Manual Kjeldahl Digestion Block

What is the Manual Kjeldahl Digestion Block?

A manual Kjeldahl digestion block heats digestion tubes in a controlled aluminium block for acid digestion of food and feed samples. It is a simple, low-cost entry into the Kjeldahl method for low-throughput laboratories.

Overview

The block holds six to twelve 250 mL or 400 mL digestion tubes in an aluminium or graphite insert heated by embedded elements, with a controller holding the set temperature, usually 350 to 420 °C, to within a few degrees. The operator adds acid, catalyst and sample, places the tubes in the block and raises a fume manifold over them to draw acid vapours to an exhaust system. Digestion is complete when the liquid clears and no charring remains, typically after 45 to 120 minutes depending on the matrix. Because temperature ramps and timing are manual, results depend on operator technique, and the block must be used with a proper scrubber or fume hood to protect staff.

How does it work?

The Kjeldahl method converts nitrogen in the sample to ammonium sulfate by heating with concentrated sulfuric acid and a catalyst, usually copper or selenium with potassium sulfate to raise the boiling point. Organic matter is oxidised and the nitrogen is retained as ammonium. The digest is then made alkaline with sodium hydroxide, releasing ammonia, which is steam-distilled into a receiver containing boric acid or a standard acid. The amount of ammonia is determined by titration, and nitrogen content is calculated from the titre. A conversion factor, typically 6.25 for food, 6.38 for milk or 5.7 for wheat flour, converts nitrogen into protein. Blank determinations, catalyst choice and digestion completeness are the main factors that determine accuracy, and the method is the traditional reference for protein content.

What are the key features?

• Six to twelve positions for 250 or 400 mL tubes
• Aluminium or graphite block with embedded heaters
• Temperature controller holding 350–420 °C
• Fume manifold connected to exhaust or scrubber
• Simple design with minimal electronics
• Optional tube racks and lift for safer handling

What are the technical specifications?

A typical manual kjeldahl digestion block configuration offers positions of 6–12 tubes, tube volume of 250 mL or 400 mL, temperature range of Ambient to 450 °C, temperature stability of ±2 °C, heating block of Aluminium or graphite and digestion time of 45–120 min depending on matrix. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Manual Kjeldahl Digestion Block is used in low-volume food and feed protein testing, teaching and method development, water and soil nitrogen digestion and backup digestion capacity for peak loads. Across the wider Kjeldahl Systems class, laboratories also deploy it for reference protein determination in food and feed, milk and dairy nitrogen testing and cereal, flour and grain protein measurement.

Which standards and certifications apply?

Relevant standards and methods include ISO 1871, ISO 8968-1, ISO 5983-2, AOAC 2001.11, AOAC 920.87 and ISO/IEC 17025. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Kjeldahl work involves concentrated sulfuric acid, hot sodium hydroxide and noxious fumes, so a fume hood, an exhaust scrubber and acid-resistant surfaces are required, along with gloves, goggles and a face shield. Digestion must be complete and free of charring; adding hydrogen peroxide or using a catalyst mixture helps, and selenium catalysts need careful handling because of toxicity. The distillation apparatus must be steam-tight and conditioned before the first sample to avoid low recoveries, and blank values should be subtracted for each batch. The standard acid and sodium hydroxide solutions need regular standardisation, and titrators should be checked with certified materials. Document temperatures, times, reagent batches and catalyst lots.

How do I select the right Manual Kjeldahl Digestion Block?

Select a manual digestion block when throughput is low and budget is limited, provided a fume hood or scrubber and trained staff are available. Confirm the tube size and position count for your sample sizes, and check that the temperature range covers the method you follow.

How does it compare with other Kjeldahl Systems variants?

Choose the scale of the system from your sample numbers and the accuracy you need. Manual digestion blocks with six to twelve positions suit low-throughput laboratories and teaching use but demand careful timing and fume control. Automated digestion blocks add temperature programming, exhaust scrubbing and unattended runs, which improve safety and reproducibility. Steam distillation units with automatic alkali addition and titration cut operator time and variability and are the core of routine Kjeldahl work. Integrated analyzers link digestion, distillation and titration in one workflow and are convenient where many samples are processed daily. Micro-Kjeldahl systems use smaller sample and reagent volumes, which reduces waste and is useful when sample material is limited. Whatever the scale, confirm recovery with reference materials.