Tropical Sources of Starches

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2. Cassava Starch Characteristics

Among the vast sources of starch outlined above only cassava and maize starches have been commercially exploited since long and continue to be major source of starch. The extraction of maize starch is slightly complicated due to the need for steeping the dried cobs and using some whitening agents. On the other hand, cassava starch is easily extractable since the tubers contain very low quantity of proteins, fats etc. Hence the extraction process is simple and the starch obtained is pure white in colour if extraction is carried out properly. Since the lipid content in the starch is very little (<0.1%), the starch and its derivatives have a non-cereal taste very desirable in many food products.

Cassava starch granules are mostly round ^14-16 with a flat surface on one side containing a conical pit, which extends to a well-defined eccentric hilum. Some granules appear to be compound. Under polarised light, a well defined cross is observed. The granules exhibit wide variation in size ranging from 5-40mu and variation in granular size distribution among varieties and during growth period and during different seasons has been reported [17, 18, 19]. Cassava starch has been assigned ‘A’ and ‘C’ XRD patterns by various workers¹⁸

The starch has very little lipid and phosphorus content. The amylose content in the starch is in the range of 20-27% similar to most other starches. The amylose content shows difference among varieties, but age of the crop and envirornmental factors do not affect the amylose content to any large extent.

GPC analysis of debranched starch of different varieties did not reveal much variation among the varieties ^18,20.The soluble amylose content forms approximately 40% of the total amylose.

Thermal characteristics of cassava starch have been studied in detail using Differential Scanning Calorimetry which has become very important in characterisation of starch gelatinisation.

DSC analysis of starch extracted from five varieties of cassava possessing different organoleptic quality showed that varietal differences manifest themselves in the DSC patterns Fig 1.

The characteristic peak shape could be traced to structural differences among the varieties. The values reported for T_onsetand T_end exhibited wide variation showing that varietal differences, environmental conditions, experimental conditions used for DSC measurement all influence these factors.

Asaoka et al. ²¹ have examined the gelatinisation characteristics of four cultivars harvested at different seasons and their results indicated that both genetic constitution and environmental conditions affected the DSC parameters. Starch of one cultivar consistently displayed highest gelatinisation temperature through all the harvesting seasons (Tab. 2 ).

Defloor et al. ¹⁹ studied the DSC characteristics of five varieties harvested during dry and rainy seasons and found that although within each planting season for each genotype and for each harvest time, significant differences in gelatinisation temperatures were noticed, no systematic changes as a function of genotype or harvest time was noticed. Starch samples harvested at six months stage in dry season had higher onset, peak and conclusion temperatures compared to that from rainy season, but the reverse was true for the tubers harvested at 12, 15 and 18 months. Even the temperature of drying affected the DSC gelatinisation temperatures.

The gelatinisation parameters of cassava starch extracted using SO₂ incorporated water were enhanced from 59.6 to 62.0 for T_onset and 84.7 to 87.2°C for T_end ²².

Gelatinisation enthalpy depends on a number of factors like crystallinity, intermolecular bonding, rate od heating of the starch suspension, presence of other chemicals etc. For cassava starch, the values reported in the literature range all the way from 4.8 ²³ to 16 Jg^{-1 24}, Tab. 1.

Genetic and environmental factors also affect the gelatinisation enthalpy as illustrated by Asaoka et al. ²¹ and Defloor et al. ¹⁹ from studies using different varieties, time of harvest and seasonal variations. SO₂ treatment slightly enhanced the gelatinisation enthalpy of cassava starch from 18.1 to 19.1 J g^{-1 22}.

Among different tuber starches, cassava starch possesses the lowest gelatinisation temperatures. Varietal difference is also evident for the reported data from various sources. Correlation between granule size and gelatinization temperature was not obtained ¹⁹. The gelatinisation temperatures of cassava starch determined microscopically by various workers ranged from 49-64°C ²⁵ to 62-73°C ¹⁵. In a Brabender Viscographic study involving starch of different varieties, pasting temperature of H 165 starch was slightly lower than those for most of the other varieties, and M4 starch had the highest range of pasting temperature ¹⁸.

The values were quite close to DSC values of 66 and 78°C for T_onset and T_peak respectively. SO₂ treatment lowered the pasting temperature of cassava starch from 92 to 89°C.

Gelatinization temperature of starch was enhanced tremendously in glycerol and ethanediol, while in DMSO and formalin, only slight increase was noticed. The high values observed in the first two solvents can be attributed to steric factors ²⁶.

Viscosity is an important starch property on which many applications are based. Studies on viscosity of cassava starch have been carried out extensively and almost all their studies indicate high viscosity level for cassava starch compared most other tuber starches and the cereal starches. The viscosity characteristics are influenced by varietal differences, environmental factors, rate of heating, other ingredients present in the system etc.]. The Brabender Viscographs of starch of different varieties showed three main peak patterns viz. single stage gelatinisation with high peak viscosity and high viscosity breakdown , two-stage gelatinization with high peak viscosity and breakdown and broad two-stage gelatinization with medium viscosity and medium breakdown. These patterns seem to be genetically controlled as the patterns were maintained by these starches irrespective of the environmental factors, though there was variation in the viscosity values[12. In a comparative study of five cassava varieties having different cooking quality, it was observed that H-1687 starch had a medium peak viscosity and low viscosity breakdown but high set-back viscosity.

M4 starch had slightly lower peak viscosity and setback viscosity. On the other hand, H-165 starch had a very high peak viscosity and the breakdown was also quite large. The viscographs clearly indicated that for H-165 starch, the set-back viscosity was much lower compared to peak viscosity, whereas for H-1687 starch, the reverse was true (Fig. 2). The result indicates a possible relationship between cooking quality and starch rheology, as tubers of variety H-1687 have reasonably good cooking quality, while H-165 is poor in its culinary quality. Tubers of variety M4 starch, whose starch behaves somewhat similar to H 1687 starch in its rheology, has excellent organoleptic quality ¹⁸. Olorunda et al. ²⁷ found that mealier cassava varieties had slightly higher peak viscosity for their starches.

Rickard et al. ¹⁵ reported wide variation among the viscosity data for cassava starch from various sources ^28-30 (Tab. 1). However,Rosenthal et al. ³¹ reported only minor variations among a few Brazilian varieties. Asaoka et al. ²¹ has compared the viscosity data of cassava starch using Brabender Viscograph and Rapid Visco Analyser (RVA) and the results are presented in Table 2. Breakdown of viscosity is another important factor which has a bearing on starch application in food. When the starch granules swell and they are subjected to heat and shear, the starch undergoes fragmentation and the resulting reduction in viscosity indicates the breakdown for the starch. Breakdown is not desirable since it leads to uneven viscosity and also leads to a cohesive nature for the starch paste. Cassava starch has a higher breakdown compared to most other starches, especially the cereal starches and hence is a major drawback attributed to the starch in some food applications.

Padmanabhan and Lonsane ³² observed a slight reduction in peak viscosity and breakdown in cassava starch extracted by an enzymatic method. The viscosity of cassava starch extracted from inoculum provided fermentation was lowered due to presence of fibrous matter ³³. However, the breakdown was also correspondingly reduced due to the cementing of the granules by fibrous materials and its rheology was similar to cassava flour.

Indent Another factor related to viscosity is Setback which is attributed to the retrogradation of starch during cooling. Cassava starch has relatively low set back and this may be due to lower amylose content and also the structure of the amylopectin.

Swelling power and solubility of starch provide evidence of non-covalent bonding between starch molecules. Factors like amylose-amylopectin ratio, chain length and molecular weight distribution, degree / length of branching and conformation decide the swelling and solubility Cassava starch has medium swelling power compared to potato and cereal starches – a property in conformity with its observed viscosity. The reported values for the swelling power of cassava starch vary considerably from 42-71 (Tab. 1) ¹⁵. The swelling volume of different varieties of cassava varied from 25.5 to 41. 8 ml g^-1 of starch.

It was observed that during the growth period, starch of two varieties H-2304 and M4 maintained their swelling volumes within small ranges, while that for some varieties such as H-165 expressed wide variations which indicate that these varieties are very much susceptible to environmental influences ³⁴ and also point to possible relationship between cooking quality and swelling volumes. Soni et al. ³⁵ have reported a two stage swelling for cassava starch and attributed it to the two types of forces, which require different energy input to cause relaxation of the starch molecules. Asaoka et al. ²¹ have determined the swelling volume of four varieties of cassava harvested during two seasons and observed that swelling power was higher in samples harvested in November compared to those harvested in August.

Cassava starch has a higher solubility compared to the other tuber crop starches and the higher solubility can be attributed partly to the high swelling it undergoes during gelatinisation and the reported values ranged from 25 to 48% (Tab. 1) ¹⁵. The solubility of starch of different cassava varieties varied from 17.2 to 27.2%. However no direct correlation between swelling and solubility could be observed . The values for solubility of starch from different varieties during the growth period also indicated that starch of varieties H.2304 and M4 had good stability in their solubility, whereas the others had medium or poor stability ³⁴. Among a few non-aqueous solvents studied for solubility of starch, maximum solubility was obtained in DMSO and formalin, while in glycerol it was moderate. Starch was insoluble in anisole and methyl cellosolve.

The solubility data indicate that starch is more soluble in polar solvents or solvents with affinity towards water ²⁶.

The high clarity of starch has much relevance in food and textile applications and depends on the associative bonds between the starch molecules in the granules. Cassava starch having weaker associative forces compared to cereal starches has better clarity.

Another starch property which has relevance in food applications is sol stability which is decided by the retrogradation of starch molecules. During cooling and storage, the starch molecules associate leading to settling of the starch gel.

This settling is not desirable in food products especially those canned and subjected to freezing and thawing. In this respect cassava starch has fair sol stability compared to the cereal starches.

The digestibility of cassava starch in native and gelatinised forms are quite high both in vivo and in vitro studies. It was over 60% in in vivo studies on small animals.

The properties of cassava starch reveal that the starch can find use in various food products. First, the bland taste of the starch is an advantage over the cereal starches which have a cereal flavour due to the lipids present in the starch. Sago pearls of best quality are obtained using cassava starch in view of its white colour, bland taste and easy gelatinisation.

The starch has a very low gelatinisation temperature- lower than most other root starches and the cereal starches and hence cooking is easier In addition the low gelatinisation temperature can be useful if some of the ingredients added to the product is heat-labile at high temperature. The high viscosity of the starch paste is also very useful in many food products which require body. This is exemplified by the preference for this starch in puddings and in fact in USA, tapioca pudding is an important food item. Though the stability of the viscosity is poor, some food products do require a cohesive texture like some gravies used in the orient and here cassava starch finds preference . Another important property which is very desirable in food products is the relatively good sol stability of the starch paste. This is of utmost importance in starch based products which are stored for long periods. The starch contains relatively lower content of amylose compared to cereal starches and its retrogradation is very low.

The clarity of the cassava starch paste is excellent and hence finds favour in pie fillings where the products appear more appealing when the fruits are clearly visible. Thus the starch has a lot of desirable qualities which makes it widely used in food products. However, the starch is not used in some products due to the cohesive nature brought about by the breakdown of starch under heat and shear and poor freeze thaw stability. But with a number of modification techniques available, the undesirable properties can be rectified while maintaing the desired ones. ^35-37