||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (November 2011)|
Chocolate bloom is a whitish coating that can appear on the surface of chocolate. This effect is one of the main concerns in the production of chocolate. There are two types of bloom: fat bloom, arising from changes in the fat in the chocolate; and sugar bloom, formed by the action of moisture on the sugar ingredients. The unsightly crystals of fat and sugar bloom limit the shelf life of many chocolates. Chocolate that has "bloomed" is still safe to eat, but may have an unappetizing appearance and surface texture.
Fat bloom 
A great deal has been written about chocolate fat bloom. To date, it is generally accepted that visual fat bloom in chocolate is the cocoa butter that has separated toward the surface. It is caused by:
- Poor (incorrect or incomplete) tempering of the chocolate
- Incorrect cooling methods, including covering cold centers
- The presence of soft fats in the centers of chocolate-covered units
- Warm storage conditions
- The addition to chocolate of fats incompatible with cocoa butter
- Abrasion and finger marking
There is no single universally accepted theory that describes fat bloom in chocolate. Current theories about fat bloom fall into two categories: polymorphic transformation and phase separation theory.
Phase separation theory 
According to this theory, storage bloom is due to phase separation of the triglycerides within the crystalline structure of cocoa butter. This theory is based on the fact that butter is a mixture of diverse triglycerides and each type of triglyceride has a unique melting temperature. At higher temperatures, the lower melting point triglycerides in cocoa butter will melt and this liquid phase occupies a greater volume than the solid. The liquid phase will then be squeezed out to the surface through fissures or pores in the chocolate. After the liquid phase reaches the surface, the triglycerides will recrystallize into the most stable form, with bloom occurring.
Polymorphic transformation 
In cocoa butter, there are six essential polymorphic forms. The polymorphic transformation theory is based on the fact that bloomed chocolates are always found to contain the most stable VI polymorph of cocoa butter. According to this theory, bloom occurs through the uncontrolled polymorphic transformation of cocoa butter from a less stable form (form IV or V) to the most stable form (form VI). The technology of good chocolate production depends on ensuring that only the stable form of the cocoa butter ingredient exists in the final product. In poorly tempered chocolate, form IV will transform to V and eventually form VI, resulting in bloom, while in well-tempered chocolate, form V will transform to form VI and bloom may also occur. Elevated and/or fluctuating temperatures promote the transformation rate and thus also promote the rate of bloom formation. However, it is found that the production of form VI from form V does not always lead to bloom; a direct relationship cannot be established between fat bloom and the evolution of the crystalline forms. The onset of transformation from form V to form VI crystals is not the cause of visual bloom; rather, this transition should be considered as an aspect of cocoa butter recrystallization that may result in bloom.
Methods to minimize fat bloom 
- Maintain an appropriate storage temperature for chocolate products
- Use an appropriate recipe for the fat
- Use a fat–based barrier coating between two domains of differing fat content and composition
- Add a bloom inhibitor
- Add seeding crystals
Sugar bloom 
While the most common is fat bloom, sugar bloom also occurs and is not necessarily distinguishable from the fat bloom by appearance. In freshly sugar bloomed samples, it is often easy to feel the surface difference; sugar bloom feels dry and does not melt to the touch, while fat bloom feels slick and melts. One can often see the difference by touching a small droplet of water to the surface. With fat bloom, the droplet simply beads up. With sugar bloom, the droplet quickly flattens and spreads, as the water dissolves the microscopic sugar particles on the surface (Seguine, 2001). Alternatively, gentle warming of the surface (with a hairdryer, for example) will cause the crystals of fat bloom to melt, removing the appearance of bloom, while leaving sugar bloom unchanged.
Sugar bloom is caused by moisture. Condensation on to the surface of the chocolate or moisture in the chocolate coating causes the sugar to absorb the moisture and dissolve. When the moisture evaporates, the sugar forms larger crystals, leaving a dusty layer. It is caused by:
- Storage of chocolates in damp conditions
- Deposit of "dew" during manufacture from damp cooler air or allowing chocolates to enter a packing room at a temperature below the dew point of that room
- Use of hygroscopic ingredients (low grade or brown sugars)
- High-temperature storage conditions of chocolate-covered confectionary, where the centers have a high relative humidity and the water vapor given off is trapped in the wrappings
Method to minimize sugar bloom 
- Maintain an appropriate storage temperature for chocolate products. A psychrometric chart is a valuable tool in determining the temperature above which food must be maintained in order to avoid condensation.
- "Why does chocolate turn gray sometimes? Is it still safe to eat?". HowStuffWorks, Inc. Retrieved 6 November 2011.
- McCarthy, J.; David, S. and Daijing, Wei (2003). "Fat Bloom in Chocolate". The Manufacturing Confectioner: 89–93.
- Beckett, T. Stephen 2000. The Science of Chocolate, pp 85–103
- Hartel, W. Richard 1999. Chocolate: Fat Bloom During Storage, The Influence of Structural Elements in The Manufacturing Confectioner, 89-93
- Minifie, W. Bernard 1989. Chocolate, Cocoa and Confectionery, pp 646–663