The relationships between mechanical or textural properties and structure ob- served for extrudates are potentially applicable to other brittle-porous products, such as baked flat breads, crackers, and popcorn. Furthermore, many baked goods, while plastic, are cellular and therefore subject to similar relationships between mechanical strength and structure or between mechanical properties and texture. Such correspondence between structure, failure properties, and function- ality provides a convenient means of ‘‘tailoring’’ foods to possess desired attri- butes and acceptance.
ACKNOWLEDGMENT
This work was supported by the U.S. Army SBCCOM, Natick Soldier Center, and was conducted as part of ration research and development efforts.
REFERENCES
1. Alvarez-Martinez, L., Kondury, K.P., and Harper, J.M. 1988. A general model for expansion of extruded products. J. Food Science 53(2):609–615.
2. Launey, B., and Lisch, J.M. 1983. Twin-screw extrusion cooking of starches: flow
behavior of starch pastes, expansion and mechanical properties of extrudates. J. Food Engineering 2:259–280.
3. Hicsasmaz, Z., and Clayton, J.T. 1992. Characterization of the pore structure of starch-based food materials. Food Structure 11:115–132.
4. Barrett, A.H., Ross, E.W., and Taub, I.A. 1990. Simulation of the vacuum infusion process using idealized components: effects of pore size and suspension concentra- tion. 1990. J. Food Science 55(4):989–993.
5. Barrett, A.H., and Ross, E.W. 1990. Correlation of extrudate infusibility with bulk properties using image analysis. J. Food Science 55(5):1378–1382.
6. Gibson, L., and Ashby, M.F. 1988. Cellular Solids. New York: Pergamon Press.
7. Moore, D., Sanei, A., Van Hecke, E., and Bouvier, J.M. 1990. Effect of ingredients on physical/structural properties of extrudates. J. Food Science 55(5):1383–1387, 1402.
8. Russ, J.C., Stewart, W.D., and Russ, J.C. 1988. The measurement of macroscopic images. Food Technology, February:94–102.
9. Gao, X., and Tan, J. 1996. Analysis of expanded food texture by image processing.
J. Food Process Engineering 19:425–456.
10. Tan, J., Gao, X., and Hseih, F. 1994. Extrudate characterization by image processing.
J. Food Science 59(6):1247–1250.
11. Smolarz, A., Van Hecke, E., and Bouvier, J.M. 1989. Computerized image analysis and texture of extruded biscuits. J. Texture Studies 20:223–234.
12. Barrett, A.H., and Peleg, M. 1992. Cell size distributions of puffed corn extrudates.
J. Food Science 57(1):146–149, 154.
13. Barrett, A.H., Cardello, A.V., Lesher, L.L., and Taub, I.A. 1994. Cellularity, me- chanical failure, and textural perception of corn meal extrudates. J. Texture Studies 25:77–95.
14. Cohen, S., Voyle, C., Harniman, R., Rufner, R., Barrett, A.H., and Hintlian, C. 1989.
Microstructural evaluation of porous nutritional sustainment module extrudates. U.S.
Army Natick RD&E Center Technical Report, TR-89-034.
15. Barrett, A.H., and Peleg, M. 1995. Applications of fractal analysis to food structure.
Food Science and Technology 28:553–563.
16. Barrett, A.H., Kaletunc, G., Rosenberg, S., and Breslauer, K. 1995. Effect of sucrose on the structure, mechanical strength and thermal properties of corn extrudates. Car- bohydrate Polymer 26:261–269.
17. Kim, C.H., and Maga, J.A. 1993. Influence of starch type, starch/protein composition and extrusion parameters on resulting extrudate expansion. In: G. Charalambous, ed. Food Flavors, Ingredients and Composition. New York: Elsevier Science, pp 957–964.
18. Faubion, J.M., and Hoseney, R.C. 1982. High-temperature short-time extrusion cooking of wheat starch and flour. I. Effect of moisture and flour type on extrudate properties. Cereal Chemistry 59(6):529–537.
19. Fan, J., Mitchell, J.R., and Blanshard, J.M.V. 1994. A computer simulation of the dynamics of bubble growth and shrinkage during extrudate expansion. J. Food Engi- neering 23:337–356.
20. Chinnaswamy, R., and Hanna, M.A. 1988. Optimum extrusion-cooking conditions for maximum expansion of corn starch. J. Food Science 53(3):834–836, 840.
21. Barrett, A.H., and Peleg, M. 1992. Extrudate cell structure–texture relationships. J.
Food Science 57(5):1253–1257.
22. Lai, C.S., Guetzlaff, J., and Hoseney, R.C. 1989. Role of sodium bicarbonate and trapped air in extrusion. Cereal Chemistry 66(2):69–73.
23. Koh, B.K., Karwe, M.V., and Schiach, K.M. 1996. Effects of cysteine on free radical production and protein modification in extruded wheat flour. Cereal Chemistry 73(1):115–122.
24. Barrett, A.H. Normand, M.D., Peleg, M., and Ross, E.W. 1992. Characterization of the jagged stress–strain relationships of puffed extrudates using the fast Fourier transform and fractal analysis. J. Food Science 57(1):227–232, 235.
25. Barrett, A.H., Rosenberg, S., and Ross, E.W. 1994. Fracture intensity distributions during compression of puffed corn meal extrudates: method for quantifying fractura- bility. J. Food Science 59(3):617–620.
26. Rhode, F., Normand, M.D., and Peleg, M. 1993. Effect of equilibrium relative hu- midity on the mechanical signatures of brittle food materials. Biotechnology Prog- ress 9:497–501.
27. Peleg, M., and Normand, M.D. 1993. Determination of the fractal dimension of the irregular, compressive stress-strain relationships of brittle, crumbly particulates.
Particle and Particle Systems Characterization, 10:301–307.
12
Understanding Microstructural Changes in Biopolymers Using Light and Electron Microscopy
Karin Autio and Marjatta Salmenkallio-Marttila VTT Biotechnology, Espoo, Finland
I. INTRODUCTION
Most foods are derived from raw materials that have a well-organized microstruc- ture, e.g., cereal products from grains. Processing, including malting, milling, dough mixing, and heating, produce great microstructural changes in proteins, cell wall components, and starch. These changes can have a large effect on the quality of the end product. The microstructure determines the appearance, texture, taste perception, and stability of the final product. A variety of microscopic tech- niques is available for studying the different chemical components of grains and cereal products (1). Bright-field and fluorescence microscopic methods are fre- quently used because they allow selective staining of different chemical compo- nents. These staining systems can also be used in confocal scanning laser micros- copy (CSLM). One of the main advantages of this technique is the minimal degree of sample preparation that is required (2, 3). And CSLM is well suited for high- fat doughs, which are difficult to prepare for conventional microscopy. Compared with light microscopy (LM), the resolution is considerably improved with elec- tron microscopy. Scanning electron microscopy is used to examine surfaces and transmission electron microscopy the internal structure of food (1).
Although the morphologies of cereal grains share many similar features, differences exist, especially in chemical composition and the distribution of com- ponents. Wheat is rich in protein and has very thin walls, especially in the outer
endosperm. Rye is rich in cell walls, and the cell walls are thick throughout the kernel. Barley may also have thick cell walls throughout the kernel, but there are great variations in thickness between varieties. In barley, wheat, and rye, the size distribution of starch granules, A- and B-type, vary from one sample to an other. In oat, cell walls can be very thick in the subaleurone layer (4). This concentration of cell wall material can be of benefit for milling of fiber-rich oat bran. Oat bran has a higher protein content than the brans of other cereals. The starch granules of oat grain are smaller than in other cereals and are clustered into a greater unit.
Germination and malting induce great changes in the microstructure of cell walls, proteins, and starch granules. The most important quality requirement of malted barley is rapid and even modification of the grain. Although in Western countries high viscosities of cereal cell wall components are largely beneficial in bread baking and in human nutrition, in brewing these same properties may decrease the rate of wort separation and beer filtration.
During milling the outer layers of the grain and the embryo must be sepa- rated from the starchy endosperm to produce a high yield of wheat flour (5). Grain properties affect the milling quality of different cultivars. Flour yield, amount of starch damage, losses of storage proteins, and efficiency of embryo removal dur- ing milling can vary from one wheat to another. In baking, the major purpose of wheat dough mixing is to blend the flour and water into a homogeneous mix- ture, to develop the gluten matrix. The major microstructural changes that take place during the heating of wheat dough are starch gelatinization, protein cross- linking, melting of fat crystals, and sometimes fragmentation of cell walls. Al- though most wheat breads are made from refined flour rather than from whole grain flour, the raw material of traditional rye breads is whole grain flour. In rye dough, the cell wall components make a greater contribution to structure forma- tion than do the proteins.
Pasta products have gained popularity in many countries. Both the raw material, typically durum semolina, durum flour, and hard wheat flour, and the pasta process affect the quality of dry and cooked pasta. To a great extent, the microstructure of fresh, dry, and cooked pasta determines the textural properties, such as stickiness. Protein quality and quantity are important in the production of high-quality pasta products. However, additional research is needed on the role of starch, the major constituent of flour (6).
Oat products have captured the attention of the food industry because the soluble β-glucans abundant in oat bran have been shown to have cholesterol- lowering effects in rats and humans (7). Several commercial products have come to the market: oat bran, oat flour, oat flakes, rolled oats, oat chips, oat biscuits, oat breads, etc. The nutritional value of the dietary fiber components of the other cereals is also receiving great attention.
Table 1 Chemical Composition of Cereal Grains (% Dry Weight)
Wheat Rye Oat Barley
Fat 2.1–3.8 2.0–3.5 3.1–11.6 0.9–4.6
Protein 9–17 8–12 11–15 8–13
Starch 60–73 50–63 39–55 53–67
Pentosan 5.5–7.8 8.0–10.0 3.2–12.6 4.0–11.0
β-Glucan 0.5–3.8 1.0–3.5 2.2–5.4 3.0–10.6