Resveratrol (3,5,4’-trihydroxystilbene) is a phenolic phytoalexin produced by grapevines in response to fungal infection [
1]. It exists in
trans- and
cis-isomeric forms, with the former being much more abundant in wine. In red wine, resveratrol also can be found in its glycoside form (resveratrol-3-
O-
β-mono-D-glucoside; piceid).
Trans-resveratrol and piceid are the major active constituents of red wine.
In vitro research indicates that resveratrol has chemopreventive effects against cardiovascular disease, aging, and cancer.
Trans-piceid is present in red wine to a greater extent than its aglycone, but hydrolysis of this glycosylated derivative may occur in the small intestine and liver, which would increase the amount of biologically active
trans-resveratrol. The average content of piceid was ten times higher than that of resveratrol in red wine. In addition, piceid was the most abundant form of resveratrol in nature. Other glycosylated derivatives of resveratrol could possibly be present in wine, but the lack of literature data on their contribution to the total amount of resveratrol suggests their minor importance [
2]. A number of studies suggest that piceid may have similar bioactivity to resveratrol, e.g., anticarcinogenic effects, inhibition of platelet aggregation, and antioxidant activity. For this reason, it was of interest to analyze piceid in our innovative wine. It acts as a pro-drug and is stable during transport from the mouth to the small intestine, where it undergoes metabolic conversion to active
trans-resveratrol and is reabsorbed into the blood plasma. Piceid preserves resveratrol from degradation in the gastrointestinal tract [
2]. In Bordeaux varieties,
trans-resveratrol was not found in measurable amounts, but only
trans-piceid, in amounts between 0.26 mg/L and 1.25 mg/L [
3]. The influence of winemaking techniques and grape varieties on resveratrol content, total phenolic content and antioxidant potential of red wines was previously studied on 10 commercial Serbian red wines. It was clearly found that resveratrol content was very low (0.18-1.31 mg/L) in all studied wines [
4]. It was also demonstrated that winemaking techniques influence the amount of phenolic compounds. The highest average resveratrol and total phenolic contents were found in Merlot (4.85 mg/L; 1208 mg/L GAE) and Cabernet Sauvignon (3.78 mg/L; 1410 mg/L GAE) wines [
5]. Quercetin has unique biological properties that may improve mental/physical performance and reduce the risk of infection. These properties form the basis for potential benefits to overall health and disease resistance, including anticarcinogenic, anti-inflammatory, antiviral, antioxidant, and psychostimulant activities, as well as the ability to inhibit lipid peroxidation, platelet aggregation, and capillary permeability, and to stimulate mitochondrial biogenesis. The first study on the pharmacokinetics of quercetin in humans showed very low oral bioavailability after a single oral dose (~2%) [
6]. In red winemaking, maceration with skin and seeds during fermentation results in a higher concentration of resveratrol and quercetin in red wines compared to white wines. Resveratrol and quercetin content in wine depends on many different factors, including grape variety, harvest year, climatic conditions, UV light, winemaking technique, selected yeast strain, and aging [
5,
7]. The techniques of skin extraction and enzymatic hydrolysis of the glucoside forms also play an important role in the resulting resveratrol and quercetin concentrations. One way to control phenolic content in wine is through the choice of yeast strain. Not only do yeasts play a role in alcoholic fermentation, but they are also responsible for biochemical, enzymatic and physical reactions during the process, and thus exert a significant influence on wine phenolic composition of the wine. Thus, resveratrol, which is present in glycoside form (piceid), can be hydrolyzed by
β-glucosidases, resulting in an increased concentration of free resveratrol [
8]. Fermentation of must with yeast strains of
Saccharomyces cerevisiae (SC) and fermentation with mixed cultures of
Saccharomyces/non-
Saccharomyces and lactic acid bacteria are widely used in modern winemaking [
9]. On the other hand, many studies have highlighted the positive influence of non-
Saccharomyces yeast strains on the chemical composition of wine [
8,
10,
11]. Recent studies with non-
Saccharomyces species described the intense effect of some strains on anthocyanin color and subsequent stability by greatly lowering pH of wine during fermentation [
12], as well as co-inoculation with lactic acid bacteria [
10].
Torulaspora delbrueckii is probably the non-
Saccharomyces yeast currently most commonly used for winemaking because of its good fermentation performance compared to other non-
Saccharomyces yeasts considered for winemaking [
13]. The fermentative capacity of
T. delbrueckii allows it to be used at the beginning of the fermentation process, unlike other strictly oxidising non-
Saccharomyces yeasts [
14].
Regarding the influence of non-
Saccharomyces yeasts on stilbene composition,
T. delbrueckii obtained the best results, especially when compared to
S. cerevisiae. Moreover,
T. delbrueckii resulted in higher total phenolic content without increasing some negative organoleptic properties such as acetic acid content [
15]. According to Minnaar et al. (2015) [
17], the measured anthocyanins and flavanols in Pinotage wines produced with
T. delbrueckii were consistently higher than those in Pinotage grape must inoculated with
S. cerevisiae. Glycosylated anthocyanins and acetylated anthocyanins were also highest in Pinotage wines produced with
T. delbrueckii. The main focus of this study was to find out the contribution of the innovative technology (co-inoculation with mixed cultures) to increase the content of stilbenes and quercetin in wine. In addition to these compounds, which are of great benefit to human health, the enrichment of wine with other phenolics by co-inoculation with mixed cultures was also investigated. A new winemaking process could improve wine bioactivity, provide guidance to future wine producers, and identify knowledge gaps for future research.