Rheological properties of chocolate are important in manufacturing process for obtaining high-quality products with well-defined texture and are directly influenced by composition and their refining time. Soy protein benefits from nutritional and functional properties to be used in different foods. Effects of different replacement levels of skim milk powder by soy flour, (from 0 to 100%) and ball mill refining time (105 and 135 minutes) on particle size and rheological properties of compound chocolate were investigated. Considering four rheological models, it was found that compound chocolate behaved as a Casson fluid .Overall, the results indicated that Casson plastic viscosity values ranged between 1.36 and 5.47 (Pa s) and replacing skim milk powder with soy flour led to a significant (P< 0.05) increase in Casson plastic viscosity in either of the refining time treatments. Casson plastic viscosity and apparent viscosity also increased for all the replacement levels with increase in refining time from 105 to 135 minutes. Values of Casson yield ranged from 11.23 to 38.88 (Pa). By replacing soy flour, Casson yield value increased significantly (P<0.05). Casson yield value also increased with increase in refining time in samples containing only skim milk powder, but it decreased in samples containing soy flour.

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