Starch Nanoparticles:A Review
De´borah Le Corre,Julien Bras,and Alain Dufresne*,†
The International School of Paper,Print Media and Biomaterials(Pagora),Grenoble Institute of
Technology-Laboratory of Pulp and Paper Sciences(LGP2)-461,Rue de la Papeterie BP65,
38402Saint Martin d’He`res Cedex,France
Received December14,2009;Revised Manuscript Received April6,2010
Starch is a natural,renewable,and biodegradable polymer produced by many plants as a source of stored energy. It is the second most abundant biomass material in nature.The starch structure has been under research for years, and because of its complexity,an universally accepted model is still lacking(Buleon,A.;et al.Int.J.Biol. Macromol.1998,23,85-112).However,the predominant model for starch is a concentric semicrystalline multiscale structure that allows the production of new nanoelements:(i)starch nanocrystals resulting from the disruption of amorphous domains from semicrystalline granules by acid hydrolysis and(ii)starch nanoparticles produced from gelatinized starch.This paper intends to give a clear overview of starch nanoparticle preparation,characterization,
properties,and applications.Recent studies have shown that they could be used asfillers to improve mechanical and barrier properties of biocomposites.Their use for industrial packaging,continuously looking for innovative solutions for efficient and sustainable systems,is being investigated.Therefore,recently,starch nanoparticles have been the focus of an exponentially increasing number of works devoted to develop biocomposites by blending starch nanoparticles with different biopolymeric matrices.To our knowledge,this topic has never been reviewed, despite several published strategies and conclusions.
Introduction
Starch is a natural,renewable,and biodegradable polymer produced by many plants as a source of stored energy.It is the second most abundant biomass material in nature.It is found in plant roots,stalks,crop seeds,and staple crops such as rice, corn,wheat,tapioca,and potato.1,2The starch industry extracts and refines starches by wet grinding,sieving,and drying.It is either used as extracted from the plant and is called“native starch”,or it undergoes one or more chemical modifications to reach specific properties and is called“modified starch”. Worldwide,the main sources of starch are maize(82%),wheat (8%),potatoes(5%),and cassava(5%)from which tapioca starch is derived.3In2000,the world starch market was estimated to be48.5million tons,including native and mod
ified starches.The value of the output is worth€15billion per year, explaining the industrialists and researchers seeking new proper-ties or high value application.
Meanwhile,nanocomposites show unique properties,because of the nanometric size effect,compared to conventional composite even at lowfiller content.4Nanofillers have strong reinforcing effects,and studies have also shown their positive impact in barrier packaging.However,for decades,studies have been conducted with nonrenewable inorganicfillers and a petroleum-based matrix.Increasing environmental concerns have led to developing newflexible barrier biobased packaging and investigating the potential uses of renewable resources for such an application.However,the uses of these materials have been limited by their poor performances,such as brittleness and poor gas and moisture barrier.
Polysaccharides are good candidates for renewable nanofillers because they have partly crystalline structures conferring interesting properties.Recent reviews have been published on cellulose nanocrystals,5-7which is by far the most studied polysaccharide for nanoparticles.However,as far as we know, nothing similar has been done for starch nanocrystals except for mentioning in book chapters.8,9For these reasons,starch nanoparticles will be presented in detail with a complete overview of their preparation,characterization,and applications. Authors would like to underline that dis
cussions will deal with flexible barrier packaging applications and not food applications. Preliminary chapters on nanocomposites and starch structure will be presented to avoid misunderstanding.
Barrier Properties.Barrier properties increase shelf life duration by protecting the inside product from deteriorations such as oxidation,humidity,and bacteria.Unfortunately,one given material rarely answered to all properties.A material with good barrier properties to water vapor usually has low barrier properties to oxygen and vice versa.The permeation mecha-nisms depend on the chemical nature of the polymer and the penetrant characteristics,explaining why barrier properties are classified according to the type of penetrant.10Most important external parameters are temperature and humidity.They both greatly affect the behavior and the structure of both the polymer and the penetrate.This is why it is very important to know in which conditions tests have been carried out.Unfortunately,this information is often forgotten in the literature.Given the current interest toward biopolymers,water vapor and oxygen perme-ability of the most common ones are shown in Table1.Data available for some synthetic polymers are added for comparison. Even though results from different research teams are difficult to compare due to the differences in methods and measuring conditions,the data show the tendency for most common biopolymers.Several researchers are trying tofind solutions to counter these tendencies and some focus their research on nanocomposites.Tortuo
sity is one of the most important parameters that increases molecule migration pathways and consequently limits permeability.This property can be obtained
*To whom correspondence should be addressed.E-mail:alain.dufresne@
†Present address:Universidade Federal do Rio de Janeiro(UFRJ),
Departamento de Engenharia Metalurgica e de Materiais,Coppe,Rio de
Janeiro,Brazil.
Biomacromolecules2010,11,1139–11531139
10.1021/bm901428y 2010American Chemical Society
Published on Web04/20/2010
with nanofillers and explains why nanocomposites are developed for barrier application.
Nanocomposites
Composites are materials made of two types of components:the matrix whose role is to support and protect the filler materials and transmit and distribute the applied load to them and the mentioned fillers,which are the stronger and stiffer components reinforcing the matrix.Now,in the era of nanotechnologies,the reinforcing materials are nanoscaled.The pioneer work on nanocomposites that was initiated by researchers at Toyota in the early 1990′s created nanoclay reinforced polymers,opening a new research path on composites.Shortly after,researchers started working on cellulose whiskers reinforced polymers.11-13
Nanoparticles (fillers)not only enhance mechanical properties,but also physical properties,such as permeability or fire retardancy.Their properties depend on the nature and effective-ness of interactions at the interfacial region,that is,on both the surface area and the dispersion of the particles.The surface area depends on the dimensions of the dispersed particles from 0.5to 250g/m 2for natural fibers and up to 1000g/m 2for cellulose nanofibrils,exfoliated clays,and carbon nanotubes.9
One of the most wide sprayed classifications of nanopar-ticles is made according to particle shape:(i)Particulate nanocomposites,such as metallic nanoparticles or carbon black-reinforced composites,are generally iso-dimensional and show moderate reinforcement due to their low aspect ratio.They are used to enhance resistance to flammability and decrease permeability or costs.(ii)Elong
ated particles that show better mechanical properties thanks to their high aspect ratio.Such particles include cellulose nanofibrils (also called whiskers or nanocrystals)and carbon nanotubes.(iii)Layered particles,like nanoclays,also referred to as LPN:layered polymer nanocomposites.This latter family is the most used industrially and can show different degrees of dispersion,as shown in Figure 1,namely,intercalated nanocomposites (intercalated polymer chains between layered nanocomposites),exfoliated nanocomposites (separation of individual layers),and flocculated or phase-separated nano-composites,which are also called microcomposites and consequently show the poorer physical properties.Exfoliation is sought by nanocomposite producers as it gives,by far,the best results.In light of reviewed classifications for fillers,starch nanocrystals fit the last category,“layered particles”.
Table 1.Water Vapor Permeability (WVP)of Biopolymer Materials Used in Packaging in Temperate (23°C,50%RH)and Mild Conditions (25°C,75%RH)
material (film thickness)T (°C)RH (%)WVP [1011g/m ·s ·Pa]
PO2[(cm 3·µm)/(day ·m 2·kPa)]
ref.amylose +40%glycerol 23501197110amylopectin +40%glycerol 235014414
110chitosan +40%glycerol
20750.1-04
111
pure chitosan (15.2µm/2µm))20/23b 75/0b 4.50.5203cm 3/m 2·d 111-113corn starch (63.1µm)257517.7112wheat starch (500µm)20n.a. 1.2096a
114cassava (300-4000µm)
255845a
115cassava +20%glycerol (100µm)2350240116waxy maize (300-4000µm)255838a 115pea starch +6.5%sorbitol
221099.4a 117pea starch +10.87%%sorbitol 2210186.1a 117high amylose
255034118zein-coated high amylose 25501170119
PLA 2350 1.34160
120,121PHA
2350150121PHB (61µm)25500.245122ecovio ASTM F12490.92a 1426a 123ecoflex ASTM F1249 1.7a 3329a 123LDPE ASTM D14340.016897a 124PET 230/95b 0.28153.2120NR
2350
0.0107a
1.67a
30
a
Recalculated from the original paper n.a.Data nonavailable in original paper.b
Differing conditions for O 2
permeability.
Figure 1.Possible dispersion of layered particles in a polymeric matrix.Reproduced with permission from ref 135.Copyright 2000Elsevier.
1140Biomacromolecules,Vol.11,No.5,2010Le Corre et al.
There are several techniques for preparing such nanocom-posites.For all techniques there are two steps:mixing and processing,which often occur at the same time.Processing methods are usually the same as for pure polymers:extrusion, injection molding,and casting or compression molding.How-ever,special attention must be brought to the processing temperature when working with organicfillers.The choice of the matrix depends on several parameters such as the application, the compatibility between components,the process,and the costs.However,the current tendency is to use eco-friendly polymers such as PLA or starch blends.For food packaging applications together with mechanical properties,barrier proper-ties are the most important.Therefore,platelet-shaped particles are preferred because they are thought to alter the diffusion path of penetrant molecules and improve the barrier properties of the material.Clays and starch nanocrystals could therefore both befitted forflexible food packaging applications.
Polymer matrix/clay-based nanocomposites have largely dominated the polymer literature,14-19since theirfirst applica-tions as reinforcement in the automotive industry.The packaging industry has focused its attention mainly on layered inorganic solids like clays and silicates due to their availability,low cost, significant enhancement,and relatively simple processability.20 Several studies have reported the effectiveness of nanoclay to decrease water vapor21-24and oxygen permeabilities15,16,25and improv
e mechanical properties.14,26The most widely studied type of clay is montmorillonite(MMT).Attention has been more recently directed toward organic and renewablefillers.
Since thefirst announcement of using cellulose whiskers as a reinforcing phase by Favier et al.in1995,13new nanocom-posite materials with original properties were obtained using cellulose whiskers and microfibrillated cellulose and led to the development of studies on chitin whiskers27-29and starch nanocrystals3,12,30-34by analogy.In comparison to nanoclay, literature on these bionanofillers is scarce.However,most studies report improved mechanical properties and water vapor perme-ability(WVP),21,35,36although a negative impact of microcrys-talline cellulose in a PLA matrix was reported for oxygen permeability despite improved mechanical properties.35This phenomenon could be explained by the already high degree of crystallinity of the PLAfilm.Table2summarizes WVP values of polymeric matricesfilled with mineralfillers compared to organicfillers.Values are in the same order of magnitude. However,for the same polymeric matrix more MCC is needed to reach values comparable to nano ZnO.36
As previously stated,starch is the second most studied organic material for producing nanocrystals.To understand how these nanocrystals are extracted,it isfirst necessary to give more details on starch and its structure.
Starch
After its extraction from plants,starch occurs as aflour-like white powder insoluble in cold water.This powder consists of microscopic granules with diameters ranging from2to100µm, depending on the botanic origin,and with a density of1.5.1 The basic formula of this polymer is(C6H10O5)n,and the glucose monomer is called R-D-glycopyranose(or R-D-glycose)when in cycle.Depending on their botanic origin,starch raw materials have different conversion factors,size,shape,and chemical content,as described in Table3.
Interesting and detailed reviews1,37,38and articles39-41on starch structure have been published and authors recommend referring to them for more details.Starch’s composition was first determined by studying the residue of its total acid hydrolysis.It consists of mainly two glucosidic macromolecules: amylose and amylopectin.In most common types of starch the weight percentages of amylose range between72and82%,and the amylopectins range from18to28%.However,some mutant types of starch have very high amylose content(up to70%and more for amylomaize)and some very low amylose content(1% for waxy maize).
Amylose is defined as a linear molecule of glucose units linked by(1-4)R-D-glycoside bonds,slightly br
anched by (1-6)R-linkages.Amylopectin is a highly branched polymer consisting of relatively short branches of R-D-(1-4)glycopy-
Table2.Water Vapor Permeability(WVP)of Polymeric Matrix Filled Either Mineral Fillers or Organic Fillers a
matrixfiller
filler
content%T(°C)RH(%)
WVP
[1011g/m·s·Pa]
relative
WVP ref.
mineralfillers glycerol plasticized pea starch nano ZnO0257547.6 1.0036
0.5257537.5b0.79
1257527.5b0.58
2257525b0.53
3257522.10.46
4257521.80.46 glycerol plasticized chitosan unfilled27.176.2131 1.0021
Na-MMT526.478.8980.75
cloisite30B524.378.2920.70
nano silver524.578.1950.73
Ag-ion522.377.3960.73 PLA unfilled2550 1.8 1.0022
cloisite Na+52550 2.08 1.16
cloisite30B52550 1.70.94
cloisite20A52550 1.150.64 organicfillers glycerol plasticized pea starch microcrystalline cellulose0257550.1 1.00125
2.5257533b0.66
6257527.5a0.55
8.5257525.50.51
12257530b0.60
xylan sulfonated cellulose whiskers[gmil/hm2
(WVTR·film thickness)]02575304 1.0024 1025751740.57
natural rubber waxy maize starch nanocrystals02550 3.41 1.0030
52550 2.41c0.71
102550 2.36c0.69
202550 1.88c0.55
a Relative WVP refers to WVP values divided by the value obtained for the unfilled matrix.
b Recalculated from thefigure.
c Recalculate
d from th
e original paper.
Starch Nanoparticles Biomacromolecules,Vol.11,No.5,20101141
ranose that are interlinked by R -D -(1-6)-glycosidic linkages
approximately every 22glucose units.8The multiplicity in branching lead Peat et al.42to describe the basic organization of the chains in terms of A,B and C chains.The single C chain per molecule,with a mean degree of polymerization (DP)above 60,carries other chains as branches and contains the terminal reducing end of the amylopectin macromolecule.The A chains are glycosidically linked to the rest of the molecule by their reducing group trough C6of a glucose residue.The B chains are defined as bearing other chains as branches.They are linked to the rest of the molecule by their reducing grou
p on one side and by a R -(1-6)linkage on the other,thus being the backbone of the grape-like macromolecule.From then,several models have been proposed,all referring to the cluster model presented in Figure 2.
Minor components associated with starch granules are of three types:(i)cell-wall fragments,(ii)surface components,and (iii)internal components.The main constituents of surface compo-nents are proteins,enzymes,amino acids,and nucleic acids,whereas internal components are composed mainly of lipids.The proportion of these components depends on the botanical origin.
Starch Nanocrystal Preparation Protocols.Starch structure has been under research for years,and because of its complexity,a universally accepted model is still lacking.1However,in this past decade a model seems predominant.It is a multiscale structure,shown in Figure 3,consisting of the (a)granule (2-100µm)into which we find (b)growth rings (120-500nm)composed of (d)blocklets (20-50nm)made of (c)amorphous and crystalline lamellae (9nm)41containing (g)amylopectin and (h)amylose chains (0.1-1nm).
The shape and particle size of granules depends strongly on its botanic origin.On the surface,pores can be observed,as can be seen in Figure 3a.They are thought to be going through the growth rings to
the hilum (center of the granule).Observed under a microscope and polarized light,starch shows birefrin-gence.The refracted “Maltese cross”corresponding to the crystalline region is characteristic of a radial orientation of the macromolecules.1An X-ray diffraction study showed that starch is a semicrystalline polymer.43Starch granules consist of concentric alternating amorphous and semicrystalline growth rings.They grow by apposition from the hilum of the granule.The number and thickness of these layers depends on the botanical origin of starch.They are thought to be 120-400nm thick.44Details on the structure of the amorphous growth ring are not found in literature.
The blocklet concept was developed in the 1930′s by Hanson and Katz 43but quickly opposed to the fibrillar concept developed by Sterling 48and the following cluster organization model developed by Nikuni et al.,45one reducing group at the hilum and,thus,one macromolecule of amylopectin for the whole granule.It was later corrected by French 44and Lineback,46representing starch layers as clusters of short chains.More recently SEM observations have enabled the observation of blocklets structure.47-49Although the blocklet concept is not commonly mentioned in the literature,it was heavily supported and brought back to discussion by Gallant et al.41They suggested that both semicrystalline and amorphous growth rings are subdivided into respectively large (diameter 20-500nm for wheat)and small (25nm)spherical blocklets.More recently,Tang et al.50supposed the blocklets of the amorphous r
egion and the surface pores (as mentioned earlier)to be defective blocklets,with lower branching molecules.On average,two end-to-end blocklets would constitute a single semicrystalline growth ring.These blocklets have an average size of 100nm in diameter and are proposed to contain 280amylopectin side chain clusters.37Schematically,the semicrystalline growth rings consist of a stack of repeated crystalline and amorphous lamellae (Figure 3).The thickness of the combined layers is 9nm 41regardless the botanic origin.In reality,it is believed that the crystalline region is created by the interwining of chains with a linear length above 10glucose units to form double helixes 38that are packed and form the crystallites,and the amorphous region corresponds to branching points.
Crystallization or double helixes formation can occur either in the same amylopectin branch cluster or between adjacent clusters in three dimensions and is called the superhelical structure.Although this model follows the fibrillar concept,Gallant et al.41supported the helical lamellar model because studies showed that amylopectin lamella were also observed within blocklets (about 10).They calculated that assuming that an amylopectin side chain cluster is 10nm,a small blocklet
Table 3.Conversion Factor and Characteristics of Starches from Different Botanic Origins
botanic source maize wheat rice waxy high amylose cassava potato sweet potato smooth pea wrinkled pea ref.
1,126,1271,126,127371,1271,126,127115,126,128126,127
126,129,130131131conversion factor
0.650.0.30.avg granular size (µm)30302-7155-253-3040-1003-272-4017-30amylose content (%)25-2825-2920-250.560-732820-2519-2233-4860-80crystallinity (%)393638-513919n.r 25n.stalline type
A A A A
B
B B
C
C
B
a
<)not
reported.
Figure 2.Amylopectin cluster model.Reproduced with permission from ref 41.Copyright 1997Elsevier.
1142Biomacromolecules,Vol.11,No.5,2010Le Corre et al.
(20-50nm)is composed of about 2-5side chain clusters.Tang et al.50illustrated this model,as shown in Figure 3d,making amylopectin the backbone of the blocklet structure.Amylose molecules are thought to occur in the granule as individual molecules,randomly interspersed among amylopectin molecules a
nd in close proximity with one another,in both the crystalline and amorphous regions.38Depending on the botanic origin of starch,amylose is preferably found in the amorphous region ,wheat starch),interspersed among amylopectin clusters in both the amorphous and the crystalline regions (e.g.,normal maize starch),in bundles between amylopectin clusters,51or cocrystallized with amylopectin (e.g.,potato starch).51
Native starches contain between 15and 45%of crystalline material.Depending on their X-ray diffraction pattern,starches are categorized in three crystalline types called A,B,and C.Hizukuri et al.39,52postulated that amylopectin chain length was a determining factor for crystalline polymorphism.Imberty et al.53,54proposed a model for the double helices packing configuration to explain difference between A and B types starches.A-type structures are closely packed with water molecules between each double helical structure,whereas B-types are more open and water molecules are located in the central cavity formed by six double helices,as shown in Figure 4.It was later envisaged that branching patterns of the different types of starch may also differ.55It was suggested that the B-type amylopectin branching points are clustered,forming a smaller amorphous lamella,whereas A-type amylopectin branching points are scattered in both the amorphous and the crystalline regions,giving more flexibility to double helixes to pack closely.Gerard et al.56recently confir
med that the distance between two R -(1-6)linkages and the branching density inside each cluster are determining factors for the development of crystallinity in starch granules.Clusters with numerous short chains and short linkage distance produce densely packed structures which crystallizes into the A allomorphic type.Longer chains and
distances lead to a B-type.The C-type starch pattern has been considered to be a mixture of both A-and B-types because its X-ray diffraction pattern can be resolved as a combination of the previous two.It has been suggested by Bogracheva et al.57that C-type starch granules contain both types of polymorph:the B-type at the center of the granule and the A-type at the surrounding.
Several attempts of structural characterization of C-type starch were conducted using acid hydrolysis by Wang et al.58,59They revealed that the core part of C-type starch was preferably hydrolyzed and that hydrolyzed starch showed an A-type diffraction pattern,suggesting that B-type polymorphs constitute mainly the amorphous regions and are more readily hydrolyzed than A-types constituting mainly the crystalline region.This is in agreement with the previous conclusions of Jane et al.55that B-type starches are more acid-resistant than A-types.These conclusions are of importance for starch nanocrystal production.Another V-type was also identified as the result of amylose being complexed with other substances such as iodine,fatty acid,emulsifiers,or butanol.This crystalline form is character-
ized by a simple left helix with six glucose units per turn.60
Cheetman et al.61established a correlation between the maize starch amylose content (0-84%)and both the average chain length of amylopectin and the ratio of short chains to long chains (S/L).Higher amylose content granules display a higher
average
Figure 3.Starch multiscale structure:(a)starch granules from normal maize (30µm),(b)amorphous and semicrystalline growth rings (120-500nm),(c)amorphous and crystalline lamellae (9nm),magnified details of the semicrystalline growth ring,(d)blocklets (20-50nm)constituting a unit of the growth rings,(e)amylopectin double helixes forming the crystalline lamellae of the blocklets,(f)nanocrystals:other representation of the crystalline lamellae called starch nanocrystals when separated by acid hydrolysis,(g)amylopectin’s molecular structure,and (h)amylose’s molecular structure (0.1-1nm).Reproduced with permissions from refs 41and 50.Copyright 1997Elsevier;ref 136.Copyright 1997Science and Technology Facilities
Council.
Figure 4.Double helixes packing configuration according to crystalline type.
Starch Nanoparticles Biomacromolecules,Vol.11,No.5,20101143
chain length and a lower ratio than lower amylose granules.The results showed a transition of crystalline type from A through C to B,accompanying a decrease in the degree of crystallinity from 41.8to 17.2%across a range of apparent amylose content from 0to 84%.These findings are summarized in Table 4together with the conclusions of Imberty et al.53,54and Gerard et al.56mentioned earlier.Jenkins and Donalds 40concluded that an increase in amylose content has the effect of increasing the size of the crystalline lamella and acts to disrupt their packing.Two mechanisms to explain this disrupting have been introduced:first,cocrystallization between amylose and amylopectin chains and,second,the penetration of amylose into amorphous regions.
Starch Nanocrystals (SNC)
First of all,it is important to clarify the terms commonly used.Starch crystallite,starch nanocrystal,microcrystalline starch,and hydrolyzed starch all refer to the crystalline part of starch obtai
ned by hydrolysis but to a different extent (from the most to the least).It has to be distinguished from starch nanoparticles,presented later,which can be amorphous.
First interest in starch nanocrystals has been studied by analogy with cellulose whiskers to be used as reinforcing fillers in a matrix.In 1996,Dufresne et al.12reported a method for producing what they called at the time “microcrystalline starch”and which they reported to be agglomerated particles of a few tens of nanometres in diameter.The procedure consisted of hydrolyzing starch (5wt %)in a 2.2N HCl suspension for 15days.Because it was shown that classical models for polymers containing spherical particles could not explain the reinforcing effect of microcrystals,further studies on the morphology of these microcrystals were conducted by Dufresne and Cavaille ´62in light of aggregate formation and percolation concept.In 2003,
Putaux et al.34revealed the morphology of “nanocrystals resulting from the disruption of the waxy maize starch granules by acid hydrolysis”.After 6weeks of hydrolysis,TEM observations (Figure 5)showed (a)a longitudinal view of lamellar fragments consisting of a stack of elongated elements with a width of 5-7nm and (b)a planar view of an individualized platelet after hydrolysis.Shapes and lateral dimensions were derived from the observation of individual platelets in planar view:a marked 60-65°acute angles for parallelepipedal blocks with a length of 20-40nm and a width of 15-30nm.However,
amber刘逸云more recent publications report bigger starch nanocrystals (40-70nm for potato SNC;6330-8064and 60-150nm 65,66for pea SNC;and 5067and 70-100nm 68for waxy SNC),with round edges 69and found as grape-like aggregates of 1-5µm.The heterogeneity in particle size could be explained by the differences in starch types and also by the difficulty to obtain well-defined pictures of nonaggregated nanocrystals.Contrary to cellulose nanocrystals,starch nano-crystals are not almost 100%crystalline,but rather 45%crystalline,with variations depending on the botanic origin,as recently presented by Le Corre et al.70
Acid hydrolysis has been used for a long time to modify starch and its properties.Nageli 71reported the obtaining of a low molecular weight acid-resistant fraction after the hydrolysis of potato starch at room temperature during 30days in a 15%H 2SO 4suspension.The fraction would be known as Nageli amylodextrin.Lintner 72also gave his name to a hydrolysis process consisting of a 7.5%(w/v)HCl suspension of potato starch at 30-40°C to produce a high molecular weight starch suspension called “lintnerized starch”.Most recent publications use either one of these two acidic conditions.In the industry,starch slurry is treated with dilute HCl or H 2SO 4at 25-55°C for various periods of time to produce “acid-thinned”
starch
Figure 5.First TEM observations of starch nanocrystals:longitudinal view and planar view.Reproduced with permission from ref 34.Copyright 2003American Chemical Society.
Table 4.Correlation between Crystalline Type and Nanostructure of Maize Starch
crystalline type
A
C B
ref.amylose content (%)low (0-30%)40%
high (50-90%)61amylopectin CL
short
long
52L/S amylopectin chains high (10mol %)low (4mol %)61crystallinity
high (30-40%)
low (17-20%)
61double helices structure closed packed arrangement with water molecules between each double helical structure
more open arrangement with water molecules are located in the central cavity formed by six double helices
54branching density and distance between R (1-6)linkages high density and short distance low density and long distance 56
crystalline lamellae
thin
large
1144Biomacromolecules,Vol.11,No.5,2010Le Corre et al.
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