NIR spectroscopy for dairy QC

The requirements of quality control are constantly growing within the food production and processing industry. The laboratories must master not only new tasks and a higher sample volume, but also supply faster results, since nowadays less, but larger production plants with higher capacity are in operation. Modern NIR technology offers a fast and easy quality control in the laboratory as well as online.

However, up to recently, NIR was used in routine exclusively for analyzing finished milk products like cheese and milk powder. Recently more and more interest is shown in the analysis of milk by NIR in order to utilize only one instrument in the laboratory for all kind of samples. NIR instruments are an alternative in general as a back-up for the FT-IR based milk analyzers or for small companies to invest in only one instrument like the MPA from Bruker Optics.

Milk Testing

Milk is the starting point of any dairy product and is one of the best controlled food products in the world. The composition of milk is changing from season to season and even from cow to cow which makes a standardization step necessary to maintain milk quality for all further process steps in dairy production.

Commonly, dedicated analyzers are used based on FT-IR technology with transmission flow cells with 25µm path length. In such small dimensions milk is a heterogeneous emulsion of fat in water and a homogenizer is required to reduce the size of fat droplets especially in raw milk.

NIR spectroscopy can utilize a longer path length of 1mm and a larger sample spot compared to FT-IR. This allows milk analysis even of raw milk without homogenization. A typical setup can be found in Figure 1.

Figure 1. Measuring liquid milk with FT-NIR spectroscopy

The analysis with NIR spectroscopy delivers comparable results as the more expensive FT-IR milk analyzers. The fat content of raw milk can e.g. be determined in a range from 1.3 - 7.7% with an accuracy of 0.05%. Parameters like protein, lactose or SNF (solids non fat) can be determined with a comparable accuracy.

Not only milk, but also liquid milk derivatives, viscous samples and those containing sugar can be easily measured using the same setup independent of the composition of the product.

Besides different milk types the following sample types can be measured without any pre-processing:

• Whey and whey concentrate
• Whey protein concentrate
• Cream and whey cream
• Lactose concentrate
• Condensed milk, sweetened condensed milk
• Flavoured milk drinks

Analysis of Milk and Whey Powder

Not only liquid milk and whey can be tested with NIR spectroscopy, but also the spray dried powders. This measurement is really simple, with the powder being filled in a cup with quartz glass bottom (Figure 2). By measuring from below, a reproducible sampling is possible as such a fine powder will give a homogeneous and compact surface in contact with the glass.

Most of the milk powders are very homogeneous, so that with a measurement spot of 15 mm, a representative sample amount can be measured. However, the sample cup is normally rotated eccentrically during the measurement, in order to collect more sample information and to level out slight in-homogeneities.

Figure 2. NIR measurement of milk powder in a rotating cup with quartz window on the bottom

Guaranteeing optimal cheese quality

A further example is the raw goods examination for processed cheese production. The most important parameters of processed cheese are the fat content and the dry matter, which is indicated later than fat in dry matter (FDM). Since depending on availability a mixture of different cheese types is cooked together with emulsifying salts, a production planning for the recipe is essential. It is particularly important to determine the fat content and the dry matter of the incoming cheese material. The more exactly the mixture for the melt can be determined, the better the addition of expensive fat components such as butter can be controlled.

Since many different cheese types from most different sources are processed, the sample number in the incoming goods laboratory is high and a fast and simple procedure such as NIR is required. The samples are filled into a standard Petri dish. The NIR light is now directed through the sample to the external detector (Figure 3). This way, more sample amount is measured at a time. The Petri dish rotator is used in order to analyze a higher sample amount.

Figure 3. NIR measurement of grinded cheese in a plastic Petri dish

Different types of cheese can be integrated into a single calibration, so that the effort of creating a calibration can be limited. Samples with 15.4 to 34.5% fat and dry matter values between 48.5 and 66% were measured. A mean deviation of the NIR from the reference method from 0.3% for fat and 0.35% for dry matter could be found.

In-line control of butter production

From an economic point of view, it is crucial to keep the water content as closely as possible to the statutory limit of 16 %, as water is of course much cheaper than the butter fat.. The aim of the process control should therefore be to use an accurate analysis technique to achieve this.

To monitor the moisture content with NIR spectroscopy, a measurement probe is built into the butter stream via a flange. In parallel, the salt content of salted butter can be determined.

Figure 4. Bruker Optics' MPA for the dairy industry

For an inline calibration of water, samples in a range from 15.2 to 16.2% were measured. For salt samples from 1.1 to 1.5% were used for the calibration. For water a mean deviation of 0.06% from the reference values was achieved, for salt 0.03%. The high accuracy makes it possible for the process to get as close as approximately. 0.1% to the critical water content of 16%. Taking the high throughput and 24/7 operation into account, an investment into this technology will pay off in a short period of time.

Why NIR?

NIR is definitely a powerful tool that should have its place in the dairy industry. Common parameters like dry matter, protein, fat and can be monitored quickly, easily and reliably by NIR spectroscopy at every stage of the operation. The implementation of NIR spectroscopy can help the dairy industry to keep raw materials, production processes and finished products in control. It is a tool of the future that will replace some of the older and slower techniques like wet chemistry.