Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Mechanisms of flux decline in skim milk ultrafiltration: A review Kenneth S.Y. Ng, Malavika Haribabu, Dalton J.E. Harvie, Dave E. Dunstan, Gregory J.O. Martin ⁎ Department of Chemical & Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia ARTICLE INFO Keywords: Ultrafiltration Skim milk Concentration polarisation Fouling Modelling ABSTRACT Skim milk ultrafiltration (UF), used for milk protein concentration, is one of the most important unit operations in dairy processing. However UF performance is severely reduced by flux decline resulting from concentration polarisation (CP) and fouling, the mechanisms of which are still not fully understood for the complex colloidal fluid of skim milk. In this review, we analyse published observations of CP and fouling of relevance to skim milk UF to examine the underlying mechanisms. Current approaches for modelling the flux decline caused by CP and fouling are reviewed. Focussed discussion is given to the current state of understanding of CP and fouling in relation to the physicochemical properties of skim milk and the effectiveness of chemical cleaning. This review identifies the roles of various milk components in CP and fouling behaviour and offers insights into the mechanisms that govern flux decline. 1. 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 [2,4–7] and milk protein concentrates (MPC) [5–9], and also for protein standardisation [5– 8,10]. Skim milk UF is particularly susceptible to poor operational efficiency [8] due to flux decline resulting from concentration polarisa- tion (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 mem- brane [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 mem- brane 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 http://dx.doi.org/10.1016/j.memsci.2016.09.036 Received 21 June 2016; Received in revised form 21 September 2016; Accepted 23 September 2016 ⁎ Corresponding author. E-mail address: gjmartin@unimelb.edu.au (G.J.O. Martin). Abbreviations: AAS, atomic absorption spectroscopy; ATR-FTIR, attenuated total reflection Fourier transform infrared spectroscopy; BSA, bovine serum albumin; CCP, colloidal calcium phosphate; CFD, computational fluid dynamics; CFV, cross-flow velocity; CIP, cleaning-in-place; CM, casein micelle; CN, casein; CP, concentration polarisation; DF, diafiltration; EDTA, ethylenediaminetetraacetic acid; EDX, energy dispersive X-ray; GISAXS, grazing incidence small-angle X-ray scattering; IEP, isoelectric point; MF, microfiltration; MPC, milk protein concentrate; MWCO, molecular weight cutoff; NPC, native phosphocaseinate; PAN, polyacrylonitrile; PEG, polyethylene glycol; PES, polyethersulfone; PSf, polysulfone; PVDF, polyvinylidene difluoride; SAXS, small-angle X-ray scattering; SEM, scanning electron microscopy; SMUF, simulated milk ultrafiltrate; TMP, transmembrane pressure; TN, total nitrogen; UHT, ultra-high temperature; UF, ultrafiltration; VRR, volume reduction ratio (or volume concentration factor, VCF); WP, whey proteins; WPI, whey protein isolate Journal of Membrane Science 523 (2017) 144–162 0376-7388/ © 2016 Elsevier B.V. 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