A study of the mechanisms of chemical cleaning of milk protein fouling deposits using a model material (whey protein concentrate gel)

Abstract

It is crucial to understand the fundamental mechanisms of cleaning milk protein fouling to optimise Cleaning-in-place (CIP) process. Using Whey Protein Concentrate (WPC) gel as a model material and a rapid ultraviolet (UV) spectrophotometry, a comprehensive laboratory study on the cleaning of the WPC gel deposits from hard surface with alkaline cleaning solutions has been conducted. The kinetics of the cleaning process has been established and mathematical models have been developed in order to elucidate the influences of various parameters on cleaning process. This study has provided sound evidence that whey protein concentrate gel is a reliable simulation of the whey protein fouling deposits used in most milk protein fouling and cleaning studies. Based on treating denatured whey protein gels as biopolymers, a chemical reaction controlled polymer dissolution cleaning mechanism has been proposed. The polymer dissolution plays a major role of removing proteinaceous deposits when treated with alkaline solutions under the flow conditions tested. Similar to the diffusion of cleaning chemicals and chemical reactions, the reptation (induction) is also one of essential steps for the dissolution of WPC gels in alkaline solutions. The disengagement of intermediate reaction products (altered protein molecules) from a gel-solution interface and subsequent mass transfer of these reaction products to the bulk cleaning solutions are the rate-limiting steps for the cleaning process. The typical dissolution cleaning rate curve of WPC gels in alkaline solutions includes swelling, uniform and decay cleaning stages. This study on cleaning kinetics shows that increasing the cleaning temperature can improve the cleaning efficiency. The apparent activation energy for these three stages is 32.6, 40.5, and 38.3 kJ/mol, respectively, which is in agreement with previous research works. Increasing flow velocity enhances the cleaning process. However, this effect could be reduced when and the cleaning process gradually changes from a mass transfer-controlled process to a disengagement-controlled process, where the flow velocity is very high. The introduction of the hydrolysis, β-elimination reactions and some competing chemical reactions have highlighted the complex of chemical reactions involved in cleaning of proteinaceous fouling using alkaline solutions. The changes in molecular mass distribution and SH content of WPC gel dissolved at various temperatures observed has confirmed the assumption that all these chemical reactions are temperature dependent. The investigation on the swelling, microstructural and mechanical properties of WPC gels treated with alkaline solutions also illustrates the concentration dependency of these chemical reactions. The mechanical property studies demonstrate that the chemical treatment could make WPC gel weaker and easier to be destroyed. However, the relationship between the mechanical properties and the cleaning process needs to be further studied. Based on the polymer dissolution and mass transfer theory, a mathematical model of chemical cleaning has been proposed. Various parameters, such as tr (reptation time), Rm, (constant cleaning rate), mc, (the critical mass), ξ (rate constant in swelling stage), kA (rate constant in decay stage) and Ψ (a dimensionless parameter) have been used to characterise the whole cleaning process. Among the parameters used in the cleaning model, the constant cleaning rate (Rm) is the most important one and determines the overall efficiency of a cleaning process, which has been further predicted and expressed as a product of mass transfer coefficient and solubility of disengaged protein molecules. The successful model formulations for the cleaning rate and cleaning time under various operation conditions are a good outcome of the rational mechanisms proposed for the removal of proteinaceous fouling. This research has provided a good foundation for further fundamental research in this area and for optimising the cleaning processes.