Thickens sauces and gravies and is used for baking.
A Brookfield Powder Flow Tester, equipped with Powder Flow Pro software for automated instrument control and data acquisition, was used to test this name brand corn starch. The corn starch was scooped into the trough, and the scraping tool was then used to evenly distribute the powder throughout the trough. After recording the sample weight and entering it into the software, a standard flow function test and then a wall friction test were run. Time required for each test was 35 minutes and 20 minutes respectively.
Figure 1: Corn Starch Flow Function Graph
Figure 1 shows the flowability of the corn starch at different levels of consolidating stress. These results show that the corn starch is generally cohesive at low levels of consolidating stress from about 1.4 kPa to about 6 kPa. At levels of consolidating stress higher than 6 kPa, the corn starch then lies in the easy flowing range. On the other hand, at very low consolidating stresses below 1.4 kPa, the starch falls into the very cohesive range.
Note: The Flow Function data is indicated by the red line. The other lines are references (or "Standard Flow Indices"), which distinguish the different types of flow behavior, ranging from "non-flowing" to "free flowing".
Figure 2: Corn Starch Wall Friction Graph
Figure 2 represents the angles of wall friction at different levels of normal stress. Angles of wall friction represent the friction between the sliding powder and the wall of the hopper or chute at the onset of flow. In this test a stainless steel lid was used, illustrating what the friction would be like if the corn starch was in a stainless steel hopper. At a low normal stress of about .5 kPa, the effective angle of wall friction is about 35º and goes down to about 29º at higher levels of normal stress (4.75 kPa).
Figure 3: Corn Starch Bulk Density Graph
Figure 3 shows the bulk density of the material at different levels of consolidating stress. This graph tells us that the brownie mix has a fill density of about 475 kg/m³ and rises to about 660 kg/m³ at around 4.75 kPa of consolidating stress. In general, a free flowing powder will show very small changes in bulk density, while a cohesive or poor flowing powder will generally show a large increase in bulk density. This brownie mix shows a large increase in bulk density which is another indicator that this powder is very cohesive.
The corn starch is a very cohesive powder at low consolidation stress levels (below 1 kPa), cohesive at consolidation stress levels from 1 kPa to 6 kPa, and easy flowing at high consolidation stress levels. This means that the corn starch may have flowability issues as the hopper empties. Possible problems include arching (when the powder forms a cohesive bridge over the outlet) and ratholing (when the powder flows out only from the center leaving the rest of the material static against the walls). The critical arching dimension, of 2.5 inches (63.1 mm) provides a conservative estimate to prevent arching from happening, provided the minimum outlet dimension of the hopper exceeds this value. The critical ratholing dimension is dependent on the diameter of the bin. The rathole diameter can be automatically calculated by Powder Flow Pro once the bin diameter is entered.