tumoricidal properties. Nowadays, the development of physical (spray drying, lyophilization), physic-chemical and chemical methods of encapsulation allows to obtain healthier and more acceptable bioactive compounds from horseradish, which have good perspectives of application for food technologies in the context of ensuring sustainable development of the country's agricultural sector [14]. It has been established that microencapsulated horseradish juice is effective for extending the shelf life and stabilizing the quality of raw pork [15]. The addition of horseradish processing products to butter biscuits (leaf cake and microencapsulated horseradish root and leaf juice) is proposed for enrichment bakery products with bioactive compounds [16]. At the same time, the potential of this plant is still not used to the full.

Focusing on the health benefits of horseradish, the main purpose of this review was to discuss some relevant topics for new research that could be essential for agricultural and industrial production and impact the regional economy and sustainable development of the country's agricultural sector. Thus, the consolidation of the conducted research on horseradish as a promising crop lays a theoretical basis for more conscious smart consumption and can stimulate new directions in research aimed at strengthening the field of production of horseradish and its processing products and identifying the issues that still need to be clarified.

The main purpose of the article is to study the peculiarities of the use of biologically active substances of horseradish in the system of sustainable development of the agricultural sector of the country. The structure of our article can be divided into several parts: part of the analysis of the literature, part of the description of the methodology and main methods, part of the presentation of the results of the study, part of the discussion of the results of the study, part of the description of opinions in the form of conclusions and subsequent studies.

3.1 Effect of processing technologies on glucosinolates and isothiocyanates

Technological processing has a complex effect on the content of nutritious and non-nutritious substances in the vegetables. Due to highly volatile nature of ITCs, their amount depends on numerous variables related to the storage and preparation of horseradish products.

After harvesting and before processing, horseradish roots are usually stored in a cold room for a long time. During the 10 months’ period of storage at the temperature of 0-1℃, the total content of isothiacyanates decreases in the root of horseradish of the Polish type from 1,651 mg⋅kg^-1 of raw weight (f.w.) to 1,424, while the isothiacyanates content in the the Hungarian type horseradish falls from 1028 mg⋅kg^-1 f.w to 923. At the same time, the average level of these compounds remains relatively high, which is important for the vegetable processing and the product quality [36].

Fine crushing or grinding horseradish root into powder is an important step in making the horseradish sauce. Conventional grinding as a rule is done at room temperature, which accelerates the evaporation of ITCs. After slicing and holding the horseradish at 5℃ for 12 hours, the profile of volatile compounds changed and new compounds were found. Some of these decreased in concentration, with 2-phenylethyl isothiocyanate becoming the most common compound. According to Wang et al. [36], when horseradish is ground in a ball mill, which can be done at the temperature of 4℃, the number of ITCs increases within 15, 30 and 60 min of grinding. When the grinding time is increased to 120 min, the content of isothiocyanates decreases due to the acceleration of evaporation and degradation. Ground horseradish slowly loses its sharp flavor and taste even when cooled. Kosson and Horbowicz showed that the industrially produced horseradish cream, which is not subject to heat treatment, contains from 645 to 631 mg⋅kg^-1 f.w. isothiocyanates. When the finished product is stored, the concentration of ITCs decreases. However, lower storage temperatures of 2℃ slow down the degradation of ITCs compared to storage at 8℃ and 18℃ [37].

GLS are believed to undergo thermal degradation above 100℃ [38]. However, the degree of GLS degradation is affected by the method of heat treatment. Steaming, microwaving and roasting preserve the GLS content of Brassica vegetables. In contrast, according to other data, the antioxidant activity of individual ITCs is triggered when heated above 100℃. Thermal treatment of sulforaphane and erucin leads to the formation of organosulfur antioxidants that can increase the oxidative stability of food. Cedrowski et al. [34] suggest that the thermal decomposition of ITCs can form secondary compounds capable of scavenging peroxyl radicals. As reported by Tomsone et al. [39], the major volatile compound in fresh horseradish leaf extracts was allyl isothiocyanate. On the other hand, when the pomace was dried by different methods, it was not found in any of the dried samples, although the antioxidant activity is pertinent to dried pomace [39].

The behavior of GLS and ITCs under the influence of high temperatures in different ways of processing plant tissues still needs additional research.

Phenolic substances show high biological activity. These compounds are classified into several groups, such as phenolic acids, flavonoids, xanthones, stilbenes, lignins, tannins, etc., as well as into various subgroups. Various phenolic acids (gallic, protocatechin, caffeic, ferulic, isoferulic, chlorogenic, p-hydroxybenzoic, p-coumaric, syringic and sinapic acids) and flavonoids (catechin, epicatechin, quercetin, kaempferol, luteolin, apigenin, isorhamnetin and rutin) have been identified in the roots and leaves of horseradish [32]. These compounds lay a protective role for such degenerative diseases as cardiovascular diseases, cancer, diabetes, inflammation and many others [40]. Their impact on health is widely known due to their strong antioxidant effect. Scientists particularly highlight the antioxidant activity of flavonoids.

Horseradish roots and leaves are characterized with a rather high total content of phenolic сompounds in general (TPC) and high total flavonoid content (TFC) in particular (Table 2).

Table 2. Content of phenolic compounds in horseradish

Table compiled from data of study by Galova et al. [40]. Total phenolic content was expressed as mg of gallic acid equivalents (GAE)×g^-1 dry mass (DW) of sample. Total flavonoids content was expressed as mg of quercetin equivalent (QE)×g^-1 dry mass of sample.

However, the biosynthesis of phenolic compounds in plants depends on such factors as genotype, agricultural cultivation techniques, the stage of plant development, soil composition, environmental conditions and other abiotic and biotic factors [41].

Different genotypes of horseradish show significant difference in content of polyphenolic compounds. According to Tomsone et al. [42], out of 9 analyzed horseradish genotypes collected in different regions of Latvia and Belarus, the highest TPC was found in the root of horseradish genotype G280 (Malnava region, Latvia), collected in September (503.54 mg GAE ×100 g^-1 DW). Instead, the lowest TPC was found in the root of horseradish genotype G26B, grown in the same region and also collected in September (160.14 mg GAE×100 g^-1 DW) [42]. Later, the same author analyzed other horseradish genotypes and found that the horseradish root of the GM genotype (Marupe region, Latvia) contains only 109.93 mg GAE ×100 g^-1 DW. Despite the fact that horseradish roots can be harvested from mid-summer until the frosty weather, it is believed that the best time for harvesting is September. Comparing harvesting dates, the TPC of horseradish roots fluctuated, and in September the highest TPC was found only in 6 of the 12 horseradish genotypes researched. That is, the general trend of TPC differences in horseradish roots depending on the genotype and time of harvest was not revealed [42].

The content of phenolic compounds in leaves is 7-10 times higher than in horseradish roots. Tomsone [43] notes that the genotype with the highest TPC in the leaf does not have the highest TPC in the roots. Out of 12 horseradish genotypes, the highest TPC was identified in the leaves of horseradish genotype G12B Preili region, Latvia (5406.2 mg GAE×100 g^-1 DW), and the lowest one was found in the leaves of genotype GJ Jelgava region, Latvia (710.89 mg GAE×100 g^-1 DW). The season of collection has a strong influence on the formation of the TPC complex in horseradish leaves. The highest content of phenolic compounds in horseradish was in the period from May to June, which coincides with the period of development of horseradish before flowering and during the period of flowering [43].

The data regarding the content of flavonoids in the roots and leaves of horseradish, received by different authors, differ significantly. So, Alazzam et al. [44] show that TFC in leaves is just slightly higher than in roots (Table 2). Latvian researchers demonstrate that TFC in leaves reaches 5889.85 catehin equivalents (CE) mg CE ×100 g^-1 DW and is ten times higher than in roots, achieving the level of 242.32 mg CE×100 g^-1 DW. Biosynthesis of flavonoids is induced by UV radiation, light intensity, exposure to ozone, which explains the higher content of flavonoids in aerial parts of plants. In horseradish leaves, collected in August, depending on their genotype, TFC varied from 2485.8 to 15510 mg CE ×100 g^-1 DW, which confirms the opinion about the significant influence of varieties, species and genotypes on the processes of flavonoid synthesis.

3.2 Effect of processing technologies on the phenolic compounds

Technological processes can significantly affect TPC and individual phenolic compounds. TPC and TFC of horseradish roots differ significantly depending on the applied technological processes. Freezing horseradish roots leads to 11-12% TPC and 21% TFC increase in almost all groups of phenolic compounds compared to their content in the fresh product, whereas the total content of phenolic acids does not change significantly [16]. Instead, there are changes in individual phenolic compounds. The content of kaempferol and luteolin decreases, quercetin, which is present in fresh samples, is not identified in the processed ones. Freezing triggers the growth of the amount of gallic acid, whereas the content of caffeic acid decreases.

The processes of drying are held at elevated temperatures, which causes thermal oxidation of phenolic substances. Microwave-vacuum drying reduces TPC by 19.6% and freeze-drying by 29.5%. Similar trends are observed in the technological processing of horseradish leaves. After freezing horseradish leaves the content of phenolic compounds increases and drying leads to significant losses of phenolic compounds [45].

Drying horseradish leaves pomace obtained after pressing the leaves for juice resulted in a significant decrease in TPC and TFC and depended on the method of drying. Freeze drying causes a 29% reduction in phenolic compounds, while convective drying results in a 53-59% reduction. Freeze drying is also a more effective method for preserving flavonoids.

The effect of the horseradish root ball mill grinding time on TPC is similar to that on ITCs. When the grinding time was shorter than 60 min, the TPC increased with the increase of the grinding time. Grinding in a lower temperature environment may promote the release and simultaneous retention of phenolic compounds. However, when milling time increased to 120 min, TPC tended to decrease due to oxidative degradation of phenolic compounds. In contrast to TPC, due to the decrease in horseradish powder particle size, TFC increased with milling time.

Let's highlight the key theses of the research results (Table 3).

Table 3. The key theses of the research results

Thus, the use of technological methods for processing horseradish roots and leaves leads to changes in the complex of biologically active compounds of horseradish, and the nature and direction of these changes depend on many factors and require additional research. The information on the quantitative and qualitative transformation of biologically active compounds in various horseradish processing products is insufficient.