Staff PhD

LSI pasteurisation of milk and effect on microbiology, cheese maturation and protein denaturation

Jonatan Ahrens Dickow

Background

Traditionally, milk is pasteurised by indirect heat-transfer in plate or tube heat exchangers. Besides ensuring a microbiologically safe food product, through elimination of heat labile bacteria and fungi, the heat treatment also inactivates milk enzymes and causes a certain degree of protein denaturation, which is of importance for both functionality and sensory properties of milk products. By new pasteurisation technologies such as Lenient Steam Injection (LSI) a direct heat transfer from steam to the milk is used. Small steam-droplets are injected into the milk followed by a flashdestillation and the hot product evaporates under vacuum, which results in a short time boiling as energy is released when the steam condensates. Preliminary data have shown that LSI-pasteurisation appears to be gentler regarding protein denaturation and degree of denaturation using steam-pasteurisation is lower than when using traditional indirect low pasteurisation or UHT treatment (Silveira & Abreu, 2003). Furthermore, the content of methylketones caused by lipase activity is higher in cheese produces from milk pasteurised by direct steam heat transfer than from indirect low pasteurised milk (Sbampato et al., 2000). Most bacteria are usually eliminated by pasteurisation, while spores and heat-resistant bacteria can survive, as well as heat stabile bacterial enzymes are not inactivated. Especially, Bacillus spores are a problem in milk and these can be destroyed by steam injection at 134°C for 4-12 sec. (Blake et al. 1995) or reduced by 3-5 log units by LSI treatment at 120°C for < 1 sec (Danielsson, 2008).

Aim

The aim of the project is to investigate and to describe the influence of LSI pasteurization on milk and milk products.

The project main hypothesis is that LSI is a technology, which by its direct heat transfer can be used for milk pasteurisation to provide a gentler treatment in comparison with traditional indirect plate/tube pasteurisation, resulting in an efficient bacterial elimination, reduction of spore-forming bacteria, lower degree of protein denaturation of the whey proteins, and better preservation of the milk lipoprotein lipase activity, which are of importance for respectively the functional properties of whey protein concentrate (WPC) and the maturation and sensory properties of blue cheese.

Research Outline

Initial experiments with the pilot equipment of LSI processing will be conducted in order to get acquainted with the equipment and experience the possibilities and limits of heat treatment of milk. Furthermore, an external engineering university is affiliated on the task of developing a precise method for calculation or measurement of the very short holding time of the process.

Secondly, the effect of different conditions during milk pasteurisation e.g. time, temperature, steam pressure, preheating etc. are studied for cheese milk and WPC and compared with traditional pasteurisation technologies. In cheese milk, the activity of lipoprotein lipase in relation to free fatty acids, fatty acid composition and quantification of methylketones are analysed. In WPC, the effects on whey protein denaturation, aggregation and lactosylation, surface hydrophobicity, activity of plasmin and microbial proteolytic enzymes are studied in relation to pasteurisation and a subsequent spraydrying of WPC.

The parameters of LSI-pasteurisation found on cheese milk are optimised in order to preserve a high lipoprotein lipase activity in relation to blue cheese. Blue cheeses are produces in collaboration with Arla Foods amba and the sensory properties are evaluated after a maturation period.

The parameters of WPC pasteurisation are optimised in regard to the functional properties of WPC as ingredient. The protein denaturation and enzymatic activities are studied and correlated with known application analysis performed by Arla Food amba.