NIR spectroscopy says it's freezing

By utilising multipoint NIR spectroscopy, in which multiple NIR probes are linked by optical fibres to a single spectrometer, Finnish scientists have been able to monitor the freeze-drying process at various different locations on a freezer shelf.

Freeze-drying is a dehydration process used to preserve a wide variety of perishable materials, especially food and drugs. It involves freezing the material and then reducing the surrounding pressure, which causes the frozen water in the material to sublimate directly into water vapour. In this way, water is driven out of the material, preventing it being degraded by microbes and enzymes.

Because driving all the water out of perishable materials such as drugs can damage them, this process needs to be finely monitored to ensure that just the right amount of water is driven off. Making this trickier is the fact that freeze-drying is usually accomplished by placing numerous samples onto a shelf in a freeze-dryer. Unfortunately, the freeze-drying process tends to take place at slightly different rates on different parts of the shelf, happening faster at the outer edges than in the middle.

Although NIR spectroscopy has already been used to monitor the moisture content of perishable materials during freeze-drying, it is restricted by the fact that it can only monitor the samples in one specific location on the shelf, meaning it misses the different drying rates happening at other locations. Now, however, a team of Finnish scientists led by Ari Kauppinen at the University of Eastern Finland in Kuopio has used multipoint NIR spectroscopy to monitor the freeze-drying process at three distinct locations on the shelf.

As they reveal in a forthcoming paper in Analytical Chemistry, they placed three NIR probes at places in the shelf where the rate of freeze-drying displayed the greatest variation: at the far corner of the shelf, at the centre of the shelf, and half-way between the edge and the centre. These probes were then linked by fibre optic cables to a single spectrometer. Finally, they filled the shelf with vials of sugar solution and conducted freeze-drying.

After first calibrating the NIR data with a much slower, off-line titration technique for determining moisture content, Kauppinen and his team found that they could accurately monitor the moisture content at the three locations during the freeze-drying process. According to Kauppinen, this on-line monitoring should help scientists determine the optimum freeze-drying process for a specific material and also help manufacturers of freeze-dryers identify potential 'weak spots' of uneven drying in their systems.