Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2007
Publication Date: 9/27/2007
Citation: Davis, J.P., Sanders, T.H. 2007. Liquid to Semisolid Rheological Transition of Normal and High-Oleic Peanut Oils Upon Cooling to Refrigeration Temperatures. Journal of the American Oil Chemists' Society vol. 84 No. 11 pg 979-987. Interpretive Summary: Peanut oil is generally considered to crystallize at relatively high temperatures, which may or may not be desirable depending on the application. However, with the continuous release of new cultivars with differing fatty acid profiles, it is important to document variation in crystallization behaviors among these oils. Small strain oscillatory rheological analyses were applied to monitor liquid to solid-like transitions in 9 commercially available peanut oils upon cooling to refrigeration temperatures. There was indeed substantial variation in crystallization behaviors among the 9 oils, with high oleic varieties being the last to crystallize upon equivalent cooling treatments. Furthermore, good agreement was observed between rheological analyses and differential scanning calorimetry measurements of crystallization. This work is valuable from a practical perspective for those working with peanut oils, i.e. food technologists, who may or may not want a peanut oil that readily crystallizes. This work also provides fundamental insights into peanut oil crystallization at sub-ambient temperatures.
Technical Abstract: Rheological transitions of peanut oils cooled from 20 to 3ºC at 0.5ºC/min were monitored via small strain oscillatory measurements at 0.1 Hz and 1 Pa. Oils were from 9 different cultivars of peanut, and 3 oils were classified as high-oleic (approximately 80% oleic acid). High-oleic oils maintained an overall liquid-like character at 3ºC for 2 hr. In contrast, several normal (non high-oleic) peanut oils displayed a predominantly elastic (solid-like) response after 2 hr at 3ºC. Increases in viscoelasticity were associated with lipid crystallization events as confirmed by DSC. The higher (p<0.001) liquid viscosities and increased (p<0.001) contents of oleic acid, which has a more non-linear structure as compared to other fatty acids typical in these oils, were hypothesized to hinder crystallization in high-oleic oils. Changes in viscoelasticity at 3°C were greatest for 3 normal oils that had the significantly (p<0.001) highest content of C20:0 and/or C22:0 fatty acids, and these long, saturated hydrocarbon chains are hypothesized to promote crystallization. No peanut oil maintained clarity after 5.5 hr at 0°C (modified cold test used to screen salad oils); however, these data as a whole suggest strategies for breeding and/or processing peanut oils for enhanced resistance to crystallization.