Biomedical Engineering Reference
In-Depth Information
(a)
(b)
1 0 u
1 u
Figure 2.2 SEM images of entire (a) and fractured (b) starch coatings isolated from a jet-cooked starch-
lipid composite, dehydrated with ethanol, and critical-point dried.
rheological properties (Byars et al ., 2003), and the effect of added oil on dispersion
properties (Byars et al ., 2006 ).
In addition to widespread use of steam jet cooking for starch utilization in the papermaking
and textile industries, jet cooking has also been used for producing starch gels for food use
(Edwards et al ., 1998), for cooking cereal flours (Rankin et al ., 1976 ), for preparing
complexes of starch with poly(ethylene-co-acrylic acid) (Christianson et al ., 1992 ), and for
forming composites of starch with natural gums (Fanta and Christianson, 1996). More
versatile and sophisticated jet-cooking devices continue to be developed (Shroyer, 1997;
Cummins and Perry, 1998 ).
A significant discovery was made in the 1990s when a mixture of soy oil and corn starch
was passed through an excess steam jet cooker. The resulting starch dispersion contained
soy oil droplets about 1 to 10 μm in diameter; and the droplets did not coalesce with time,
even after prolonged storage (Fanta and Eskins, 1995). The stability and lack of coalescence
of the oil droplets in both liquid dispersions and dried solids was demonstrated, suggesting
a specific starch-oil interaction (Knutson et al ., 1996 ). Dickinson ( 1988 ) proposed that
starch separates from water solution and adsorbs onto the hydrophobic surfaces of oil
droplets because water is a thermodynamically poor solvent for starch, and the adsorption
of starch at the lipid-water interface leads to a reduction in the interfacial tension of the two-
phase aqueous oil system.
The development of this technology was significantly enhanced by the issuance of
two patents covering a wide range of starches, lipophilic materials, and processing
conditions (Eskins and Fanta, 1997, 1999). Light microscopy of the composites revealed
a boundary layer around the oil droplets (Eskins et al ., 1996 ); this absorbed layer was
later shown to consist of starch and starch-lipid complexes (Fanta et al ., 1999 a). Efficient
complex formation was demonstrated in jet-cooked mixtures of high amylose starch and
fatty acids (Fanta et al ., 1999b). The nature and composition of the adsorbed starch
layers (or shells) was described (Fanta et al ., 2001a), and a patent on the starch shells and
starch-coated droplets was issued (Fanta et al ., 2001b). SEM images of the adsorbed
starch shells (Figure 2.2) were obtained by isolating the starch-coated oil droplets by
centrifugation, extracting the oil with ethanol, and critical-point drying the remaining
adsorbed starch. Further characterization of starch-lipid composite properties included
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