Who yellowed my sugar?

Over the years, numerous studies have looked at the many reasons why sugar changes colour during storage. 

Everything from moisture levels, syrup pH and sulfite content to the Maillard reaction (a chemical reaction that creates browning) and high warehouse temperatures have been blamed for the darkening or yellow of sugar – which, in turn, leads to higher ICUMSA scores (the grading scale is a type of yellowness index from the International Commission for Uniform Methods of Sugar Analysis). Higher ICUMSA scores for refined sugar means a lower value product. 

A recent study, however, has revealed that the temperature at which sugar is packaged is one of the leading and most underestimated influences on a sugar’s final colour.  

Researchers from the José Simeón Cañas Central American University in El Savador have found a direct relationship between the packaging temperature and the darkening of in sugar during storage.  

"It has been observed that, during storage, the temperature profile does not drop significantly if the sugar has been packaged at an elevated temperature,” reported study authors Oscar Alfaro, Ricardo Garcia Cárcamo and José Carlos Arrué.  

They also examined correlations with other parameters in more than 50 experiments, and from this, packaging temperature was identified as the main parameter that needs to be controlled. 

"As most colour generation reactions are temperature-catalyzed, this parameter has been shown to be directly proportional to the packaging temperature, and to effectively minimize this increase it is recommended to reduce the packaging temperature below 35°C. 

"Thus, cooling the sugar before it is packaged is one of the main measures that can be taken to avoid increasing the color in storage" 

The researchers found the ICUMSA color can even double in value over time in extreme cases, as we normally see in amorphous sugar refineries, due to the smaller crystal size and higher operating temperature of the dryers. 

If sugar is not being cooled below 35°C before being packaged, the risk of declassification over time increases exponentially. This can lead to a lower market value and an increase in manufacturing costs due to the supply of coloured sugar that is well below that originally specified. 

Advances in sugar cooling technology 

Direct-contact heat exchangers such as rotary drums and fluidized bed coolers have traditionally been used to dry and cool sugar prior to storage. Sugar drying removes excess moisture from the sugar using heat, while cooling the sugar brings it to an ideal temperature for storage and transport. 

Over the past 30 years, Solex Thermal Science has perfected the application of indirect heat transfer technology to and provides refineries with a more efficient way to produce sugars at optimal and uniform temperature prior to packaging. 

The patented technology utilizes vertically oriented plates with internal channels to provide a large heat transfer area within a compact unit. The sugar crystals flow by gravity between the plates, while a cooling fluid (e.g., water) flows counter-current to the sugar crystals through the plate’s internal channels. This eliminates any contact between the sugar and the cooling fluid, providing a high-quality product while providing efficient and uniform cooling. 

Solex uses a proprietary modeling software called ThermaPro® to accurately predict how long the sugar crystals need to stay in the cooler to reach the desired cooling temperature. It does this by performing detailed calculations based on material thermal properties and process requirements including the sugar’s bulk density, specific heat, thermal conductivity, input temperature and flow rate. 

Solex’s technology has been featured in more than 35 different facilities around the world, including two dozen sugar beet installations in Europe, Russia and the U.S., nearly a dozen sugar cane plants in Mexico and South America, along with refineries and sweetener locations in Europe and Asia-Pacific. 

Learn more about how Solex’s applies indirect heat transfer technology to drying and cooling sugar. 

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About the author: Caio Raineri is an Operations Consultant with Solex Thermal Science.