Suitable for all beverages
The energy value or caloric value of beer is calculated according to Council Directive 90/496/EEC and is based upon the amount of protein, carbohydrate, alcohol, glycerin and organic acids. For performing the calculation, caloric values are attributed to 1 g of each of the following: 17 kJ (4 kcal) of protein or utilizable carbohydrate, 29 kJ (7 kcal) of ethanol, 13 kJ (3 kcal) of organic acid, and 10 kJ (2.4 kcal) of higher alcohols. (Fat is not included in the calculation for beer.)
Hydrolytic cleavage of carbohydrates to determine total glucose from the glucose already present and that formed by hydrolysis.
Suitable for all beverages
Carbohydrates are split by acid hydrolysis in the boiling heat under reflux with acid. After neutralization and filtration of the hydrolysis mixture, the glucose content is determined enzymatically after dilution. In this process, glucose is phosphorylated by the enzyme hexokinase (HK) and adenosine 5'-triphosphate (ATP) to glucose 6-phosphate (G-6-P):
\(\text{Glucose + ATP} \space ^{\underrightarrow{\text{HK}}} \space \text{G-6-P + ADP}\)
In the presence of the enzyme glucose-6-phosphate dehydrogenase (G6P-DH), G-6-P is oxidized by nicotinamide adenine dinucleotide phosphate (NADP) to gluconate-6-phosphate. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is formed:
\(\text{G-6-P + NADP}^+ \space ^{\underrightarrow{\text{G6P-DH}}} \space \text{NADPH + H}^+\)
The amount of NADPH formed during the reaction is equivalent to the amount of glucose. NADPH is determined based upon its absorbance at 334, 340 or 365 nm.