Physicochemical properties of enzymatically separated starch from sweet potato
S.N.Moorthy and C.Balagopalan
Central Tuber Crops Research Institute, Sreekariyam, Thiruvananthapuram-695 017, Kerala, India.
Abstract The physicochemical properties of starch extracted from sweet potato tubers using five different concentrations of the enzyme system cellulase-pectinase was studied. The starch content of the extract was 90-93% on a dry weight basis. The reducing values were not noticeably increased by the enzyme treatment. The peak viscosity values of the extracted starches showed an increase up to 0.05% concentration of enzyme, and thereafter registered a minor fall. The viscosity stability also showed a small reduction at enzyme concentrations above 0.05%. The swelling volumes exhibited a slight decrease and solubility was almost doubled at the highest concentration of the enzyme. The SEM photographs did not indicate any major change in the surface morphology of the extracted starch. Thus the enzyme treatment does not adversely affect the starch properties up to 0.05% concentration.
Keywords: sweet potato, starch, cellulase, pectinase, physicochemical properties.
Sweet potato (Ipomea batatas L.) is cultivated more widely than other trophical tuber crops, and it occupies third position in terms of calories produced per square metre (Leung et al 1972). However, the extraction of starch from sweet potato tubers has been practiced in only a few countries, particularly China and Japan. The reason for this is the difficulty of getting a good yield of starch. The tubers contain 15-30% of starch, but the yield is usually less than 15%, even for the high starch varieties. In contrast, with cassava, over 80% of the starch is obtained by simple extraction. The low recovery makes the starch more expensive. The reason for the low extraction rate is probably difficulty in breaking down non-starchy constituents like cellulose, hemicellulose and pectin, which restrict the starch granules from moving into the aqueous phase during the conventional process (Balagopalan et al. 1996). With the increased availability of commercial enzymes which break down cellulose and pectin, attempts have been made to use them to improve the extractability of starch. Kallabinski and Balagopalan (1991) studied the effect of cellulolytic and pectinolytic enzymes on the extraction of starch from sweet potato tubers: the yield showed a substantial increase.
The physicochemical and functional properties of sweet potato starch have been summarized by Tian et al. (1991): they vary with variety, climate, environment, etc. when cassava is fermented before extraction, the starch undergoes minor changes in its properties but the main structural features are unaffected (Moorthy et al. 1993). This work aimed to establish whether enzymatic separation affected the physicochemical properties of sweet potato starch, on which the food and industrial uses depend.
Sweet potato tubers were washed and the outer skin was peeled with a knife. The extraction of starch was by the procedure of Kallabinski and Balagopalan (1991), using the enzymes Celluclase and Pectinex (Novo, Denmark) at 0.01, 0.025, 0.05, 0.1 and 0.2%. The starch content in the extracted starch was monitored titrimetrically (Moorthy et al. 1996). The reducing values were determined by the method of Schoch (1964a). Viscosity, viscosity stability and pasting temperatures were obtained from Brabender runs with 5,6 and 7% starch is distilled water and a heating rate of 1.5° C/ min. The swelling volumes, solubility and swelling power were determined by the methods of Schoch (1964b). Swelling volumes were based on the sedimentation volumes, while the solubility was calculated from the weight of residue left on drying a fixed volume of the supernatant. The scanning electron microscopic analysis of the samples was on a Joel unit after coating the starch with gold in vacuum, at three magnifications, 1500x, 2000x and 4500x.
Results and discussion
The extract from sweet potato with different concentrations of enzyme contained 90-93% of starch (Table 1). This indicates that only a small quantity of fibrous material is being extracted with the starch, even with the enzyme at 0.2%.Padmanabhan and Lonsane (1992) found that in an enzymic extraction of cassava starch, the total ash content was lower than in the conventional procedure, and this was attributed to the liberation of minerals from the root cells. When fermentation was carried out on cassava tubers using and inoculum provided culture, the resulting starch had significant fibre content, which increased with the fermentation time (Moorthy et al. 1993). The absence of large amounts of fibre in the enzymatically separated starch from sweet potato indicates that the non-starchy polysaccharides are completely broken down and do not contaminate the starch, so the enzymes appear more efficient than the cultures in breaking down the pectins and cellulosic components. The reducing values of the starch from the enzyme treatments were small (Table 1), as expected, since the enzymes are pectinolytic and cellulolytic, and should not affect the starch granules.
Table 1. Properties of enzymatically separated sweet potato starch
|Enzyme Concentration (%)||Starch Content (%)||Reducing Value||Swelling Volume (%)||Solubility (%)|
The viscosity data of the starches from enzymatic and conventional extraction are in Table 2. the peak viscosity varied depending on the concentration of starch used. With 5 and 6% pastes, the peak viscosity increased for the extracts obtained with increasing amounts of the enzymes, up to 0.025 or 0.05%, and with 0.2% of enzyme it dropped noticeably. With the 7% paste, there was a fairly steady drop in peak viscosity as the enzyme concentration increased. This is probably due to a weakening of the associate forces rather than to a breakdown of the starch granules. The breakdown in viscosity also increased with higher levels of the enzymes. When the concentration of the enzyme was 0.1%, the breakdown was large and it became very significant at 7% starch concentration. A reduction in the breakdown was observed with 0.2% enzyme, attributable to the correspondingly lower peak viscosity. These results also show that the strength of the associative forces is somewhat affected at higher concentrations of the enzyme. The presence of fibre reduces the breakdown of starch viscosity by protecting the starch granules against heat and shear (Moorthy et al. 1994). In the enzymic extraction of cassava starch, the peak viscosity showed a slight reduction while the breakdown increased marginally (Padmanabhan and Lonsane 1992), as in our work. The pasting temperature did not show any definite pattern, but generally there was a shift to lower temperatures with increasing concentrations of the enzyme. This can also be explained on the basis of a weakening of the associative forces rather than the presence of fibrous residues, which would have led to higher pasting temperatures, as observed with cassava fermentation (Moorthy et al. 1993), Padmanabhan and Lonsane (1992) did not notice such an effect in the enzymic extraction of cassava starch.
Table 2. Pasting temperature and viscosity of enzymatically separated sweet potato starch.
|Starch Concentration (%)||Enzyme Concentration (%)||Pasting Temperature (°C)||Peak Viscosity (BU)||Viscosity Breakdown (BU)|
The enzymatically separated starch had a slightly lower swelling volume at 95°C than the control (Table 1), and there was no relationship between the reduction in swelling volume and the concentration of enzyme used. Padmanabhan and Lonsane (1992) found a slight reduction in the swelling volume in enzymatically extracted cassava starch and attributed this to a small reduction in Ph. The solubility of enzymatically extracted starch was less than the control at low enzyme concentrations, but a higher levels it increased substantially. This could be explained by the weakening of associative forces at higher concentrations. A similar increase in solubility was observed with cassava starch (Padmanabhan and Lonsane 1992).
Scanning electron microscopy of the starch granules showed that there was no marked change in the surface morphology upto the 0.1% enzyme level (Figure 1a-d). Above 0.1% some minor fissures appeared on the surface (Figure 1e and f). The effect appears to be restricted to the surface of the granules, since the other granular properties were affected to only a small extent. Padmanabhan and Lonsane (1992) did not observe any difference between conventionally and enzymatically extracted starch from cassava.
The starch separated from sweet potato using pectinase and cellulase does not undergo any major breakdown up to and enzyme concentration of 0.05%, but above that some level some weakening of the associative forces takes place.
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Figure 1. SEM photographs of starch granules of sweet potato extracted using different enzyme concentrations. (a) 0% enzyme; (b) 0.01% enzyme; (c) 0.025% enzyme; (d) 0.05% enzyme; (e) 0.1% enzyme; (f) 0.2% enzyme.