Characterization of operating parameters for a mini extruder used for processing of soybeans
Abstract
Mini-extrusion technology is particularly suited for use in developing countries or remote areas, where traditional large-scale processes are infeasible due to either cost or practicality. As a high-temperature, short-time process, extrusion can yield a nutritious soy flour product with high efficiency and control. Heat treatment of de-hulled whole soy chips via a prototype mini-extruder with a throughput of 60 lb/hr was performed, in order to inactivate antinutritional factors naturally present in soy. The level of heat treatment in soy protein products is typically determined by assay of the activity of an indicator enzyme, urease. Previous studies have shown a strong correlation between urease inactivation and the inactivation of other antinutrional factors by steaming. However, the relationship between the thermal inactivation of Trypsin Inhibitors, a key antinutritional factor in soy, and urease, a thermal inactivation curve was determined for extrusion cooking, showing the relationship between extruder die temperature and deactivation. Results show that there is a difference between Trypsin Inhibitor activity and urease activity at extrusion temperature of 130 165 °C. It was found that a die temperature of 160 °C was necessary to achieve 90% inactivation of Trypsin Inhibitor activity, a common standard for full-fat soy flours. In addition, operating data from the prototype mini-extruder was collected. It was found that adjustment of the soybean feed moisture content had a linear effect on die temperature in the extruder, due to increased friction and shear in the extruder. Oil removal using a perforated die was shown to increase at higher temperatures, with a maximum removal of 58% at 165 °C. A study of rheological characteristics of soy within the extruder was also performed. The upper operating limit of apparent viscosity in the extruder was approximately 3900 Pa*s. By this observation, a method for achieving operating temperatures above 145 °C was developed, wherein the extruder is preheated with soybeans of 8-9% moisture content. While running, the feed material is transitioned to soybeans of 6-7% moisture content, normally too viscous to cold-start in an extruder. The maximum stable operating temperature using this method was 165 °C. Shear rate within the extruder was shown to be heavily dependent on soy moisture and oil contents.
Degree
M.S.E.
Advisors
Okos, Purdue University.
Subject Area
Food Science|Agricultural engineering
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