Tropical Sources of Starches

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3. Other starches

2. Yam and Aroid Starches

   The extraction of starch from yams and aroids is difficult due to presence of mucilage in the tubers. This is epecially true for Colocasia which contains large quantity of mucilage. However use of dilute ammonia was found to facilitate extraction of starch from these tubers. The yield was enhanced and the quality of the resultant starch was equal or superior to the starch extracted using water ⁵⁵.

   These starches also have low lipid content. However the yam starches have higher phosphorus contents which contribute to their viscosity and gel properties. Yam starches have a large variability in shape viz., round, triangular, oval and elliptical. In fact, the species exhibit dramatic variability in granular size ranging from very tiny granules of D. esculenta (1-5um) to the very large ones observed for D. alata (16-110um). Farhat et al. ⁵⁶ have examined the distribution in size of different yam species. D. rotundata and D. cayensis starches showed similar patterns with a single symmetrical distribution centred at 32 and 35 mm respectively while D. alata starch exhibited a non symmetrical wider particle distribution around 31 mm and D. dumetorum starch had uneven distribution. Use polarised light microscopy confirmed the findings and corroborated the results of Rasper and Coursey ⁵⁷ on granule sizes. Variation in granule size could not be observed among the three yam species viz D. rotundata, D. alata and D. esculenta. The edible Dioscorea starches (viz. D. alata, D.esculenta and D. rotundata), D. abysinica and D. cayensis starch possesssed ‘B’ XRD patterns. However ‘A’ and ‘C’ patterns has been reported for D. dumetorum starch. It was found that the XRD pattern of extracted starch is the same throughout the growth period of D. rotundata (Fig. 3). The macromolecular characteristics of10 yam species have been studied in detail using HSEC-MAILS-DRI and it was found that the physicochemical and functional properties could be related to the macromolecular characteristics ⁵⁸

   Amylose content in yam starches also varied considerably according to various reports. Farhat et al. ⁵⁶ have obtained the following values for amylose content in starches of different Dioscorea species – D. alata – 25%, D. rotundata and D. cayensis – 23.8% and D. dumetorum -12.6%. A value of 29.7% was reported for the amylose content of D. abyssinica starch from Ethiopia, while Soni et al. ¹⁵ obtained 24.1% for D. ballophylla starch. Only very little variation in the amylose content was observed with age of the crop for D. esculenta, D. alata and D. rotundata starches Yam starches gelatinised over a temperature range of around 20°C and gelatinisation continued even after 95°C showing strong intermolecular linkages. The RVA results on pasting of different yam species indicate the values to range between 75 and 83°C, the highest being for D dumetorum and lowest for D. cayensis starch ⁵⁶. D. abyssinica starch had a gelatinisation temperature of 73°C. Nkala et al. ⁵⁹ examined the starch of D. dumetorum from dry and wet seasons and found the values to be similar, ie. 83°C. The large range for yam starches may be attributed to the presence of phosphate linkages in these starches (similar to potato starch).

   All the yam starches showed a characteristic pattern of slow rise in viscosity and even after 95°C, an increase in viscosity was noticed. This implies that all the granules do not gelatinise at 95°C and some of them gelatinise only during the holding period. In this respect the yam starches resemble potato starch. The viscosities of starch of four varieties of D. esculenta extracted using water and ammonia solution showed only minor difference between the varieties (800-950 BU), but the viscosities of starch extracted using ammonia solution were much higher than those obtained by water extraction. For D.alata starch, there were no clear peak viscosity values. As observed for the D. esculenta starch, there was no noticeable breakdown in viscosity on heating and stirring For most yam starches, the viscosity breakdown was quite low in spite of the high viscosity levels. The yam starches contain three to four times more phosphorus than that in cassava and aroid starches. It has been reported that the phosphate linkages in potato starch are responsible for its high viscosity and such effects may also be important in the yam starches.

   Gebre-Mariam and Schmidt ⁶⁰ have obtained 781, 756, 1282 BU for peak viscosity, hot paste viscosity and cold viscosity respectively for D. cayensis starch showing low breakdown, but high setback for this starch. Farhat et al. ⁵⁶ have examined the viscosity properties of four yam species (Fig. 4). The peak viscosities ranged from 2028 mPas for D dumetorum to 3893 mPas for D. cayensis compared to 8900 mPas for potato and 3134 mPas for cassava starches. The breakdown in viscosity was much lower compared to cassava and potato starches and in conformity with the results obtained with the Brabender Viscograph. The final viscosity was slightly higher than peak viscosity indicating a tendency to retrograde. Among these species, D. dumetorum had all the viscometric parameters lower than the other three and in support of earlier results ⁶⁰. Nkala et al. ⁵⁹ did not observe much difference in the viscosity parameters of D. dumetorum starch during the dry and wet seasons. The RVA profiles of starch of some varieties of D. alata, D. esculenta and D. rotundata harvested at different maturity have revealed that maturity did not affect the rheological properties to any major extent

   Swelling volume values of starch of six clonal selections of D. rotundata showed only slight differences among them. The swelling volume of D. abyssinica starch increased from 10 to 23ml g^-1 as temperature was increased from 65 to 85°C. The relatively lower swelling of Dioscorea starch compared to potato starch has been attributed to the higher lipid content in the starch and also higher inter-associative forces compared to potato starch. The solubility of D. abyssinica starch was found to be enhanced with temperature and the values resembled those of maize starch ⁶⁰. The solubility of starch of yams pretreated with various chemicals was affected to different extent by the chemicals used and also the concentration. The values were between 18-32%.

   Dioscorea starches have almost equal clarity as cassava starch, indicating that their associative forces are similar. Variation was not observed among different varieties of Dioscorea alata, D. esculenta and D. rotundata starches.

   The Dioscorea starches have relatively poor digestibility in the native state (20%) similar to potato starch. However gelatinisation leads to much higher digestibility.

   Colocasia granules are among the smallest of starches observed in the plant kingdom making them useful in various applications eg. as a filler in biodegradable plastics, in toilet formulations, aerosol etc ⁶¹. Strauss and Griffin ⁶² have examined a large number of taro cultivars and found maximum granules having size of 5.10 mm and least in the size 1.79 mm with a mean value of 3.34 mm.

   Unlike other tuber crop starches, which do not exhibit any significant variability in size with varietal differences, Colocasia starch was found to exhibit varietal difference. Studies on 10 varieties revealed a significant difference in average granule size (Fig 5)⁶³. The average granule size and the distribution of the granule sizes showed only minor difference between the corms and cormels of four cultivars of Colocasia. Although variation existed among the cultivars, no significant variability was noticed within a cultivar during the growth period. Colocasia starch has an ‘A’XRD pattern similar to cassava and sweet potato starches.

   Colocasia esculenta starch showed a wide range in the amylose content and noticeable relationship between the amylose content and granule size was observed. The variety C-9 having the largest granule size possessed the highest amylose content among 10 varieties examined ⁶³. Strauss and Griffin ⁶² found that the maximum value for amylose content was 43% and minimum 3% and a mean value of 24.04%. They could not obtain correlation between amylose content and granule size.

   DSC studies indicated that Colocasia starch has T_onset values over 80°C and T_end values above 85.0°C, these being the highest among the starches ¹⁸. Strauss and Griffin ⁸⁷ found the gelatinisation temperature of different varieties of Colocasia starch to range from 69 to 74°C. Pasting temperatures by Brabender viscography of different cultivars of Colocasia esculenta showed only very slight difference between the varieties, but were distinctly higher than those of cassava and sweet potato starches. Considerable difference in peak viscosity values among the accessions was observed. It was also interesting to observe that starch from the C9 variety having the highest granule size and amylose content had the highest peak viscosity. There was only nominal breakdown in viscosity even at the highest concentrations pointing to the possibility of using this starch in various applications, which require paste stability.

   Considerable variation in swelling volume of different varieties of Colocasia has also been reported. The values ranged from 26.5 to 60 ml g^-1 – which indicates a high degree of variability. Inverse relationship was noticed between the granule size and swelling volume of ten accessions of taro. The clarity of colocasia starch is, as expected, poor and the values are closer to that of cereal starches.

   In vivo digestibility of colocasia starch was found to be good both in native and gelatinisd forms Xanthosoma starch possessed granular size ranging from 10-50um (Tab.4) and the starch granules are round. The starch possesses an ‘A’ RXD pattern and the amylose content is in the range 15-25%, similar to most other tuber starches. The gelatinisation temperature of the starch ranged from 68-95 °C by RVA and 79-92 °C by DSC. Valetudie et al ⁶⁵ found that starch of fresh and freeze dried tubers had similar gelatinisation temperatures. The T_onset and T_end for the starch were between 68-90 °C obtained by various workers. The gelatinisation enthalpy ranged from 9-15 J/g ¹⁸.

   The Brabender viscosity values showed the starch to have medium viscosity (600 BU for a 5% paste) with low breakdown. Varietal difference was not evident. The starch has a swelling volume of 20ml/g . In vivo digestibility of the starch was found to be quite good both in native and gelatinised form being over 60%.

   Amorphophallus starch granules are round or polygonal with a granular size of 3-30um. The starch has an ‘A’ XRD pattern and there was very little variation in the amylose content among different varieties ⁶⁶ Variability in the viscosity properties of Amorphophallus paeoniifolius starch extracted from ten accessions was quite minor.

   The viscosity breakdown for the starch samples was very low, similar to other aroid starches and in the same range as the cereal starches. Soni et al. ³⁵ observed a value of 440 BU at 5% concentration and a breakdown of 40 BU, while Wankhede and Sajjan ⁶⁷ reported peak viscosity of 1570 BU at 10% concentration with a breakdown of 270BU.