The Physicochemical properties of starch of some accessions of amorphophallus paeoniifolius

S.N Moorthy, M. Unnikrishnan and K.R. Lakshmi

Central Tuber Crops Research Institute, Trivandrum 695 017, India

Abstract : Starch was extracted from the tubers of ten accessions of Amorphophallus paeoniifolius of the same maturity, with a yield varying from 7 to 14 % on a fresh weight basis. The starch was pure white in color. There was no significant difference in granule size between the different accessions. The starch granule size and viscosity values were lower in A. paeonifolius than in Dioscorea spp. And cassava, but the viscosity was higher than that of cereal starches. The viscosity stability was good, indicating suitability for many food applications.

Keywords : elephant yam, aroids, Amorphophallus paeoniifolius, starch properties.

Introduction

Amorphophallus paeoniifolius is an important tropical tuber crop grown in many parts of India, Philippines, Malaysia, Indonesia and Sri Lanka (Ramachandran 1977; Ghosh et al. 1988). The subterraneous tuber is hemispheric, globose and weighs 2-8 kg at harvest. The flesh of the tuber is often yellow in color and normally contains 8-18% starch ( fresh weight basis) (Rajendran et al. 1977). The tubers are consumed as a vegetable after cooking well to remove the acrid properties present in them. There have been few reports on the properties of the starch of Amorphophallus (Wankhede and sajjan 1981; Soni et al. 1985) and the studies are restricted to a single local variety. There is little morphological variation among the different accessions of Amorphophallus available in the CTCRI Germplasm.

Materials and Methods

The accessions used (Am 2, Am 5, Am 14, Am 15, Am 27, Am 32, Am 36, Am 43 and Am 51) were grown at the CTCRI farm under standard practices and harvested after 10 months. Starch was extracted from the tubers by the method of Moorthy (1991). Tubers were washed, peeled and cut into small pieces, then 1 kg was washed again and disintegrated in a mixer fro 1 min with 0.03 M ammonia solution. The starch milk was passed through 80- and 260-mesh sieves and the suspension allowed to settle overnight. The supernatant was withdrawn and resettled fro 6-8 h. The combined residues were resuspended in water and allowed to stand for 6 h. The resulting wet starch cake was crushed manually, spread out to dry in the sun for 8 h and then oven dried at 500C for 12 h. The sampling was duplicated to give the yield of starch.

Granule size was measured at 10x and 45x magnification using an ocular micrometer. A total of 100 granules were randomly selected from five fields ( 20 granules per field ) and the mean was calculated. Distilled water was used for mounting and 0.1% iodine solution for staining the granules.

The total amylose and soluble amylose contents were determined by the procedures of Sowbhagya and Bhattacharya (1971) and Shanthy et al. (1980), with six replications. Swelling volumes were obtained by Schoch’s method (1964) at concentrations of 0.5, 1.0, 1,5, and 2.0% w/v; three replications were carried out.

The paste viscosity was monitored on a Brabender viscoamylograph, Model 801002, using a 350 µg cartridge. The starch suspension was heated from 500C to 970C at the rate of 1.50C/min . Concentrations were 5, 6 and 7%. After holding the paste at 970C for 30 min, it was allowed to cool to 500C. The pasting temperatures were read from the viscosity curves. Viscosity of 2% starch solutions were also determined using Redwood viscometer (ISI 1969) in triplicate.

Clarity and paste stability were obtained from a 2% solution using the absorbance of the sample at 500 nm compared to water and the time taken by the starch to start settling from the solution respectively. The X-ray diffraction pattern was obtained on a Philips PW 1730 X-ray system using monochrome CuK? radiation.

Results and discussion

The yields of starch (Table 1) varied from 7.0 to 14.3%, the highest being for Am 51. The starch content in Amorphophallus is reported by Kay (1973) and Rajendran et al. (1977) to be in the range 4-8%. This is low compared with cassava and Diosorea spp., but the extraction is easy and settling of starch is not hampered by presence of mucilage in the tubers, unlike Colocasia and Dioscorea alata tubers. The starch yield is also comparable to the 10-12% of potato. The colour of the starch is pure white and is not tinged with the yellow colour present in the raw tubers.

The granule size of all the accessions was between 1 and 10 ocular divisions (3.34-33.4 µm) with 53% in the classes 6.68 and 10.02 µm. Grains above 30 µm below 5 µm were few. Average grain size of the different accessions was between 9.6 and 13.03 µm, the variety not being statistically significant (Table 1). Wankhede and Sajjan (1981) obtained 7-30 µm length and 5-24 µm width for Amorphophallus starch, while Kay (1973) gave a range of 5.5-18.7 µm. The starch granule size is thus less than that of Dioscorea spp. or cassava, but higher than that of Colocasia. The granules were mostly round in shape and no fissures were observed.

The total amylose content of starch of different accessions showed only very minor variation, from 21.9 to 23.5% (Table 1). Wankhede and Sajjan (1981) reported 24.5-2.50% while Soni et al. (1985) found 18.6%. There were no earlier reports on the soluble amylose content of the Amorphophallus starch. The soluble amylose, whish is starch of Amorphophallus paeoniifolius supported by amylose, which is supposed to be present in the amorphous regions of the starch granules is considered responsible for the undesirable cohesive character of many cooked starches (Hoover and Hadziyev 1982). The soluble amylose content formed 40-50% of the total amylose content, similar to other tuber crop starches.

Table 1 : Yield of starch, granule size and amylose content of Amorphophallus starch

  Yield (%) Average Granule Size * µm Total Amylose † (%) Soluble Amylose † (%)
Am 2 7.0 13.03 23.2 ± 1.01 10.5 ± 0.06
Am 5 10.0 12.49 23.5 ± 0.09 11.0 ± 0.09
Am 14 8.1 10.32 22.9 ± 0.7 9.9 ± 0.05
Am 15 12.3 11.12 23.3 ± 1.0 10.5 ± 0.07
Am 27 11.1 9.62 23.3 ± 1.1 9.9 ± 0.07
Am 32 9.9 10.69 23.2 ± 0.7 9.9 ± 0.06
Am 34 12.2 11.69 22.9 ± 0.7 9.4 ± 0.05
Am 36 10.5 10.35 23.9 ± 0.9 9.9 ± 0.07
Am 43 10.5 10.19 21.9 ± 1.0 8.9 ± 0.07
Am 51 14.3 9.85 23.2 ± 0.07 9.9 ± 0.10

*Mean of five replications
† Mean of six replications

Supported by amylose, which is supposed to be present in the amorphous regions of the starch granules is considered responsible for the undesirable cohesive character of many cooked starches (Hoover and Hadziyev 1982). The soluble amylose content formed 40-50% of the total amylose content, similar to other tuber crop starches.

No noticeable differences were seen in the swelling volumes of the starch of different accessions. The values increased steadily with increase in concentration and showed no abnormal behaviour at higher concentrations (Table 2). The viscosity properties (Table 3) are similar to those of Xanthisima and Colocasia starches. Generally they had lower viscosity values than Dioscorea and cassava starches, but were slightly higher than cereal starches. The different accessions did not vary significantly. The increase in viscosity with concentration was regular, with no noticeable change in viscosity pattern. The viscosity values at 970C and after holding for 30 min indicate very low break down of viscosity at 5% and 6% concentrations. At 7% concentration, the breakdown was around 100 Brabender units. The low breakdown in viscosity is a very desirable property of the starch since it gives a short non-cohesive paste suitable in many food and industrial applications.

Table 2 : Swelling volume of Amorphophallus starch

  Concentration
  0.5mI/g 1.0mI/g 1.5mI/g 2.0mI/g 4.0mI/g
Am 2 12.50 22.50 35.0 48.50 75
Am 5 12.25 21.50 34.5 47.50 75
Am 14 12.25 21.50 56.2 47.50 75
Am 15 13.00 22.50 37.5 50.50 75
Am 27 12.50 21.50 35.0 48.25 75
Am 32 13.20 24.20 35.0 47.50 75
Am 34 12.10 23.80 36.2 47.50 75
Am 36 11.70 23.75 35.0 46.90 75
Am 43 12.50 23.20 35.6 45.00 75
Am 51 12.5 24.40 34.4 46.25 75

*Mean value of three replicates

Table 3 : Rheollogical properties of Amorphophallus starch

  Peak Viscosity under (Barbender units) Viscosity at 970C (Barbender units) Viscosity after holding at 970C (Barbender units) Viscosity breakdown (Barbender units) Pasting Temperature (0C)
Conc. (%) 5 6 7 5 6 7 5 6 7 5 6 7  
Am 2 280 450 770 280 450 760 280 450 660 0 0 110 82-85
Am 5 320 480 810 320 480 810 320 480 680 0 0 130 82-85
Am 14 310 480 820 310 470 810 310 470 710 0 10 110 82-85
Am 15 380 620 900 380 600 880 380 600 700 0 20 200 82-85
Am 27 330 575 880 330 570 870 330 570 800 0 5 80 81-84
Am 32 320 570 820 320 570 820 320 570 720 0 0 100 82-85
Am 34 300 580 870 300 570 860 300 570 690 0 10 170 82-84
Am 36 320 600 840 310 600 840 310 600 700 10 0 140 82-85
Am 43 340 560 850 330 550 850 330 550 630 10 10 220 81-84
Am 51 300 500 780 300 500 760 300 500 600 0 0 180 81-84

Table 4 : 2% Viscocity, clarity and paste stability of Amorphophallus starch

  Viscosity (s) Clarity* (absorbance) Paste stability (h)
Am 2 39.5 0.83 24
Am 5 37.5 0.82 30
Am 14 37.5 0.85 24
Am 15 41.0 0.78 24
Am 27 38.0 0.80 30
Am 32 43.0 0.80 30
Am 34 44.5 0.75 24
Am 36 44.0 0.77 24
Am 43 44.5 0.81 30
Am 51 39.5 0.71 24

*Relative to water = 0

The pasting temperature was nearly the same for all the accessions (81-850C), higher than cassava or potato starch but similar to cereal and Colocasia starches. The gelatinization temperature determined microscopically by the earlier workers in 73-800C (Wankhede and Sajjan 1981; Soni et al. 1985). The higher gelatinization temperature for this starch can be attributed to the strong associative forces found in the granules. The delayed gelatinization will provide a uniform paste.

The 2% solution viscosity (obtained in Redwood viscometer) was 37-46 s. Although the Redwood values do not reflect the viscosity stability, they are higher than cereal starches but lower than cassava starch. The clarity of the 2% solution was less than cassava or Dioscorea starches, though the amylose content is almost the same (Table 4). Intermolecular associative bonds may contribute to a large extent, and may also be responsible for the good viscosity stability. The paste stability of the starch of the different accessions was 24-30 h (Table 4). The starch possessed lower stability than cassava or Colocasia starches but higher than cereal starches. The gel strength at 6% was quite high, indicating that association between the starch molecules becomes quite strong on cooling.

The X-ray diffractionpattern of the starch of Amorphophallus wasan ‘A’ pattern. Peaks were observed at 7o 40′ , 8o 30′ and 12o. The pattern follows the other aroid starches, which also posses a typical ‘A’ pattern.

The study indicates that Amorphophallus starch can be used in many starch based foods. It is easily extractable and possesses a pure white colour and good viscosity stability, suitable for many applications in the food industry.