The experiments were carried out in two leading terroirs of Eger Crown Winehouse Ltd., in Eger wine region. The vineyard of the experiment was planted in 2003 in brown forest soil formed on volcanic rhyolite tuff. The plantation is located in South-West orientation the rows are in North-South direction. The examined grape varieties were Blaufränkisch and Merlot. The applied cultivation method was umbrella cultivation combined with arced, cane fixing method. The trunk height is 80 - 90 cm. The row-and vine distance in the plantation is 240 × 100 cm. Yield regulation experiments were carried out in three following years (2011-2012-2013), at 100%, 80% and 60% load levels. In the experiment control vines without yield regulation (100%) were compared with vines of different load of 80% and 60% that was achieved by cluster selection. In the case of the control (100%) there was no cluster removal but at the 80% load level there was 20% yield regulation accomplished. At the 60% load level the yield regulation was 40% namely 40% of all the clusters was removed at sprouting, at the end of July, beginning of August.

Both Blaufränkisch and Merlot grapes were harvested in full ripeness with 20˚ and 21˚ sugar degree of the must. Wine was made from each grape by identical microvinification methods, therefore differences could be only resulted by yield regulation. The grape was treated with sulfur (15 g/hL), then after destemming yeast (doses of 30g/hL) was added to start fermentation in 50 L covered plastic barrels. Fermentation was carried out at cellar temperature (12˚C - 13˚C), sugar decrease was regularly checked. In case of all wines the skin maceration time was 20 days. After complete fermentation sulfurization was done to set free sulfurous acid level to 30 mg/L, then the barrels were sealed and after self-clarification wines were bottled for analyses.

Absorbance measurements were recorded on a Beckmann DU 530 spectrophotometer. Total polyphenolic content in wine samples was determined by the Folin-Ciocalteu’s reagent [18] and the results were expressed in mg/L of gallic acid equivalent. Anthocyanins were estimated according to Ribéreau and Stonestreet [19] and were expressed in mg/L. The content of leucoantocyanins was measured after heating wine with 40:60 hydrochloric acid and butanol, containing 300 mg/L ferro-sulphate [20] . Catechin was determined by using the REBELEIN method [21] , after diluting wine with alcohol and reacting with vanillin sulfuric acid. Folin-Cio- calteu’s reagent of analytic purity, sodium carbonate, 99% gallic acid (3,4,5-trihidroxi-benzoic acid) and all the reagents were purchased from Sigma-Aldrich Ltd. (Budapest, Hungary).

Concentrations of stilbenes were determined by using high-performance liquid chromatography (HPLC) method with isocratic elution with a Thermo Scientific Dionex UltiMate 3000 Instrument equipped diode array UV- visible detector. The isocratic solvent consisted of 90% water, 5% methanol and 5% acetonitrile. All wines were analyzed in terms of four stilbenes: trans-resveratrol, cis-resveratrol, trans-piceid and cis-piceid [15] . The wine samples were analyzed after filtration by Minisart Syringe Filter (Minisart RC Hydrophilic, 15 mm, 0.2 µm) and kept from light at 4˚C. The circumstances of the HPLC-measurement were as follows: The system was equipped with a Hypersil GOLD column C18 (250 × 4 mm, 3 µm particle size) with a pre-column (10 × 4 mm, 3 µm particle size) of the same material, maintained at 35˚C. Mobile phase consisted of a water/methanol/acetic acid mixture (volume ratio 64.6/33.4/2) with a flow rate of 1.0 mL/min. Injection volume was 20 µL. Detection was performed at 310 nm wavelength, run time was 15 min. Quantification was based on peak area. The content of total resveratrol was expressed in mg/L of wines. Analysis was based on calibration with commercially available standards (Sigma Aldrich, St. Louis, MO, USA). Detection limit was 0.1 mg/L for all components.

Trans-Resveratrol and Piceid standards of analytical purity were purchased from Sigma-Aldrich Ltd. (Budapest, Hungary). Both the standards of resveratrol and piceid were trans-isomers, cis-isomers were synthesized by treating the trans-isomers for 30 minutes by UV irradiation. Methanol, HPLC solvents, acetic acid were purchased from VWR (Debrecen, Hungary).

The sensory examination and evaluation of wines was carried out by wine tasting. To ensure impartial judgment of wine, they were served blind for 8 experts, who set up the order in the case of each different wine. The best one from each variety was analyzed according to the commonly used 100 scale wine tasting method and the results were displayed with the help of radar chart.

The statistical evaluation was carried out by the commonly used Student’s t-test. A two-sample location test of the null hypothesis of two different yield regulations has an equal result in the aspect of total polyphenol content, color intensity or color tone.

The measured total polyphenol values are shown in Figure 1 and Figure 2.

In the case of Blaufränkisch and Merlot wines similar values for total polyphenol content can be seen.

Both figures show that the year of vintage definitely determines the polyphenol content of wine. Year 2012 was weaker, than the 2011 and 2013 vintages. The total polyphenol content of 2012 wines was lower even at 60% yield regulations, than without yield regulation in the other two years (2011 and 2013). 80% and 60% yield regulations were proved to be effective in all three years, in the case of both red wines. Regarding the increase of

total polyphenol concentration in Blaufränkisch and Merlot wines at 60% load level managed to achieve the highest total polyphenol concentration in the produced wines. Organoleptic analyses were in accordance with the total polyphenol content of Blaufränkisch and Merlot wines. According to our experiments it can be summarized that the application of yield regulation leads to increased total polyphenol content. In the case of different load levels the amount of polyphenols did not show linearity, while the concentration of the appropriate phenolic components can be higher but even lower as it is shown later in our paper.

Anthocyanin, leucoanthocyanin, catechin contents of Blaufränkisch and Merlot wines are shown in Figure 3. According to the year of vintage and 60% - 80% yield regulations different values were observed for the above mentioned polyphenols. The analysis of data proves the idea that besides vintage anthocyanin, leucoanthocyanin and catechin content of wines is in correlation with the variety of grape. In the case of the results of Merlot wine an increasing effect of yield regulation can be seen but in the case of Blaufränkisch rather stagnation can be observed, and leucoanthocyanin content in 2012 vintage was even decreased. Summarizing our results yield regulation did not lead to similar change of the different polyphenolic components (catechin, anthocyanin, leucoanthocyanin), besides increase of their concentration even concentration decrease was obtained. Winemakers can make decision according to our findings whether the amount of which phenolic component they intend to increase in the case of next vintage. They have to adjust the appropriate parameters in next year yield regulations to achieve the best quality wines.

The measured and calculated color intensity and color tone values are presented in Figure 4. Results of the experiments represent that yield regulation does not have significant effect on color intensity and color tone of the produced red wine.

According to the results of our measurements increasing color intensity leads to decreasing color tone value.

Increasing value of color tone results in decreasing color intensity since color intensity and color tone values complement each other.

In the skin of grape only trans-piceid can be found which can be transformed to cis-piceid by UV irradiation. A small amount of cis-piceid may also appear in the skin of well ripen grape. Trans-resveratrol is a biologically active component of red grapes and wines, its biosynthesis performs from trans-piceid. Most of trans-resveratrol is formed in alcoholic fermentation from trans-piceid. Trans-resveratrol can be totally transformed to cis-resver- atrol by UV treatment. During production of wines light should be avoided. It is known from literature that not only trans-piceid is the source of trans-resveratrol but there is another source in the berry that is viniferin. This is the reason that no equal material balance can be made between piceid and resveratrol.

Figure 5 contains the data of resveratrol and piceid concentrations of wines from our experiments. Based on the measurements on load levels of the three years it can be stated that trans-resveratrol content can be increased by applying yield regulation in the vineyard. The concentration of trans-piceid can also be increased by yield regulation, the highest concentration of trans-resveratrol and trans-piceid can be achieved at 60% load level. The concentration of resveratrol and piceid depends not only on yield regulation but also on the vintage. The effect of vintage cannot be forgotten, but in the case of the examined red wines it was not considerable. According to our previous results the effect of vintage on the quality of wines is more definite in white wines.