Wheat flour extrudates

Thiamine is particularly sensitive to thermal processing the published retention values range from 0 to 95% (Killeit, 1994). Wheat flour extruded with no added water suffered large thiamine losses, and increased barrel temperature decreased the vitamin only at the slowest feed rate used (250 g/min versus 500 and 750 g/min) (Andersson and Hedlund, 1990). In the same study, extrusion conditions did not affect the content of riboflavin (B2) or niacin ascorbic acid (C) decreased with higher temperatures at 10% moisture. Since enriched wheat flour is an important source of B vitamins in the United States, care must be taken to protect these vitamins. Relatively little is known about the stability of synthetic vitamins compared with natural sources, or the effects of added vitamins as opposed to endogenous nutrients. 

EPR signals from wheat flour extrudates were a mixture of nitrogen and sulfur centered radicals, with evidence of alkoxyl or peroxyl radicals as well (Figure 2). They varied quantitatively and qualitatively with extrusion conditions. The dominant signal in all extrudates was from nitrogen-centered radicals widi broadened shoulders consistent with oxyl radicals. Weaker downfield peaks (left of the central line) from sulfur radicals appeared in some extrudates (1). 

Figure 4. EPR signals of wheat flour extruded at 185 X die temperature, 16% moisture, 225 g/min mass flow rate, and 500 rpm screw speed, then ground for analysis by various methods. Sulfur peaks in EPR signals show that shear stress (grinding) produces sulfur radicals via scission of S-S bonds.

Figure 7. Protein solubility increases in wheat flour extrudates as a junction oj level of cysteine added as a free radical scavenger during extrusion. From Koh (3), used with permission. Copyright American Association oJ Cereal Chemists, 1996.

Figure 10, Glucoamylase digestion releases proteins complexed with starch and results in essentially complete solubilization of proteins in wheat flour extrudates. Other extrusion conditions (see Figure 11) show comparable behavior. Adapted from (4) with permission. Copyright American Assoc, of Cereal Chemists, 1996,...

Coextrusion is the process of extruding two or more materials simultaneously or in tandem. It allows a combination of an ingredient such as wheat flour, which is inexpensive and easily enriched with vitamins and minerals, with dairy protein, which provides functionality and texture. For example, an early coextrusion of wheat flour and rennet casein was performed by van de Voort et al. (1984), who obtained products with varying characteristics depending on process parameters. 

A product resembling bread can be made using 75% of wheat flour and 25% of flour milled from extruded rice. The importance of extruding the rice is that some at least of the starch would have gelatinised. 

J.A. Menjivar, E.N. Chang and L. Maciel, Extrudate Swell Behaviour of Wheat Flour Doughs, in "Theoretical and Applied Rheology", P.Moldenaers and R. Kennings (eds.), Elsevier Science B.V., 1992, pp. 714-716. 

The amount and diversity of pyrazines formed on extruding a wheat flour system can be substantially enhanced by the addition of ammonium bicarbonate and 2-oxo-propanal.241... 

Figure 5. Cumulative MWDs of the high protein wheat flour control and an extruded sample (G39).

Hwang, H.I. et al., Aroma generation in extruded and heated wheat flour, in Lipids in Food Flavors, Ho, C.T. and Hartman, T.G., Eds., American Chemical Society, Washington, D.C., 1994, p. 144. 

Andersson, Y. Hedlund, B. Extruded wheat flour correlation between processing and product quality parameters. Food Qual. Prefer. 1990, 2, 201—216. 

Table 1. Volatiles generated in extruded wheat flour systems with added precursors...

UNEX=Unextruded wheat flour, EX= Extruded Wheat Flour, AB= Ammonium Bicarbonate added, " PA= Pyruvaldehyde added. 

This discovery led us to study free radical production systematically in extruded wheat flour to determine whether free radical production was related to extrusion conditions and chemical changes in wheat proteins, and to distinguish the relative contributions of heat, shear, and oxidizit lipids in formation of protein radicals. 

Extrusion was conducted on a Werner and Pfleiderer ZSK30 twin screw extruder (Werner and Pfleiderer, Ramsey, NJ) equipped with two co-rotating self-wiping screws. Commercial hard red wheat flours (Bay State Milling Co., Minneapolis, MN) were extruded under puffing conditions according to conditions described by Schaich (1), Rebello (2), Koh et al (3), and Wang ( 4). 

Figure 2. Low temperature EPR spectra from extruded wheat flour, g-values of major peaks and associated radical species are noted. Number sequences at right of spectra — % protein, moisture, temperature, SME. G37-G46 extrusion run code numbers. Adapted from (1) with permission. Copyright American Assoi of Cereal Chemists, 1999.

To test this Working Hypothesis, 0%, 0.5%, 1% and 2% cysteine were added to wheat flour as a radical scavenger during extrusion (3). The original Bouncer flour (14% protein) was extruded at process moisture 16% (w/w), constant die temperature 185 C, 225 g/min mass flow rate, and screw speed 500 rpm conditions that provided optimum wheat flour expansion, flavor and textural quality in the earlier experiments. 

Schmid AH, Dolan KD, Ng PKW (2005) Effect of extruding wheat flour at lowta- tempruatures on physical attributes of extrudates and on thiamin loss when using carbon dioxide gas as a puffing agent. Cereal Chem 82 305-313... 

Eairweather-Tait, S. J., Porhvood, D. E., Symss, L. L., Eagles, J., and Minski, M. J. (1989). Iron and zinc absorption in human subjects from a mixed meal of extruded and nonextruded wheat bran and flour. Am. ]. Clin. Nutr. 49, 151-155. 

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