The absorbance of a juice or wine indicates the color of a sample. For white wines, a single wavelength reading at 420 nm (yellow) is sufficient. For red wines, the color is usually determined at three different wavelengths: 420 nm (yellow), 520 nm (red), and 620 nm (blue).
Acetic acid is the main volatile acid in wine. It can be formed by several yeasts and bacteria and is the main microbial spoilage indicator during winemaking. There is a legal limit in wine (1.2 g/L for white wine, 1.4 g/L for red wine; late-harvest wines with >28 Brix: 1.5 g/L for whites and 1.7 g/L for reds).
While grape juice typically only contains tartaric and malic acid, finished wine will also have lactic, citric and acetic acid. The individual concentrations and ratios between these organic acids allow for the identification of fermentation problems, potential off-flavors and stability / shelf-life issues.
Ethanol is the main alcohol in wine and is formed during fermentation from glucose and fructose. Wineries are legally required to display the ethanol concentration in % (v/v) on the label.
Volatile compounds in wine can be analyzed by gas chromatography and provide information about the aroma composition of the product. Esters, terpenes, alcohols, and aldehydes can be quantified to estimate the aroma quality.
The total concentration of soluble solids is commonly expressed in % Brix or ° Brix. In grape juice, where the majority of soluble solids are sugars, Brix is used as an indicator for sugar concentration.
Calories in wine come primarily from alcohol and sugar and can be calculated based on analytical data. The information can be added to the wine label for the consumer. Some countries require that data for export certifications.
Citric acid is formed by the yeast during fermentation or can be added to the wine. It is microbially unstable and can be the source for stylistic off-flavor production.
Density describes the mass of compounds in solution in relationship with the solvent. Water has a relative density of 1 g/cm3, so before fermentation the sugar leads to a high density. Alcohol lowers the reading which leads to final densities around 0.995 g/cm3 in dry wines and 0.920 g/cm3 in distilled spirits.
The sum of glucose and fructose in grape juice results in the concentration of total fermentable sugar. In finished wine, this sum indicates residual sugar.
Iso-alpha acids from hops are causing the bitterness sensation in beer. The measurement of IBU is a chemical bitterness, typically between 0 and 100, that can be used to communicate this stylistic element to the consumer.
Lactic acid can be produced from sugar by lactic acid bacteria and serves as a spoilage indicator in grape juice. It is also a product of malic acid degradation during malolactic fermentation, so if found in finished wine, it can have different implications.
Malic acid is one of the two main acids in grape juice. It is microbially unstable and can be metabolized into lactic acid by different organisms at different stages of the winemaking process. The grapevine also uses malic acid for respiration, making it an important climate indicator for grapes.
Methanol is a by-product of fermenting fruit that is high in pectin such as grapes and apples. It is not causing problems in wine or cider but methanol is concentrated during distillation and needs to be removed in the heads fraction. Legally, fruit brandy must contain less than 3.5 g/L methanol.
pH measures the relative concentration of free hydrogen ions in solution, providing an estimate of acidity strength. It ranges from 0 to 14 with 7 being neutral. Wines typically have an acidic pH between 2.9 and 4.0.
Most 6 and 5-carbon sugars in fruit and wine have reducing properties but not all of the sugars are fermentable. Saccharomyces yeast can ferment glucose and fructose and is leaving other reducing sugars in the medium. A typical concentration of non-fermentable reducing sugars in a dry wine is 1-2 g/L.
Wine stability should be assessed before bottling. Cold stability determines the risk of tartrate crystallization in the bottle, while heat stability estimates protein instabilities that can lead to hazy wine. Fining trials can be performed to address any other instability issues.
Sulfur dioxide is the main antioxidant and preservative in wine. It can bind to several different wine components and should be analyzed as a combination of free and total sulfur dioxide (SO2). The recommended dosage to wine depends on pH and there is a legal limit for total SO2 (350 mg/L). Wines containing more than 10 mg/L total SO2 must bear a “contains sulfites” label warning.
Tartaric acid is one of the two main acids in grape juice. It is microbially stable but can be modified using chemical or physical methods. It is also the main source of crystal precipitation in wine when combined with potassium or calcium.
The sum of all organic and inorganic acids in juice and wine can be expressed as titratable acidity. Since some of the organic acids are relatively weak, the sample is titrated beyond neutrality to an endpoint of pH 8.2 and expressed as tartaric acid equivalents using a molar equivalence factor.
Phenolic acids, flavan-3-ols, hydroxycinnamic acids, anthocyanins, and flavonols are examples of compounds that are classified as polyphenols. The Folin Ciocalteu method summarizes all polyphenols and quantifies them as gallic acid equivalents.
Tannins are polymeric molecules formed from polyphenols, mostly flavan-3-ols and phenolic acids. Their molecule size and structure determine their effect on stability and sensory. Total tannins are quantified and expressed as catechin equivalents.
The combination of amino acids (NOPA) and ammonium is referred to as YAN. The target for a successful fermentation is 10 ppm of YAN per % Brix in grape juice.
