Effect of pore size and process temperature on flux, microbial reduction and fouling mechanisms during sweet whey cross-flow microfiltration by ceramic membranes

Abstract

In this study, operating conditions for the cross flow microfiltration (CFMF) of sweet whey were optimised. Filtrations were performed for 65 min at 20, 40 and 50 °C using ceramic membranes with different nominal pore sizes (0.1, 0.5 and 0.8 μm). Periodically, samples of whey retentate and permeate were taken and analysed for microbiological quality and physical and chemical properties. The best microbial reduction rates were achieved during filtration using a 0.1 μm membrane at 50 °C. The highest flux rates were achieved during filtration at 50 °C with all tested membranes. Fouling intensity was the lowest after filtration using a 0.5 μm membrane at 20 °C. According to all results obtained, the membrane with the nominal pore size of 0.5 μm appeared to be optimal for purposes of preserving the nutritional value, minimising membrane fouling and achieving appropriate microbiological quality of sweet whey.

Introduction

Whey has excellent nutritional value, mostly due to whey proteins. It has been proven that whey proteins, especially α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), are a source of bioactive peptides with several health effects, such as antimicrobial, anti-hypertensive and anticarcinogenic properties (Chatterton, Smithers, Roupas, & Brodkorb, 2006). Additionally, whey proteins have excellent functional properties and are widely used in the food industry (Foegeding, Davis, Doucet, & McGuffey, 2002). Since fresh whey is very susceptible to microbial spoilage, heat treatments like pasteurisation are obligatory. Whey proteins tend to denaturation and precipitate at temperatures above 60 °C (Parris, Purcell, & Ptashkin, 1991), so sediments of proteins and some salts are likely to occur during heat treatment (Jeličić, Božanić, & Tratnik, 2008), causing a reduction in the nutritional and sensory quality of whey. Also, as a result of denaturation, the potential to further modify functional properties of whey proteins is often lost (Kulozik & Kersten, 2002).

The world whey production is over 160 million tons per year, with an annual growth rate of 1–2% (Guimarães, Teixeira, & Domingues, 2010). About 70% of total whey is processed into different products, but about 30% of whey is being utilised for pig feeding or other similar purposes (Jelen, 2003). To increase whey utilisation for human nutrition, alternative processing techniques such as membrane processes, i.e., microfiltration (MF) or ultrafiltration, are needed.

MF is commonly used to remove the microorganisms present, as well as their thermo stable spores, fat globules and other lipid components (e.g., lipoproteins), and casein residues (Goulas & Grandison, 2008). Kaufmann and Kulozik (2006) pointed out that, regardless of the processing parameters, an average microbial reduction of vegetative cells from approximately 3–4 log cycles can be observed during MF of skimmed milk.

The high variability of whey composition, the high water content and the high numbers of lactic acid bacteria make microfiltration of fresh whey slightly more complicated than milk, in terms of membrane fouling. Calcium phosphate salts, as well as whey proteins, naturally tend to precipitate at higher temperatures (e.g., 50 °C), and are considered to be the main contributors to membrane fouling (Goulas and Grandison, 2008, Kelly and Zydney, 1997, Koutsoukos, 2007). In recent years, a lot of efforts have been made to improve MF technology. However, the main disadvantage characterising most of the proposed methods is that they cannot be applied in a large scale. In addition to that, some also negatively affect the quality of the filtered product (Brans, Schroën, van der Sman, & Boom, 2004).

Only a few comprehensive studies have been focused on optimising the efficiency of sweet whey cross flow microfiltration (CFMF) in terms of the microbial reduction, the whey protein recovery, and membrane fouling. Hence, the aim of this study was to investigate CFMF of sweet whey by using ceramic membranes with different nominal pore size in relation to the process temperature. Microbial reduction, flux, membrane fouling parameters, whey protein retention and calcium permeation were monitored. Also, to compare CFMF as a possible alternative for purposes of microbiological stabilisation of whey, microbiological reduction and whey protein recovery have been compared with that obtained by the conventional pasteurisation.