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Knowing the exact amount of the ingredients in a product improves the quality and food safety for both consumers and producers. As the contents and ingredients have to be clearly listed on product labels, it is important for producers of dairy products to accurately determine the lactose content of the raw materials they use.
In the EU, food manufacturers are obligated by law to declare all the ingredients on the product label, clearly listed in descending order depending on the quantity. This includes lactose whenever it is knowingly used in food processing and production. The same applies if lactose is solely used as a carrier substance, for example for aromas or spice blends. As a consequence, the determination of the lactose content is highly important in the inspection of incoming goods, during the production process and in the final product to ensure quality and food safety, and to conform to legal requirements.
Lactose, a characteristic carbohydrate in milk, is metabolized by all mammals via an enzymatic reaction to form glucose and galactose, which the body needs for energy. Milk sugar, as lactose is also commonly referred to, is contained in the milk of mammals in various amounts, from about 4.8% in cow milk to approximately 6.3% in sheep’s milk. Human milk contains 7.0% lactose. Also, the lactose content of dairy products varies depending on the processing. In concentrated and dried products, for example, it increases proportionally to the dry substance, whereas it is lower in fermented products. Knowledge of this is an essential part of manufacturing dairy products, particularly as dry lactose powder is often used (e.g. in the production of cheese, yoghurt, chocolate). Sometimes, however, this knowledge is not enough and further information is required.
Polarimetry makes it possible to determine the lactose content in powders prepared from milk, whey or from raw lactose. However, this method cannot be used to determine the lactose content in milk or lactose-free milk.
Before starting the polarimetric measurement, the powders need to be dissolved in water. Fats and proteins, which basically cause the typical white color in milk, have to be removed as they would otherwise interfere with the light beam in the polarimeter. After this nothing stands in the way of determining the lactose content polarimetrically based on the optical activity of lactose. A precise measurement can be obtained with for instance an MCP polarimeter from Anton Paar with accuracy better than 0.002° optical rotation. These measurement results are then used to list the ingredients for different food products.
In dry powders like raw lactose often not only the percentage of lactose but also its crystallization stage is of interest. Here there is a difference between alpha lactose and beta lactose. After removing fats and proteins by using a Carrez solution as the clarifying agent, the polarimetric measurement of the optical rotation is done directly or even the next day. From this measurement data the content of alpha lactose and beta lactose, as well as the content of alpha-lactose monohydrate in the milk powder can be determined.
Determining the lactose content requires the sample to be free of other optically active sugars. Therefore, this method is not suitable for the determination of powders based on lactose-free milk because these powders contain glucose and galactose as a result of enzymatic lactose cleavage.
For all consumers, the determination of lactose content in dry powders contributes to the product quality as well as the food safety of resulting products.
The determination of the lactose content can be done using MCP polarimeters from Anton Paar. This ensures the quality of many dairy products based on milk powders and guarantees the correct declaration of the ingredients that also provides information on the lactose content for consumers.
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2. The polarimetric approach
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