Mechanisms of flux decline in skim milk ultrafiltration: A review
Graphical abstract
Introduction
Skim milk ultrafiltration (UF) is a key unit operation in dairy processing in which milk proteins are concentrated through the removal of lactose, salts, peptides and other solutes, and water. UF exploits size differences in the milk components to preferentially concentrate the proteins, which cannot be achieved by evaporation alone, allowing more flexible control over product composition. In addition, filtration is considerably less energy-intensive [1], [2], and avoids prolonged heat exposure, allowing the functional properties and sensory qualities of milk to be well preserved [3]. It is used extensively in the production of cheese [2], [4], [5], [6], [7] and milk protein concentrates (MPC) [5], [6], [7], [8], [9], and also for protein standardisation [5], [6], [7], [8], [10].
Skim milk UF is particularly susceptible to poor operational efficiency [8] due to flux decline resulting from concentration polarisation (CP) and fouling. CP is the accumulation of retained particles at the membrane surface, while fouling occurs due to adsorption or deposition of colloidal particles on the membrane surface and in the membrane pores [2], [11]. CP and fouling contribute resistance to permeation flow, and can be responsible for severe reductions in flux and changes in rejection properties, ultimately resulting in lower throughput and altered product quality. The CP layer is a direct result of flux and is usually reversible in the sense that it will quickly diffuse if flux across the membrane is halted. However, severe CP can also result in the formation of a gel layer formed through particle-particle interactions and such a layer dissipates slowly, if at all, when the flux is halted. From an operational perspective CP is unavoidable but it can be minimised by improving particle convection away from the membrane [2], [12], [13]. On the other hand, fouling is irreversible (upon cessation of flux), and its removal requires back washing or often even chemical cleaning. This interrupts operation, lowers productivity, consumes large amounts of water and chemicals, and reduces membrane life [2].
The optimisation of skim milk UF (both operation and cleaning) requires an understanding of the mechanisms of flux decline in relation to the physicochemical properties of skim milk, however this is still not fully understood despite the widespread use of skim milk UF for over 30 years. In recent years substantial progress has been made in understanding milk chemistry, and separately, the physics of CP and membrane fouling by proteins. In addition significant advances have been made in the development of mathematical models to describe filtration behaviour, in particular via computational fluid dynamics (CFD). However, this whole body of work has yet to be brought together in the context of skim milk UF and relatively few studies have been made specifically investigating CP and fouling mechanisms in skim milk UF. Past reviews on skim milk UF or dairy filtration have mostly focused on applications and avenues for product development, and very little has been dedicated to the discussion of CP and fouling in relation to the physicochemical properties of skim milk.
In this review we bring together results from recent studies of CP, membrane fouling, and filtration modelling with existing knowledge to extend our understanding of the fundamental mechanisms of flux decline in relation to the physicochemical properties of skim milk. Aspects of membrane cleaning and processing factors affecting flux decline are also discussed. Through the analysis of the available literature, we hope to derive new insights that will allow further optimisation of skim milk UF.
Section snippets
The skim milk system
Some knowledge of milk chemistry is necessary for discussing how flux decline behaviour can be affected by changes in milk properties. This section provides an overview of the major milk proteins, mineral equilibria, and casein micelle structure and interactions with the surrounding milk serum.
Skim milk is a complex colloidal suspension of proteins in an aqueous solution of lactose and minerals, and some minor components including residual milk fat globules. Milk proteins make up 3.5 wt% of
Fouling
The term ‘membrane fouling’ is used here to describe changes brought about by interactions between colloidal particles and the membrane (e.g. protein adsorption and mineral precipitation). The resultant membrane-fouling conglomerate has different physicochemical properties compared to the membrane, for instance altered surface charge [48], [49], [50] and reduced pore sizes [51]. Consequently, its separation/fractionation properties are also changed. Fouling can manifest as complete or partial
Concentration polarisation and gel layer formation
During filtration, there is an accumulation of retained colloidal particles at the membrane surface giving rise to a concentration gradient of particles perpendicular to the membrane surface, known as concentration polarisation (CP) [2], [8], [11], [13]. This concentration gradient is the driving force for diffusion of the particles back to the bulk, which at steady state, is in balance with the bulk movement of particles to surface. This layer of concentrated particles generates a resistance
Mathematical modelling of skim milk UF
Mathematical models that can describe filtration behaviour from knowledge of fluid and particle properties are required for predicting and optimising filtration performance. Central to describing this performance, the flux has to be related to the hydraulic resistance resulting from fouling and CP. A large body of work has been devoted to developing mathematical models of cross flow filtration. In this section, basic filtration models are first briefly described and discussed. These models are
Processing factors affecting flux decline
The purpose of this section is to discuss how processing can affect the properties of skim milk, and how in turn filtration behaviour is affected by these changes.
Membrane cleaning
Membrane cleaning is an integral part of membrane processing operations. Fouling not only reduces filtration productivity, it also promotes microbial growth posing a contamination risk for the processed product [8]. As such, regular chemical cleaning is necessary during skim milk UF operation. Fouled membranes are commonly rejuvenated via cleaning in-place (CIP) procedures. A cleaning regime in the dairy industry typically involves an initial water rinse to remove residual milk and loosely
Conclusions
Recent advances in our understanding of milk chemistry, membrane science, and mathematical modelling can yield new insights into the mechanisms of flux decline in skim milk UF. In particular, the past decade has seen significant advances towards characterisation of the CP layer in skim milk UF. The quantification of the intrinsic dewatering properties of casein micelles via osmotic stressing, and the ability to directly observe the behaviour of CMs during filtration have revealed the importance
Acknowledgements
This work was supported by Dairy Innovation Australia Limited (DIAL) and the Australian Research Council (ARC) through LP110200570.
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