More particularly, to convert from total acidity expressed in H2SO4 to its expression in tartaric acid, add half of the value to the original value:
In the other direction, a third of the value must be subtracted.
The French also continue to express volatile acidity in equivalent weight of sulfuric acid. More generally, in other countries, volatile acidity is expressed as acetic acid. It is rarely expressed in milliequivalents per liter. The table above also allows simple conversion from one expression to another.
The expression of volatile acidity as acetic acid is approximately 20% higher than as sulfuric acid.
Evaluating the Sugar Concentration of Musts
This measurement is important for tracking grape ripening, monitoring fermentation kinetics, and, if necessary, determining the need for chaptalization.
This measurement is always determined by physical, densimetric, or refractometric analysis. The expression of the results can be given according to several scales: some are rarely used today (Baumé scale and Oechsle scale). At present, two systems exist (Volume 1, Section 10.4.3):
1 The potential ethanol content (PEC) of musts can be read directly on equipment that is graduated using a scale corresponding to 17.5 or 17 g/l of sugar for 1% volume of alcohol. Today, the EU recommends using 16.83 g/l as the conversion factor. The mustimeter is a hydrometer containing two graduated scales: one shows specific gravity and the other gives a direct reading of the PEC. Different methods varying in precision exist to calculate the PEC from a specific gravity reading. These methods take various elements of must composition into account (Boulton et al., 1995).
2 Degrees Brix express the percentage of sugar by weight. By multiplying degrees Brix by 10, the weight of sugar in 1 kg (or slightly less than 1 l) of must is obtained. A conversion table between degrees Brix and PEC is shown in Volume 1, Section 10.4.3. Seventeen (17) degrees Brix corresponds to an approximate PEC of 10%, and 20° Brix corresponds to a PEC of about 12%. Within the alcohol range most relevant to enology, degree Brix can be multiplied by 10 and then divided by 17 to obtain a fairly good approximation of the PEC.
In any case, the determination of the Brix or PEC of a must is approximate. First of all, it is not always possible to obtain a representative grape or must sample for analysis. Secondly, although physical, densimetric, or refractometric measurements are extremely precise and rigorously express the sugar concentration of a sugar and water mixture, these measurements are affected by other substances released into the sample from the grape and other sources. Furthermore, the concentrations of these substances are different for every grape or must sample. Finally, the conversion rate of sugar into alcohol (approximately 17–18 g/l per 1% alcohol by vol.) varies and depends on fermentation conditions and yeast properties. The widespread use of selected yeast strains has lowered the sugar conversion rate.
Measurements Using Visible and Ultraviolet Spectrometry
The measurement of optical density (or absorbance) is widely used to determine wine color (Volume 2, Section 6.4.5) and total phenolic compounds (Volume 2, Section 6.4.1). In these works, optical density is noted as OD: OD 420 (yellow), OD 520 (red), OD 620 (blue), or OD 280 (absorption in ultraviolet spectrum) to indicate the optical density at the indicated wavelengths.
Wine color intensity is expressed as: CI = OD420 + OD 520 + OD 620, or is sometimes expressed in a more simplified form: CI = OD 420 + OD 520.
Hue is expressed as:
The total phenolics concentration is expressed by OD 280. The analytical methods are described in Chapter 6 of Volume 2.
CHAPTER 1 Yeasts
4 1.4 The Cytoplasm and Its Organelles
6 1.6 Reproduction and the Yeast Biological Cycle
8 1.8 Classification of Yeast Species
9 1.9 Identification of WineYeast Strains
10 1.10 Ecology of Grape and Wine Yeasts
1.1 Introduction
Man has been making bread and fermented beverages since the beginning of recordedhistory. Yet the role of yeasts in alcoholic fermentation, particularly in the transformation of grapes into wine, was only clearly established in the middle of the 19th century. The ancients explained the boiling during fermentation (from the Latin fervere, to boil) as a reaction between substances that come into contact with each other during crushing to produce effervescence. In 1680, a Dutch cloth merchant, Antonie van Leeuwenhoek, first observed yeasts in beer wort using a microscope that he designed and produced. He did not, however, establish a relationship between these corpuscles and alcoholic fermentation. It was not until the end of the 18th century that Antoine Lavoisier
