The changes were initially proposed at the end of last year and confirmed last month. The firm's public relations manager Mark Jephcott said: "We are now in a position to confirm that these plans will go ahead."
He said that a new pasteuriser had recently been fitted at the Cumbernauld site last month as part of the investment. "We intend to significantly improve the Cumbernauld site. These improvements will take place over the next couple of years and will help to significantly increase capacity."
At the end of last year the firm also launched a consultation with employees at the Mansfield plant. "Following the launch of this consultation, we can now confirm that we will close this site in early 2011," added Jephcott. The move will affect around 100 jobs.
He added that through continued investment at its third site in Tredegar, Rubicon was now successfully integrated into the business after it was acquired at the end of 2008. "We are planning some exciting new launches for the Rubicon brand within the next few months," he added.