Progress 09/01/03 to 08/31/06
Outputs
A base formulation of corn starch was used with various combinations of material and process variables in a factorial experimental design. Specific mechanical energy (SME) was directly controlled by varying the extruder screw speed (200-400 RPM) and in-barrel moisture content (MC, 23-34% on wet basis). Material formulation was varied by adding 0-15% whey protein concentrate (WPC), 0-18% whey protein isolate (WPI), and 0-6% sucrose. X-ray microtomography (XMT) was used for non-invasive imaging of sample cross-sections at various depths, and facilitated accurate and hitherto impossible measurement of features like true cell size distribution, average cell diameter (0.79 - 6.32 mm), open wall area ratio (0.100 - 0.146), cell wall thickness (0.09 - 0.23 mm), and true void fraction (0.62 - 0.84). Material properties of formulations were measured, including glass transition temperature (Tg, ?? - ??) by DSC, and softening and flow temperatures (Ts, 38-93oC; Tf, 55-135oC) by
phase transition analyzer (PTA). Mechanical properties (including compression modulus, 0.34 - 7.9 MPa and crushing stress, 40-240 kPa) and acoustic features were measured using texture analyzer and sound acquisition apparatus.Lower MC resulted in higher Ts, higher expansion and weaker mechanical strength; higher WPI gave lower Ts, higher expansion, greater cell numbers with smaller diameters and softer texture; effect of WPC on Ts and expansion showed interactions with MC; higher sucrose led to less expansion, less cell numbers with smaller diameters, and harder texture. In general, SME was positively correlated with RPM and, in most cases, with the material properties Ts or Tf. Good relationships were observed between structural features (piece density, cell density and ratio of cell wall thickness to cell size) and mechanical properties, and also between acoustic-mechanical signals and sensory attributes (hardness and fracturability). A variety of brittle foams were also produced
using supercritical fluid extrusion (SCFX) based on pregel corn starch and with three SC-CO2 injection rates (0.25, 0.50 and 0.74%). Cell diameter and porosity from transverse cross-sections of extrudates decreased with radial distance from the center indicating presence of a relatively non-porous skin, which was absent in SBX extrudates. This skin-effect in SCFX was more prominent for less CO2 injection rates. Average cell size of SCFX foams ranged from 0.34-0.40 mm, which was more than 10 times smaller than SBX foams. Average cell wall thickness of SCFX foams varied from 127-160 μm and was lower than SBX foams. The cell density of SCFX extrudates was approximately 100-fold greater than that for SBX extrudates. SCFX extrudates showed higher polydispersity indices, indicating more uniform cell size distribution. SC-CO2 concentration was found to be critical for controlling the microstructure of SCFX extrudates, whereas SME primarily governed the morphology of SBX extrudates.
Impacts
This project investigated production of brittle biopolymeric foams with varying microstructure and texture using two fundamentally different extrusion processing technologies - steam-based extrusion (SBX) and supercritical fluid extrusion (SCFX). The foaming mechanism and resultant microstructure were studied using X-ray microtomography (XMT), a novel three-dimensional and non-invasive imaging technique that enabled characterization of foam morphology in an objective and accurate manner. Critical material properties of formulations were identified using new (phase transition analysis or PTA) and traditional (DSC) methods. The dynamics of foaming by SBX and SCFX could be understood, and relationships between material properties, processing parameters, and microstructure and texture properties of brittle foams were established. A significant outcome of this project is the development of a conceptual model for structure formation and microstructure-texture relationships
in both steam-based extrusion and SCFX processes, leading to an improvement over current knowledge in the area. Results from this study can be applied towards development of a new generation of cereal-based food products which are highly nutritious and have desired microstructure and texture characteristics. These results can also be utilized for improvement of the production process for biopolymer industrial foams. The new microstructure and material characterization techniques (XMT and PTA) explored in this project can be applied to a whole range of food and industrial products.
Publications
- Agbisit, R. Alavi, S., Trater. A.M., Cheng, E., and Herald, T.J. 2006. Relationships between microstructure and mechanical properties of cellular corn starch extrudates. Journal of Texture Studies. Accepted for publication.
- Cheng, E., Alavi, S., Pearson, T., and Agbisit, R. 2006. Mechanical-acoustic and sensory evaluations of corn starch-whey protein isolate extrudates. Journal of Texture Studies. Accepted for publication.
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Progress 01/01/05 to 12/31/05
Outputs
Research was focused on studying effect of process parameters (screw speed and in-barrel moisture) and raw material composition (corn starch with different levels of whey protein isolate or WPI and sugar) on the micro-structure and textural properties of extruded brittle foams. X-ray microtomography (XMT) was utilized for non-invasive imaging of sample cross-sections at various depths, and facilitated hitherto impossible measurement of features like true cell size distribution, average cell diameter (0.582 to 2.27 mm), open wall area ratio (0.100 to 0.146), cell wall thickness (0.047 to 0.087 mm), and true void fraction (0.629 to 0.842). Material properties, including the glass transition temperature (Tg) and softening and flowing points (Ts and Tf, respectively), of formulations were measured using a phase transition analyzer and differential scanning calorimeter. Significant relationships were observed between Td and Tf and physical and microstructure parameters.
Good relationships were observed between piece density, cell density and ratio of cell wall thickness to cell size, and compression modulus and crushing stress. High correlations between acoustic-mechanical signals and sensory attributes (sensory hardness, crispness work and fracturability) of the foams were found. A variety of foams were also produced using supercritical fluid extrusion (SCFX) with three different SC-CO2 injection rates and different levels of WPI. Results indicated that cell diameters and porosity decreased from center to the periphery, leading to a 'skin-effect' which was more prominent for lesser CO2 injection rates. Material properties of the raw and extruded formulations were evaluated.
Impacts
Results are being used for developing a conceptual model for structure formation and microstructure-texture relationships in both steam-based and supercritical fluid extrusion processes, which will present a significant improvement over current knowledge in the area.
Publications
- Trater, A.M., Alavi, S, and Rizvi, S.S.H. 2005. Use of non-invasive X-ray microtomography for characterizing microstructure of extruded biopolymer foams. Food Res. Int. 38: 709-719.
- Cheng, E., Agbisit, R., Alavi, S., and Pearson, T. 2005. Mechanical-acoustic and sensory evaluations of extruded food foams. AACC Annual Meeting, September 2005, Orlando, FL. Book of Abstracts. (Oral).
- Trater, A. M., Cheng, E., and Alavi, S. 2005. Thermal flow analysis as a predictive tool for expansion and microstructure formation in extruded biopolymer foams. Submitted to J. Food Science.
- Trater, A. M., Cheng, E., and Alavi, S. 2005. A conceptual model for microstructure evolution in extruded biopolymeric foams. Under preparation for J. of Carb. Polymers.
- Katnas, S., Katnas, Z., Alavi, S., and Rizvi, S. S. H. 2005. Investigation of microstructure evolution in biopolymeric foams produced by supercritical fluid extrusion. Under preparation for Food Res. Int.
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Progress 01/01/04 to 12/31/04
Outputs
Foaming, or incorporation of bubbles in materials, is of great significance in both food and non-food applications. The microstructure controls texture and mouth feel of food foams and expanded snack products. This study aims at using material phase transition properties, cellular structure information and resulting texture properties to arrive at this understanding. X-ray microtomography (XMT) was utilized to investigate biopolymer foam microstructure. Thermal flow analysis was performed on raw materials using a phase transition Analyzer (PTA). Mechanical properties of extruded foams were obtained using a Texture Analyzer (TAXT2). To measure material pasting and thermal flow properties (softening temperature, Ts and flowing temperature Tf), and relate these parameters to extrudate cellular structure, corn starch-based foams were produced using four levels of in-barrel moisture contents (22.7, 24.8, 26.8 and 28.8%) and three screw speeds (200, 300, and 400 RPM).
Product temperature behind the die (Td) was recorded, and its differential with Ts and Tf (Tds and Tdf, respectively) obtained via thermal flow analysis. Tds ranged between 67.4 and 86.4 oC and Tdf between 31.3 and 64.6oC, and in general larger values of Tds and Tdf led to greater expansion. Significant relationships existed between Tdf and average cell diameter (R2=0.94) and physical void fraction (R2=0.80). More focus was put on relationship between foam microstructure and texture attributes. Increasing in-barrel moisture content led to increased compression modulus (2.2 to 5.3 MPa), crushing stress (77.6 to 170 KPa) and other crispness attributes (average specific force and crispness work). Positive relationships were observed between piece density, nucleation density, and ratio of cell wall thickness to cell size and compression modulus and crushing stress A variety of foams were produced using supercritical fluid extrusion (SCFX) using a base formulation mainly comprising of
pregel corn starch (50%), pregel potato starch (25%) and sugar (25%), in-barrel moisture content of 35% (w.b.), and three different SC-CO2 inhection rates, namely 0.4, 0.8 and 1.2% (w/w dry mix). Samples were scanned non-invasively using XMT, and their cellular structure analyzed using imaging software. Results indicated that cell diameters and porosity decreased from the center of extrudates to the periphery, leading to a presence of a 'skin-effect' which was more prominent for lesser CO2 injection rates. The skin-effect was explained on the basis of the underlying physical processes of CO2 diffusion and surface cooling of melt to temperatures below Tg+40oC.
Impacts
Bioploymeric foams with both food and non-food applications are increasingly being produced by a variety of techniques ranging from traditional processes like steam-based extrusion to newly developed technologies such as supercritical fluid extrusion. This study will significantly expand our understanding of the foam dynamics and mechanics, and will be put to practical use in a priori design of products with unique textures and formulations, for example, expanded starch-based foams containing very high levels of whey protein. Another major outcome of this study will be development of a new and non-invasive methodology for measuring the microstructural features of food foams.
Publications
- No publications reported this period
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Progress 01/01/03 to 12/31/03
Outputs
Foaming, or incorporation of bubbles in materials, is of great significance in both food and non-food applications. The dynamics of foaming and the resultant microstructure control important attributes like mechanical strength of non-food foams, and texture and mouth feel of food foams (like breads, cakes, puffed breakfast cereal and expanded snack products). Despite several innovations in foaming technology, a basic understanding of the dynamics of structure formation is lacking. This study aims at using material phase transition properties and cellular structure information to arrive at this understanding. The potential of a promising new technology, called X-ray tomography imaging (XTI), for imaging of biopolymer foams was investiagted. Corn starch based expanded products containing 24 or 34% moisture and 5 or 15% whey protein concentrate (WPC-34) were made using twin-screw extrusion, and subsequently scanned using XTI. X-ray tomography allowed imaging of sample
cross-sections at various depths in a non-invasive manner, and facilitated accurate measurements of features like true cell size (0.764 to 1.367 mm), open surface area ratio (which was till now impossible to measure) (0.100 to 0.146) and cell wall thickness (0.047 to 0.087 mm). Results indicated that XTI is a significant improvement over conventional methods for measuring foam microstructure such as optical microscopy and scanning electron microscopy. Ongoing work involves an expanded experimental design for study of phase transition properties like Tg and Tm, and characterization of extrudate structure using XTI.
Impacts
Bioploymeric foams with both food and non-food applications are increasingly being produced by a variety of techniques ranging from traditional processes like steam-based extrusion to newly developed technologies such as supercritical fluid extrusion. This study will significantly expand our understanding of the foam dynamics and mechanics, and will be put to practical use in a priori design of products with unique textures and formulations, for example, expanded starch-based foams containing very high levels of whey protein. Another major outcome of this study will be development of a new and non-invasive methodology for measuring the microstructural features of food foams.
Publications
- Alavi, S., and Trater, A. 2003. Microstructure characterization of biopolymer foams using non-invasive X-ray tomography. Proceedings of the Eight Conference of Food Engineering. 2003 AIChE Annual Meeting (Nov. 16-21, 2003). AIChE Pub. No. 192.
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