Physicochemical Parameters for Biological and Synthetic Surfactants Used as Detergents

Abstract: Biological and synthetic surfactants were compared in terms of their ability to reduce interfacial tension， change the thermodynamic characteristics of a pre-conditioned surface， and to modify the rheological properties of their respective formulations at two different temperatures. Both classes of surfactants were able to reduce the interfacial tension of their formulations to a similar level. However， the biosurfactants were more effective than the synthetics surfactants. Biosurfactants also altered the surface properties of stainless steel， rendering it hydrophilic. Microbial adhesion to stainless steel conditioned with biosurfactants was found to be thermodynamically unfavorable for all microbial strains tested. A linear relationship between shear stress and shear rate was obtained across a range of experimental conditions for all surfactant mixtures， indicating that all formulations behaved as Newtonian fluids.

Key words: Surfactants， biosurfactants， interfacial tension， microbial adhesion， stainless steel.

1. Introduction

The contamination of surfaces by spoilage and pathogenic microorganisms is a concern in the food industry. The development of biofilms in food processing environments results in product spoilage and possible risks to public health， in addition to create a number of serious problems for industrial fluid processing operations [1-3]. The presence of organisms such as Salmonella spp.， Listeria monocytogenes and Escherichia coli in food processing environments can be a persistent source of contamination [4].

To reduce or eliminate microorganisms found on food contact surfaces， cleaning and disinfection procedures using physical and chemical methods have been extensively used over the years [5]. An interesting strategy is the pre-treatment of surfaces using surface-active compounds， also known as surfactants.

Surfactants can be classified into two main groups: synthetic surfactants and biosurfactants. Synthetic surfactants are produced by organic chemical reactions， whereas biosurfactants are produced by biological processes， being excreted extracellularly by microorganisms such as bacteria， fungi， and yeast [6]. Biosurfactants are produced using relatively cheap hydrophilic and hydrophobic substrates such as carbohydrates， vegetable oils， or even waste from the food industry [7]. When compared to synthetic surfactants， biosurfactants have several advantages， including high biodegradability， low toxicity， low irritancy， and compatibility with human skin [8， 9]. Because of these superior properties， biosurfactants have many potential uses in the food， pharmaceutical， and cosmetic industries [10].

Some of the most extensively studied biosurfactants are the rhamnolipids， primarily produced by Pseudomonas aeruginosa [11]. The surfactant properties of a rhamnolipid mixture depend on its composition. This factor， in turn， varies according to the bacterial strain， culture conditions， and medium composition used for rhamnolipid production [12].

Nitschke et al. [13] investigated the effect of the biosurfactant surfactin on the adhesion of the food pathogens Listeria monocytogenes， Enterobacter sakazakii and Salmonella Enteritidis to stainless steel and polypropylene surfaces. The pre-conditioning of stainless steel surfaces with surfactin caused a reduction in the number of adhered cells of Enterobacter sakazakii and Listeria monocytogenes. The most significant result was obtained with L. monocytogenes， where the number of adhered cells was reduced by 102 CFU/cm2. On polypropylene surfaces， surfactin significantly decreased the adhesion of all strains.

In the literature， there are no references that describe the thermodynamic properties of surfaces that have been pre-conditioned with surfactants. This work aimed to compare biological and synthetic surfactants in terms of their ability to reduce interfacial tension， change the hydrophobicity of pre-conditioned surface and alter the rheological properties of their respective formulations at two different temperatures.

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