With fast scanning FT-IR interferometers, multi subsampling is possible, an innovative alternative to the traditional homogenizers in overcoming the challenge of fat globule variation in MIR spectroscopy to measure fat in milk.
Challenges in Measuring Fat in Milk
When using infrared (MIR) spectroscopy to measure fat in milk, very narrow cuvettes with pathlengths between 25 and 50 µm are used. This is to achieve a clear results as milk has a lot of water content, a strong absorber. However in turn, only a small volume of typically around 2.5 mm3 , or 2.5 µL is measured. Furthermore in unhomogenized milk, fat globules come in a range of sizes (0.1 – 10 µm), making it difficult to get a representative sample in just 2.5 µL.
Hence a common practice is to use a homogenizer since MIR measurements on milk were introduced in the 1970s. However, it does have its pro and cons.
| Pro | Cons |
| Fat Globule reduced, hence leads to excellent repeatability | Homogenizer plugs are wear parts that must be replaced at regular intervals |
| Variation in globule reduced, absorption peaks have consistent shape | Difficult to transfer prediction models with varying homogenizers |
| Very fast (6 seconds/sample) | Sample types, particularly cream, becomes very viscous if subjected to homogenizers |
A New Alternative: Multiple Subsampling
A new alternative is to measuring a larger volume of the sample, which can be done by letting the sample flow through the cuvette while performing a number of FT-IR scans. Due to fast scanning FT-IR interferometers, it is possible to perform, for example, 40 scans within a very short period of time (10 seconds).
| Pro | Cons |
| Reduces the need for maintaining the hardware, and replacing expensive parts | cannot be applied to filter instruments as it requires a full MIR spectrum to produce accurate prediction models |
| Products like cream are easier to measure as they are not thickened by the homogeniser | |
| Prediction model transferability between instruments is improved as the unpredictable effect of the fat globule size distribution does not affect the spectra |
On balance, the use of multiple subsamples results in comparable or improved measurement capability with only a minor compromise in terms of measurement speed. At the same time, the need for a homogeniser unit and associated routine maintenance is effectively avoided, simplifying and strengthening the already well-proven mid-infrared analyser concept.
