AN RTD DETERMINATION METHOD FOR EXTRUSION COOKING

Effects of Operating Conditions on the Cellular Structure and the Thermal Properties of Extruded Foams Made From Corn

The characteristics of a corn extrudate generally appear similar to commercial plastic foam cushioning materials. If extruded foams from whole yellow dent corn are used for loose fill cushioning material, low cost will be realized. In addition, this extruded foam is biodegradable and might even be recoverable as animal feed or as an ethanol feedstock. It could also be spread on farm fields as a soil amendment. Extrusion technology has been applied in cornstarch processing for many years. Different categories of extruders have been developed to give various advantages and disadvantages as used for production. The low-moisture (low-cost) extruder cooker is a good choice for producing low cost expanded corn for packaging use. The objective of this study is to focus on the feed compositions and screw configurations affecting the cellular structure and the specific heat of extruded foams and to evaluate corn/ PVA extrudates as packaging material.

Experimental results indicated that:

1. The size of air cells became smaller and more uniform, and the air cell wall became thinner with the increase in screw pitch and screw speed. Moreover, using the food grade corn grits also made the size of cells smaller and more uniform.

2. Treatments 5 and 22 had the highest specific heats. For all treatments, the specific heat increased linearly as the temperature increased.

3. The extrudates from Treatments 5 and 22 have potential applications as thermal insulation applications for their high specific heat values.

Effects of Die Shapes and Additives on the Physical and Mechanical Properties, and Cellular Structure of Biodegradable Cushioning Extruded Foams

Efforts are focused on the effects of operating parameters, i.e., die shapes (DS), PVA ratio (PR), and additives (AT), on the physical and mechanical properties of corn-PVA extrudates in this study. The cellular structure of biodegradable extruded foams are studied and the extrudates are evaluated as cushioning materials. The process variables are the operating conditions, DS (O, ∞, Δ, and 0), PR (40, 50, 60%), and AT (8%) (sodium bicarbonate (NaHCO3), calcium carbonate (CaCO3), calcium hydroxide (Ca(OH)2)). The experimental setup is a 4 × 3 × 3 factorial design. Samples of each treatment are collected and the physical and mechanical properties are measured and analyzed using the PROC ANOVA and PROC CORR of the SAS 6.0 software package. Moreover, the cellular structures of the extrudates are observed by using a scanning electron microscope. The experimental results indicate that: (a) the change of DS has a significant effect on the longitudinal expansion, bulk density, and compressibility of the extrudates; (b) the change of PR has a significant effect on the longitudinal expansion, bulk density, and compressibility of the extrudates; (c) the change of AT has a significant effect on the longitudinal expansion, bulk density, and compressibility of the extrudates; (d) the most uniform cell is obtained with the O-shaped die, as compared with other die shapes. A moderate cell wall is obtained with the O-shaped die. The largest cell sizes are found in PVA 60% with CaCO3. However, the smallest cell sizes are found in PVA 60% with NaHCO3. Changing the types of additives from NaHCO3 to CaCO3, and CaCO3 to Ca(OH)2 increases the cell size and cell wall thickness.

Effects of combined shear and thermal forces on destruction of Microbacterium lacticum

A twin-screw extruder and a rotational rheometer were used to generate shear forces in concentrated gelatin inoculated with a heat-resistant isolate of a vegetative bacterial species, Microbacterium lacticum. Shear forces in the extruder were mainly controlled by varying the water feed rate. The water content of the extrudates changed between 19 and 45% (wet weight basis). Higher shear forces generated at low water contents and the calculated die wall shear stress correlated strongly with bacterial destruction. No surviving microorganisms could be detected at the highest wall shear stress of 409 kPa, giving log reduction of 5.3 (minimum detection level, 2 x 10(4) CFU/sample). The mean residence time of the microorganism in the extruder was 49 to 58 s, and the maximum temperature measured in the end of the die was 73 degrees C. The D(75 degrees C) of the microorganism in gelatin at 65% water content was 20 min. It is concluded that the physical forces generated in the reverse screw element and the extruder die rather than heat played a major part in cell destruction. In a rotational rheometer, after shearing of a mix of microorganisms with gelatin at 65% (wt/wt) moisture content for 4 min at a shear stress of 2.8 kPa and a temperature of 75 degrees C, the number of surviving microorganisms in the sheared sample was 5.2 x 10(6) CFU/g of sample compared with 1.4 x 10(8) CFU/g of sample in the nonsheared control. The relative effectiveness of physical forces in the killing of bacteria and destruction of starch granules is discussed.