Optimization study of sulfur dioxide application in processing of sultana raisins

The current use of sulfur dioxide (SO2) fumigation to bleach Sultana raisins has a number of limitations including poor absorption into the product and air pollution problems. A study was conducted to improve the sulfitation process by use of SO2 solutions as an alternative. A factorial design was used to evaluate the effect of concentration, temperature and raisin/solution ratio on the absorption of SO2 by raisins. It was found that the most important variables were concentration and temperature of the used solutions. The amount of SO2 absorbed by the raisins was time dependent, but the absorption rate declined after the first 15 min of contact. The use of SO2 solutions was found to be more efficient than fumigation with raisins retaining from 9-21% of applied SO2 in solution compared to 3-5% of SO2 applied as a gas. Higher final SO2 tissue concentrations could be achieved in the liquid application method compared to the gas method. There were no negative consequences of the proposed solution-based system with respect to post-treatment drying time and the storability of the raisins.