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PH, a scale of measurement of the concentration of the effective, active acidity in a solution and an important statistic, of relevance to how vines grow, how grapes ripen, and how wine tastes, looks, and lasts. (The technical definition is that pH is the negative logarithm of the all-important hydrogen ion activity or concentration.) Low values of pH indicate high concentrations of acidity and the tart or sour taste that occurs in lemon juice, for example. Values near 7 are effectively neutral; drinking waters have pH values near 7. Values between 7 and 14 are found in basic or alkaline solutions such as caustic or washing soda. Grape must and wine are acidic, with pHs generally between 3 and 4. The scale is logarithmic so a solution with a pH value of 3 has 10 times as much hydrogen ion activity as one whose pH value is 4.
The pH of soils is of some relevance to the resultant wine, although the effect is not direct. More important are effects on vine growth. Soils high in limestone tend to have high pH values, between 8 and 9, and in general limit plant growth, although the grapevine is one of the domesticated plants most tolerant of such inhospitable soils. Poorly drained soils high in organic matter tend to have acid pH values, between 5 and 6, which are also unfavourable to vine growth. Irrigation and soils high in nitrogen encourage vines to produce excessively large crops and can yield grapes with lower concentrations of organic acids and therefore higher pH values. See also soil acidity and soil alkalinity. The pH of grapes as well as wines can vary enormously since temperature, rainfall, soil type (see soil types), viticultural practices, and vine varieties can all influence the different natural organic acids and minerals of mature grapes. In general, cool regions produce wines with low pH and hot regions produce wines with high pH. Part of the reason why white wines generally have a lower pH than red wines is that red wines have higher levels of potassium, which is extracted from the grape skin, where this ion is concentrated. See also grape and acidity. The pH of grape juice is now well established as a factor affecting wine quality. In particular, high pH values are associated with high concentrations of potassium and low acidity, and red wine quality in particular is diminished (see below). Among factors known to affect the potassium concentration of grapes are potassium content of soils and shade within the canopy. The pH range of most wines is between 2.9 and 4.2 (which incidentally, since the pH of the normal stomach is about 2, means that wines are 10 to 100 times less concentrated in the acid hydrogen ion than is the stomach interior). Wines with low pHs taste very tart while those with high pHs taste flat, or `flabby'. Wines whose pH is between 3.2 and 3.5 not only tend to taste refreshingly rather than piercingly acid, they are also more resistant to harmful bacteria, age better, and have a clearer, brighter colour (see colour of wines) (see below). Wines with pH values higher than this suffer from tasting flat, looking dull, and also from being more susceptible to bacterial attack. While it is possible to manipulate pH values, with grapes and wines it is difficult because of the wine's high `buffer capacity', which roughly correlates with total acidity. The pH can be increased by decreasing the concentration of hydrogen ions, however, and vice versa. (See acidification and deacidification for discussion of the legal and practical aspects of these operations.) The wine-maker is interested in both the pH and the total acidity (both the fixed and volatile acids) of both the grape juice and the resultant wine for several reasons. What one tastes in wines as the tart or sour sensation is influenced both by the total amount of acids present and by the concentration of hydrogen ions in the solution. Different yeasts and bacteria have varying tolerances for hydrogen ion concentration and for the nature and concentration of the acid. Finally, the resistance of the wine to changes of effective acidity (hydrogen ion concentration) during processing and stabilization depends mainly on the total acid concentration. Keeping wine pH values low is of further importance because the hydrogen ion concentration of the wine controls the effectiveness of sulphur dioxide. Sulphur dioxide gas, when dissolved in wine, reacts with the water in the wine to form sulphurous acid, the form of the compound that is best at inhibiting bacteria and wild yeasts and countering oxidation. Sulphurous acid breaks down partially into hydrogen ions and bisulphite ions, a form having little effect on micro-organisms such as bacteria and wild yeasts. High hydrogen ion concentrations (low pH values) in the wine tend to combine with the bisulphite ions and thus keep more of the sulphur dioxide in the effective, anti-microbial form. pH is also important in wine-making because the pigmented tannins that colour red wines exist (like the monomeric anthocyanins from which they are formed) in several forms of different colours. At low pH values, the high concentration of hydrogen ions forces the pigment molecule into a form with a positive charge and a bright red colour. As pH increases (and hydrogen ion concentration decreases), the pigment molecules tend more and more to change through dull purple to blue, and ultimately greyish forms. The net result in the several pigments of red wine is a passage from bright to purplish red and finally to a dull brownish red as pH increases. Measurement of pH is a familiar operation to those who maintain a garden, swimming pool, or aquarium. Probably the earliest measurement technique was the use of indicator solutions or papers dependent on the fact that many dyes change molecular form and colour as the hydrogen ion concentration, or pH, changes. More precise laboratory measurements use glass electrodes. References acidification acidity anthocyanins bacteria colour of wines deacidification grape limestone nitrogen organic matter oxidation pigmented tannins potassium rainfall shade soil acidity soil alkalinity soil types stabilization sulphur dioxide temperature total acidity vine varieties yeast
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