White wines phenolic compounds

Whilst sulfate appears to be fundamental to haze formation, other wine components such as phenolic compounds remain as candidate haze modulators. One possibility is that white wine phenolic compounds affect the particle size of denatured aggregated proteins, possibly through crosslinking. Several researchers (Oh et al. 1980 Siebert et al. 1996b) have suggested a hydrophobic mechanism for the interaction between phenolic compounds and proteins, in which the protein has a fixed number of phenolic binding sites. More of these sites are exposed when the protein is denatured. 

Red wines are macerated wines. Maceration is responsible for all of the specific characteristics of sight, smell and taste that differentiate red wines from white wines. Phenolic compounds (antho-cyanins and tannins) are primarily extracted, participating in the color and overall structure of wine. Yet aromas and aroma precmsors, nitrogen compounds, polysaccharides (in particular, pectins) and minerals are also liberated in the must or wine dm-ing maceration. 

A reversed-phase liquid chromatographic method was developed for simultaneous determination of carboxylic acids, phenolic compounds, and SA in white wines (84). The diluted samples are injected into a Spherisorb ODS-2 column with a gradient of sulphuric acid (pH 2.5)/methanol as mobile phase. A diode array detector is used, set at 210 nm for carboxylic acids and altered to 278 nm, during the run, for phenolics and SA. The identification of compounds is based on retention time and UV spectra. Some cleanup methods (Sep-Pak C18 and an ion-exchange column) were tested and did not improve the results. The analysis was considered simple, with no sample preparation. Application of this method was illustrated by analyses of Brazilian Welchriesling wines (84). 

MT Benassi, HM Cecchi. Method development for the simultaneous determination of carboxylic acids, phenolic compounds, and sorbic acid in white wines. J Liquid Chromatogr Related Technol 21 491-501, 1998. 

Many of the recent studies on the BAs in wines are from the research group of Busto. In 1994 this group developed a method suitable for the determination of 19 BAs in wine (48) that involves the removal of phenolic compounds with polyvinylpirrolidone (PVP), derivatization with DNS, SPE extraction with C18 cartridges, concentration, and HPLC-UV analysis reaching DLs between 50 and 150 /rg/L. The method was applied to five different red and white wines from Catalonia. 

Ortega, A. F., Lopez-Toledano, A., Mayen, M., Merida, J., and Medina, M. (2003). Changes in color and phenolic compounds during oxidative aging of sherry white wines. /. Food Sci. 68, 2461-2468. 

Due to the long maceration on skin, red wines are particularly rich in phenolic compounds, having higher antioxidant capacity and also higher resveratrol content than white wines. Of the high number of studies on this field, only a few focused on the botrytized white wines. 

The content of phenolic compounds found in the Madeira wine samples assayed is represented in Table 7.4. As can be easily observed, the phenolics analyzed are about six times more abundant in red than in white wines. The fact that polyphenols content is higher in red wines was widely described before in the literature (Kuroda and Hara, 1999). 

MPs are constituted by 90% of mannose, protein and phosphoric acids and represent the 35% of total polysaccharides in wine (Vidal et al. 2003). MPs combined with phenolic compounds have shown an indirect effect on astringency, although their stabilizing effect on protein precipitation in white wine and tartrate crystallization in both red and white wines are their main function (Ribereau-Gayon et al. 2006). Glucomannoproteinshscve been also detected in wines in lower amount than MPs (Ribereau-Gayon et al. 2006). 

Grape phenolics compounds are important to wine colour, flavour, astringency and bitterness, with red wines generally containing 1200-1800 mg gallic acid equiv-alents/L of total phenolics, six- to ninefold more than present in white wines (Kennedy et al. 2006). Hydroxycinnamic acids (non-flavonoid phenolics) are major phenolic compounds of white wines and are responsible for their colour. Other non-flavonoid phenolics contribute flavour, such as vanillin, vinyl phenols and gallic acid. Vinyl and ethyl phenols, which can be present to variable extents, elicit phenolics, medical, Bandaid , barnyard and spicy characters in wine, which are generally... 

Pozo-Bayon, M. A., Hernandez, M. T, Martin-Alvarez, P. J., Polo, M. C. (2003). Study of low molecular weight phenolic compounds during the aging of sparkling wines manufactured with red and white grape varieties. J. Agric. Food Chem., 51, 2089-2095. 

Among polyphenolic compounds, two types of flavonoids, the anthocyanins and flavanols (i.e., catechins, proanthocyanidins, condensed tannins), are particularly relevant to the quality of red wines, as they are key compounds for color definition and astringency. Other flavonoids such as flavonols may have some influence on color and bitterness, although they are present in red wines in much lower amounts. Phenolic acids and hydrolysable tannins, released from barrel wood, may also have an influence on wine taste and color, and hydroxycinnamoyl derivatives from grape must are involved in the oxidative browning of white wines together with flavanols. Besides, some of these perceptions may be modified by other sensory characteristics (e.g. sourness, sweetness) related to other wine components (Preys et al. 2006). 

Ethanethiol has a very weak perception threshold (1.1 p-g/L) according to Goniak and Noble (1987), and has never been detected in wines without reduction defects. In some of the wines with a very high H2S concentration (estimated at around 20 pg/L), the ethanethiol concentrations are very low (lower than 2 pg/L). Also, concentrations in some white wines are higher than those found in red wines (Lav-igne 1996). The very different chemical composition of a white wine compared to a red wine (especially in relation to the presence of phenolic compounds) could explain the difference in results. On the other hand, no author has demonstrated the formation of ethanethiol from H2S, ethanol or ethanal in a red wine. 

A general method for the evaluation of phenolic compounds in fermented beverages, fruit juices and plant extracts was developed using gradient HPLC and coulometric detection. In a 10 p,L injection it was possible to identify and determine 36 different flavonoids and simple and complex phenols, without sample extraction, purification or concentration, in several kinds of beers, red and white wines, lemon juice and soya, forsythia and tobacco extracts. This may also be useful for the characterization of beverages and extracts . 

Phenolic compounds play a major role in enol-ogy. They are responsible for all the differences between red and white wines, especially the color and flavor of red wines. They have interesting, healthful properties, responsible for the French paradox . They have bactericide, antioxidant and vitamin properties that apparently protect consumers from cardiovascular disease. 

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