Role of lactose phosphate in lactose containing dairy products

Abstract

The overall objectives of this project were to develop an understanding of lactose phosphate (LP), to implement practical methods to identify its level in dairy products and to determine its impact in conditions relevant to the industrial lactose manufacture. Little was known about LP, its origin and its impact on lactose functionality. The study of LP was limited by several difficulties: obtaining the appropriate lactose phosphate mixture, sourcing pure lactose free of LP and other impurities, directly analysing LP, and carrying out crystallisation studies under controlled cooling seeded batch conditions. This project has investigated new approaches to solve these problems. A new, direct and rapid capillary electrophoresis (CE) method was developed which was capable of analysing LP in lactose-rich dairy products. The LP contents of different lactose powders were analysed by CE and were found to vary between 144 and 252 mg/ kg lactose. The CE method was then used as a tool to study the origin of LP along a dairy processing line and the inhibition of lactose crystal growth by LP. The comparison of the new direct CE method with the traditional indirect method led to the discovery that other forms of phosphorylated impurities were contaminating lactose in addition to LP. The core of this work focused on studying the effects of LP addition on the growth kinetics of lactose crystals during isothermal and cooling seeded batch crystallisations. The action of LP was also compared with riboflavin, glucose-6-phosphate and potassium chloride. Based on experimental data, two techniques were used to define the growth rate constant kg of a power law function describing the crystallisation kinetics of pure and impure lactose feeds. The first technique was based on previous research while a new technique was developed to include two independent variables instead of one, and consider temperature variations. For a pure feed, over the experimental temperature range (20 to 45°C), increasing the temperature increased median size and yield of the end-product. Increasing the initial median size of the seeds increased the median size but decreased the yield of crystallisation. For all conditions, provided the seed crystals had a narrow particle size distribution, the power of the supersaturation n was equal to 2.8, which differed from the published value of 2. This result was consistent with the observations of Visser (1984, 1988) who demonstrated that pure lactose, free of lactose phosphate, had a faster growth rate. For an impure feed, at 30°C, the value of kg decreased with increasing LP concentration, following a linear Langmuir adsorption isotherm up to 60 ppm LP. Compared to the control, LP addition significantly decreased kg hence lactose crystals could grow two times faster in the absence of LP. The particle size distribution was affected for the first 2 to 8 hours and a proportion of crystals had their shape modified. Riboflavin produced changes comparable to LP for growth and size distribution while the other two compounds showed smaller effects. The results for isothermal crystallisations with LP addition were confirmed in the controlled cooling operation. It was shown that LP had a stronger influence on growth than on nucleation. The impact of LP on lactose crystallisation kinetics under conditions relevant to the industrial situation was demonstrated. The advantages of using a pure feed were highlighted such as faster crystallisation kinetic, better predictions and control of the final product's specifications and improvements in the manufacturing process.

Date of Award	2007
Original language	English