The method is suitable for the determination of water vapor volatile aroma compounds in beer.
Volatile aroma compounds are driven out of the sample through steam distillation. The ethanolic distillate is saturated with NaCl. Potassium hydrogen sulfite is added to separate carbonyl groups that might interfere with the analysis. The extraction of the aroma compounds is performed by shaking out with dichloromethane and the phases separated by centrifuging.
Determination of organic acids by means of reversed phase chromatography/ion chromatography
This method is suitable for wine, fruit juice and other non-alcoholic beverages.
The organic acids are separated using two combined columns, reversed-phase HPLC and an ion exchange column and are then determined using a UV detector.
The method is suitable for beer brewed to any original gravity or to any alcohol content.
Volatile compounds in beer are concentrated through distillation and extracted with dichloromethane. The solvent phase is analyzed with a gas chromatograph. The linearity of the detector and the determination of the concentrations of analytes in the sample are achieved by using multiple concentration levels within the relevant range and through evaluation of the relative area under the peaks.
Determination of the total acidity through titration
This method is used to determine the total titratable acids in beverages and concentrates.
Titratable acidity represents the sum of the free acids present in a beverage, with the exception of the dissolved carbon dioxide (carbonic acid). In fruit juices and the beverages prepared from them, they usually consist of malic acid, citric acid and tartaric acid.
The titration of the degassed beverage sample (freed from carbonic acid) is carried out potentiometrically using 0.25 mol/l sodium hydroxide solution either to a pH of 7.0 calculated as tartaric acid or to a pH of 8.1 calculated as citric acid.
Determination of the steam-volatile acids by means of titration
This method serves as a means for determining the titratable steam-volatile acids in beverages and concentrates
Volatile acids are distilled using steam, and the distillate is analyzed through titration. Sulfurous acid present in the distillate is determined iodometrically and subtracted from the total.
Determination of citric acid by enzymatic means
This analysis is suitable for malt, wort, beer, beer-based beverages and soft drinks
Fruit juices:
The acid spectrum typical of certain types of fruit are used, along with other criteria, as a basis for recognizing unadulterated fruit juices. Tartaric acid, citric acid and L-malic acid are recorded here, which, with a few exceptions, determine the total acidity of the fruit.
Citric acid occurs as the primary acid in citrus juices and other juices. Orange juice usually contains 3–17 g/l citric acid (AIJN).
In citrus juices, an addition of citric acid can be detected via the citric acid/D-isocitric acid ratio, as this lies within relatively narrow limits. In orange juice, values below 130 are found.
D-isocitric acid is partly present in fruit products as a lactone. The lactone must first be saponified prior to enzymatic determination in order to detect the total D-isocitric acid content.
Malt, wort and beer:
Citric acid is an organic acid and is present in malt and wort and is also produced during fermentation.
Citric acid (citrate) is converted to oxaloacetic acid and acetic acid catalyzed by the enzyme citrate lyase (CL):
Citrate oxaloacetic \(^{\underrightarrow{CL}}\) acid + acetate
In the presence of the enzymes malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), oxaloacetic acid and its decarboxylation product pyruvic acid are reduced to L-malic acid and L-lactic acid, respectively, by reduced nicotinamide adenine dinucleotide (NADH):
Oxaloacetate + NADH + H+ \(^{\underrightarrow{L-MDH}}\) L-malate + NAD+
Pyruvate + NADH + H+ \(^{\underrightarrow{L-LDH}}\)L-lactate + NAD+
The sum of the quantity of NADH consumed during the reaction is equivalent to the quantity of citric acid. The absorbance is determined photometrically at 334, 340 or 365 nm.