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.