Clear sample solutions in analysis of fruit and vegetable juice

Dr Achim Gessler looks at the problems associated with control and monitoring during the process analysis of fruit and vegetable juices.

Process analysis during the manufacture of fruit and vegetable juices is important for controlling and monitoring the manufacturing process. Analytical methods with a very short analysis time are required in this sector. This allows the determined values to be quickly utilised and ensures that production processes are safely controlled, monitored and optimised.

A particular advantage is gained when the monitored parameters are obtained directly, either inline or online. If this is not possible then external measurements must be carried out in a laboratory.

Photometric and liquid chromatography can be employed as quick, reliable methods for determining the parameters necessary in process control. In the manufacture of carrot juice, the nitrite from microorganisms should not be present in the final product. In order to guarantee nitrite-free carrot juice parameter must be monitored continuously during processing.

In the photometric determination of nitrite, sulphanilamide is reacted with N-(1-naphthyl)-ethylenediamine-dihydrochloride to form a red azo dye; this compound can be detected photometrically at 546 nm. The test can be carried out quickly and is very sensitive; this makes it an excellent choice for process analysis. A content of less than 0.1 mg/l can be detected reliably.

However, as the determination can only be performed on a clear solution, a certain delay in carrying out the test is unavoidable due to the need to clarify the sample. The turbidity that is found in carrot juice is caused by polymeric constituents, which mainly consist of pectins, hemicelluloses and glycolipids together with the carotinoids bound to them. A Carrez clarification is very effective in removing the turbidity, but this also introduces a delay. Substances contained in the juice that interfere with the determination of nitrite cannot be removed by filtration and centrifugation.

Filtration through a membrane filter holder with a pore size of 0.45 µm gives a very good result, although in constitution filter holders the polymeric contamination blocks the membrane after only a few microliters. Thus, insufficient clarified sample material is obtained through a 0.45 µm membrane alone.

These issues were the catalyst behind development and optimisation of the Celtron 30/0.45CN-GF51filter holder in the Schleicher & Schuell application laboratory. It consists of a special glass fibre prefilter with a 0.45 µm cellulose nitrate membrane filter and is capable of clarifying up to 5 ml of carrot juice directly, without any preliminary treatment.

The Celtron 30/0.45CN-GF51 makes it possible to reduce sample preparation time for nitrite determination from several minutes to only a few seconds. In practical daily use in the laboratory the Celtron 30/0.45CN-GF51 has evolved into an universal sample pre-treatment system for the clarification of turbid fruit and vegetable juices. Delays in subsequent analysis by photometric, enzymatic and liquid chromatography methodsare reduced.

Sample Distilled water Sulphanilamide-NED solution

0.5 ml 1.5 ml 0.5 ml

0.5 ml 2.0 ml ---

--- 2.0 ml 0.5 ml

Pipette into a cuvette: (Sample, Sample blank, Reagent blank).

Mix cuvette contents with a spatula stirrer or seal cuvette with Parafilm and mix by repeated inversion. After two minutes measure the extinction at 546 nm against the reagent blank.

Instructions for the determination of nitrite in carrot juice. Measuring range: 0.1 to 3 g/l

Instruments: Spectrophotometer measuring at 546 nm; Plastic cuvettes; Ready-to-use Celtron 30/0.45CN-GF51 filter holder.

Reagents: ortho-Phosphoric acid 85 per cent, analytical grade (Merck #573); Sulphanilamide, analytical grade (Merck #11799); N-(1-Naphthyl)ethylenediamine dihydrochloride (NED), analytical grade (Merck #6237); Sodium nitrite, analytical grade (Merck #6549).

Preparing the solutions: Sulphanilamide-NED solution: 5 ml phosphoric acid, 2 g sulphanilamide and 0.2 g NED are made up to 50 ml with distilled water.

Shelf life if stored in amber bottles at 2°C to 8°C: approximately one month. Nitrite stock solution with 1 g nitrite/l: 1.50 g sodium nitrite is made up to 1 l with distilled water. Nitrite working standard 1.0 mg/l: 1000 µl stock solution is made up to 1 l with distilled water. This standard should be used within approximately one week.

Procedure: The carrot juice sample is filtered through the ready-to-use Celtron 30/0.45CN-GF51 filter holder. The functioning of the reagents is checked by using the 1mg/l working standard; this is analysed in the same way as the sample solution.

Result: Nitrite mg/L = (sample extinction ­ sample blank extinction) x 6.08.


Dr Achim Gessler is with Schleicher & SchuellGmbH, Dassel, Germany.

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