Physicochemical Properties of Canna edulis Ker Starch on Heat-Moisture Treatment

Abstract

Canna edulis Ker starch was modified by heat-moisture treatment at moisture levels ranging from 18 to 27 g/100 g starch and its physicochemical properties were investigated. Amylose content, swelling power, solubility as well as water and oil absorption capacity in native starch were higher than in all treated starches. However, alkaline water retention and acid susceptibility of native starch were lower, along with different extent of amylose leaching. The result in the X-ray diffraction measurement revealed that the crystalline type of the starch gradually changed from B-type to A-type, and the degree of crystallinity changed. Investigation on thermal properties showed that the gelatinization enthalpy decreased, whereas the onset temperature, peak temperature, concluding temperature and transition temperature range increased in modified starch than in native starch. In addition, all modified starches exhibited remarkably low values of peak viscosity, hot pasting viscosity and final viscosity, compared to those of native starch.

Keywords:

• Canna edulis
• Heat-moisture treatment
• Physicochemical properties

INTRODUCTION

Canna edulis Ker belongs to the genus Canna (Cannceae), which is mainly cultivated in South America, Vietnam, Thailand and China. The dry rhizome of C. edulis contains abundant starches (70–80 g/100 g dry rhizome), which are reported more digestible than other kinds of starches. Due to climatic adaptability, C. edulis is largely cultivated for starch production in many regions all over the world. Consequently, there is a great demand to exploit this plant into valuable foods or related products.

In order to get a wider application, starch could be modified physically by heat-moisture treatment (HMT), which is a process that involves heating of starch at elevated temperature with incubation of starch granules in varying moisture levels for a certain period, and this physical treatment can change starch properties using simple and environmentally safe process. As reported, heat-moisture treated starches display increased paste stability and excellent freeze-thaw stability, and benefit for human health in view of decreased digestibility.[1–5]

To explore and develop C. edulis as a novel alternative starch source, more comprehensive information on chemical and physical characteristics of C. edulis starch is necessary. However, investigations on this topic are very limited, and almost no work has been reported on the influence of HMT on this starch.[6–10] Therefore, such a study will be important to extensive utilization of C. edulis starch. In the present work, starch was extracted and purified from rhizomes of C. edulis, and then modified by HMT. Moreover, the impacts of HMT on physicochemical properties of this starch, including amylose content, swelling power, solubility, alkaline water retention, acid susceptibility, extent of amylose leaching, freeze–thaw stability, crystalline and thermal characteristics, pasting properties as well as water and oil absorption capacities, were determined.

MATERIALS AND METHODS

Sample and Reagents

Rhizomes of C. edulis were obtained from China Guizhou Ziyun Jiahe Chemical Co. Ltd. All the chemicals used in the experiments were of analytical grade, and water was doubly distilled.

Starch Isolation and Purification

The slurry of C. edulis rhizomes was prepared by adding ascorbic acid solution (0.1%) into approximately 1-cm cubes of cleaned rhizome and crushing by food processing machine (JYL-390, Joyoung Co. Ltd, Beijing, China). The pulp in slurry was removed by screening through gauze, and the suspension obtained was filtered through a 200 μm sieve. The filtrate was allowed to settle until a dense, firm starch layer was deposited. Starch cake was rewashed three times, dried in the air for 48 h at 30 ± 2°C and stored at room temperature in a desiccator for further analysis.

Starch Modification

According to the method of Franco et al.,[11] the moisture levels of the starch were adjusted to 18, 21, 24, and 27 g/100 g starch by adding appropriate amount of distilled water. The mixtures were well stirred and sealed in glass jars, followed by keeping for 24 h at room temperature and heating in an air oven at 110°C for 16 h. After cooling, the jars were opened, and the starch samples were dried in the air to uniform moisture content.

Granule Morphology and Size

Starch granules were observed under normal and polarized light microscope (Nikon Eclipse E200, Japan) equipped with a camera set (Daheng Image DH-M3100UC, China), and the granule sizes were analyzed by image analysis software.

Swelling Power and Solubility

Effect of temperature on swelling power and solubility

The experiment was carried out according to the method of Olayinka et al.[5] In brief, starch sample (1.0 g) was accurately weighed (W₁ ) and dispersed in 50 ml of water. The slurries were heated at the desired temperature 60, 70, 80 and 90°C for 30 min with continuous shaking by Water-bathing Constant Temperature Vibrator, and then centrifuged (1870 × g, 30 min). 5 ml of supernatant was dried to a constant weight at 110°C. The residue after drying the supernatant represented the amount of starches solubilized in water. Solubility was calculated as grams per 100 g of starch on a dry weight basis. The residue obtained after centrifuged was weighed (W₂ ). Then the swelling power of starch was calculated by:

(1)

Effect of pH on swelling power and solubility

As above described, starch sample (1 g) was accurately weighed and dispersed in 50 ml water. The pH was adjusted to the desired values (2, 4, 6, 8, 10, and 12) with 0.1 M HCl or NaOH. The slurry was allowed to stand at 30 ± 2°C for 30 min and centrifuged (1870 × g, 30 min). The swelling power and solubility were determined by Eq. (1).

The Retention of Alkaline Water and the Capacities of Water and Oil Absorption

The methods of Beuchat[12] and Olayinka et al.[5] were employed to study alkaline water retention, as well as water and oil absorption capacities of native and heat-moisture treated starches.

Extent of Amylose Leaching

Starches (25 mg) in water (5 ml) were heated in tubes (70–90°C) for 30 min. The tubes were cooled to room temperature and centrifuged at 1870 × g for 20 min. The supernatant liquid (1 ml) was withdrawn, and its amylose content was determined by the method of Chrastil.[13]

Freeze–thaw Stability

Aqueous suspensions including water (50 ml) and starches (3 g) were rapidly heated to 95°C with constant shaking by Water-bathing Constant Temperature Vibrator. These suspensions were maintained at 95°C for 30 min before being cooled to room temperature. The gels obtained were subjected to cold storage at 4°C for 16 h and frozen at −16°C for 24 h. To measure freeze–thaw stability, the gels were thawed at room temperature for 6 h and refrozen at −16°C. Ten cycles of freeze-thaw were performed. The water excluded (V) was determined through centrifuging at 1870 × g for 20 min. The extent of freeze-thaw stability was calculated by:

(2)

Acid Susceptibility

Starches (1% w/v) were hydrolyzed with 1.35 M H₂SO₄ at 37°C for periods. At 24 h intervals, aliquots of the reaction mixtures were collected and centrifuged at 1870 × g for 20 min. The supernatant liquid was assayed for carbohydrates by phenol-sulfuric method.[14] The extent of hydrolysis was determined by expressing the solubilized carbohydrates as a percentage of the initial starches.

X-ray Powder Diffraction

X-ray diffractograms were obtained with a Rigaku D/max-2200/PC X-ray diffractometer with a chart speed of 10 mm/min. The starch powder was scanned through the 2θ range of 5–40°. Traces were obtained using a Cu-Kα radiation detector with a nickel filter and scintillation counter operating under the following conditions: 40 KV, 20 mA, 1/2°/1/2° divergence slit/scattering slit, 0.30 mm receiving slit, 1 s time constant and scanning rate of 3°/min. The degree of crystallinity of samples was quantitatively estimated following the method of Nara and Komiya.[15]

Differential Scanning Calorimeter

Gelatinization characteristics of native and heat-moisture treated starch granules were determined using a Perkin-Elmer DSC-1 differential scanning calorimeter (Norwalk, CT), equipped with a thermal analysis software. The mixture of water and starches (1: 3.5) was kept overnight at room temperature and placed into pre-weighed DSC pan, which was sealed, reweighed and allowed to stand for 2 h at room temperature. The scanning temperature range and the heating rate were, respectively, 25–120°C and 5°C/min. An empty pan was used as reference for all measurements. The transition temperatures reported were the onset (T_o ), peak (T_p ) and conclusion (T_c ) temperature of gelatinization endotherm. Indium was used for calibration. The enthalpy (ΔH) of gelatinization endotherm was estimated by integrating the area between the thermogram and a base line under the peak and expressed in terms of joules per unit weight of starch (J/g).

Pasting Properties

Pasting profiles of C. edulis starch slurries were investigated by Rapid Visco-Analyzer (RVA) model 3D (Newport Scientific Pvt. Ltd., Warriewood, Australia) equipped with thermocline software. A mixture of starches (3.5 g) and water (25.0 m1) was held at 50°C for 1 min, heated to 95°C at 12.2°C/min and further held at 95°C for 5 min. Subsequently the mixture was cooled to 50°C with the same temperature speed and kept for 2.1 min. The viscosity of starch paste including peak viscosity (PV), hot pasting viscosity at 95°C (HPV), final viscosity at 50°C (FV) and gelatinization temperature were recorded, and breakdown (BDV) and setback (SBV) of starch paste viscosity were also further calculated.

Statistical Analysis

All determinations were triplicates, and mean values and standard deviations were calculated. Analysis of variance (ANOVA) was performed, and the mean separation was done by LSD (P ≤ 0.05) using SPSS 13.0 program for windows (SPSS Inc., IL, USA).

RESULTS AND DISCUSSION

Granule Morphology and Size

As shown in Fig. 1, the starch was mainly consisted of simple granules with obvious layers, and their shapes were elliptical, oval, spherical and polygonal. Some of granules were fragmented into pieces of varying sizes, indicating that fragmentation occurred in the process of starch extraction and isolation. For the modified starches, the surface of some granules showed deep cracks, and some granules appeared to be broken up into a great number of separated pieces after HMT. Under the polarized light microscope, all starches showed birefringence, confirming the presence of non-gelatinized granules. Furthermore, for all the starches, the granule sizes showed a wide distribution range from 20 to 280 μm, and the most of granules were sized 50 to 100 μm.

Figure 1 Light micrographs: Native starch: (A₁), CHMT₁₈: (B₁), CHMT₂₁: (C₁), CHMT₂₄: (D₁), CHMT₂₇: (E₁) and polarized light micrographs: Native starch: (A₂), CHMT₁₈: (B₂), CHMT₂₁: (C₂), CHMT₂₄: (D₂), CHMT₂₇: (E₂) of native and heat-moisture treated Canna edulis Ker starches.

Swelling Power and Solubility

Effect of temperature on swelling power and solubility

Temperature exerted a pronounced effect on swelling power and solubility of native and modified starches as shown in Table 1. With the increase of temperature, swelling power and solubility of all starches obviously increased, which could be attributed that temperature might accelerate water entry into amorphous regions or inhibit formation of amylose-lipid complexes. The increased rate in native starch was much higher than in modified starches. However, with the increase of moisture in the process of HMT, the swelling power and solubility of modified starches gradually decreased. Moreover, the swelling power and solubility values of modified starches were lower than those obtained for native starch. It suggested that additional interaction between starch chains within amorphous and crystalline regions of granules came up as a result of the formation of some of double helices in the process of HMT. In addition, the amount of lipid-complexed amylose chains, responsible for the decrease in the swelling capacity and solubility, might increase during HMT, which had been reported in cereal starches.[16] Other authors have reported similar results using red millet, triticale, arrowroot, cassava, corn, potato barley, green arrow pea, eston lentil, othello pinto bean, blank bean, express field pea and white sorghum starches.[5,17]

Table 1 Effect of temperature on swelling power and solubility of native and heat-moisture treated Canna edulis Ker starches

Temperature (°C)	Native CNS ^α	CHMT18 ^β	CHMT21 ^β	CHMT24 ^β	CHMT27 ^β
Swelling power (g/g)
60	5.22 ± 0.35^a	4.35 ± 0.16^a	4.25 ± 0.06^a	3.68 ± 0.08^a	3.69 ± 0.17^a
70	11.84 ± 0.25^b	6.57 ± 0.18^b	6.05 ± 0.03^b	5.94 ± 0.17^b	5.37 ± 0.09^b
80	15.64 ± 0.62^c	8.64 ± 0.37^c	8.39 ± 0.43^c	7.43 ± 0.21^c	7.29 ± 0.15^c
90	17.08 ± 0.53^c	8.81 ± 0.32^d	8.64 ± 0.99^c	8.19 ± 0.42^d	8.06 ± 0.67^d
Solubility (g/100g)
60	9.08 ± 0.06^a	8.58 ± 0.46^a	8.11 ± 0.12^a	7.39 ± 0.09^a	2.20 ± 0.08^a
70	38.17 ± 0.28^b	33.11 ± 0.06^b	25.27 ± 0.84^b	25.33 ± 1.25^b	15.40 ± 1.07^b
80	49.96 ± 0.64^c	48.54 ± 0.77^c	45.52 ± 2.16^c	45.46 ± 1.48^c	38.11 ± 1.07^c
90	67.22 ± 3.82^d	65.93 ± 0.64^d	55.02 ± 1.46^d	52.86 ± 1.11^d	54.06 ± 1.74^d
All values are means of triplicate determinations ± S.D. Means within columns with different superscripts are significantly different (P ≤ 0.05). ^α: Canna edulis Ker native starch. ^β: Canna edulis Ker heat-moisture treated starches.

Effect of pH on swelling power and solubility

The effect of pH on swelling power and solubility of native and heat-moisture treated C. edulis starches were presented in Table 2. It could be seen that swelling capacity and solubility in native starch were higher than in all modified starches under any pH condition, both in acid and alkaline, implying the formation of strengthened associations among starch chains during HMT. The pH almost had no effect on swelling capacity for all starches in acidic region, while increase in pH leaded to the increase in the swelling power in alkaline region, in qualitative accordance with the results obtained for red and white sorghum starches.[5, 18] This could be due to the interaction between protein and starch at alkaline pH. Additionally, the swelling power also clearly increased with increase of moisture at all pH for the treated starches. This observation might be ascribed that the amount of hydrogen bonding increased with increase of moisture in the treatment, which was beneficial for integration of water molecule into interior structure of starch granules.

Table 2 Effect of pH on swelling power and solubility of native and heat-moisture treated Canna edulis Ker starches

pH	Native CNS ^α	CHMT18 ^β	CHMT21 ^β	CHMT24 ^β	CHMT27 ^β
Swelling power (g/g)
2	3.96 ± 0.03^a	2.70 ± 0.04^a	2.89 ± 0.11^a	3.01 ± 0.04^a	3.39 ± 0.09^ab
4	3.04 ± 0.05^a	2.73 ± 0.46^a	2.75 ± 0.05^b	3.02 ± 0.04^ba	3.35 ± 0.19^ab
6	3.93 ± 0.06^a	2.80 ± 0.06^a	2.87 ± 0.08^ac	3.01 ± 0.06^a	3.47 ± 0.09^bac
8	3.00 ± 0.04^a	2.76 ± 0.02^a	2.78 ± 0.01^ac	3.12 ± 0.06^c	3.35 ± 0.07^ab
10	3.94 ± 0.05^a	2.75 ± 0.09^a	2.80 ± 0.08^ac	2.87 ± 0.15^ad	3.42 ± 0.05^ab
12	3.98 ± 0.11^a	2.91 ± 0.01^a	3.15 ± 0.04^ac	3.22 ± 0.02^ec	3.62 ± 0.09^cb
Solubility (g/100g)
2	8.85 ± 0.26^a	6.42 ± 0.26^af	4.85 ± 0.36^a	5.09 ± 0.12^a	6.11 ± 0.67^a
4	14.25 ± 0.33^b	10.48 ± 1.29^b	11.25 ± 0.63^b	10.01 ± 1.78^b	11.31 ± 0.53^b
6	3.76 ± 0.21^cd	2.03 ± 0.14^cd	1.36 ± 0.23^cd	1.58 ± 0.16^cd	2.11 ± 0.12^cd
8	4.09 ± 0.45^dc	1.78 ± 0.10^dce	2.06 ± 0.05^dc	2.38 ± 0.28^dce	2.62 ± 0.34^dc
10	5.46 ± 0.11^e	3.84 ± 0.12^ed	3.46 ± 0.31^e	3.67 ± 0.28^ed	4.54 ± 0.20^e
12	9.36 ± 0.43^f	6.12 ± 0.78^fa	7.46 ± 0.53^f	7.28 ± 0.39^f	7.46 ± 0.53^f
All values are means of triplicate determinations ±S.D. Means within columns with different superscripts are significantly different (P ≤ 0.05). ^α: Canna edulis Ker native starch. ^β: Canna edulis Ker heat-moisture treated starches.

Interestingly, for native and modified starches, solubility values jumped to highest level at acidic pH (pH = 4), implying that stability of starch granules attained to be the best. So it could be well postulated that hydrogen ions were easy to access amorphous and crystalline regions of starches, or might combine phosphate groups located mostly in long B-chains of amylopectin molecules to attain stable state.[19] Otherwise, the increase in pH leaded to increase of solubility for all starches in alkaline region (pH = 8–12), similar to swelling power. This indicated that factors that influenced the increase in swelling power also influenced the increase in solubility.

Water and Oil Absorption Capacities

Water and oil absorption capacities for native and heat-moisture treated starches were shown in Fig. 2. Water and oil absorption capacities in native starch were higher than in all modified starches, as a result of strengthened interactions between starch chains contributing to inhibition for water and oil molecules entry into amorphous region. The results would be in conflict with the report for white sorghum starch.[5] This phenomena demonstrated that hydrophilic tendency increased with increase of treated moisture from CHMT₁₈ to CHMT₂₇, but hydrophobic tendency almost remained unchanged. In other words, hydrophilic nature of outer covering of starches was deeply influenced by HMT. These observations could be explained by combination of increased hydroxyl groups into the outer covering of starches with increasing severity of moisture conditioning, leading to increase of hydrophilic tendency.

Figure 2 Water and oil absorption capacities for native and heat-moisture treated Canna edulis Ker starches.

Alkaline Water Retention

All modified starches with average value of 1.05 g/g showed increase in alkaline water retention compared with that of native starch (0.14 g/g), in agreement with results for red and white sorghum starches.[5,20] This increase was partially attributed to surface area of starch phase and excessive dilution of continuous phase.[21] Moreover, with the increase of moisture, alkaline water retention also increased from 0.84 to 1.27 g/g. This result indicated that C. edulis starch could be a good material for cookie-making.

Acid Susceptibility

As exhibited in Table 3, for all treated starches, acid susceptibility was higher than that of native starch, and HMT increased susceptibility of starches towards acid hydrolysis. Moreover, with the increase of moisture, acid hydrolysis became easier, attributing to increasing hydrogen bonds, which might be destroyed under the acidic condition, leading to easy entry of hydrogen ion into amorphous regions, as a result of the ruin of strengthened chains demonstrated in the measurement of swelling power and solubility. This result was in agreement with those reported for black bean, pinto bean, green arrow pea, field pea, eston lentil and oat starches, whereas different from results obtained for normal maize, waxy maize, high amylose maize, laird lentil and pigeon pea starches, which were reported that acid hydrolysis decreased slightly after HMT.[16,17]

Table 3 Acid susceptibility of native and heat-moisture treated Canna. edulis Ker starches

Time (h)	CNS ^α	CHMT18 ^β	CHMT21 ^β	CHMT24 ^β	CHMT27 ^β
24	2.56 ± 0.23^a	4.75 ± 0.21^a	5.28 ± 0.56^a	5.45 ± 0.19^a	5.79 ± 0.25^a
48	5.71 ± 0.19^b	6.08 ± 0.55^b	5.74 ± 0.32^a	6.12 ± 0.15^a	6.16 ± 0.15^ab
72	6.23 ± 0.14^c	6.07 ± 0.42^b	6.32 ± 0.38^a	6.39 ± 0.78^a	6.53 ± 0.50^b
All values are means of triplicate determinations ±S.D. Means within columns with different superscripts are significantly different (P ≤ 0.05). ^α: Canna edulis Ker native starch. ^β: Canna edulis Ker heat-moisture treated starches.

Extent of Amylose Leaching

The extent of amylose leaching (AML) of native and heat-moisture treated starches were presented in Table 4. For all starches, AML values attained to the highest at 80°C, suggesting that the temperature was advantageous to amylose leaching. AML value of native starch was higher than those of all modified starches at 70°C, thereafter, followed by lower value when temperature rose to 80 and 90°C, different from those reported by Hoover et al.[16] The decrease of AML values at 70°C after HMT might be attributed to strengthened interaction between starch chains in amorphous domains of all modified starches approved by measurement of swelling capacity and solubility. In addition, decrease of AML values at 90°C could be ascribed for intense gelatinization, which restrained AML for all starches. Moreover, AML value of native starch dropped rapidly as a result of utmost gelatinization at 90°C due to high pasting viscosity. As exhibited in the RVA measurement, pasting temperature of native starch (66.9°C) was distinctly lower than values for all modified starches (89.95–95.25°C). Therefore, the extent of native starch gelatinization would be remarkably high compared to that of treated starches, which would be contributed to lower AML value in the native starch than in all modified starches at both 80 and 90°C.

Table 4 Extent of amylose leaching of native and heat-moisture treated Canna edulis Ker starches

Temperature (°C)	Native CNS ^α	CHMT18 ^β	CHMT21 ^β	CHMT24 ^β	CHMT27 ^β
70	55.59 ± 5.35^ab	39.49 ± 9.12^a	24.96 ± 1.71^a	19.69 ± 0.96^a	24.29 ± 3.30^a
80	61.83 ± 2.57^ba	64.87 ± 5.65^b	67.41 ± 9.49^b	63.95 ± 3.18^b	66.54 ± 1.84^b
90	35.18 ± 9.43^c	47.60 ± 0.75^ab	52.22 ± 0.49^c	59.07 ± 2.05^c	50.70 ± 2.89^c
All values are means of triplicate determinations ±S.D. Means within columns with different superscripts are significantly different (P ≤ 0.05). ^α: Canna edulis Ker native starch. ^β: Canna edulis Ker heat-moisture treated starches.

Freeze-thaw Stability

Freeze-thaw stability for all starches was exhibited in Fig. 3. As shown, freeze-thaw stability in the treated starches was higher than in native starch, signifying that hydrogen bonds formed during HMT could integrate water molecules and restricted the amount of water exuded. Moreover, with the severity of moisture treatment, modified starches exhibited better freeze-thaw stability. The freeze-thaw stability of all starches decreased with the time delay in the period of storage. Thus, it could draw the conclusion that HMT increased freeze-thaw stability of C. edulis starch.

Figure 3 Freeze-thaw stability for native and heat-moisture treated Canna edulis Ker starches.

X-ray Diffraction

As shown in Table 5, X-ray spectrum of native starch was B-type representative of tuber starches with two main peaks at around 2θ of 16.97° and 22.15°. However, the peaks 2θ at 5.57° and 24.05° were not present. With the increase of moisture in the treatment, X-ray pattern of starches gradually transformed from B-type to A-type after HMT. When moisture content attained to 27 g/100 g starch, the peak at around 2θ of 16.97° was split into two small peaks at 16.97° and 17.98°, indicating that B-type starch had been transformed to a great extent. In addition, the peak at 26.56°, characteristic peak of A-type, was shown on the X-ray spectrum of CHMT₂₇. These also suggested that A-type polymorphs became to the major component in the treated starch. The change in X-ray pattern has been attributed to rearrangement of double helices into a crystalline array that contains an amylosic helix (characteristic of ‘A’ type unit cells) in the central channel of the unit cell.[22,23]

Table 5 X-Ray diffraction intensities of major peaks and crystallinity of native and heat-moisture treated Canna edulis Ker starches

Starch	2θ (°) with intensities (CPS) ^a					Crystallinity (%)
Native CNS ^b	–	16.95(345)	–	22.95(255)	–	25.45
CHMT18 ^c	–	16.85 (381)	–	22.95 (265)	–	26.92
CHMT21 ^c	14.89 (226)	17.04 (313)	–	22.88 (235)	–	27.57
CHMT24 ^c	15.09 (265)	16.91 (339)	–	22.78 (259)	–	26.87
CHMT27 ^c	14.90 (257)	17.00 (336)	17.92 (283)	22.92 (261)	26.65 (200)	29.33
^a: Counts per second; ^b: Canna edulis Ker native starch; and ^c: Canna edulis Ker heat-moisture treated starches.

With the exception of CHMT₂₄, the degree of crystallinity of all modified starches increased with the increase of moisture content during HMT. The peaks around 2θ of 15°, 17°, and 23° became sharper in treated starch than in native starch, and an additional peak 2θ at 26.65° came up. HMT can increase X-ray intensities in C. edulis starch, different from previous studies, which have shown that X-ray intensities of all tuber (‘B’ type) heat-moisture treated starches decreased.[9] The increase in X-ray intensities of all treated starches indicated that small crystalline region of starch granules became perfect, or additional crystallites, which were probably better arrayed to diffract X-rays than those present within the native granule, might have formed. It also implied that crystallite reorientation might have occurred on HMT, resulting in a crystalline array that can diffract X-rays strongly.

Thermal Stability Properties

Thermal transition behaviors of native and heat-moisture treated starches were presented in Table 6. The values of T _o, T _p, T _c, and transition temperature range (T _c-T _o) in all modified starches were higher than in native starch. This result compared favourably with those obtained for cereal and tuber starches.[16] The increases in T _o, T _p, and T _c may be explained that amylose molecules located in amorphous regions interacted with the branched segments of amylopectin in the crystalline regions, and these interactions reduced the mobility of amylopectin chains and increased transition temperature for melting.[16] Likewise, it was possible that the penetration of water molecules into the crystalline region took more time as a result of strengthened interactions between amylose-amylose and amylose-amylopectin chains and thus made the temperature range (T_c-T_o ) wider to complete the gelatinization process.[24] A progressive increase in T _o, T _p, T _c and T _c-T _o was observed from CHMT₁₈ to CHMT₂₇, in consistent with results reported by Pukkahuta et al.[25] However, HMT decreased the ΔH of all treated starches, which was similar to the result for potato starch.

Table 6 Differential scanning calorimetry characteristics of native and heat-moisture treated Canna edulis Ker starches

Starch	T o ^a (°C)	T p ^a (°C)	T c ^a (°C)	T c -T o (°C)	ΔH ^b (J/g)
Native CNS ^c	59.707	64.185	71.860	12.153	9.508
CHMT18 ^d	61.203	65.356	74.563	12.360	9.273
CHMT21 ^d	63.563	65.987	76.365	12.802	9.265
CHMT24 ^d	64.264	67.750	77.764	13.500	9.230
CHMT27 ^d	65.203	68.654	79.763	14.56	9.024
^a: T o, T p, and T c indicate the temperature of the onset, midpoint, and end of gelatinization. ^b: Enthalpy of gelatinization. ^c: Canna edulis Ker native starch; and ^d: Canna edulis Ker heat-moisture treated starches.

Pasting Properties

As shown in Fig. 4, RVA viscoamylograph of native C. edulis starch was a Type A pasting profile by a high PV (2488 rvu), with a high BDV (1716 rvu) and low FV (1191 rvu). For all modified starches, Type C pasting profiles were characterized by low values of PV (205–231 rvu) and BDV (20–47 rvu) as well as high values of FV (221–288 rvu). Compared with those of native starch, all modified starches exhibited remarkably low values of PV, HPV, BDV, FV, and SBV, whereas past temperature and peak time increased, in qualitative accordance with those observed for corn and potato starches.[25,26] These decreases could be the result of increased inter- and intra-molecular hydrogen bonding due to association of amylose and amylopectin chains in the process of HMT.

Figure 4 Pasting profiles of native (a) and modified (b) Canna edulis Ker starches.

It is well known that SBV value is a measure of retrogradation tendency. For native starch, amylose molecules were randomly dispersed and orient themselves in parallel fashion to from gel, leading to high value of SBV (419 rvu). However, for modified starches, low SBV (56–82 rvu) values suggested that associated tendency on cooling had been eliminated because of the involvement of the hydroxyl groups into the starch molecules.[27] High pasting temperature (88.35–95.25°C) indicated that more associate forces and crosslinks were present within the modified starch granules. This was also proved by low BDV values (20–47 rvu), which were a measure of the ease with which swollen starch granule can be disintegrated, an indication of the degree of starch granule organization.[28] Likewise, the low BDV values in the viscosity showed that granules were quite strong and resisted breakdown under shear and heat. These were also consistent with low swelling capacity and high gelatinization temperature in all modified starches.

CONCLUSION

HMT had a great impact on physicochemical properties of C. edulis starch. Compared with those of all treated starches, swelling power, solubility, water and oil absorption of native starch were higher, along with low alkaline water retention and acid susceptibility, in the company of different extent of amylose leaching. The result in the XRD measurement revealed that crystalline type of C. edulis starch gradually changed from B-type to A-type with increase of moisture, and the degree of crystallinity also changed. HMT decreased the ΔH, conversely, and increased the T _o, T _p, T _c and T _c -T _o. In addition, native starch had a Type A pasting profile. However, all modified starches, Type B pasting profile, exhibited remarkably low values of PV, HPV, BDV, FV and SBV, whereas past temperature and peak time were higher than those of native starch. Low BDV and SBV values showed that HMT increased paste stability and reduced retrogradation of C. edulis starch, which would be beneficial to research food ingredient. To sum up, the range of characteristics observed made C. edulis starch amenable to different applications based on their properties. The effects of various combinations of temperature and moisture content should be more thoroughly studied to identify conditions for development of favorable C. edulis starch functionality.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant No: 20676051), the Major Project Regarding Scientific and Technological Development of Science and Technology Commission of Shanghai Municipality (07DZ195080) the Technology Standard Project of Science and Technology Commission of Shanghai Municipality (07DZ05019) and The Innovation Fund for Graduate Student of Shanghai Jiao Tong University.