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These bitter substances are extracted from the acidified sample using iso-octane. Certain disruptive substances are eliminated through washing the extract with acidified methanol. The concentration iso-α-acids as well as α-acids is determined by measuring the absorbance in alkaline methanol at 255 nm and 360 nm.

Volatile substances in beer are concentrated through distillation and the distillate is extracted using dichloromethane. The solvent phase is concentrated in the rotary vacuum evaporator and subsequently analyzed using a gas chromatograph. The linearity of the detector and the determination of the concentrations of analytes in the sample is achieved by using multiple concentration levels within the relevant range and through evaluation of the relative area under the peaks.

CO₂ is introduced into the beer so that a specific volume of foam is produced. The mean retention time of the bubbles in the foam serves as a measure for the foam stability, which is calculated as the relationship between the time required for the foam to collapse and the logarithm of the relationship between the volume of the collapsed foam and the foam still present [1–3].

This method is frequently applied in instances when the influence of carbon dioxide content on foam formation in the beer is to be eliminated.

In determining the foam stability with the Steinfurth Foamtester SFT, the time required for the foam to collapse in a graduated cylinder is measured.

The pressurization is carried out in such a way that the container does not require degassing. The attempered beer is injected into the measuring cylinder through a nozzle, and in the process, foam is formed. The foam stability is determined by several optical sensors detecting the time necessary for the beer/foam interface to pass by them. The precise metering of a predetermined quantity of beer, the entire process of executing the analysis and displaying the results as well as cleaning and conditioning of the measurement cylinder are performed automatically.

The measurement with the NIBEM Cling Meter (fig. 1) is normally performed immediately after the NIBEM foam stability measurement and serves to provide additional information on foam quality. 

After the preset timer has expired, a scan head is introduced into the Haffmans standard glass while the glass is rotated slowly at a fixed level, in order to ensure that the foam has collapsed a minimum of 48 mm. An LED on the back of the scan head emits light. The diffused light, which reflects off of the inside of the glass, is read by a sensor and recorded.

Fruit juices

The positive effect of fermented beverages on the human body has been known for centuries. Current beverage trends, like kvass (Russia) and kombucha (Asia), stem from traditions with roots deep in the past. They have always been consumed as healing beverages. Non-alcoholic forms of fermentation employ microorganisms, such as lactic and acetic acid bacteria. They produce organic acids like lactic acid and gluconic acid, which promote digestion and metabolism. Due for the most part to their slightly acidic flavor, these kinds of fermented beverages are popular with consumers as a healthy natural refreshment.

Malt, fruit juice and tea serve as a base for fermented beverages.

As a rule, fermented beverages contain 0.5 – 15 g/l D-gluconic acid.

D-gluconic acid is phosphorylated by adenosine 5'-triphosphate (ATP) in the presence of gluconate kinase to gluconate-6-phosphate

 D-Gluconate + ATP \(^{\underrightarrow{\text{gluconate kinase}}}\) D-gluconate-6-P + ADP

The enzyme 6-phosphogluconate dehydrogenase (6-PGDH) catalyzes the oxidation of gluconate-6-phosphate to ribulose-5-phosphate with nicotinamide adenine dinucleotide phosphate (NADP): 

D-Gluconate-6-phosphate + NADP⁺ \(^{\underrightarrow{6-PGDH}}\) ribulose-5-phosphate + NADPH + H⁺ + CO₂

The amount of NADPH formed during the reaction is proportional to the amount of D-gluconic acid.