Sulphur removal from juice

For many wineries and grape juice processors the grape harvest exceeds the immediately available fermentation or evaporation capacity. The temporary excess of single strength juice needs to be stored and this is handled in one of the following ways:

• Chilled storage of single strength juice
• Ambient storage of single strength sulphited juice

Chilled storage for prolonged periods is extremely expensive while conventional techniques for desulphiting can result in unnecessary thermal damage. Flavourtech’s Spinning Cone Column (SCC) technology offers the economic benefits of a low-energy desulphiting solution, while eliminating undesirable thermal damage of the juice.

Chemistry of Desulphiting

After sulphiting, sulphur dioxide is present in the grape juice in three forms:

• Molecular sulphur dioxide
• Free sulphur dioxide
• Combined sulphur dioxide

The sum of these forms is known as the total sulphur dioxide content of the juice. To be removed by a desulphiting plant the sulphur dioxide must be converted to the molecular form, SO2.

The SCC for Desulphiting

The diagram below illustrates the various stages in the Flavourtech desulphiting process, and includes the option to recover grape aroma normally lost in the traditional sulphiting/desulphiting process.

The process begins with the pressing of freshly harvested grapes. At this early stage it is possible to recover the aroma from the juice by processing the juice through the SCC. If the aroma is not recovered it will be lost during the subsequent desulphiting step. Recovery of the aroma is particularly desirable for grape varieties, such as Muscat, where the distinct varietal aroma is an important feature of the grape concentrate and finished wine. If aroma is recovered, it is stored in a concentrated form, under conditions which allow for preservation of the delicate aromatic character.

The next step in the process is the addition of sulphur dioxide to the grape juice. Typically the total sulphur dioxide content is raised to between 1,000 and 2,000 ppm. Once the sulphur addition has been made, storage of the sulphited juice at ambient temperature often lasts several months. In some cases the sulphited juice may be stored over several years without microbial spoilage.

When the juice is required for use it is desulphited to the desired residual concentration by the SCC. If the juice is to be fermented the total sulphur is typically reduced to a concentration between 50 and 80 ppm. If the juice is for soft drink production the concentration may be reduced to less than 10 ppm.

Benefits of the SCC

Key benefits of the SCC for desulphiting include:

• Reduced steam usage compared with other column types, such as packed, bubble cap and tray columns, due to the SCC’s higher mass transfer efficiency.
  
• Minimal thermal damage due to narrow residence time distribution and low average residence time (about 20 to 30 seconds) in the SCC.
  
• Aroma recovery from juice prior to sulphiting is possible.
  
• Easy to control process.
  
• Flexibility - SCC can be used in a number of other winery applications.
  
• Hygienic design with Clean In Place (CIP).

Click here to find out more about Flavourtech’s Spinning Cone Column technology.