A review of nonlinear oscillatory shear tests: Analysis and application of large amplitude oscillatory shear (LAOS)

Structuring liquid oils has become an active area of research in the past decade, mainly due to pressures to reduce saturated fat intake and eliminate trans fats from our diets. However, replacing hard fats with liquid oil can lead to major changes in the quality of food products. Recent strategies to impart solid-fat functionality to liquid oils include the addition of unusual compounds to oil, leading to its gelation. These include small-molecule organogelators such as phytosterols and 12-hydroxystearic acid, which self-assemble into crystalline fibers which trap oil. Other crystalline additives include waxes, ceramides, monoacylglycerides, and other surfactants. Recently, the polymer ethyl cellulose was reported to form a polymer gel in triacylglyceride (TAG) oils. Other non-conventional strategies also include the formation of protein-stabilized cellular solids with oil trapped within the cells. In this review, we summarize the research on each one of these components in order to provide a comprehensive overview of the state of the area in oleogel research and provide future perspectives.

Organogels: An Alternative Edible Oil-Structuring Method

The structure of many food products is based on networks of crystalline triacylglycerol (TAG) hardstock. These contain high levels of saturated fatty acids, which are among the factors contributing to cardiovascular diseases. Therefore it would be desirable to have alternative routes to structure edible oils. This review starts with general structuring concepts and building blocks which can be found in organic solvents. Subsequently it focuses on the field of edible oil structuring. Among the alternative structuring agents that have received attention in recent years are mono- and diglycerides, fatty acids, fatty alcohols, waxes, wax esters, sorbitan esters, and phytosterols. It is found that the most efficient structuring agents form small, preferably non-spherical particles. Mixing of components often provides superior structuring over use of pure components. Straightforward screening of pure components might be feasible. However, the number of options provided by mixing requires guided screening, for which understanding of the underlying mechanisms is essential. Analytical methods helping to characterise oil structuring and elucidate the structuring mechanisms are discussed.

Structuring of edible oils by alternatives to crystalline fat

The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude.

Dynamic properties of shear thickening colloidal suspensions

The “no-slip” is a fundamental assumption and generally-accepted boundary condition in rheology, tribology, and fluid mechanics with strong experimental support. The violations of this condition, however, are widely recognized in many situations, especially in the flow of non-Newtonian fluids. Wall slip could lead to large errors and flow instabilities, such as sharkskin formation and spurt flow, and hence complicates the analysis of fluid systems and introduces serious practical difficulties. In this article, we discuss slip at fluid-solid interface in an attempt to highlight the main issues related to this diverse complex phenomenon and its implications.

Slip at Fluid-Solid Interface

The rheological properties of two types of commercial peanut butter have been studied. Both products are concentrated suspensions, and differ by the presence of additives. The first type, referred to as “100% peanuts,” is an unstabilized suspension consisting of solid peanut particles in peanut oil which is a Newtonian fluid. The second type, referred to as “smooth,” consists of the same suspension stabilized with a vegetable oil and contains other ingredients such as salt and sugar in very small quantities. A mean volume particle diameter of 6.6 μm has been determined, the particle diameter distribution was found to be narrow, and the solids volume fraction was estimated to be 0.6. Slip encountered in rheometry was greatly reduced by gluing sandpaper to the parallel plates of the rheometer. Both samples behaved like plastic materials and apparent yield stresses of 24 Pa and 370 Pa have been determined for the unstabilized and the stabilized suspensions, respectively. No linear domain was found for both suspensions and the non-linearity was confirmed by deformed Lissajous curves and higher odd harmonics in the output signal of small amplitude oscillatory shear experiments. The stabilized suspension behaved more like a solid, the elastic modulus being larger than the loss modulus and almost independent of the frequency. This solid-like behavior is supposedly caused by strong repulsive (steric) forces induced by the stabilizing agent.

Guillaume P. Citerne

Papers

Rheological properties of peanut butter