Morphological, Thermal, and Rheological Properties of Starches from

Article pubs.acs.org/JAFC

Morphological, Thermal, and Rheological Properties of Starches from Maize Mutants Deficient in Starch Synthase III Fan Zhu,*,† Eric Bertoft,§ and Guantian Li† †

School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, Minnesota 55455, United States

§

ABSTRACT: Morphological, thermal, and rheological properties of starches from maize mutants deficient in starch synthase III (SSIII) with a common genetic background (W64A) were studied and compared with the wild type. SSIII deficiency reduced granule size of the starches from 16.7 to ∼11 μm (volume-weighted mean). Thermal analysis showed that SSIII deficiency decreased the enthalpy change of starch during gelatinization. Steady shear analysis showed that SSIII deficiency decreased the consistency coefficient and yield stress during steady shearing, whereas additional deficiency in granule-bound starch synthase (GBSS) increased these values. Dynamic oscillatory analysis showed that SSIII deficiency decreased G′ at 90 °C during heating and increased it when the paste was cooled to 25 °C at 40 Hz during a frequency sweep. Additional GBSS deficiency further decreased the G′. Structural and compositional bases responsible for these changes in physical properties of the starches are discussed. This study highlighted the relationship between SSIII and some physicochemical properties of maize starch. KEYWORDS: dull maize mutant, starch synthase III, thermal property, rheological property, granule morphology

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INTRODUCTION Starch is a major ingredient in our diet. It is also widely used in various food and nonfood industries. Sometimes, native starches from various botanical origins are chemically, physically, and enzymatically modified to suit various applications.1 Physical modifications are green and desired by some consumers, but are limited in their capacity to expand the range of starch functionalities. Chemical and enzymatic modifications are not widely used. However, achieving regulatory approval of these modified starches (especially chemicaly modified starches) is almost impossible for the food industry. Starches from mutant genotypes of major crops may provide an alternative source for desired functional properties.1,2 They may either be complementary to or replace the chemically modified starches, reducing the cost and being environmentally friendly. The altered properties of starch by genetic means may be positive or negative, depending on specific applications. Maize is a major starch-producing crop. World production reached over 1 billion tonnes in 2014.3 There are diverse maize mutants that produce starches with altered composition, structures, and properties.2,4 The mutations can be genetically single, double, or triple. The interactions between different genes of the mutants may be synergistic or antagonistic for desired structures and physicochemical properties.4−6 The outcome of the genetic mutation also depends on the genetic background of the plant.7 Mutations of the waxy (wx) locus, which encodes granule-bound starch synthase (GBSS), result in waxy appearance of the maize kernels.8 GBSS deficiency results in the absence of amylose in starch. Mutations of the dull1 (du1) locus, which encodes starch synthase III (SSIII), result in dull, tarnished, and glassy kernels of maize.9 SSIII deficiency increased the amylose contents of maize starch.10 For the amylopectin, the deficiency decreased the amount of short Afpchains, increased the amount of short Bfp-chains, and shortened © XXXX American Chemical Society

external and internal chain lengths. At the cluster level, the deficiency resulted in larger clusters with higher numbers of chains and building blocks per cluster. The percentage of group 2 building blocks consisting of two chains also increased.10,11 These structural changes led to altered physicochemical properties of maize starch as shown in the present study, which is still to be better explored for potential applications. Knowledge of diverse aspects of gelatinization and retrogradation of starch, for example, the structural basis, is fundamental for diverse applications. Various aspects of gelatinization and retrogradation can be probed by different methods such as differential scanning calorimetry and rheology. They are complementary to each other for a holistic view of the process. In this paper, the influence of dull mutation on granule morphological, thermal, and rheological properties of maize starch is studied. The structure−property relationships are discussed. Genetic information on these dull maize mutants with a common genetic background (W64A) was well described previously.9 The composition and amylopectin molecular structures of these starches were characterized.10 The cluster and building block structures of the amylopectins were also detailed.11 The nomenclature and building block backbone concept of amylopectin structure employed in this paper for discussion have been reviewed in detail.12 Readers are strongly encouraged to refer to these previous publications to gain background information for the present study.

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MATERIALS AND METHODS

Starches. The maize samples were one wild type (W64A), two single dull1 mutants (du1-M3 and du1-ref), and one double dull1-waxy

Received: March 19, 2016 Revised: August 12, 2016 Accepted: August 13, 2016

A

DOI: 10.1021/acs.jafc.6b01265 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Article

Journal of Agricultural and Food Chemistry Table 1. Composition and Amylopectin Structures of Starches from Maize dull1 Mutants of W64A Inbred Line9,10a cluster genotype

AAM (%)

TAM (%)

Afp-chains (mol %)

Bfp-chains (mol %)

ECL

ICL

CL

DP

W64A du1-ref du1-M3 du1-wx

29.6 36.5 40.5 0.8

21.5 25.4 30.2 0.8

6.4 4.3 4.3 4.5

19.8 21.1 21.3 21.8

13.1 12.5 11.9 11.9

5.6 5.2 5.1 5.3

19.7 18.7 18.0 18.2

68.2 87.9 88.1 80.6

NC

IBCL

NBbl

group 2 (mol %)

groups 4−6 (mol %)

12.5 15.2 15 14.5

6.2 6.2 6.2 6.5

4.2 5.6 5.5 5.2

51.1 55.3 53.1 55.1

21.8 19.8 20.4 18.7

a AAM, apparent amylose content; TAM, true amylose content; Afp-chain, fingerprint A-chains; Bfp-chains, fingerprint B-chains; ECL, external chain length; ICL, internal chain length; CL, average chain length; DP, degree of polymerization; NC, number of chains; IB-CL, interblock chain length; NBbl, number of building blocks; groups 2 and 4−6, building blocks with number of chains per block as 2, 4−6, respectively.

°C and from 95 to 25 °C at a rate of 2 °C/min. The paste was equilibrated at 25 °C for 5 min before frequency sweep from 0.1 to 40 Hz at 25 °C. Storage modulus (G′, solid component), loss modulus (G″, liquid component), and loss tangent (tan δ = G″/G′) were recorded. Data of frequency sweep were fitted by the equations

mutant (du1-wx). They had the same genetic background (W64A) and were the same samples as described in a previous paper.9 Spontaneous mutations were screened by the dull phenotype appearance on segregating ears of self-pollinated F1 plants.9 All of the maize crops were field grown in summer nurseries at Iowa State University. The genetic locations of the mutations were detailed previously.9 The du1 mutations caused the loss of SSIII enzyme activity. A truncated SSIII was expressed at a low level in du1-ref, whereas no SSIII was produced in du1-M3 and du1-wx. The du1-wx contained no amylose. The pericarp and embryo of maize kernels were removed, and the remaining endosperms were steeped in a sodium metabisulfite (0.3%) and lactic acid (1%) solution before grinding to a fine slurry. Starch was isolated by centrifugation and washing as described previously.13 The composition and molecular structures of the starches were detailed in previous papers and are summarized here (Table 1).10,11 Starch Surface Morphology. The surface morphology of the starch was examined by using a Hitachi S-570 scanning electron microscope (Hitachi Scientific Instruments, Rexdale, Canada). The starch was sputtered with gold dust (15 nm) before examination. The working distance used was 15 mm, and the voltage was 10 kV. Starch Granule Size Distribution. The particle size distribution of starch granules was measured by the Mastersizer 2000 particle size analyzer (Malvern Instruments Ltd., Worcestershire, UK). The refractive index for the aqueous continuous phase was 1.33, and that for the starch particles was 1.523. The sample absorption was 0.001. Differential Scanning Calorimetry. Thermal properties were analyzed by using a differential scanning calorimeter (DSC) (Q1000 Series, TA Instruments, New Castle, DE, USA). Starch (2.5 mg, db) was weighed into an aluminum pan, and double-deionized water (7.5 μL) was added. The sealed pan was equilibrated at room temperature for 1 h before heating from 25 to 90 °C at a rate of 10 °C/min. An empty pan was used as reference. Gelatinization parameters including onset (To), peak (Tp), and conclusion (Tc) temperatures and enthalpy change (ΔH) were recorded. Rheology. Rheological properties were measured by a Physica MCR 301 rheometer (Anton Paar, Graz, Austria) using parallel metal plates with a diameter of 40 mm and a gap of 500 μm. The edge of the gap was covered with a thin layer of sunflower oil to minimize evaporation. Steady shear and dynamic oscillatory analysis were done following the methods used previously with some modification as outlined below.14 Steady Shear Analysis. A gelatinized starch suspension (5% solid content) was conditioned at 25 °C for 1 min. Thereafter, the sample was sheared from 0.1 to 1000 s−1 (upward) and then from 1000 to 0.1 s−1 (downward) at 25 °C to describe the steady state flow behavior. The power law (eq 1) and Herschel−Bulkley (eq 2) equations were employed for data modeling.

δ = K × γn

(3)

log G″ = KG ″ × log f

(4)

where f is the frequency (Hz) and KG′ and KG″ are the slope. Statistical Analysis. Tests were performed in triplicate, and data were analyzed by SPSS software (version 22.0, IBM Corp., Armonk, NY, USA). Differences between means of data were compared by least significant difference at a significance level of p < 0.05. For the flow modeling by Herschel−Bulkley equation, the starting values for δ0, K, and n were set as 1, 1, and 0.1, respectively.

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RESULTS AND DISCUSSION Morphology. The granule diameters and morphology of starches from the wild type (W64A) and the mutants generally agreed with the literature (Table 2; Figures 1 and 2).4,15 The

Table 2. Granule Size of Starches from Maize dull1 Mutants of W64A Inbred Linea genotype

D[4,3]

D[3,2]

d(0.5)

d(0.9)


16.733a 10.772c 10.92c 11.609b

10.305a 6.811b 6.467c 6.667bc

15.917a 10.316c 10.336c 10.917b

24.265a 15.338d 16.163c 17.516b

a

D[4,3], volume-weighted mean; D[3,2], surface-weighted mean; d(0.5), volume median diameter; d(0.9), 90% of the particles are smaller than this diameter. Values with different letters in the same column indicate significant difference (p < 0.05).

granule size of the wild type was on average larger with a wider size distribution than that of du1 mutants as revealed by SEM (Figure 1) and particle size distribution analysis (Figure 2). This agreed with a previous study by Wang et al., who showed that the dull mutation (Oh43 inbred line) decreased the average size of maize starch from 11.6 to 7.8 μm.4 It also agreed with previous studies on rice and Arabidopsis leaf starches.5,16 Previous studies showed that increasing amylose content may increase the irregularity of starch granules.4 However, the shapes of the single du1 mutants appeared similar to that of the wild type. It appeared that the type of SSIII deficiency (du1-ref vs du1-M3) had little effect on granule shape and size. The double mutant du1-wx, with only trace amount of amylose, had the most irregular shape (with some depressions in the surface) among the maize starches analyzed here, suggesting interactions between SSIII and GBSS in determining the shape of granules. The SSIII appeared much more dominant over the size of the

(1)

δ = δ0 + K × γ n

log G′ = KG ′ × log f

(2) −1

δ is shear stress (Pa), γ shear rate (s ), K consistency coefficient (Pa· sn), n flow behavior index (dimensionless), and δ0 yield stress (Pa). Dynamic Oscillatory Analysis. The strain was 2%, and the frequency was 1 Hz. Sample (20% solid content) was equilibrated for 2 min at 40 °C. Then the temperature was ramped from 40 to 95 B


Article

Journal of Agricultural and Food Chemistry

Thermal Properties. The dull mutation greatly altered the thermal properties of maize starch (Table 3). The mutants had Table 3. Thermal Properties of Starches from Maize dull1 Mutants of W64A Inbred Line As Measured by DSCa genotype

To (°C)

Tp (°C)

Tc (°C)

ΔH (J/g)


62.2c 63.1bc 65.0a 64.1ab

69.9ab 69.0c 69.6bc 71.7a

75.7c 76.2c 77.2b 78.7a

14.7a 10.9c 10.4c 12.7b

a To, onset temperature (°C); Tp, peak temperature (°C); Tc, conclusion temperature (°C); ΔH, enthalpy change (J/g). Values with different letters in the same column indicate significant difference (p < 0.05).

increased To and reduced ΔH compared to the wild type. Compared with single mutants, du1-wx had higher Tp, Tc, and ΔH. The results partially agreed with a previous study on maize starch with the genetic background of Oh43.4 It was assumed that Afp-chains may cause structural defects in the crystalline regions in the granules, reducing the crystal stability.18 Indeed, W64A, which had a higher amount of Afp-chains than the mutants, possessed somewhat lower To and Tc than the mutants. The external unit chains of amylopectin are believed to interact to form double helices and crystals. The reduced length of external chains (ECL) of the mutant samples decreased the amount of ordered structure in the granules as shown by a lower ΔH. The amylose component is believed to be mostly amorphous in the granules. The absence of amylose in du1-wx possibly contributed to a higher degree of ordered structure. Therefore, ΔH of du1-wx was higher than that of du1-ref and du1-M3. The interblock chain length (IB-CL) of du1-wx was somewhat longer than that of the others, and du1-wx had the highest gelatinization temperatures among the samples. It was found that IB-CL was positively linked with the onset gelatinization temperature.19 Longer IB-CL could facilitate parallel alignment of external chains of clusters to form a more perfect structure.19

Figure 1. SEM of maize starch granules.

maize starch than GBSS. Indeed, normal and waxy maize starches have rather similar morphological characteristics.17 The morphology of starch can be a critical factor in the functional properties as shown below.

Figure 2. Granule size distribution of maize starches. C


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Journal of Agricultural and Food Chemistry

Table 4. Power Law and Herschel−Bulkley Models for Steady Shear Properties of Starches from Maize Dull1 Mutants of W64A Inbred Linea upward (0.1−1000 s−1) power law

downward (1000−0.1 s−1)

Herschel−Bulkley

power law

Herschel−Bulkley

genotype

K

n

R2

δ0

K

n

R2

K

n

R2

δ0

K

n

R2


6.69b 0.988c 0.65c 15.22a

0.32b 0.54a 0.54a 0.25c

0.98 0.99 0.98 0.97

5.227b 0.094c 0.168c 8.247a

2.15b 0.934c 0.423c 6.16a

0.540b 0.545b 0.619a 0.401c

0.99 0.99 0.99 0.99

5.82a 1.33b 0.69b 4.60a

0.349c 0.471ab 0.530a 0.418bc

0.98 0.99 0.97 0.99

3.723a 0.417c 0.269c 2.389b

2.362a 0.822b 0.433b 2.15a

0.520c 0.566b 0.62a 0.553b

0.99 0.99 0.99 0.99

K, consistency coefficient (Pa·sn); n, flow behavior index (dimensionless); δ0 yield stress (Pa); R2, coefficient of determination. Values with different letters in the same column indicate significant difference (p < 0.05). a

Figure 3. Flow curves of maize starch pastes during steady shearing from 0 to 1000 s−1.

reduced K, n, and δ0 values of starch pastes during both upward (0.1−1000 s−1) and downward (1000−0.1 s−1) shearing. δ0 and K of du1-wx were higher than those of the wild type (W64A) during upward shearing and lower than those of the wild type during downward shearing. This agreed with a previous study on normal and waxy maize starches.21 Therefore, the flow properties of maize starch are much more determined by the GBSS deficiency than by the SSIII deficiency for the double mutant. The flow properties of starch paste are determined by solid concentration, starch type, temperature, and gel preparation method.22 Amylose can be an important component,21−23 and the structural basis for the flow behaviors of W64A, du1-ref, and du1-M3 may be different from that of du1-wx as amylose tends to play a role in the rheological properties of starch. In this study, the preparation method of the starch paste with no agitation might have much retained the granule shape (ghost structure).24 A previous study showed that a good preservation of granule shape after pasting gave a

A similar effect would be expected for segments between the clusters, which generally are longer than the interblock segments (the intercluster chain length being ≥9 glucosyl residues).12 Large clusters suggest proportionally fewer intercluster segments and, thus, the larger cluster size and higher amounts of group 2 building blocks of the mutants contributed to less efficient packing of the external chains and crystals into ordered structure.20 Rheological Properties. Flow. Both power law (eq 1) and Herschel−Bulkley (eq 2) models well described the flow behaviors of starch pastes (R2 > 0.97) (Table 4; Figure 3). The data could be better fitted by the latter as reflected by higher R2 (R2 = 0.99), and the Herschel−Bulkley model is used for discussion here. This agreed with previous studies on other starches.14 All of the starch pastes showed shear-thinning and non-Newtonian flow behavior as expected.14,21,22 Dull mutation greatly altered the flow behavior of maize starch. For the single mutants (du1-M3 and du1-ref), the mutation significantly D


Article

Journal of Agricultural and Food Chemistry Table 5. Dynamic Oscillatory Rheology of Starches from Maize dull1 Mutants of W64A Inbred Linea heating (40−90 °C)

cooling (90−25 °C)

frequency sweep (0.1−40 Hz)

genotype

TG′max

G′max

tan δG′max

G′90C

tan δ90C

G′25C

tan δ25C

G′40Hz

tan δ40Hz

KG′

R2G′

KG″

R2G″


72.4b 76.4a 77.4a 71.9b

4090a 3710b 2410c 1140d

0.0868b 0.0751c 0.0700c 0.144a

1835a 1620b 1020c 283d

0.0695c 0.0880b 0.0922b 0.124a

6290b 7645a 6400b 276c

0.0283c 0.222a 0.150b 0.136b

6660c 8795a 7350b 564d

0.0712c 0.234b 0.226b 0.393a

0.0209c 0.0490b 0.0546b 0.1351a

0.95 0.80 0.88 0.89

0.2295b 0.0931d 0.1377c 0.3770a

0.98 0.90 0.99 0.98

a TG′max, temperature when G′ reaches the maximum during heating; G′max, maximum G′ during heating; tan δG′max, tan δ when G′ reaches the maximum during heating; G′90C, G′ at 90 °C during heating; tan δ90C, tan δ at 90 °C during heating; G′25C, G′ at 25 °C during cooling; tan δ25C, tan δ at 25 °C during cooling; G′40Hz, G′ at 40 Hz during frequency sweep; tan δ40Hz, tan δ at 40 Hz during frequency sweep; R2, coefficient of determination. Values with different letters in the same column indicate significant difference (p < 0.05).

Figure 4. Development of G′ of maize starches during heating cycle of dynamic oscillatory rheology.

stronger mechanical strength to the starch paste.25 The larger granule size of W64A (therefore with more ghost structure) may be responsible for the higher δ0 and K during shearing as compared with du1-ref and du1-M3. This agreed with Wang et al., who showed that dull mutation greatly reduced the gel firmness of maize starch (genetic background Oh43).4 The amylose contents of the single mutants were higher than that of the wild type, and du1-M3 had a higher amylose content than du1-ref by 4−5% (Table 1). During pasting, the starch molecules (mostly amylose) leached out of the granules, forming a matrix, which embedded the granule remnants.26 More amylose better re-inforced the paste. Therefore, the higher n values of du1-M3 (less shear-thinning) may be explained by assuming that this is the case for the maize starches of this study. The granules of W64A were larger, and the granular remnants after pasting rendered resistance to shearing. The structural basis for the changes in n remains to be better explored using simpler systems. The higher δ0 and K of du1-wx may be mostly due to the lack of a matrix of leached amylose, which resulted in closer associations between swollen granules. Upon shearing to 1000 s−1, this type of structure without amylose broke down much more easily, resulting in lower δ0 of du1-wx (2.4 Pa) than the wild type (3.7 Pa). This is also reflected by the lower n of du1wx than that of the wild type during the upward shearing.

Despite the dominant roles of amylose and granule size in the flow behavior of starch paste, interactions of amylopectin unit chains may contribute to paste rheology.27,28 Dull mutation results in larger clusters with higher number of chains per cluster (Table 1). The intercluster chain length of amylopectin is longer than the interblock chain length.12 The chains of amylopectin with large clusters are, therefore, positioned closer to each other than if the clusters are small. This may facilitate interactions of disordered amylopectin unit chains through entanglements, and the buildup of a more coherent structure. This hypothesis concerning the intramolecular interactions remains to be tested by using starches with more defined structure and composition. The intermolecular interactions between amylopectin molecules may be more related to the intercluster chain length, which remains to be studied. Dynamic Oscillatory Rheology. Dull mutation greatly changed the dynamic rheological properties of maize starches (Table 5; Figure 4). During heating (40−90 °C), starches of du1-ref and du1-M3 mutants had higher TG′max and tan δ90C and lower G′max and tan δG′max than the wild type. Additional wx mutation (du1-wx) further decreased the G′max and G′90C and increased the tan δG′max and tan δ90C. During cooling (90−40 °C), starches of du1-ref and du1-M3 mutants had higher tan δ25C, whereas the double mutant (du1-wx) had a much lower E


Article

Journal of Agricultural and Food Chemistry G′25C, as compared with the wild type. During frequency sweep (0.1−40 Hz), single mutants (du1-ref and du1-M3) had higher G′40Hz and tan δ40Hz and du1-wx had lower G′40Hz and higher tan δ40Hz, as compared with W64A. Regression analysis of G′ and G″ by eqs 3 and 4 well described dynamic rheology during frequency sweep (R2 > 0.8). Starch pastes of all the mutants had higher KG′ than W64A. Single mutants (du1-ref and du1M3) had lower KG″, whereas du1-wx had higher KG″, as compared with the wild type. Previous studies employing a Rapid Visco-Analyzer (RVA) showed that dull mutations in maize samples of various genetic background decreased various viscosities of the pasting event.29,30 Indeed, the G′max values of both du1-ref and du1-M3 were lower than that of the wild type. The larger granule size was probably responsible for higher G′ of the wild type. The higher amount of Afp-chains may further contribute to the structural defects in the granules,18 leading to a lower TG′max and the temperature when the G′ starts to take off. This agreed with the gelatinization temperatures of the samples as measured by DSC (Table 3). The double mutant du1-wx had the lowest G′max among the samples. Therefore, the additional deficiency in GBSS further reduced the G′ during heating. Indeed, amylose leaches out during heating, filling the space between the swollen granules and re-inforcing the network.31 During cooling, the starch molecules interact with each other and reassociate for gelation. Starches of du1-ref and du1-M3 had higher G′25C and tan δ25C than the wild type, whereas the double mutant had the lowest G′25C. Amylose reordering is mostly responsible for the increasing G′.31 The higher amylose contents of du1-ref and du1-M3 mutants led to higher G′25C and solid components of the gel. The gel was further subjected to frequency sweep (0.1−40 Hz) to reveal the viscoelastic properties. G′40Hz values of du1-ref and du1-M3 were higher than that of the wild type, further confirming the role of amylose in gelation. Values of tan δ40Hz of all the mutant samples (du1-ref, du1-M3, and du1-wx) were higher than that of the wild type. Amylose and amylopectin with different structures may lead to different extents of phase separation during gelation.23 The higher percentage of solid component of the mutant samples may be attributed to their amylopectin structure. The larger cluster with more chains per cluster may facilitate the unit chain interactions, increasing the amount of hydrogen bonding and solid component in the system. The higher percentage of group 2 building blocks and lower amount of large blocks (less steric hindrance due to branching) may be more suitable for the formation of junction zones, which can be a major solid component. In conclusion, morphological and physical properties of maize starch were greatly altered by SSIII deficiency. SSIII deficiency decreased the granule size of maize starch and the ΔH of gelatinization. SSIII deficiency decreased yield stress and the consistency coefficient of flow behavior while increasing the G′ of dynamic oscillatory tests during cooling and frequency sweeping. The altered physical properties could be attributed to changes in the composition and structure of starch and amylopectin clusters. The deficiency in both SSIII and GBSS led to rather different rheological behaviors. Therefore, the structural bases for the physicochemical changes of single and double mutants are different. The relatively small differences in physicochemical properties between du1-ref and du1-M3 mutants indicate possible manipulation of different du1 alleles for dull mutations. The results of this study further confirmed that factors affecting the gelatinization and rheological properties include granule size, starch composition, and amylopectin

molecular structure and that the effects depend on the specific technique used for gelatinization measurement.

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AUTHOR INFORMATION

Corresponding Author

*(F.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Alan M. Myers kindly provided the starch samples. Bruce Manion kindly helped with the particle size distribution analysis.

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DEDICATION Dedicated to the memory of Dr. Koushik Seetharaman (1966− 2014). REFERENCES

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Morphological, Thermal, and Rheological Properties of Starches from

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