New and cleaner enzymes to improve the quality of long-life lactose-free milk
Long-life milk (ultra-high-temperature (UHT) processed milk) is a growing global market. However, especially long-life lactose-free milk is exposed to challenges in relation to shelf-life, stability and quality, including taste and appearance, when stored unrefrigerated for a longer period, which is the case in many parts of the world. Researchers from Aarhus University are currently studying how so-called lactase enzymes – enzymes added to lactose-free UHT milk, in order for people suffering from lactose intolerance to be able to drink it – affect the milk, and if these can be improved.
The lactase enzyme helps infants break down the natural lactose in breast milk into glucose and galactose, which may be absorbed by the intestine contrary to lactose. In the Western part of the world, in particular the part where milk drinking is common, this enzyme remains present and active in our digestive system, more specifically the small intestine. In other parts of the world such as major parts of Asia, the gene, that keeps the enzyme active, is “turned off” in a major part of the population resulting in lactose intolerance. Therefore, the enzyme is added to UHT milk in order to split the lactose allowing people that suffer from lactose intolerance to drink milk.
The Maillard reaction – a complex challenge
For the industrial applications, the lactase enzyme is concentrated from a fermentation process, and depending on this isolation procedure may contain additional undesirable microbial enzyme activities. These can cause changes in the milk protein part as these additional, unwanted enzyme activities may “break” some of the protein structures. In addition, the glucose and galactose resulting from the lactase treatment may react with the milk proteins during heating. This process is known as the Maillard reaction and may cause a slight browning of the milk. This is a well-known reaction also witnessed in other foods, e.g. the brown color appearing when we roast meat or fry an egg. With time, these processes may interact and result in changed appearance and quality changes due changes in aroma components and the protein backbones.
The undesirable breakdown process may result in disadvantages as to sensory perception and shelf life, and means among other things, that UHT milk, with added lactase, has a shorter shelf life than UHT milk without added lactase. Within the framework of the project, the researchers will investigate shelf life, quality changes and underlying mechanisms in lactose-free UHT milk produced at a dairy by adding different lactase enzymes in a realistic production process. The researchers cooperate with DSM, an international enzyme company located in the Netherlands, which supplies enzymes for feed and foods, and which is a world-leader within lactase enzymes. Within the framework of Aarhus University’s interdisciplinary iFOOD Center, DSM has co-founded a PhD position.
Dr. Peter J.T. Dekker, Biochemist and Senior Researcher in the DSM Biotechnology Center in Delft, the Netherlands, explains about this cooperation: “Working with world-class academic partners such as the Department of Food Sciences of the University Aarhus, as well as cooperating in public-private partnerships is extremely important to DSM, as it is an important route to build and share knowledge to develop new technologies. This new PhD project will explore the changes in physical properties and chemical composition of lactose-free UHT milk, which relate to shelf life and freshness, after storage of these products at various conditions.”
Unrefrigerated milk
Lotte Juul Knudsen, PhD student in the project, explains: “In the project, we add the enzymes either prior to or after heating in order to test at which point it seems most appropriate to add them. Next, the samples are stored at different temperatures, 25 and 35 degrees respectively, for a period of up to 1 year in order to take into account the fact that UHT milk is often not refrigerated contrary to fresh milk as we know it from the refrigerated counter. The shelf life of lactose-free milk is approx. 6 months, which is considerably shorter than the shelf-life of ordinary UHT milk which is approximately 1 year.”
Subsequently, the researchers will characterize the product changes that take place, such as e.g. coagulation of proteins which may cause sedimentation, just as they will learn more about interacting conditions in order to achieve a complete picture of quality changes and the reasons for these. Project efforts further include the use of new enzymes as well as improved purification of the enzymes added. Specific focus is given to a new enzyme with a high activity, which will break down lactose faster than the enzymes used so far.
Project partners also comprise researchers from the iNANO Centre, Aarhus University, who study protein stability.
The protein-chemical toolbox ensures improved understanding
Professor and Project Manager Lotte Bach Larsen, Department of Food Science, Aarhus University, says: “Generally, what we do is to develop and apply a toolbox consisting of very advanced protein-chemical methods, including proteomics and mass spectrometry to characterize protein breakdown and molecular changes in milk protein during storage. This means that we try to elucidate the underlying food-chemical compositions, and the interaction that may exist between the processes, and how these factors can influence protein modifications, aggregate and bitter peptide formation, color changes and sedimentation in the final product.”
Arla Foods is also a project partner and produces the milk samples with the enzymes that are used in the project. Senior Researcher Valentin Rauh, Arla Foods, says: “Via this project, we hope to get a better understanding of the shelf life of our lactose free milk under different conditions and what impact the enzyme quality has on shelf life.”