Cassava flour contains fiber, sugars, and smaller quantities of lipids and other components. It exhibits properties different from those of cassava starch, which cooks to a more cohesive paste. The gelatinization characteristics of starch and flour, extracted from selected cassava cultivars, were examined. The peak viscosity of flour was generally lower than that of starch, although more stable. Swelling volumes were also correspondingly lower. The gelatinization temperature of flour, whether ascertained by differential scanning calorimetry or viscography, was consistently several degrees higher than that of starch. The lower peak viscosity and higher gelatinization temperature probably contribute significantly to the textural differences between flour and starch. Defatting and ethanol extraction had little influence on the gelatinization characteristics of either starch or flour, indicating that fiber, rather than lipids or sugars, probably makes the most important contribution to flour texture. The importance of these findings to the texture of food products made from cassava flour and starch is discussed.
Introduction
Cassava is an important root crop in many tropical countries, where the starchy and tuberous roots are eaten in various forms, including as starch and flour. The starch is extracted by a wet process and the flour obtained by milling dried chips.
The texture of cooked roots differs widely between cultivars, and considerable work has been carried out to identify reasons for this variability (Asaoka et al., 1992; Kawano et al., 1987; Moorthy et al., 1993a; Safo-Katanka and Owusu-Nipah, 1992; Wheatley et al., 1993). Few conclusions have been reached but the quantity and quality of the starch in the root and the presence of various nonstarchy polysaccharides are considered important.
Several differences also exist in the rheological and functional properties between starch and flour. We attempted to identify the reasons for these differences.
Materials and Methods
Starch and flour were obtained from five cultivars of freshly harvested cassava roots (M-4, H-165, H-1687, S-856, and H-97), each having different cooking qualities. The main constituents of the samples—starch, fiber, lipids, and sugars—were determined by standard procedures. To assess the influence of lipids, samples were defatted by extraction (Soxhlet), using petroleum ether (40-60 °C). Ethanol extraction was similarly undertaken, with 80% ethanol (Soxhlet, 6 h), to examine the influence of sugars and ethanol-soluble components.
Differential scanning calorimetry (DSC) data were obtained by using Perkin Elmer DSC-2 equipment with Indium as a standard (temperature range 25-100 °C, at a heating rate of 10 °C/min). Gelatinization profiles of the samples (5%) were obtained on a Brabender Viscoamylograph (350 cmg [torque] sensitivity cartridge, heating rate 1.5 °C/min). Swelling volumes were determined at 95 °C (Schoch, 1964).
Results and Discussion
Table 1 presents the results of the chemical analyses of starch and flour from the five cassava varieties. Starch content on a dry weight basis was 98% or more in all the starch samples and between 79.1% and 86.0% in the flour samples. Crude fiber content was 0.13% or less in the starch samples, whereas it ranged from 1.50% to 2.98% in the flour. Earlier studies show similar starch and fiber compositions (Abraham et al., 1979).
| Variety | Product | Starch (%) | Sugar (%) | Lipids (%) | Crude fiber (%) |
| M-4 | Starch Flour | 98.1 86.0 | – 2.20 | 0.11 0.45 | 0.11 1.50 |
| H-165 | Starch Flour | 98.0 79.1 | – 79.1 | 0.22 0.27 | 0.13 2.98 |
| H-1687 | Starch Flour | 98.2 80.5 | – 2.72 | 0.18 0.29 | 0.15 2.23 |
| S-856 | Starch Flour | 98.5 81.2 | – 3.23 | 0.20 0.56 | 0.12 2.56 |
| H-97 | Starch Flour | 98.3 82.7 | – 3.05 | 0.20 0.25 | 0.11 2.70 |
The lipid content, by nature much lower than that of cereals, varied from 0.11% to 0.22% in the starches and from 0.25% to 0.56% in the flours. Lipids, in common with many surfactants, significantly affect starch by complexing strongly with amylose and amylopectin side chains, rendering these less labile (Krog, 1973). This capacity has been exploited for reducing the cohesiveness of potato starch products (Hoover and Hadziyev, 1981, 1982). Low levels of surfactants can have a profound effect on cassava starch (Moorthy, 1985). Ethanol-soluble constituents in the flours ranged from 2.5% to 3.7% and only 0.9% to 1.3% in the starch samples (data not shown). The predominant sugar in cassava flour has been identified as sucrose.
The recorded gelatinization temperatures (Table 2) reveal a consistent difference between the flour and starch samples. Comparing values for initial, maximum, and end temperatures, the results for flour are each 2-3 °C higher than for the corresponding starches. Components within the flour, by restricting access of water into the starch granules, can delay gelatinization. Surfactants and lipids, by forming complexes, are known to raise gelatinization temperatures (Osman, 1967). However, the defatted and ethanol-extracted flours had the same values as native flour, indicating that neither lipids nor sugars were responsible for enhanced gelatinization temperatures. Recent experiments on cassava starch show correlation between higher fiber content and higher gelatinization temperatures (Moorthy et al., 1993a). Thus, the elevation in gelatinization temperature of flour may be attributed to the fiber.
| Variety | Product | Temperature (°C) | ∆Ha (cal/g) | ||
| Initial | Maximum | End | |||
| M-4 | Starch Flour | 68.10 71.11 | 73.24 75.67 | 78.54 81.29 | 2.95 2.02 |
| H-165 | Starch Flour | 65.35 68.65 | 69.22 72.02 | 74.86 77.19 | 3.27 2.14 |
| H-1687 | Starch Flour | 67.12 70.02 | 71.45 73.90 | 75.39 79.11 | 2.15 2.22 |
| S-856 | Starch Flour | 65.62 68.72 | 70.14 72.92 | 74.94 76.95 | 2.65 2.09 |
| H-97 | Starch Flour | 69.36 71.82 | 72.29 75.02 | 77.13 79.92 | 3.43 2.27 |
The DSC peak patterns of starch and flour from the same cultivar were similar. ‘H-97’ starch and flour had a characteristic shoulder in their peaks, whereas ‘M-4’ starch and flour had typically broad peaks. The peak patterns were not modified by defatting or by ethanol extraction, indicating dependence on the starch granular structure. The enthalpy of gelatinization of flour was lower than that for starch for every variety and neither defatting nor ethanol extraction affected the values to any noticeable extent. However, the lower enthalpy for the flour can be attributed in part to the lower starch content of the samples. The enhanced gelatinization temperatures in the DSC results for the flour samples are supported by the Brabender viscographic data (Table 3), which show that pasting temperatures for flours were 3-5 °C higher than for the respective starches.
| Variety | Producta | Viscosity (BU)b | Break-down | Pasting temp. (°C) | Swelling vol. (ml/g) | |
| V97 | VH | |||||
| M-4 | Starch | 540 | 380 | 160 | 68-73 | 32.0 |
| Flour | 380 | 320 | 60 | 71-74 | 28.0 | |
| Starch (d.) | 580 | 440 | 140 | 70-76 | 33.5 | |
| Flour (d.) | 380 | 310 | 70 | 72-75 | 29.5 | |
| Starch (e.) | 560 | 420 | 140 | 70-75 | 33.0 | |
| Flour (e.) | 410 | 380 | 30 | 72-76 | 29.0 | |
| H-165 | Starch | 940 | 480 | 460 | 66-78 | 38.5 |
| Flour | 460 | 380 | 80 | 71-75 | 32.0 | |
| Starch (d.) | 1,000 | 580 | 420 | 69-82 | 39.5 | |
| Flour (d.) | 440 | 360 | 80 | 72-76 | 33.5 | |
| Starch (e.) | 1,000 | 520 | 480 | 69-83 | 39.5 | |
| Flour (e.) | 470 | 380 | 90 | 71-78 | 33.0 | |
| H-1687 | Starch | 540 | 480 | 60 | 70-81 | 33.5 |
| Flour | 460 | 440 | 20 | 71-83 | 29.5 | |
| Starch (d.) | 570 | 510 | 60 | 71-82 | 33.5 | |
| Flour (d.) | 440 | 390 | 50 | 0-80 | 29.0 | |
| Starch (e.) | 520 | 480 | 40 | 70-83 | 34.0 | |
| Flour (e.) | 440 | 400 | 40 | 71-83 | 29.5 | |
| S-856 | Starch | 500 | 340 | 160 | 67-80 | 33.0 |
| Flour | 440 | 360 | 80 | 71-86 | 29.0 | |
| Starch (d.) | 580 | 390 | 190 | 69-75 | 33.5 | |
| Flour (d.) | 470 | 380 | 90 | 70-85 | 29.5 | |
| Starch (e.) | 490 | 360 | 130 | 69-75 | 34.0 | |
| Flour (e.) | 740 | 390 | 80 | 70-89 | 30.0 |
The peak viscosity and viscosity breakdown for each flour were different from those of the corresponding starch, and most pronounced in the comparatively much lower peak viscosity of flour from ‘H-165’. Again, the lower starch content in the flour samples can account in part for the low readings. However, while the viscosity was lower, it was more consistent throughout the temperature program. Stabilization occurs through the presence of nonstarchy components in the flour. Lipids, although known to stabilize starch viscosity (Krog, 1973), had little effect here. The absence of stabilization in the ethanol-extracted samples indicates that sugars were not involved either. The reduced viscosity noted in all varieties was most pronounced in ‘H-165’, which had the highest crude fiber content. Stability may therefore result from the fiber affecting starch granule expansion and breakdown.
Defatting and ethanol extraction slightly enhanced paste viscosity in starch, whereas flour samples remained unaffected. In contrast, the slight increase in the flours’ pasting temperature was probably due to the continuing presence of fiber in the defatted ethanol-extracted samples. Similar results have been obtained in a fiber-rich starchy flour extracted from fermented roots (Moorthy et al., 1993b). According to Osman (1967), high levels of sugars are needed to bring about perceptible changes in the viscosity of starch. The absence of significant changes in the peak viscosity of flours and starches thus indicates that sugars do not greatly influence viscosity in cassava.
Neither defatting nor ethanol extraction affected swelling volumes of the starches and flours, further indicating the influence of fiber in modifying starch rheological properties in the flour.
Cassava starch, cooked in water, generally gives a cohesive, long paste, whereas flour texture is less cohesive. Cohesiveness is attributed to the breakdown of starch molecules during heating and stirring. Early gelatinization can render starch more susceptible to breakdown because it undergoes a longer period of shear. High swelling necessitates the weakening of associative forces and thus easier breakdown of starch. The fiber may act as a barrier to earlier gelatinization and to higher swelling, reducing the cohesiveness of the paste.
Starch can act, for example, as a binder, thickener, or glazing agent in different foods (Smith, 1982[?]). In products where a cohesive texture is desired, such as gravies and puddings, starch would be favored, whereas in products where a nonsticky consistency is sought, flour would be more suitable. These mirror the findings of comparative studies conducted at the Central Tuber Crops Research Institute (CTCRI) for starch and flour in locally produced extruded products (when starch was used, the product tended to be hard and oily; with flour, the same product was crisp and nonsticky). Similarly, food items prepared from starchy flour made from fermented roots had a higher fiber content and better texture.
The study thus clearly indicates that fiber is a significant determinant of the characteristics and functional properties of cassava starches and flours. Future work should examine specific fiber components (e.g., cellulose and hemicellulose) and their interaction with starch. It should also focus on how rheological characteristics can lead to different functional properties in starch and flour.