Physicochemical and Functional Properties of Starch from Two Species of Curcuma

Abstract

The properties of starch extracted from two species of Curcuma viz. Curcuma zedoaria and Curcuma malabarica were studied. The starch from C. malabarica tubers was white in color, while that from C. zedoaria was slightly yellowish due to the presence of the yellow pigment, curcumin. The granule size and shape, amylose content and solubility did not show noticeable difference between starches from the two species. Both the starches possess “B” type x‐ray diffraction pattern. Curcuma zedoaria starch showed a lower peak viscosity and swelling volume when compared to C. malabarica starch. The complete removal of curcumin from C. zedoaria starch by alcohol extraction, resulted in an increase in the swelling and viscosity values close to those of C. malabarica starch. The breakdown in viscosity was quite low for both the starches and setback was higher when compared to cassava and sweet potato starches. The experiments showed that Curcuma starch resembles Dioscorea starches in most of its properties. Differential scanning calorimetry (DSC) data showed that the onset of gelatinisation was earlier for C. malabarica starch. The enthalpy of gelatinization was almost same for both the starches. The phosphorus content of Curcuma starch was quite high and similar to that of potato starch. The Curcuma starch was found to be easily digestible like arrowroot starch as seen from the in vitro α‐amylase digestibility patterns. Owing to the high viscosity stability and digestibility, it can be a potential source of starch for food and industry.

Keywords:

• Curcuma zedoaria
• Curcuma malabarica
• Curcumin
• Starch properties

Introduction

There are a number of root crops, which have not received much attention, in spite of being grown in different parts of the tropical belt. Curcuma species is one such crop, which has been traditionally used as a medicinal crop. The genus Curcuma belonging to the family Zingiberacea occurs in the tropics of Asia and extends to southern China, Sri Lanka, Indonesia, Philippines, Africa, and Australia.[1],[2] Curcuma is reported to be used as a source of an easily digested starch, which is rather similar to that of arrowroot. In India, it is utilized on a cottage industry basis for the preparation of invalid and baby foods.

The Curcuma plants are rhizomatus perennial herbs having primary and secondary rhizomes, shape of which varies from spherical to slightly conical, hemispherical, and cylindrical.[2] The flesh color of the tubers from Curcuma zedoaria species is yellowish white due to the presence of curcumin. The Curcuma malabarica tubers have a bluish flesh color. There are only some very old reports on the composition of C. zedoaria tubers and its starch.[1],[3] As very little information is available on the starch properties of this important crop, a systematic study on the properties of starch extracted from two species of curcuma viz. C. zedoaria and C. malabarica available in CTCRI germplasm collection was conducted.

Materials and Methods

Curcuma tubers were obtained from Central Tuber Crops Research Institute farm. The tubers were harvested 10 months after planting and starch was extracted according to standard procedure.[4] The complete removal of the colorant, curcumin from C. zedoaria starch was brought about by refluxing the starch with absolute ethanol at 85°C for 5–6 hrs. It was filtered and the residue was washed with alcohol followed by distilled water and dried. The properties of the decolorised starch were also studied in order to investigate whether the presence of curcumin affects starch properties.

The starch granule size and shape were determined microscopically using an optical microscope (LEICA DMLB Research Microscope) with a magnification factor of 400×. The swelling volume, solubility and clarity of the samples were determined according to standard procedure of Schoch.[5] The amylose content of samples were determined using non defatted starch samples since it is well documented that tuber crops contain only small quantities of lipids in them and therefore expressed as apparent amylose. The apparent and soluble amylose contents were determined iodimetrically.[6],[7] A Seiko II 6200‐DSC equipment was used to study the thermal characteristics of starch by Differential Scanning Calorimetry (DSC). The x‐ray diffraction patterns of the starch samples were obtained using a Phillips XRD equipment with CuK_α source and a wavelength range of 2θ = 4 to 60°. The pasting behavior was studied using a Rapid Visco Analyser (RVA‐4, Newport Scientific, Australia) at a starch concentration of 10% and rotation speed of 160 rpm. The in vitro α‐amylase digestibility was studied using bacterial α‐amylase (Bacillus sp.), having an activity of 22.5 units/mg solid, according to the method of Raja.[8] Starch was gelatinised in citrate–phosphate buffer (pH‐5.6) and 1 ml aliquot was transferred to test tubes. Equal volume of enzyme prepared by dissolving 25 mg of α‐amylase in 25 ml buffer and further diluting the solution by 1:9 using the same buffer was added. The tubes were incubated at 70°C for a total period of 60 min and samples were withdrawn at 15 min intervals. The reducing sugar content in each sample was determined using dinitro salicylic acid reagent.[9] Reducing value[10] and phosphorus content[11] of the starch samples also were determined according to standard procedures.

Results and Discussion

Granule Size and Shape

Microscopic studies revealed that the starch granules do not exhibit variation in size among species (Table 1). For C. malabarica starch, about 40% of starch grains had a size range of 9–30 µm while 60% of the grains were between 30–45 µm with an average granule size of 34.3 µm. For C. zedoaria starch, 50% of the granules were in the range of 8–30 µm and the rest 50% in the range of 35–60 µm. The average granule size was 33.13 µm. The granules were slightly larger than cassava, sweet potato, and aroid starches,[12] but similar to Dioscorea rotundata, Dioscorea alata, and Canna edulis starch granules.[12],[13] Curcuma starch granules were elliptical in shape like D. rotundata and Canna starch granules.

Table 1. Physicochemical properties of Curcuma starch. ^a

	Average granule size (µm)	Granule shape	Apparent amylose (kg/100 kg)	Soluble amylose (kg/100 kg)	Ferricyanide number	Phosphorus content (ppm)
C. zedoaria	33.13	Elliptical	28 ± 1	12 ± 1	1.2 ± 0.2	419 ± 6
C. zedoaria (without curcumin)	—	—	27.2 ± 0.2	11.2 ± 0.4	—	—
C. malabarica	34.30	Elliptical	25 ± 1	11.1 ± 0.5	2.1 ± 0.2	454 ± 11

a: Mean values of three replications.

X‐Ray Diffraction Pattern

The Curcuma starch resembles yam starches and C. edulis starch in possessing “B” type X‐ray diffraction pattern, in contrast to cassava, sweet potato and aroids, which exhibit “A” pattern.[12],[14] In the XRD pattern of C. malabarica starch, peaks were observed at 2θ = 15.4, 17.2, and 23–26° (Fig. 1). For C. zedoaria starch, the peaks were at 16, 18.6 and 23–25°.

Figure 1. XRD patterns of Curcuma starch.

Reducing Value (Ferricyanide Number)

The ferricyanide number, which is a measure of the number of reducing end groups in the sample, was higher for C. malabarica starch (2.1) than C. zedoaria starch (1.2) indicating slightly higher molecular weight for the latter (Table 1).

Phosphorus Content

The phosphorus content of Curcuma starch from both the species was quite high when compared to other tuber starches. It was almost similar to that of potato starch,[15] but slightly lower than C. edulis starch.[16] The high phosphorus content may be responsible for the high viscosity and gel strength of the starch paste of Curcuma starch.

Amylose Content

The apparent and soluble amylose contents showed only minor difference between the two species (Table 1), and were similar to those of D. rotundata starch,[17] but slightly lower than cassava and sweet potato starches.[12] The apparent amylose content was 25 kg/100 kg in C. malabarica starch and about 27 kg/100 kg in C. zedoaria starch before and after removal of curcumin. According to some earlier reports the amylose content of C. zedoaria starch was 31.3 kg/100 kg, which is very close to our results. Like other tuber crop starches, the soluble amylose formed nearly 35 kg/100 kg of the apparent amylose content in Curcuma starch.

Swelling Properties

Studies on the swelling properties indicated that at 1% concentration, swelling volume and solubility were higher for C. malabarica starch (Table 2). After removal of curcumin, the swelling volume of C. zedoaria starch increased and came close to that of C. malabarica starch. The swelling volume of Curcuma starch was lower than cassava, sweet potato, and aroid starches, but resemble that of D. rotundata starch (15–25 mL/g).[12] However, solubility of Curcuma starch was in the same range of other tuber starches.

Table 2. Swelling volume, solubility, and clarity of Curcuma starch samples. ^a

	Swelling volume (mL/g)	Solubility (%)	Clarity (absorbance at 700 nm)
C. zedoaria	14.8 ± 1.2	20 ± 1	1.4 ± 0.1
C. zedoaria (without curcumin)	22.5 ± 0.1	19.5 ± 0.5	0.8 ± 0.1
C. malabarica	22.3 ± 0.5	23.6 ± 0.1	0.5 ± 0.1

a: Mean of three replications.

Clarity

The optical density of the starch suspension in water (1%), measured at 700 nm was found to be much higher for C. zedoaria starch (Table 2). This can be attributed to the presence of curcumin, which absorbs in the visible region. After removal of curcumin, the value decreased from 1.36 to 0.778, making the suspension clearer.

Viscosity

Viscosity studies on Curcuma starch showed that variation exists between starches extracted from the two species (Table 3). The C. malabarica starch had a higher peak viscosity of 3.39 Pa s compared to C. zedoaria starch. Removal of curcumin lead to increase in viscosity of C. zedoaria starch to 3.41 Pa s, which was almost close to that of C. malabarica starch (Fig. 2). The breakdown in viscosity was quite low for both the starches, showing that the granules are quite strong and resist breakdown under shear and heat. In this respect, Curcuma starch resembles Dioscorea starch rather than cassava, which exhibits considerable breakdown in viscosity.[12],[18],[19] The setback viscosity of starch from both the species of Curcuma was higher compared to cassava and sweet potato starches indicating a reasonable amount of re association of granules during cooling. The high level of phosphate cross‐linkages, as reported in the case of potato and C. edulis starches can be responsible for the comparatively high setback viscosity and low viscosity breakdown of Curcuma starch.[13],[15] The high viscosity with very low breakdown is a desirable property of the starch since its paste has a non‐cohesive texture suitable for many food and industrial applications. The pasting temperatures of Curcuma starches were higher than cassava and sweet potato, but similar to cereal and aroid starches. The higher gelatinisation temperature can be attributed to strong associative forces found in the granules.

Table 3. Viscosity parameters (RVA) of Curcuma starch. ^a

	Peak I (Pa s)	Trough I (Pa s)	Breakdown viscosity (Pa s)	Final viscosity (Pa s)	Setback viscosity (Pa s)	Peak time (min)	Pasting temperature (°C)
C. zedoaria	2.02	1.95	0.07	2.60	0.65	6.87	87.20
C. zedoaria (without curcumin)	3.41	3.36	0.05	6.80	3.44	6.67	87.15
C. malabarica	3.39	2.83	0.56	4.60	1.77	5.13	80.50

a: Mean values of three replications.

Figure 2. RVA graphs of Curcuma, Dioscorea, and Cassava starches.

Differential Scanning Calorimetry Characteristics

Differential scanning calorimetry thermogram data of the starches from the two Curcuma varieties are given in Table 4. The gelatinisation peak of C. malabarica starch was found to be a doublet (Fig. 3). Multiplicity of peaks could arise due to deficiency of water during gelatinisation.[20] We have used a starch/water ratio of 1:3, which provides enough water for gelatinisation. Therefore, the splitting of peak can be attributed to the structural differences in the starch, which needs further studies. The onset of gelatinisation as indicated by T _onset was earlier for C. malabarica starch whereas T _end was nearly similar for the two starches. The range of gelatinisation of Curcuma starch was similar to yam and potato starches.[12]. The T _onset values of both the starches were similar to yam starches (75–80°C), but higher than cassava starch (65–69°C). The T _peak values also showed a similar trend. There was no noticeable difference in the enthalpy of gelatinisation of the two starches.

Table 4. Differential scanning calorimetry thermogram values of Curcuma starch

	T onset (°C) ^a	T peak (°C) ^b	T end (°C) ^c	ΔH (J/g) ^d
C. zedoaria	79.7	82.7	97.1	16.1
C. zedoaria (without curcumin)	82.3	84.2	98.2	18.0
C. malabarica	74.3	76.8	82.1	17.5

a: Onset of gelatinisation.

b: Peak minimum gelatinisation temperature.

c: End of gelatinisation.

d: Enthalpy of gelatinisation.

Figure 3. Differential scanning calorimetry gelatinisation curves of Curcuma starch.

α‐Amylase Digestibility

The in vitro α‐amylase digestibility pattern of Curcuma starch is given in Fig. 4. The digestibility of starch from both the species of Curcuma were found to be high when compared to other tuber starches like cassava, sweet potato, taro, yams etc., but was almost similar to that of arrowroot starch. The easy digestibility of this starch can be exploited for use in food for infants and invalids. The C. malabarica starch showed highest enzyme susceptibility at 15 min of incubation and released about 87% of dextrose, beyond which there was a decline. For C. zedoaria starch, before and after removal of curcumin, the digestibility was maximum at 45 min of incubation. Arrowroot starch also found to have almost same digestibility with maximum release of dextrose at 30 min of incubation. The result is in agreement with earlier reports on the digestibility property of Curcuma starch.[1] The study reveals the close resemblance of Curcuma starch with Dioscorea starch except in the digestibility properties. The digestibility of Dioscorea is found to be poor.[16],[21]

Figure 4. Alpha amylase digestibility patterns of Curcuma and arrowroot starches.

Conclusion

The results of the above study indicated that Curcuma starch resembles yam starches and C. edulis starch in most of the structural and physicochemical properties. However, the enzyme susceptibility of Curcuma starch was higher than yam and other tuber starches but almost close to that of arrowroot starch. Starch from the two Curcuma species showed variations in some of their properties, especially viscosity and swelling characteristics. However removal of curcumin resulted in modification of these properties of C. zedoaria starch. The study indicates that Curcuma starch possess good viscosity stability, gel strength and easy digestibility. Thus it can be a potential source of starch for food industry.

Acknowledgments

The authors wish to acknowledge the help and encouragement given by Dr. S. Edison, the Director, CTCRI, and Dr. C. Balagopalan, Head, Division of Crop Utilization and Biotechnology. Thanks are also due to Ms. Sunitha Rani, Guest Researcher, University of Lund, Sweden for the DSC analysis.