Introduction
The unique chocolate matrix is a mixture of sugar and cocoa particles dispersed in a cocoa butter phase, yet its specific packing structure and particle interactions make chocolate an even more intriguing and complex substance. Chocolate texture is a combination of triglyceride packing structures (polymorphs), microstructural properties, dispersed particulates, particle size distribution,and solid fat content (SFC), the ratio of solid to liquid fat in a product. Chocolate has a shelf life of approximately 12 to 24 mo; as chocolate is stored, structural changes occur (Bomba 1993; Subramanian 2000). With improper storage, these changes can be magnified, causing an increase in particle size, which is extremely
important to mouthfeel (Morgan 1994). Minimal particle sizes detected by the human tongue are 20 to 30 μm; control of particle size is essential for smooth mouthfeel, uniform melting, and proper volatile release (Rostagno 1969; Hoskin 1994). Various storage conditions may lead to development of either fat bloom or sugar bloom, both of which compromise visual and textural quality. Bloom is the main cause of quality loss in the chocolate industry (Ziegleder 1997). With total chocolate sales nearly $15 billion annually in the United States, loss due to bloom formation may be substantial (Information Resources Inc. 2006). Market loss due to fat bloom is difficult to verify, since these changes arise
many months after processing and often occur many steps down the distribution ladder. As bloom forms, particle size may also increase, but it is unclear whether microstructural and perceptual changes also occur. Very few studies have been conducted relating instrumental texture measurements to trained sensory panel results; also, data correlating sensory results with instrumental texture and flavor properties in stored chocolate are nonexistent.The purpose of this study was to relate microstructural characteristics of stored dark chocolate to instrumental and sensory texture measurements. Sensory texture and flavor were determined by a trained panel and related to instrumental texture analysis. As
chocolate blooms, color is significantly altered; therefore, color was also measured instrumentally throughout the study. Polymorphic transition from form V to VI may significantly impact the texture and flavor of chocolate and thus, was assessed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Another more recently adapted instrumental technique that may give further insights to microstructural changes is atomic force microscopy (AFM). AFM is a technique used to illustrate nanoscale changes in chocolate microstructure during storage. Surface topography is visualized by measuring the force between the tip and the sample surface as detected by a deflection of the cantilever. A photo detector measures the deflection and a map of the surface topography is assembled. Triglyceride concentrations may be impacted by lipid polymorphic transitions or fat-bloom formation; therefore, concentrations of the major
triglycerides were quantified throughout storage.