A back extrusion cell, with cylindrical plunger and 1.9-mm annulus, was built and a simple method was developed to determine the flow properties of mustard slurry, the mixture of ingredients prior to final stone milling. The first peak force from the back extrusion test was measured for nine rollermill treatment combinations of 0.3, 0.5 and 0.7-mm roller gaps and 5, 20 and 35 degrees C mustard seed temperatures. The back extrusion force was sensitive to only gross differences among the rollermill treatments, and was lowest for mustard seed cracked by a 0.3-mm gap rollermill. Lower temperature treatments produced insignificantly lower back extrusion forces.