Fouling during food processing – progress in tackling this inconvenient truth

Highlights

• Review of recent academic literature indicates that fouling is a niche subject.

• Fouling affects mainly heat transfer and membrane separation operations.

• The understanding of dairy fouling is notably more advanced.

• Fouling and cleaning processes are interlinked in a fouling-cleaning cycle.

• Many studies aim to understand or manage fouling.

Progress in understanding fouling in food processing operations is assessed, -based on a review of the literature published from 2014–2017. Quantitative mechanistic models have been refined and tested for milk heat exchanger fouling: in other heat transfer and membrane separation applications the studies have focused on identifying fouling behaviour and management of fouling. The awareness of fouling being one stage in a fouling-cleaning cycle is becoming more widespread and this is being incorporated in the selection of mitigation strategies and development of coatings or new surfaces. Analytical techniques are providing more detailed insight into how fouling deposits grow and evolve, which can be used to optimise parts of the cycle.

Introduction

Fouling refers to the unwanted deposition of material from a product stream on a surface as a result of processing. Fouling deposits are here differentiated from product residuals which remain in contact with a surface (i.e. packaging or processing line surfaces) after use: this material usually has the same composition and structure as the bulk product and remains in contact with the surface due to adhesion forces in a layer adjacent to the surface dominating over bulk (cohesive) ones, or because the rheology of the material gives a long timescale for self-drainage or disengagement of the product [1].

Fouling layers are frequently encountered in membrane separation and heat transfer operations in the food sector. This is because the conditions at the surface in such operations is well-known to promote transformations in food materials: in membranes, the concentration of rejected species at the surface will promote the formation of gels or crystallisation, while trapping of larger entities will lead to pore blockage. Heat transfer gives rise to local temperatures which promote phase changes, particularly solidification and protein rearrangement. Fouling arises when these unwelcome examples of structured material adhere and grow on the surface.

Food processing operations are particularly prone to fouling as the materials being processed, such as proteins [2], are excellent structuring agents while the potential to add components to inhibit deposition is limited by the twin criteria of consumer protection and product quality. The scope for hardware modification is often limited to units used to manufacture one, or similar, products. Where a device is used to manufacture several products, the differences in composition can promote very different reaction and adhesion behaviour.

The propensity for fouling in food processing operations means that discussions of fouling should be accompanied by consideration of cleaning [3,4]. In multi-product plants this will include product elimination to avoid cross-contamination. An active research community exists, holding regular international conferences on Fouling and Cleaning in Food Processing. These started in Lund, Sweden, in 1983 and are now held quadrennially (Cambridge, 2014; Lund, 2018).

This article reviews progress in the field, drawing on a survey of articles published in the open literature since 2014. Key concepts are highlighted. Note that little attention is given to the impact of fouling layers on process hygiene. Fouling deposits offer alternative surfaces for microbiological attachment and can shield bacteria and spores from the chemical environment in the bulk fluid. These are essential considerations for the food sector and have been reviewed elsewhere [e.g. [5]].