Detection of harmful yeasts and bacteria in weakly acidic non-alcoholic beverages.
All non-alcoholic drinks with a pH value > 4.3.
Non-alcoholic beverages with a pH value greater than 4.3, e.g. vegetable juices, mixed drinks etc., are considered to be more susceptible to contamination. In addition to the fermentable yeasts, respiratory yeasts, moulds, lactic and acetic acid bacteria, it is also advisable to record the mesophilic and thermophilic bacterial counts (aerobic and anaerobic). These also provide initial indications of possible heat-resistant spore formers such as bacilli and clostridia.
Acidulated malt intended for use in beer brewing or elsewhere in the food industry
Acidulated malt intended for use in beer brewing or elsewhere in the food industry
L-lactic acid (L-lactate) is oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of L-lactate dehydrogenase (L-LDH) to pyruvic acid. For oxidation of D-lactic acid, the enzyme D-lactate dehydrogenase (D-LDH) is required.
|
L-lactate + NAD+ |
L-LDH
|
pyruvate + NADH + H+ |
|
D-lactate + NAD+ |
D-LDH
|
pyruvate + NADH + H+ |
The equilibrium of these reactions is much closer to lactate. The equilibrium can be shifted towards the pyruvate and NADH side of the equation by removing the pyruvate with the help of the following reaction involving the enzyme glutamate-pyruvate transaminase (GPT) in the presence of L-glutamate.
|
pyruvate + L-glutamate |
GPT
|
L-alanine + α-ketoglutarate |
The amount of NADH formed during the reactions is equivalent to the amount of lactic acid or D-lactic acid; the absorbance is determined photometrically at 334, 340 or 365 nm.