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The influence of co-solutes on tribology of agar fluid gels

I. Fern andez Farr es*, I.T. Norton

Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

a r t i c l e i n f o

Article history:

Received 2 September 2014

Accepted 17 November 2014

Available online 25 November 2014

Keywords:

Lubrication

Tribology

Hydrocolloid

Fluid gel

Agar

Rheology

a b s t r a c t

The effects of glucose and glycerol on the lubrication properties of agar fluid gels have been studied using

soft tribology. A novel approach using the sediment and supernatant of centrifuged fluid gels has allowed

investigation of the distinct contributions of both the gelled particulate phase and the continuous phase

on fluid gel tribology. The friction coefficient of both the particulate phase and fluid gels was significantly

lower than that of the continuous phase across the three lubrication regimes. This indicates that particle

entrainment occurs at all entrainment speeds, enhancing lubrication by prevention of surface contact.

Softer fluid gel particles produced with intermediate levels of glycerol (up to 30%) show increased

friction as would be expected for an increased contact area between the tribological surfaces. At high

levels of glycerol, the friction does not increase. It is proposed that soft particles are produced but the

increasing friction is overcome with the increased lubrication from the more highly viscous continuous

phase. In contrast, the presence of intermediate levels of glucose (up to 30%) increases the friction of the

aqueous continuous phase but does not affect the particle properties. Texture analysis, rheology and light

scattering techniques were used to elucidate the structural changes of the fluid gels induced by the

addition of co-solutes and the influence this has upon lubrication.

(http://creativecommons.org/licenses/by/3.0/).

1. Introduction

Fluid gels are concentrated suspensions of micro-gel particles

dispersed in a continuous medium, typically water, and are produced

by applying a shear field to the hydrocolloid solution while

the solegel transition is taking place (Norton, Frith, & Ablett, 2006;

Norton, Jarvis, & Foster, 1999). Application of fluid gels is of

increasing interest to the food industry because of their ability to

generate film thickness in the flow conditions that prevail during

food processing in the mouth, potentially imparting desirable

mouthfeel and texture attributes (e.g. smoothness) (Malone,

Appelqvist, & Norton, 2003). The lubrication properties of fluid

gels, which have been shown to be greatly influenced by the

physical properties of particles (e.g. size, elasticity) (Gabriele,

Spyropoulos, & Norton, 2010; Mills, Koay, & Norton, 2013), can be

measured using tribology (thin-film rheology).

In this work, we used fluid gels produced from agar which is a

seaweed hydrocolloid composed by mixtures of neutral agarose

and charged agaropectin, in proportions that vary depending on

both the production process and the raw material (Araki, 1956;

Phillips and Williams, 2009). Agarose is the polysaccharide with

the gelling capability of agar and consists in alternating units of 3-

linked b-D-galactopyranose and 4-linked 3, 6-anhydro-a-L-galactopyranose

(Araki, 1956). The gelling temperature of agar varies

depending on the degree of methoxylation of the agarose chain

which is characteristic of the seaweed speces from which the agar

has been extracted (Armisen & Galatas, 2000).

As the gelling temperature of agar is approached, intramolecular

hydrogen bonds start to form between the agarose

chains, followed by the formation of inter-molecular hydrogen

bonds between the two distinct polysaccharide chains (Tako &

Nakamura, 1988). The mechanism for gelation involves a coil to

double helix transition and a subsequent aggregation of these helical

domains to create a three dimensional structure (Arnott et al.,

1974; Morris, 1986; Schafer & Stevens, 1995).

Upon cooling the agar solution under shear below the agar

transition temperature, kinetic competition between the process of

gel network formation and the shear induced break-up of the

network occurs. The result of these two competing mechanisms

determines the physical properties of the resulting fluid gel particles

as well as the extent of inter-particle interactions. The control

over the conformational ordering kinetics is achieved by the

applied cooling rate such that a range of viscoelastic properties can

be produced by manipulation of the processing conditions.

* Corresponding author.

E-mail address: ixf072@bham.ac.uk (I. Fern andez Farr es).

Contents lists available at ScienceDirect

Food Hydrocolloids

journal homepage: www.elsevier.com/locate/foodhyd

http://dx.doi.org/10.1016/j.foodhyd.2014.11.014

Food Hydrocolloids 45 (2015) 186e195

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