Resveratrol in Grapevine Components, Products and By-products - A Review

1. Introduction

More and more studies are focusing on biologically active compounds of plant origin. Of these, resveratrol (3,5,4′-trihydroxystilbene), a natural polyphenol of the stilbene group, is found in various parts of plant species (72 plants according to studies by [1] including Vitis vinifera L., Rubus fruticosus L., Vaccinium myrtillus L., Ribes nigrum L., Fragaria L., Corylus avellana L., Raspberries L., Pistacia vera L. etc. (Figure 1) and plant products. Resveratrol was first isolated in the roots of Veratrum grandiflorum [2]. It was later detected in the medicinal plant Polygonum Capsidatum roots, used as an essential traditional medicine in China [3]. Over time, resveratrol has been detected in several species, with grapevine being the representative (Figure 1). As for grapevine among the species used for fruit production, the most widespread worldwide is Vitis vinifera L. ssp. Sativa. Over 10,000 grape varieties belong to this species; over 90% of the world's grapes are Vitis vinifera [4]. A different amount of resveratrol characterises each component of the grapevine; its presence depends on several factors: grape variety, growing environment (climate, soil type, exposure), winemaking technology, etc. Thus, studies have been carried out evaluating the effects of some parameters (temperature, humidity, maceration/fermentation time) during the grapevine process on the variation of resveratrol levels in the skin and seeds of grape berries [5], whole grapes [6,7], annual and multiannual grape bunches [8] and wine [9,10]. [11] mentions that stilbenes accumulate with dehydration of grapes on the stalk or during the post-harvest period under controlled conditions (temperature, relative humidity (RH), and airflow).

[12] mentions that after 16 months of red wine storage, under established storage conditions, the concentration of free trans-resveratrol in wine is expected to increase with storage due to piceid hydrolysis (a bound form of resveratrol). The content of phenolic compounds is one of the main factors in the quality of black grapes; their amount and structure significantly affect the oenological potential and sensory quality of red wine, influencing the colour, astringency, stability, and ageing ability of wines [13]. Having complex physicochemical properties, the qualitative analysis of this compound is complex, and various analytical methods have been reported relying on the use of High-Performance Liquid Chromatography (HPLC), capillary electrophoresis (CE), and gas chromatography (GC) [14,15]. Resveratrol can also be regarded as a functional, novel, essential, non-toxic, and pharmacologically active prebiotic nutraceutical compound with effective properties for human health [16]. Its introduction in different composites (zinc fructoborate - ZnFB, boron recently proven to be essential for the symbiosis of the healthy microbiome) and utilisation of the nutraceutical composite (RSV - ZnFB) as an essential prebiotic will ensure healthy nutrition of the microbiome, thus positively influencing the immunity and health of the organism [17,18]. In the last decade, the technological development of analytical tools has greatly improved and expanded the knowledge about this compound, resulting in the emergence of products in the medical, nutritional, and food spheres that present resveratrol as a biologically active compound with a functional role that supports nutrition leading to increased quality of life. This literature survey aims to review and synthesise existing knowledge on aspects of resveratrol chemical structure and isomers, biological properties and factors influencing resveratrol synthesis and content in grapevine, sources of resveratrol in grapevine components, products, and by-products.

2. Resveratrol: Structure, Biological Properties, and Synthetic Factors

2.1. Chemical Structure and Isomers

Structurally, resveratrol is a lipophilic polyphenol stilbene synthesised from tyrosine following the action of tyrosine ammonia lyase (deamination), stilbene synthase (condensation with three malonyl-CoA molecules) and 4-hydroxycycinnamoyl-CoA ligase [19]. The structure of stilbene consists of a 14-carbon backbone and two benzene rings joined by an ethylene segment [20], being a simple structure with a molecular weight of 228.247 g/mol [21]. There are two isomeric forms of 1,2-diphenylethylene: (E)-stilbene (trans-stilbene), which is stable, and (Z)-stilbene (cis-stilbene), which is less stable due to steric interactions between the aromatic rings [22]. Stilbenes are represented in grapes by cis- and trans-resveratrol (3,5,4′-trihydroxystilbene) and their glucosides (cis- and trans-piceides) - Figure 2, piceatanol (3,4,3,5-tetrahydroxy-trans-stilbene) and resveratrol dimers (viniferins) [23]. The trans isomer form of resveratrol is synthesised in grapes (Vitis vinifera) in the immunological response to injury, infection or abiotic stress [24,25].

The resveratrol derivatives are of additional interest for their biological properties, especially the trans isomers, which exhibit more potent bioactivity than cis-isomers [26]. The two isomer forms act differently to environmental factors: cis-resveratrol accumulates under UV light and high pH, and trans-resveratrol is synthesised at high temperature, visible light and low pH [27]. The cis and trans configurations can be converted to each other under specific conditions due to the presence of a C-C double bond [3] (Figure 3).

Stilbene derivatives, including monomers, glucoside derivatives, dimers (viniferins), trimers and tetramers, were qualitatively and quantitatively identified in grapes by MS [28]. A scheme of resveratrol oligomer formation in grapes is shown in Figure 4 [29].

Due to oligomerisation, monomeric and polymeric stilbene are present. Over 300 resveratrol oligomers have been characterised in grapes by oligomerising resveratrol monomers [30]. Resveratrol dimers and oligomers are synthesised in grapes as an active defence against exogenous attack or are produced by extracellular enzymes released by pathogens to remove unwanted toxic compounds [31].

2.2. Factors Influencing Resveratrol Synthesis and Content in Grapevine

Factors influencing the synthesis and content of resveratrol in grapevine include genetic elements, environmental conditions, and processing techniques. Resveratrol synthesis factors have been divided into biotic and abiotic factors. Biotic factors lead to resveratrol synthesis in response to pathogens (Botrytis cinerea, Plasmopara viticola, Trichoderma viride, Erysiphe necator, Rhizopus stolonifer, Bacillus spp., Aspergillus carbonarius, Aspergillus japonicus) [32]. Abiotic factors, such as ultrasound (US) treatment, LED illumination, UV irradiation, macronutrient and fungicide application, variety, climatic conditions in the growing areas, vineyard health, vineyard practices, compound-protective winemaking techniques (such as UV-C irradiation), wine ageing method, terroir effect and harvest time, cause significant variations in resveratrol content in grapevine components, products and by-products worldwide. These factors contribute to synthesising, stimulating, and accumulating resveratrol in grapevine tissues (Table 1).

Table 1. Abiotic factors on which the resveratrol content in grapevine depends

Abiotic factors	References
Variety	[33,34,35]
Climatic conditions in the growing areas	[25,36,37,38,39,40]
Grapevine health	[41]
Vineyard management	[42,43]
Winemaking techniques	[37,44,45,46]
Wine ageing method	[12,45,47,48]
Terroir effect	[49,50,51,52,53]
Harvest time	[11,54]
Treatment with ultrasound (US), light emitting diode (LED), ultraviolet (UV) irradiation or macronutrients and fungicides	[55,56,57,58]
Storage conditions	[8,59]

Table 1. Abiotic factors on which the resveratrol content in grapevine depends

Abiotic factors	References
Variety	[33,34,35]
Climatic conditions in the growing areas	[25,36,37,38,39,40]
Grapevine health	[41]
Vineyard management	[42,43]
Winemaking techniques	[37,44,45,46]
Wine ageing method	[12,45,47,48]
Terroir effect	[49,50,51,52,53]
Harvest time	[11,54]
Treatment with ultrasound (US), light emitting diode (LED), ultraviolet (UV) irradiation or macronutrients and fungicides	[55,56,57,58]
Storage conditions	[8,59]

The study by [40] demonstrates that water stress regulates resveratrol synthesis through the enzyme activities and gene expression of PAL (phenylalanine ammonia-lyase-lyase) and STS (stilbene synthase) [60] shows that genotype could have a profound impact on the antioxidant properties of wine and phenolic composition. The study by [61] examined the influence of abiotic factors on resveratrol accumulation. It indicated that the levels of this compound may vary depending on the growing season, variety, and growing region.

The terroir effect is stronger than the variety of resveratrol-inducing capacity after ultraviolet-C (UV-C) treatment [49]. Studies show that V. vinifera varieties contain higher amounts of trans-resveratrol than V. labrusca hybrids, and wine grapes have higher concentrations than table grapes [6]. Winemaking technologies in the pre-fermentation, fermentation, and post-fermentation stages may influence the concentration of resveratrol [45]. However, the analysis of [47] revealed that the type and size of oak barrels, including barrels, did not significantly affect the trans-resveratrol content in wine. Concerning the grapevine ropes, it has been observed that during long-term storage, the ropes accumulate bioactive compounds under the influence of temperature [8] or UV-B and UV-C radiation [52]. Thus, the cords resulting from viticultural activities can be a valuable source of phenolic compounds (especially trans-resveratrol), minerals, carbohydrates, and proteins. [6] developed a molecular network for post-harvest UV-C treated grapes showing accumulation of resveratrol, with applicability for fruit storage. [62] reported an increase in trans-resveratrol in UV-exposed grapes ranging from 1.5 to 200 compared to untreated samples. Grapevine strings may thus be an accessible source of antioxidants and dietary supplements [59,63]. Detail the effects of environmental and storage conditions (temperature, light) on polyphenolic compounds in strings, showing that at 40 °C in the dark, 70% of these compounds are degraded, with trans-resveratrol decreasing by 23% after three months. Resveratrol is also found in various grapevine products and by-products, such as wine, grape juice, and lees resulting from the winemaking process. Different processing and storage conditions significantly influence the concentration of resveratrol and other bioactive compounds. In this context, innovative methods, such as the ozonisation of grapes, have increased endogenous resveratrol content in wine products and grape juice [64]. Wine lees, a microbial biomass, contains ethanol, organic acids (such as tartaric acid), phenolic compounds (29.8 mg/g dry weight), and anthocyanins (6-11.7 mg/g), along with inorganic materials [65]. It plays an essential role in interacting with polyphenols in wine and influencing its sensory characteristics. Grapevine components, products, and by-products vary in resveratrol content depending on various factors, with essential applications in industry and therapy recognised for its cardioprotective, anticarcinogenic, and antioxidant effects [40].

2.3. Biological Properties

With technological advances, there is interest in replacing non-steroidal anti-inflammatory drugs and corticosteroids with natural, less toxic alternatives with high therapeutic potential; one product with potential in this regard is resveratrol. Numerous clinical studies (in vitro, in vivo) suggest that resveratrol may induce anti-ageing health benefits, including anticarcinogenic, antidiabetic, anti-inflammatory, antioxidant, phytoestrogen, and cardioprotective, antiviral, and neuroprotective properties [66]. For example, administration of resveratrol appears to improve the metabolic profile in obese and insulin-resistant patients [67]. A study conducted by [68] on 25 obese patients (BMI ≥ 30 kg/m², age range 30-60 years) divided into a placebo group and a resveratrol-treated group (250 mg/day) showed that after 3 months, both groups showed a decrease in BMI and waist circumference. However, only the resveratrol group showed increased HDL, reduced total cholesterol, urea, creatinine, albumin, and low-density lipoprotein (VLDL). Also, [69] demonstrated the protective effect of low-dose trans-resveratrol on retinal ganglion cell degeneration in diabetic mice, reducing retinal damage and inhibiting cell apoptosis. [70] explains the positive effects of resveratrol in type 2 diabetes and on long bone strength in Wistar rats. [71] shows that a resveratrol-enriched bread diet reduced polydipsia and weight loss in rats with type II diabetes. In humans, resveratrol is promising as a prophylactic and therapeutic supplement to inhibit tumorigenesis and treatment resistance in breast cancer. It also has a role in bone tissue regeneration [72]. Studies conducted by [73,74] also suggest the potential of resveratrol as a phytoestrogen and its beneficial impact on reproductive health and pregnancy complications, paving the way for future research. Resveratrol intervenes in key inflammatory pathways, such as nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase kinases (MAPK), inhibiting the production of inflammatory cytokines and chemokines. In addition, it has been found to influence some cellular processes (cell cycle progression and immunological responses) [75]. Resveratrol is a strong candidate for developing functional products and pharmaceuticals to prevent and treat certain chronic diseases [76]. Clinical trials are currently focused on increasing resveratrol's bioavailability and maintaining resveratrol for a more extended period in the metabolic system.

3. Sources of Resveratrol from the Grapevine

Grapevine is among the most essential sources of polyphenolic compounds. More than 60 stilbenoids can be found in this species as monomers, such as trans-resveratrol or piceatannol, and oligomers, usually in their trans configuration [77]. Resveratrol, a phytoalexin with a significant active character [78], is present in smaller or larger amounts in all constituent parts of the grapevine: grapes (skin, pulp, seeds, stem), shoots, leaves, roots, products obtained from the valorisation of grapes (wine, raisins, powders, juice) as well as in by-products (grape pomace, wine lees, ropes), hence the importance of the numerous types of research that have appeared (Figure 5).

3.1. Sources of Resveratrol in Grapes and Other Grapevine Components

The grape is one of the most researched grapevine constituents regarding resveratrol content. Research has investigated the presence of resveratrol both in the whole grape and in its components (skin, seeds, pulp, and raisins) (Table 2).

Large-scale targeted metabolomic analysis showed that 82 phenolic compounds, including resveratrol, were differentially accumulated in grape seeds from melatonin-treated berries [30]. The concentration of resveratrol in grapes can occur by using purification procedures that lead to an increase in (E)-resveratrol purity from 29% to 78% (34% recovery shown [95]. Ten main compounds were identified in grape skin extracts, which contained many polyphenols, including trans-resveratrol [87]. Resveratrol was detected in both pulp and seeds, with seeds being richer in phenolic substances than pulp [82]; [100]. The highest amount of resveratrol is located in the skin of grapes, where the compound acts as a natural defence mechanism against stress factors such as UV radiation or fungal infections. In addition to skins, seeds, and raisins, although essential sources of other polyphenols contain lower amounts of resveratrol. However, they play a vital role in research on the full utilisation of grapes and by-products of winemaking (Table 2). Wines, especially red wines, also contain varying amounts of stilbenes, which are influenced by multiple factors: grape type, climatic conditions, and winemaking technology [14]. It has recently been shown that the applicability of resveratrol from plant extracts (such as those from grapevine leaves) in nutraceuticals can be increased by converting it into nanofibers. Furthermore, the use of resveratrol lacks in vivo efficacy due to its low solubility and stability, which limits its bioavailability. [101] investigates this issue and shows that improved bioaccessibility was achieved using nanofibers with resveratrol in encapsulated form; the controlled release profile of resveratrol under simulated in vitro gastrointestinal conditions increased up to 67.6% whereas native resveratrol was only about 48.1%. Recent studies also attest to the presence of resveratrol in grapevine roots (Table 2).

Each grapevine component contributes differently to the total content of resveratrol and other bioactive compounds, but with different concentrations depending on the variety, cultivation methods, and environmental conditions.

3.2. Sources of Resveratrol in Products and By-Products

Resveratrol accumulation in grape skins, juice, and wine is induced by external stimuli: microbial infection, ultrasound (US) treatment, light-emitting diode (LED), ultraviolet (UV) irradiation, elicitors or signalling compounds, macronutrients and fungicides [55]. Research on grape products has focused mainly on wine, one of the most essential and studied products in the wine industry. Wine, especially red wine, is recognised not only for its cultural and economic value but also for its impact on health due to its content of bioactive compounds, including resveratrol. Although the concentration of resveratrol in wine is much lower than that of other polyphenols, it has received much attention for its biological properties and potential therapeutic effects [102], followed by grape skin powder (Table 3). Grape skin powder is rich in phytochemicals, including anthocyanins, flavonols, and hydroxycinnamic acids. The phytochemical analysis of grape skinskin powder suggests that it contains several catechins, anthocyanins, polyphenols, and flavonols and may, therefore, represent a natural combination of resveratrol with other valuable phytonutrients. Drinks derived from grapes, such as red grape juice, have a complex array of phenolic compounds, including resveratrol and quercetin, anthocyanins, known for their antioxidant effect, prevention of oxidative reactions, and free radical formation, anti-inflammatory and antiproliferative effects [103]. Studies on raisins conducted by [104] show the presence of polyphenols, phenolic acids (caftaric acid and coumaric acid), and tannins (flavonols, quercetin, and kaempferol) with implications for consumer health (may reduce postprandial insulin response, reduce sugar absorption (glycemic index), affect specific oxidative biomarkers and promote satiety via leptin and ghrelin. Raisins introduced into the regular diet may improve heart disease. In this regard, a study undertaken by [105] shows their protective role (biochemical and histopathologic) on cardiac muscle in rats fed a high-cholesterol diet (HCD). Administration of raisins and CDH significantly reduced cholesterol, triglycerides, low-density lipoprotein, blood glucose, and insulin while increasing high-density lipoprotein levels compared to rats fed CDH alone. Results on glycemia and cardiovascular risk factors by adding raisin snacks to the diet compared to conventional snacks are also found in the study conducted by [106] on human subjects (females and males) over 12 weeks, resulting in a significant decrease in glycated haemoglobin levels as well as a reduction in systolic blood pressure (SBP). By-products of the wine industry serve as a potential economic interest, as they are sources of significant natural bioactive compounds that may exhibit biological properties related to health improvement and maintenance [41]. Thus, scientific interest is shown in the presence of resveratrol in grape pomace, annual and multiannual cords, and wine lees (Table 3). The total amount of grape pomace from winemaking generated worldwide is millions of tons per year, which may make it difficult to manage the waste from an environmental and economic point of view [107]. However, grape pomace, which is made up of seeds and skin and stalk residues left after pressing, representing 20-25% of the weight of the grapes [108], has a high polyphenol content, increasing the interest in research on these by-products for the food or pharmaceutical industry.

In addition to polyphenols, grape pomace has other essential substances such as simple sugars, alcohols, tannins, pigments, high levels of alcohol and tartaric acid, and other economically significant compounds. Grape pomace is among the industrial by-products with potential bioactive and antibacterial properties [127] that are being investigated to identify simple and affordable solutions/alternatives that can be used in the food industry. It has been found that the annual and multiannual grapevine prunings obtained from the yearly and multiannual grapevine prunings contain appreciable quantities of several substances such as phenolic compounds, carbohydrates, minerals, pigments (chlorophyll and carotenoids), etc. Studies have been carried out in this respect with the aim of sustainable utilisation of vineyard waste by applying various extraction, purification, and determination methods (Table 3). Another by-product of compositional importance for the wine industry is the wine lees obtained as a by-product of the vinification process. The wine lees obtained after primary fermentation contain proteins, lipids, anthocyanins, and beta-glucans and can be used in the food industry due to the presence of wine lees of the genus Saccharomyces cerevisiae [128]. In the food industry context, polyphenolic compounds in wine by-products can achieve lipid oxidative protection or act as antimicrobial agents against spoilage bacteria [81].

4. Conclusions

Resveratrol is found in all grapevine parts, products, and by-products in different concentrations depending on biotic and abiotic factors, increasing the importance of cultivating certain varieties of Vitis vinifera L.

The current climate change is impacting the wine-growing sector; further research into the identification and synthesis of resveratrol remains topical, relying on innovative, quality-oriented research techniques that can allow better selection of grapes with superior phenolic characteristics necessary to determine the direction of use.

Resveratrol in some grapevine components, products, and by-products is successfully used in various industrial applications (pharmaceuticals or cosmetics). In the future, it could be part of elements used to increase the functionality of some food products, such as nutraceuticals in medicine, soil, and plant bio-fertilizers, animal feed, bio-energy or biofuel. So, some of the ideas and practices developed and implemented in current research can contribute to industrial development and, at the same time, to quality of life.