Comparison of microwave processing and excess steam jet cooking for spherulite production from amylose-fatty acid inclusion complexes

RESEARCH ARTICLE

Comparison of microwave processing and excess steam jetcooking for spherulite production from amylose–fatty acidinclusion complexesy

Frederick C. Felker1, James A. Kenar1, George F. Fanta2 and Atanu Biswas2

1 Functional Foods Research Unit, USDA, Agricultural Research Service, National Center for Agricultural Utilization Research,Peoria, IL, USA

2 Plant Polymer Research Unit, USDA, Agricultural Research Service, National Center for Agricultural Utilization Research,Peoria, IL, USA

Helical inclusion complexes of amylose with fatty acids can form spherulites of various morpho-

logical types. Previous studies have described the spherulites obtained by cooling dispersions of

steam jet cooked corn starch either by itself or supplemented with various fatty acids. In light of

potential advantages of microwave processing, we investigated the use of a laboratory microwave

instrument as an alternative method for spherulite production. With native high amylose corn

starch (HAS), spherulites were formed with morphology similar to those observed previously by

steam jet cooking. Adjustments to the reaction conditions led to a slight improvement in yield over

jet cooking. Using solvent-defatted HAS supplemented with straight-chain fatty acids (C10:0 to

C22:0), microwave processing produced only small, disc-shaped spherulites in a gel matrix.

However, when defatted HAS was supplemented with either capric or palmitic acid and processed

by steam jet cooking, uniform dispersions of toroidal spherulites were obtained. These results

show that although jet cooking is not required for spherulite formation when native HAS is used,

defatted HAS requires the high-shear steam jet cooking method of heating for optimal spherulite

growth. Researchers and product developers could use the results of microwave experiments to

refine jet cooking methods for large scale spherulite production.

Received: September 26, 2012

Revised: January 30, 2013

Accepted: January 31, 2013

Keywords:

Jet cooking / Microwave / Spherulites / V-amylose

1 Introduction

When amylose forms V-type inclusion complexes with vari-

ous fatty acids, the resulting complexes can be precipitated

from aqueous solutions upon cooling to form either amor-

phous material or semi-crystalline structures or spherulites

[1, 2]. Much research has been directed towards understand-

ing the molecular interactions between amylose, the fatty acid

ligands, and water leading to the stabilization of the complex

and formation of the unit cell structures and crystalline

lamellae. However, the mechanisms involved in the for-

mation of larger supramolecular assemblies or micron-sized

spherulites are not well understood [3]. For example, no basis

has yet been suggested to explain why a V-amylose spherulite

would develop in a toroidal versus radial manner, or what

limits the growth of spherulites to yield uniform dispersions

of spherulites with a narrow size range.

Spherulites in the micron size range can be prepared by

cooling dispersions of corn starch that have been passed

through a steam jet cooker under excess steam conditions

to provide a high shear environment and a peak temperature

yMention of trade names or commercial products in this publi-cation is solely for the purpose of providing specific informationand does not imply recommendation or endorsement by the U.S.Department of Agriculture. USDA is an equal opportunity pro-vider and employer.

Correspondence: Dr. Frederick C. Felker, Functional FoodsResearch Unit, USDA, Agricultural Research Service, NationalCenter for Agricultural Utilization Research, 1815 N. UniversitySt., Peoria, IL 61604, USAE-mail: [email protected]: þ1-309-681-6685

Abbreviations: HAS, high amylose corn starch

DOI 10.1002/star.201200218864 Starch/Starke 2013, 65, 864–874

� 2013WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.starch-journal.com

of 1408C. The native lipid constituents of corn starch that

complex with amylose produce both torus/disc-shaped and

spherical/lobed spherulites of various sizes [4]. When defat-

ted corn starch, supplemented with a variety of specific fatty

acids, was similarly jet-cooked, spherulites with variations in

morphology and size were formed [5]. These spherulites are

dissimilar in several respects from the radially symmetrical

spherulites obtained from lipid-free amylose processed

at higher temperatures, which have been the subject of

extensive research in terms of their synthesis and character-

ization [6–8].

A number of factors influence the yield, size, and

morphology of V-amylose spherulites, including starch and

ligand concentrations, cooling rates, ligand structure, and the

presence or absence of native lipids. By varying these con-

ditions, spherulite yields on the order of 80%, based on

amylose content, could be obtained using excess steam jet

cooking to disperse the starch [9]. In order to enable the

consistent production of uniform spherulite dispersions in

large quantities sufficient to develop specific biobased appli-

cations, the identification of the necessary feedstock type,

ligand, and preparation conditions would be required.

Excess steam jet cooking is an efficient, scalable technique

for producing starch dispersions in which the high tempera-

ture and shear forces completely disrupt the starch granules

and slightly reduce the molecular weight of both the amylose

and amylopectin fractions [10]. It was recently shown that

steam jet cooking corn starch at a temperature as low as 1008Cresulted in significantly smaller particle size and a higher

level of starch granule degradation than hot water boiling at

908C, due to the higher shear forces of the jet cooking process[11]. Because high temperature and shear are inseparable

factors with steam jet cooking, an alternative method was

needed to provide high temperature in a low-shear processing

system in order to evaluate the role of shear in the preparation

of starch for spherulite formation.

Laboratory microwave technology offers several advan-

tages for these investigations including programmable heat-

ing, smaller batch size, and the availability of high pressure

reactor vessels. The versatility of this heating approach has

been used to advantage in the preparation of starch acetates

[12, 13]. In addition to these practical considerations, the

ability of microwave irradiation to rapidly heat and activate

a reaction sequence often leads to higher reaction rates,

yields, and efficiency [14]. Magnetic stirring can also be used

in the reaction vessels employed for microwave heating, thus

providing uniform heating in a low-shear environment

relative to that obtained in a steam jet cooker. This factor

allows the effects of the high shear component of the cooking

process on the yield and morphology of spherulites to be

evaluated. Although steam jet cooking ismore readily scalable

to commercial production requirements, the use of micro-

wave processing may allow researchers and product devel-

opers to efficiently investigate and optimize such factors as

ligand type, reagent concentration, reaction times, tempera-

ture requirements, and stirring conditions for spherulite

formation.

The purpose of this study was to determine (1) whether the

high shear method of steam jet cooking is necessary for

spherulite production, (2) whether microwave heating would

reveal factors which improve the yield of spherulites over that

obtained by jet cooking, and (3) to determine whether uniform

dispersions of spherulites could be obtained with defatted high

amylose starch supplemented with individual fatty acids.

Establishment of the usefulness of microwave processing

for spherulite formation would allow the investigation of other

potential ligands, which might not be available in the relatively

large amounts necessary for processing by jet cooking.

2 Materials and methods

2.1 Materials

High amylose corn starch (HAS) (AmyloGel 03003, contain-

ing 70% apparent amylose) was a product of Cargill

(Minneapolis, MN). Defatted HAS was prepared by sequen-

tial extraction with refluxing 85%methanol–water and 75% n-propanol–water [15]. Fatty acids and other reagents were

purchased from Sigma Chemical Co. (St. Louis, MO). Fatty

acids were either dissolved in ethanol, applied to the dry

starch, and the ethanol evaporated before jet cooking or

microwave processing of the starch, or dry mixed with the

starch before processing, as indicated in the results section.

2.2 Processing by steam jet cooking

Spherulites were prepared by jet cooking using a Penick and

Ford (Cedar Rapids, IA) Laboratory Model steam jet cooker

operating with 65 psig (550 kPa) line pressure and 40 psig

(380 kPa) back pressure (1408C) [10]. Samples were immedi-

ately transferred to a programmable water bath preheated to

958C, and allowed to cool to 408C over 22 h. Dispersions were

then diluted with water to give a 10-fold reduction in solids

concentration, and the spherulites were isolated by centrifu-

gation in a Beckman GS-6KR centrifuge (Beckman

Instruments Inc., Fullerton, CA) at 3000 rpm (approx.

1470 � g). Dissolved starch was removed from the precipi-

tated spherulites by washing two times with excess water

followed by centrifugation. Percent yield was determined

by freeze drying the water-washed spherulites. Data presented

are the mean of three experiments.

2.3 Microwave processing

An Ethos 1600 (Milestone Inc., Monroe, CT) microwave

reactor oven was used to irradiate mixtures of starch–fatty

acid mixtures dispersed in 100 mL deionized water in a

Starch/Starke 2013, 65, 864–874 865

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sealed 100 mL perfluoroalkoxy Teflon1 reactor vessel (prod-

uct code 45111T). The sample was stirred using a Teflon1

4-bar magnetic stirrer at maximum speed and heated from 0

to 1208C in 1.5 min, 120 to 1408C in 1.5 min, and held at

1408C for 10 min (Fig. 1) unless otherwise specified. The

reaction mixture was allowed to cool for 10 min to 1008C and

then maintained at 1008C for 60 min unless otherwise speci-

fied. The reactor vessel was then transferred to a program-

mable water bath preheated to 958C, and allowed to cool to

408C over 22 h. Spherulites were diluted, washed, and freeze-

dried as described above for processing by steam jet cooking.

2.4 Microscopy

Aqueous dispersions of spherulites were observed with a

Zeiss Axioskop light microscope equipped with an

Axiocam ICc 3 digital camera (Carl Zeiss, Inc.,

Thornwood, NY) using phase contrast optics. For SEM,

samples were dehydrated with ethanol, critical point dried

with CO2 onto aluminum stubs, sputter coated with gold, and

examined and photographed with a JEOL 6400V scanning

electron microscope (JEOL, Peabody, MA).

2.5 X-ray diffraction

Freeze-dried samples were equilibrated at 238C and 45%

relative humidity prior to analysis. X-ray powder diffraction

patterns were obtained with a Phillips 1820 diffractometer

operated at 40 kV with graphite-filtered Cu Ka radiation and a

u compensating slit. Data were acquired in 2u ¼ 0.058, 4 s

steps.

2.6 Differential scanning calorimetry (DSC)

Thermal properties were determined using a TA Instruments

(New Castle, DE) model Q2000 DSC calibrated against an

indium standard (156.68C, 28.86 J/g). Data was collected and

analyzed with TA Universal Analysis 2000 V3.9A software.

Freeze-dried samples for DSC were prepared at 20–25 wt%

(dry weight basis) of sample (approximately 10–12 mg) in

water using high volume stainless steel DSC pans. The pans

were hermetically sealed and allowed to equilibrate at least

4 h before analysis. Samples were referenced against an

identical DSC pan containing water and analyzed using

modulation (�0.408C every 40 s) at a heating rate (b) of

5.08C/min. Samples were analyzed by first heating from 0

to 2058C, immediately cooling back to 08C at 5.08C/min, and

then reheating from 0 to 1908C. The melting point (TM) was

taken at the peaks of the DSC curve. Exothermic signals are

reported in the upward direction.

3 Results and discussion

3.1 Formation of spherulites from Native HAS-

palmitic acid mixtures

For a direct comparison with the previously published con-

ditions used with steam jet cooking [5], aqueous dispersions

of 3.5% w/v native HAS combined with 5% palmitic acid

(based on amylose content) were processed by steam jet

cooking followed by slow cooling, giving a spherulite yield

of 82.9% (Experiment 1, Table 1). The microwave processing

method was designed to simulate as closely as possible the

conditions afforded by steam jet cooking, so a 10-min hold

period at 1408Cwas included to insure complete dispersion of

the starch and solubility of the ligand. When the dispersion

was not held for 60 min at 1008C after cooling from 1408C,the spherulite yield was 58.1% (Experiment 2, Table 1). A

single experiment was carried out with a doubling of the

palmitic acid supplementation to 10% (based on amylose),

giving only a slightly higher yield of 70.9%. This suggested

Figure 1. Temperature profile for microwave processing.

Table 1. Formation of spherulites from native high amylose starch and palmitic acid using steam jet cooking and microwave processing

Experiment # Method Min 1408C hold Min 1008C hold % Yield/amylosea)

1 Jet cooker n/a n/a 82.9 (1.7)

2 Microwave 10 0 58.1 (6.9)

3 Microwave 10 60 85.3 (3.3)

4 Microwave 0 60 88.7 (2.8)

a) Yield of spherulites based on amylose content, mean (SD) of three experiments.

866 F. C. Felker et al. Starch/Starke 2013, 65, 864–874


that the availability of dissolved palmitic acid for complex

formation was not the limiting factor using microwave proc-

essing. To determine whether the time available for inclusion

complex formation before spherulite formation was a factor,

the dispersion was held at 1008C for 60 min after reducing

the temperature from 1408C, and before beginning the 22-h

cooling period to 408C (Experiment 3, Table 1). This pro-

cedure gave a spherulite yield of 85.3%, which was essentially

the same as the yield from the steam jet cooking method

(82.9%). A slightly higher yield of 88.7% was obtained by

eliminating the 10-min hold at 1408C (Experiment 4, Table 1).

This could be due to less reduction of amylose molecular

weight resulting from less time at 1408C, and indicates that

raising the dispersion to 1408C even briefly provides suffi-

cient heat to adequately dissolve the ligand and disperse the

amylose.

Phase contrast images of representative samples from

each of the four experiments are shown in Fig. 2. In each

case, the most abundant spherulite type was toroidal, with a

much less frequent abundance of the lobed morphology

indicated by arrows in the micrographs. Very little difference

was seen between the treatments, indicating that steam jet

cooking is not essential for preparing starch for spherulite

formation. In all treatments, there was a strong tendency for

spherulites to adhere to each other and form large clumps,

especially after washing treatments involving centrifugation.

The toroidal nature of the most abundant spherulite type was

confirmed by SEM as shown in Fig. 3.

Figure 2. Phase contrast light micrographs of spherulites formed from non-defatted HAS processed with steam jet cooking or microwaveheating. Specific treatments are described in Table 1. A–D, Experiments 1–4 (Table 1). Arrows indicate spherulites of spherical/lobedmorphology.

Figure 3. Scanning electron micrographs of toroidal spherulitesobtained from Experiment 3 (Table 1).

Starch/Starke 2013, 65, 864–874 867


When the palmitic acid supplementation was omitted (in a

single experiment), the spherulite yield dropped to 19.6%,

which is about the same yield reported previously for HAS jet-

cooked without added lipids [9]. However, the morphology of

the spherulites was a mixture of smaller toroidal and lobed

forms essentially similar in shape to those observed with

added palmitic acid. This suggests that the native lipids

present in the high amylose starch may determine the spher-

ulite morphology, whereas the size and yield of those spher-

ulites are enhanced by fatty acid supplementation.

In addition to the spherulite types described above, there

were two other forms of solid material observed in these

experiments. One form appeared as expansive clusters or

networks of uniformly sized, apparently spherical particles

slightly less than 1 mm in diameter (Fig. 4A and B,). The

relative abundance of these particles varied with minor differ-

ences in stirring speed during the microwave experiments,

and they were also present to a lesser extent in samples

prepared by steam jet cooking. The other form appeared

as disorganized, amorphous masses ranging in size from

tens to hundreds of micrometers in diameter (Fig. 4C

and D). The randomly oriented sectors in these very large

particles exhibited a conspicuous striated appearance both in

phase contrast light micrographs (Fig. 4C) and in SEM

images (Fig. 4D). Such striations are characteristic of the

spherical/lobed spherulite morphology visible in the larger

spherulites designated by arrows in Fig. 2. The large masses

were never observed among spherulites produced by steam

jet cooking, and they were only found in microwave exper-

iments in which the palmitic acid was delivered with an

ethanol solution to the starch granules before processing.

They were absent in samples prepared by dry mixing the

starch and palmitic acid. These results suggest that ligands

dried onto starch granules via ethanol solutions were

unevenly distributed, resulting in ligand-rich domains during

granule gelatinization in the heating phase of the microwave

procedure, when the dispersion viscosity was presumably

highest. This would create zones very rich in complexes

which might begin to form spherulitic structures so rapidly

that they grow together, resulting in the discrete, striated

sectors observed in the large masses. In contrast, discrete

palmitic acid particles dry-mixed with the starch may have

dissolved more gradually during heating, and with stirring

there was more uniform distribution of the ligand when not

dried onto individual starch granules. Both the small,

spherical particles and the large disorganized masses exhib-

Figure 4. Phase contrast light micrographs (A and C) and SEM images (B and D) of small, spherical particles (A and B) and large,disorganized masses (C and D) observed in spherulite preparations from non-defatted HAS depending on stirring conditions or methodof ligand addition.



ited XRD patterns typical of V6 helical complexes (data not

shown), confirming their interpretation as structures prim-

arily composed of amylose complexes.

3.2 Microwave processing of defatted HASwith fatty

acids varying in chain length

The microwave processing conditions of Experiment 4

(Table 1) were used to determine the type of spherulites

obtained with defatted HAS supplemented with the following

straight-chain fatty acids: capric (C10:0), lauric (C12:0), myr-

istic (C14:0), palmitic (C16:0), stearic (C18:0), and behenic

(C22:0). Unlike the experiments carried out with native HAS,

all six saturated fatty acids yielded dispersions containing a

fragmented gel matrix in which small, flat, disc-shaped spher-

ulites were embedded (Fig. 5A–F). With capric and myristic

acid supplementation, the disc-shaped spherulites were com-

pletely embedded in the gel (Fig. 5A and C), whereas for the

other four fatty acids, individual spherulites were detached

from the gelmatrix (Fig. 5B andD–F).When embedded in the

Figure 5. Phase contrast lightmicrographs of cooled dispersions preparedwithmicrowave heating fromdefatted HAS supplementedwithcapric (A), lauric (B), myristic (C), palmitic (D), stearic (E), and behenic (F) acids. Arrows indicate disc-shaped spherulites.

Starch/Starke 2013, 65, 864–874 869


gel matrix, only those spherulites oriented vertically (edge-

wise) in the light path could be seen as straight, dark lines,

suggesting the presence of dense, needle-shaped particles.

However, when detached spherulites were seen away from

the gel matrix, many of them settled horizontally on the slide

and could be seen as round objects with a lighter center

region. Horizontally aligned disc-shaped spherulites super-

imposed with the gelled material were apparently obscured

by the mottled gel appearance. SEM showed a similar

appearance of the flat disc morphology in all six treatments

(Fig. 6A–F). Also in each experiment, the SEM images

revealed a fine textured, fibrous network associated with

the spherulites, which was probably the dehydrated remains

of the gelled material seen in the phase contrast images. The

presence of gelled material associated with the spherulites

precluded isolation of the latter, and therefore the spherulite

yield from defatted HAS preparations could not be accurately

determined.

XRD analyses were performed on the defattedHASmicro-

wave-processed samples to confirm the identity of the pre-

cipitated solid as V-amylose complexes. For reference,

the diffraction pattern for the precipitate isolated from

Figure 6. Scanning electron micrographs of cooled dispersions prepared with microwave heating from defatted HAS supplemented withcapric (A), lauric (B), myristic (C), palmitic (D), stearic (E), and behenic (F) acids. Arrows indicate disc-shaped spherulites.



native high amylose starch after microwave processing in the

absence of added fatty acid was obtained using the same

procedure as that used for the fatty acid-supplemented prep-

arations (Fig. 7A). Reflections were observed at scattering

angles of 7.58, 138, 188, 19.58, and 228. Three of these reflec-tions, 7.58, 138, and 19.58, are typical of V6 helical complexes

[16, 17], and patterns with those three reflections were also

observed for the smaller, torus-shaped spherulites isolated

from jet-cooked, non-defatted corn starch. The reflection at

188 may be due to V7-complexed amylose formed from the

more bulky components of the native lipid mixture, such as

linoleic acid. The diffraction pattern obtained from micro-

wave-processed, defatted HAS is shown in Fig. 7B. The main

reflections in this case are represented by a peak at 178 and a

broad, rounded peak at 228–248. When well-defined, this

pattern is indicative of retrograded amylose [18]. Thus, the

non-defatted high amylose starch pattern shown in Fig. 7A

indicates the presence of both retrograded starch, as revealed

by the peaks at 178 and 228–248, and inclusion complexes

from native lipids represented by the other reflection angles.

XRD patterns for the series of products prepared by

microwave processing of defatted HAS with the six fatty acids

are shown in Fig. 8A–F. In each case, peaks of varying

intensity could be seen at scattering angles normally associ-

ated with V6 complexes. Capric, lauric, myristic, palmitic, and

stearic acid-supplemented preparations showed strong peaks

at 138 and 19.58, with smaller but still distinct peaks at 7.58(Fig. 8A–E), indicating the presence of V6 complexes which

would be expected for the small disc-shaped spherulites seen

in the phase contrast and SEM images. Also discernible in

these patterns are smaller and less well-defined peaks at 178and 228, which suggests the presence of some retrograded

amylose. The behenic acid-supplemented HAS preparation

showed less prominent peaks at the typical V6 scattering

angles (Fig. 8F). Bhatnagar and Hanna [19] demonstrated

by XRD that extrusion of normal corn starch with behenic

acid (4% based on dry weight of starch) in a single screw

laboratory extruder at 1408C and 22% moisture gave an

amylose-behenic acid inclusion complex. A lower degree of

spherulite formation with behenic acid than with the other

fatty acids could be attributed to the lower solubility of the

behenic acid ligand under the conditions used in this study.

Since the V6 XRD pattern observed with the small disc

spherulites is the same as that seen with larger, toroid spher-

ulites in previous studies, and some of the disc spherulites

seen in phase contrast images have a lighter center, we

conclude that the discs represent a very early stage in the

growth of toroid spherulites. The low intensity of peaks more

typical of retrograded starch in the X-ray patterns for products

prepared from capric, lauric, myristic, palmitic, and stearic

acids suggests that some portion of the background gel

observed in these preparations may also be V6-complexed

amylose that was unable to form spherulites under the con-

ditions of microwave heating. If retrograded starch was

present in these samples, the amount was not sufficient to

form an overall, rigid gel structure typical of that normally

seen with high amylose starch. Instead, the gelled material

fell apart on dilution and readily separated into very small,

irregular fragments with clumps of embedded spherulites.

DSC has been used extensively to examine V-amylose

complexes prepared using fatty acids [20–22]. Accordingly,

the thermal behavior of the microwave-processed samples of

defatted HAS complexed with the series of fatty acids (C10:0

through C22:0) were examined by DSC. The initial heating

scans, shown in Fig. 9A–E, showed prominent melting endo-

therms with a (TM) of 112.78C (DH ¼ 21.1 J/g), 115.08C(DH ¼ 25.1 J/g), 116.98C (DH ¼ 24.6 J/g), 117.38C

Figure 7. XRD patterns of native HAS (A) and defatted HAS(B) after microwave heating.

Figure 8. XRD patterns of solid material recovered from slowlycooled dispersions prepared with microwave heating fromdefatted HAS supplemented with capric (A), lauric (B), myristic(C), palmitic (D), stearic (E), and behenic (F) acids.

Starch/Starke 2013, 65, 864–874 871


(DH ¼ 29.1 J/g), and 117.48C (DH ¼ 27.0 J/g) correspond-

ing to the fatty acid treatments C10:0, C12:0, C14:0, C16:0,

and C18:0, respectively. The C22:0 fatty acid sample (Fig. 9F)

gave two endothermic melting transitions at 108.38C(DH ¼ 13.6 J/g) and 122.68C (DH ¼ 8.1 J/g). The observed

endotherms occur in the correct temperature region

(TM � 100–1208C) for V-amylose fatty acid complexes. The

TM progressively increased as the chain length of the fatty acid

increased, in accord with previous reports [20, 23]. The higher

melting nature of these peaks suggests they may represent

Type II V-amylose fatty acid complexes analogous to those

previously reported [21, 24]. Also visible in these thermo-

grams were minor peaks corresponding to uncomplexed fatty

acid (indicated with asterisks in Fig. 9). Further evidence that

the observed endotherms represented V-amylose complexes

was obtained by examining the second reheating scans (data

not shown). Under the DSC conditions used, the first heating

scan fully dissociated the complexes to release the fatty acids,

disordered the amylose, and avoided reforming the com-

plexes upon cooling [25]. The reheating scans showed an

increase in the peaks between 31 and 808C corresponding

to themelting point of released fatty acids and no endotherms

between 112 and 1188C indicative of the amylose–fatty acid

complexes. In the case of the C22:0 fatty acid sample, both

peaks disappeared. The presence in the initial heating scan

and disappearance in the second scan of the two endotherms

for C22:0 complexes could be the result of heterogeneity of

the complexes, such as both helical sizes (6 and 7 glucosyl

units per turn) being present [26], or recrystallization of

complexes during the course of the DSC scan [27].

Typically, the endothermic transitions of retrograded amy-

lose occur in the region of �140–1608C [23]. No significant

endotherms in that temperature range were observed in

any of these samples. The method used for the isolation of

precipitated materials was such that the warm material from

the water bath was immediately diluted to avoid retrograda-

tion of uncomplexed amylose. The DSC profiles suggest that

most of the amylose was complexed by the supplemented

fatty acid ligands, and that only very minor amounts of

retrograded starch, if any, were present.

3.3 Processing of defatted HAS–fatty acid mixtures

by steam jet cooking

To determine whether the partially gelled products and small

size of the spherulites obtained from microwave processing

of defatted HAS supplemented with saturated, straight-chain

fatty acids were a result of the microwave processing method

used, defatted HAS was supplemented with lauric and pal-

mitic acids and processed by steam jet cooking for compari-

son. These preparations were then subjected to the same

cooling profile used for the microwave experiments. Phase

contrast micrographs of the resulting dispersions are shown

in Fig. 10. In contrast to the relatively small disc-shaped

spherulites embedded in gel observed with microwave proc-

essing, the predominant material in these preparations con-

sisted of toroid spherulites with about the same range of sizes

observed with non-defatted HAS processed by microwave

heating and also observed previously using steam jet cooking

[5]. The toroidal spherulites formed from capric acid

(Fig. 10A) were about 5 mm in diameter, while those from

palmitic acid (Fig. 10B) were about 8 mm in diameter. The

spherulite yield from the palmitic acid supplemented prep-

aration was 73% based on amylose, but this is not directly

comparable to the yield obtained with non-defatted HAS

(Experiment 4, Table 1), since these were uniform toroidal

spherulites with none of the spherical/lobed variety present.

As noted previously, most of the spherulites obtained with

microwave processing from non-defatted HAS plus palmitic

acid were very similar in size and morphology to those

produced by steam jet cooking (Figs. 2 and 3). The formation

of gel-embedded, relatively small, disc-shaped spherulites

observed in the series of fatty-acid supplemented, defatted

HAS microwave preparations (Figs. 5 and 6) was unexpected.

The larger size, uniformity, and abundance of toroidal spher-

ulites obtained with defatted HAS supplemented with capric

and palmitic acids processed by steam jet cooking (Fig. 10)

therefore indicates that some aspect of the microwave proc-

essing method was interfering with spherulite formation,

since the same reagents, concentrations, and cooling profiles

led to normal spherulite formation after jet-cooking. The

solubility of the fatty acid ligand under the conditions of

microwave processing is probably not the most important

factor, since with palmitic acid, both steam jet cooking and

microwave processing with non-defatted starch gave good

yields of spherulites. Since the solubility of a fatty acid in

Figure 9. DSC of solid material recovered from slowly cooleddispersions prepared with microwave heating from defattedHAS supplemented with capric (A), lauric (B), myristic (C), palmi-tic (D), stearic (E), andbehenic (F) acids. Endotherms representinguncomplexed fatty acids are indicated with asterisks.



hot water would be inversely proportional to chain length, it is

likely that the smaller fatty acids (capric, lauric, and myristic)

also be sufficiently soluble under these conditions. Both the

dispersion morphology and XRD profile of the stearic acid

supplemented defatted HAS preparation were similar to

those of the shorter chain-length ligands, suggesting suffi-

cient solubility of even the C18:0 ligand. Only the behenic acid

preparation indicated a possible reduced solubility condition

as evidenced by the smaller V6 reflections on the XRD pattern

(Fig. 8F). Therefore, we conclude that insufficient solubility of

the fatty acid ligands during microwave heating and incu-

bation is not likely to be the reason for the differences

observed in dispersion morphology.

One factor that might explain the inhibition of spherulite

formation in microwave-processed preparations with defatted

HAS is the possible modification of the properties of HAS by

solvent defatting. It is well known that HAS requiresmore heat

than normal dent corn to fully disperse or dissolve the starch

granules. In a previous study, it was also found that normal

dent corn starch granules from which lipids were completely

removed by extraction with refluxing 75/25 v/v n-propanol–

water exhibited reduced swelling under standard pasting con-

ditions as compared to native, unextracted starch [28]. This

difference suggested that the amylose remaining after lipid

removal had associated during the solvent extraction process,

thus inhibiting granule swelling and solubility. The water

content of the hot solvent would cause a partial gelatinization

followed by retrogradation during cooling. The combination of

usingHAS, which is intrinsicallymore difficult to dissolve, and

the use of hot aqueous solvent extraction may together have

raised the threshold of heat and shear required to disrupt the

granule structure and thus allow the spherulites to form.

Consistent with this possibility is the fact that jet cooking

defatted HAS supplemented with capric and palmitic acids

resulted in the formation of full-size toroidal spherulites as

expected. Alternatively, lower temperature methods of defat-

ting the HAS, or using a different type of starch as a source of

lipid-free amylose, could be used for further investigations of

spherulite formation using microwave processing.

4 Conclusions

When native, non-defatted HAS was used as the amylose

source for V-amylose spherulite formation by microwave

processing with palmitic acid supplementation, the yield of

spherulites was comparable to that obtained by steam jet

cooking. The high shear processing conditions used for steam

jet cooking are therefore not necessary for spherulite prep-

aration. However, if the HAS was defatted by extraction with

refluxing aqueous solvents and then supplemented with

specific fatty acids to form a specific spherulite morphology,

microwave processing yielded very small, disc-shaped spher-

ulites in a gelled matrix. Solvent-defatted HAS supplemented

with capric or palmitic acid and processed by steam jet cook-

ing yielded dispersions of uniform, larger toroidal spheru-

lites, suggesting that the mechanical shear of steam jet

cooking was sufficient to overcome the limitation of amylose

solubility. The practical significance of this finding is that for

large-scale production of spherulites, steam jet cooking pro-

vides a more reliable means of maximally dispersing the

starch as a prerequisite for inclusion complex formation

and spherulite growth. For research purposes, as long as

non-defatted starch is used, microwave processing would

provide a more flexible heating and cooling program useful

for investigating alternative ligands or optimizing heating and

cooling conditions. Information gained from microwave

studies could then be readily adapted to large scale processing

methods using steam jet cooking.

The assistance of Gary Grose with XRD, Arthur Thompsonwith SEM, and Jeanette Little, Janet Berfield, and JanetLingenfelter is gratefully acknowledged.

The authors have declared no conflict of interest.

Figure 10. Phase contrast micrographs of uniform toroidalspherulites prepared by steam jet cooking defatted HAS supple-mented with capric (A) and palmitic (B) acid.

Starch/Starke 2013, 65, 864–874 873


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Comparison of microwave processing and excess steam jet cooking for spherulite production from amylose-fatty acid inclusion complexes