Andrew J. Gravelle

Influence of solvent quality on the mechanical strength of ethylcellulose oleogels

Ethylcellulose (EC) is the only known food-grade polymer able to structure edible oils. The gelation process and gel properties are similar to those of polymer hydrogels, the main difference being the nature of the solvent. The present study examines the influence of solvent quality on the large deformation mechanical behavior of EC oleogels. Two alternative strategies for manipulating the mechanical response of these gels were evaluated; manipulating the bulk solvent polarity and the addition of surface active small molecules. Gel strength was positively correlated to solvent polarity when blending soybean oil with either mineral oil or castor oil. This behavior was attributed to the ability of the polar entities present in the oil phase to interact with the EC gel network. The addition of the small molecules oleic acid and oleyl alcohol resulted in a substantial enhancement in gel strength up to 10wt% addition, followed by a gradual decrease with increasing proportions. Binding interactions between EC and these molecules were successfully modeled using a Langmuir adsorption isotherm below 10wt% addition. Furthermore, the thermal behavior of stearic acid and stearyl alcohol also indicated a direct interaction between these molecules and the EC network. Differences in the mechanical behavior of gels prepared using refined, bleached, and deodorized canola or soybean oils, and those made with cold-pressed flaxseed oil could be attributed to both oil polarity, and the presence of minor components (free fatty acids). Shorter pulsed NMR T2 relaxation times were observed for stronger gels due to the more restricted mobility of the solvent when interacting with the polymer. This work has demonstrated the strong influence of the solvent composition on the mechanical properties of EC oleogels, which will allow for the tailoring of mechanical properties for various applications.

Revisiting the crystallization behavior of stearyl alcohol : stearic acid (SO : SA) mixtures in edible oil

Mixtures of stearyl alcohol and stearic acid were some of the first identified oleogelators with potential for food applications. Previously, a synergistic enhancement in gel strength was identified at a stearyl alcohol:stearic acid (SO:SA) ratio of 7:3 and 8:2, which was attributed to their needle-like crystal morphology. In the present study, we have meticulously characterized this system with a variety of techniques at different gelator ratios. Accelerated oil loss tests showed the stability of the gels mirrors the mechanical strength with ∼1 wt% oil loss in the firm gels and >10 wt% in the weak formulations. X-ray diffraction and light microscopy suggest that the crystal networks which form the hardest gels (8:2, 7:3) and weakest gels (5:5, 4:6) are similar, and thus crystal morphology and crystal size cannot solely explain the observed enhancement in mechanical strength and stability. Scanning electron micrographs clarified that all oleogels crystalized in a platelet-like, rather than needle-like microstructure. Using the scaling theory of cellular solids, the enhancement in mechanical strength of the 8:2 SO:SA oleogel was shown to be mainly due to an increase in the scaling exponent of the hardness to the mass fraction of the crystalline material, and not the total amount of crystalline network solids, the size of the platelets or the interactions between them.

Influence of particle size and interfacial interactions on the physical and mechanical properties of particle-filled myofibrillar protein gels

The physical and mechanical properties of particle-filled composite gels are influenced by a variety of factors which are often system-specific. Here, we report on the effect of solid fillers of varying sizes and surface properties in a model gel system; heat-set comminuted meat protein gels. Hydrophobic rice bran wax particles and hydrophilic glass beads were selected for their contrasting surface chemistry, which influenced the particle/gel interfacial interactions. All the composites were found to be stable up to 0.5 volume fraction filler, based on post-gelation liquid loss, light microscopy, and cryo-SEM analyses. The influence of the dispersed particles on the large deformation mechanical properties of the composites were evaluated based on particle type, size, and volume fraction of the filler. The behavior of the Youngs modulus was compared to that predicted by particle reinforcement theories proposed by van der Poel and Kerner, each with subsequent extensions. Both filler type and size were found to influence the Youngs modulus and stress at 50% strain. The recoverable energy and post-compression height recovery were found to be predominantly influenced by the filler volume fraction, and were less influenced by particle/gel interactions. Interestingly, filler type and size range were found to have no effect on the cohesiveness of the composites, as this parameter was found to be solely dependent on the volume fraction of the extensible continuous phase. The influence of the filler on the optical properties of the composites was evaluated by reflectance spectroscopy in the visible range, and interpreted based on the effect of the filler optical properties. The results from this study indicate that filler size, surface chemistry, and incorporation level can strongly influence the macroscopic physical characteristics of heat-induced comminuted myofibrillar protein composite gel systems.

Interresidue carbonyl–carbonyl polarization transfer experiments in uniformly 13C,15N-labeled peptides and proteins

In this work, we demonstrate that Homonuclear Rotary Resonance Recoupling (HORROR) can be used to reintroduce carbonyl-carbonyl interresidue dipolar interactions and to achieve efficient polarization transfer between carbonyl atoms in uniformly (13)C,(15)N-labeled peptides and proteins. We show that the HORROR condition is anisotropically broadened and overall shifted to higher radio frequency intensities because of the CSA effects. These effects are analyzed theoretically using Average Hamiltonian Theory. At spinning frequencies used in this study, 22kHz, this broadening is experimentally found to be on the order of a kilohertz at a proton field of 600MHz. To match HORROR condition over all powder orientations, variable amplitude radio frequency (RF) fields are required, and efficient direct transfers on the order of 20-30% can be straightforwardly established. Two- and three-dimensional chemical shift correlation experiments establishing long-range interresidue connectivities (e.g., (N[i]-CO[i-2])) are demonstrated on the model peptide N-acetyl-valine-leucine, and on the third immunoglobulin binding domain of protein G. Possible future developments are discussed.

Ethylcellulose Oleogels: Structure, Functionality, and Food Applications

The structuring edible oils by nontraditional means has become a popular strategy for improving the lipid profile of food products while retaining the functionality of a crystalline triglyceride network. Although numerous oleogelator systems have now been identified, the polymer gelator ethylcellulose (EC) may present the greatest potential for applications in a diverse range of food systems which require unique physical attributes and structuring properties in the fat phase. The first portion of this chapter will provide a brief overview of oleogelation strategies, outline the basic physical characteristics of the polymer EC, and describe the mechanism of gelation and some basic physical characteristics of EC oleogels. The subsequent sections will highlight different strategies which have been identified to manipulate the rheological and mechanical properties of these gels, including the addition of food-grade surfactants and other amphiphilic molecules, modulating bulk solvent polarity, and through the formation of EC/hybrid gelator systems. The final section will highlight various applications in food systems reported in the literature, outline recent work investigating the effect of structuring edible oils with EC on digestibility, and the potential applicability of these oleogels as a delivery vehicle for lipid-soluble molecules. The potential applications for EC oleogels in complex food systems are quite promising, and the strategies for manipulating their physical properties may also extend their applicability into the pharmaceutical, cosmetic, and manufacturing industries.

Food-grade filler particles as an alternative method to modify the texture and stability of myofibrillar gels

A series of food grade particles were characterized for their potential as fillers in myofibrillar gels. The fillers were separated into (i) hydrophilic, insoluble, crystalline particles and (ii) starch granules. The particles used were microcrystalline cellulose, oat fiber and walnut shell flour, as well as potato and tapioca starches. Crystalline particles increased hardness and decreased recovery properties. Although all of these fillers decreased the T2 relaxation time of water, this was dependent on particle type and size. An increase in gel strength was observed with increasing filler content, which was attributed to particle crowding. Native potato starch was the most efficient at increasing liquid retention, while native tapioca was the least effective. Gel strength increased significantly only for the native potato and modified tapioca starches, but no effect on recovery attributes were observed for any of the starch varieties. The potato starches became swollen and hydrated to a similar extent during the protein gelation process, while the native tapioca starch gelatinized at higher temperatures, and the modified tapioca showed little evidence of swelling. T2 relaxometry supported this finding, as the meat batters containing native potato starch displayed two water populations, while the remaining starches displayed only a single population.

Water immobilization by glass microspheres affects biological activity

We recently reported that the water holding capacity of myofibrillar protein hydrogels could be increased upon addition of small amounts of microparticles, particularly glass microspheres. Glass microspheres were found to decrease the spin-spin relaxation time (T2) of water protons in the gels, which was interpreted as enhanced water binding by the glass. We were thus interested in determining whether the observed effects on water proton relaxation were a direct consequence of water-glass interactions. Here we show how glass microspheres reduce the mobility of pure water, reflected in large decreases in the T2 of water protons, decreases in the self-diffusion coefficient of water molecules, a lower water activity, and strengthening of O-H bonds. Even though glass is considered an inert material, glass microspheres were shown to inhibit the growth of human embryonic kidney cells, and stimulate or inhibit the growth of leukemia and monocytic lymphoma cells in vitro, depending on dose and time. The germination of alfalfa seeds and the growth of E.coli cells were also inhibited upon exposure to glass microspheres. This work indicates that the properties and behavior of materials, even ones considered inert, can be affected by their size. These observations suggest possible toxicological consequences of exposure to microparticles, but also open us possibilities to affect cellular/organism function via modulation of macromolecular hydration.