Molecular assembly, interfacial rheology and foaming properties of oligofructose fatty acid esters.

Two major types of food-grade surfactants used to stabilize foams are proteins and low molecular weight (LMW) surfactants. Proteins lower the surface tension of interfaces and tend to unfold and stabilize the interface by the formation of a visco-elastic network, which leads to high surface moduli. In contrast, LMW surfactants lower the surface tension more than proteins, but do not form interfaces with a high modulus. Instead, they stabilize the interface through the Gibbs-Marangoni mechanism that relies on rapid diffusion of surfactants, when surface tension gradients develop as a result of deformations of the interface. A molecule than can lower the surface tension considerably, like a LMW surfactant, but also provide the interface with a high modulus, like a protein, would be an excellent foam stabilizer. In this article we will discuss molecules with those properties: oligofructose fatty acid esters, both in pure and mixed systems. First, we will address the synthesis and structural characterization of the esters. Next, we will address self-assembly and rheological properties of air/water interfaces stabilized by the esters. Subsequently, this paper will deal with mixed systems of mono-esters with either di-esters and lauric acid, or proteins. Then, the foaming functionality of the esters is discussed.

The effect of diesters and lauric acid on rheological properties of air/water interfaces stabilized by oligofructose lauric acid monoesters.

In this study, the rheological properties of interfaces stabilized by oligofructose fatty acid esters were elucidated. First, the properties of interfaces stabilized by monoesters (ME), diesters (DE), lauric acid (LA), oligofructose (OF), and mixtures of ME with DE, LA, or OF were studied. Second, the properties of interfaces stabilized by the crude product (CP) containing ME, DE, LA, and OF were studied. The dependency of the dilatational modulus on frequency and deformation amplitude indicated the possible formation of a soft glass phase for ME, and a viscous interface for DE. When ME and DE were mixed at a ratio of 0.8/0.2, the experimental results suggest that the interfacial structure consists of islands of a glass phase formed by ME, dispersed in a 2D viscous phase of DE. CP stabilized interfaces, where the ratio ME/DE was higher, lead to a different rheological response. The ratio ME/DE plays an important role in the surface properties of the CP. This may have significant consequences for applications in macroscopic systems such as foams.

Novel surface-active oligofructose fatty acid mono-esters by enzymatic esterification.

This article describes the synthesis of a series of oligofructose monoesters with fatty acids of different chain length (C8, C12, C16 and C18) to obtain food-grade surfactants with a range of amphiphilicity. Reactions were performed in a mixture of DMSO/Bu(t)OH (10/90 v/v) at 60°C and catalysed by immobilised Candida antarctica lipase B. MALDI-TOF-MS analysis showed that the crude reaction products were mixtures of unmodified oligofructose and mostly mono-esters. The conversion into mono-esters increased with the length of the fatty acid chain, reflecting the specificity of the lipase towards more lipophilic substrates. Reverse phase solid phase extraction was used to fractionate the products, which lead to sufficient purity (>93%) of the fatty acid esters for functionality testing. It was shown that derivatives of longer (C16 and C18) fatty acids were more efficient in lowering surface tension and gave a much higher dilatational modulus than derivatives of the shorter (C8 and C12) fatty acids.

Non-linear surface dilatational rheology as a tool for understanding microstructures of air/water interfaces stabilized by oligofructose fatty acid esters.

In this paper, the rheological response of air/water interfaces, stabilized by various oligofructose fatty acid esters, to oscillatory dilatational deformations was studied and compared to the response of interfaces stabilized by sucrose esters. We have followed a traditional approach to surface rheology, where the development of the modulus as a function of time is studied as well as the frequency dependence of the modulus. We also adopted a different approach where we investigate in detail the amplitude dependence of the modulus. Finally, we studied the temperature dependence. We show that for an accurate characterization of the dilatational rheology of fluid­fluid interfaces with a complex microstructure, a protocol should be used that not only involves variations of surface pressure, frequency, and temperature, but also establishes amplitude dependence. We show that Lissajous plots of surface pressure versus deformation can be useful tools to help interpret surface dilatational behavior in terms of interfacial microstructure. The rheological response of interfaces stabilized by oligofructose esters differed significantly from the response of those stabilized by sucrose esters. Sucrose esters behaved like typical low molecular weight surfactants, and gave interfaces with relatively low moduli, a frequency scaling of the dilatational modulus with an exponent close to 0.5, and displayed no asymmetries in Lissajous plots. In contrast, the oligofructose esters gave, depending on the fatty acid tail, relatively high moduli, almost independent of frequency. Significant asymmetries were observed in the Lissajous plots, with strain hardening during compression and strain softening during extension. Our results suggest that the unusual rheological properties of interfaces stabilized by oligofructose esters may be the result of the formation of a two-dimensional soft glass phase by the oligofructose part of the ester.