Crystalline stability of self-assembled fibrillar networks of 12-hydroxystearic acid in edible oils Michael A. Rogers a , Amanda J. Wright b , Alejandro G. Marangoni a, * a Department of Food Science, University of Guelph, Guelph, ON, Canada N1G2W1 b Department of Human Health and Nutritional Science, University of Guelph, Guelph, ON, Canada N1G2W1 article info Article history: Received 23 May 2008 Accepted 15 July 2008 Available online xxxx Keywords: Hydroxystearic acid Organogels Time Temperature Stability Oil mobility abstract The stability of organogels differs based on the organogelator concentration, storage temperature, and solvent type. Organogel post-crystallization annealing was monitored at 5 °C, 15 °C, 20 °C or 30 °C for up to 1 month. Gels, stored at 5 °C, had highly immobilized oil, as judged from large T 2 relaxation peaks at 50–70 ms determined by pulsed nuclear magnetic resonance (pNMR) and visual observations. When the gels were stored at 30 °C, the 50–70 ms T 2 relaxation peak shifted to longer relaxation times, indicat- ing that the oil was more mobile than at 5 °C. Along with the increase in syneresis at 30 °C, the 12HSA network’s crystallinity was also greater, having fewer inclusions of liquid oil, as determined by pNMR. The highly branched network observed at 5 °C changed more in time with regards to crystallinity although it entrained the oil to a much greater extent than the gels stored at 30 °C. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Interest in ‘‘self-assembled fibrillar networks” (SAFINs) extends beyond food and pharmaceutical research. Many SAFINs are pres- ent in nature and include materials such as fibrous actin (Greer, 2002), clathrin (Kirchhausen, 2000), tubulin (Oakley & Akkari, 1999), keratin (Fuchs, 1995), insulin (Waugh, 1946), collagen (Car- ia, Bixio, Kostyuk, & Ruggiero, 2004), silk (Jin & Kaplan, 2003), and amyloid fibrils which are found in Alzheimer’s and other neurode- generative diseases (Lui, Prausnitz, & Blanch, 2004). Organogels are a subclass of SAFINs which immobilize organic liquids, while hydrogels immobilize polar liquids. Interest in these compounds originated in the petrochemical industry to immobi- lize flammable solvents and to aid in the clean up of oil spills (Abdallah & Weiss, 2000; Bhattacharya & Krishnan-Ghosh, 2001). Recently, there has been great interest in applying this technology to food systems in order to structure edible oils without the need for high levels of saturated or trans fatty acids (Pernetti, van Mals- sen, Floter, & Bot, 2007). At this time, however, there are no known industrial applications of organogels in the food industry due to a series of issues. These include the lack of low-cost and effective SA- FINs which are food grade, as well as a lack of understanding of the physics of the systems, which is critical to the control of the gelation process. This gelation process defines gel structure and thus the appearance, structure and stability of the material. Many compounds and mixtures of specific compounds can in- duce the gelation of edible oils (Pernetti et al., 2007); however, the focus of this work is on anhydrous crystalline SAFINs that form translucent fibrillar networks at concentrations below 1% (w/w). The most prominent examples in this category include 12-hydroxy- stearic acid (Elliger, Guadagni, & Dunlap, 1972; Rogers, Wright, & Marangoni, 2008a, 2008b), phytosterol–oryzanol mixtures (Bot & Agterof, 2006), and ricinelaidic acid (Wright & Marangoni, 2006). Due to their crystalline nature, these organogels display birefrin- gence under polarized light as well as X-ray diffraction patterns. Syneresis of oil from the organogel structure is one of the most important factors that limit the use of these gels in food applica- tions. Thus, it is important to better understand the structural fac- tors which influence syneresis in an attempt to minimize it. Syneresis may be due to the two-stage nature of the 12-hydroxy- stearic acid (12HSA) gelation process (Terech, Pasquier, Bordas, & Rossat, 2000). An initial rapid increase in opacity is followed by a slight decrease in opacity over time. The decrease in opacity has been proposed to be related to fiber agglomeration into bundles resulting in larger aggregates, as well as larger pores. Fiber agglom- eration may affect both gel strength and solvent-holding capacity. Many studies on hydrogels have shown that gel strength and water-holding capacity are directly related to the microstructure of the gel (Mao, Tang, & Swanson, 2001). In agar gels, for example, a lower gel strength is correlated with more severe syneresis 0963-9969/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.07.012 * Corresponding author. Tel.: +519 824 4120x54340; fax: +519 824 6631. E-mail address: amarango@uoguelph.ca (A.G. Marangoni). Food Research International xxx (2008) xxx–xxx Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres ARTICLE IN PRESS Please cite this article in press as: Rogers, M., A., et al. Crystalline stability of self-assembled fibrillar networks of 12-hydroxystearic ... Food Research International (2008), doi:10.1016/j.foodres.2008.07.012