Starch is one of the most abundant substances in nature, a renewable and almost unlimited resource. Starch is produced from grain or root crops. It is mainly used as food, but is also readily converted chemically, physically, and biologically into many useful products to date, starch is used to produce such diverse products as food, paper, textiles, adhesives, beverages, confectionery, pharmaceuticals, and building materials. Cassava starch has many remarkable characteristics, including high paste viscosity, high paste clarity, and high freeze-thaw stability, which are advantageous to many industries.
Cassava starch is produced primarily by the wet milling of fresh cassava roots but in some countries such as Thailand it is produced from dry cassava chips. Starch is the main constituent of cassava. About 25% starch may be obtained from mature, good quality tubers. About 60 % starch may be obtained from dry cassava chips and about 10 % dry pulp may be obtained per 100 kg of cassava roots.
Fresh tubers are processed during season and dry chips during the off-season in some countries. Extraction of starch from fresh cassava roots (Fig. 1) can be divided into five main stages: preparation (peeling and washing), rasping/pulping/grating, purification (starch washing), dewatering and drying, and finishing (milling and packaging).
For cassava, the process of starch extraction is relatively simple as there are only small amounts of secondary substances, such as protein, in the roots. When cassava roots are harvested or selected for starch extraction, age and root quality are critical factors. Cassava roots need to be processed almost immediately after harvest, as the roots are highly perishable and enzymatic processes accelerate deterioration within 1-2 days. A first-rate quality starch can be obtained from cassava using only water, and this makes the processing of cassava starch and flour particularly suitable for developing countries and rural industries.
Advantages of cassava starch
Cassava has many advantages for starch production.
- High level of purity.
- Excellent thickening characteristics.
- A neutral (bland) taste.
- Desirable textural characteristics.
- A relatively cheap source of raw material containing a high concentration of starch (dry-matter basis) that can equal or surpass the properties offered by other starches (maize, wheat, sweetpotato, and rice).
- is easy to extract using a simple process (when compared to other starches) that can be carried out on a small-scale with limited capital.
- is often preferred in adhesive production as the adhesives are more viscous, work more smoothly, and provide stable glues of neutral pH
- has clear paste.
The development of both the food and non food uses of cassava starch has made much progress and continues to have a bright future. Both old and important new products, such as modified starches, starch sugars, starch-based plastics and fuel alcohol, are reviewed briefly.
Starch can be classified into two types: native and modified. Native starches are produced through the separation of naturally occurring starch from either grain or root crops, such as cassava, maize, and sweetpotato, and can be used directly in producing certain foods, such as noodles. The raw starches produced still retain the original structure and characteristics and are called “native starches”. Native starch is the basic starch product that is marketed in the dry powder form under different grades for food, and as pharmaceutical, human, and industrial raw material. Native starch has different functional properties depending on the crop source, and specific types of starch are preferred for certain applications. Native starch can be considered a primary resource that can be processed into a range of starch products.
Native starches have limited usage, mainly in the food industry, because they lack certain desired functional properties. The native starch granules hydrate easily when heated in water, they swell and gelatinize; the viscosity increases to a peak value, followed by a rapid decrease, yielding weak-boiled, stringy, and cohesive pastes of poor stability and poor tolerance to acidity, with low resistance to shear pressure, as commonly employed in modern food processing.
However, food, metallurgic, mining, fermentation, construction, cosmetics, pharmaceutical, paper and cardboard, and textiles industries among others use native starch in its traditional form.
For those characteristics, which are unattainable with native starch, modified starch can be used for other industrial applications through a series of techniques, chemical, physical, and enzymatic modification. Thus, modified starch is native starch that has been changed in its physical and/or chemical properties. Modifications may involve altering the form of the granule or changing the shape and composition of the constituent amylose and amylopectin molecules. Modifications are therefore carried out on the native starch to confer it with properties needed for specific uses. Some of these modified starches, their methods of modification and desirable properties are shown in Table 6. When a starch is modified chemically or physically, the properties of the native starch is altered. Various modifications give the starch properties that make it useful in many industries such as food, pharmaceutical, textile, petroleum, and paper pulp industries.
The different ways of modifying native starch consist in altering one or more of the following properties: paste temperature, solids/viscosity ratio, starch paste resistance to reduction of viscosity by acids, heat and or mechanical agitation (shear), retrogradation tendencies, ionic and hydrophilic nature. Modifying starch is important to provide the following properties: thickening, gelatinization, adhesiveness and/or film-formation, to improve water retention, enhance palatability and sheen and to remove or add opacity.
The reasons why native starch is modified are:
- To modify cooking characteristics (gelatinization).
- To reduce retrogradation.
- To reduce paste’s tendency to gelatinize.
- To increase paste’s stability when cooled or frozen.
- To increase transparency of pastes and gels.
- To improve texture of pastes and gels.
- To improve adhesiveness between different surfaces, such as in paper applications.
Modification can be as simple as sterilizing products required for the pharmaceutical industry or highly complex chemical processes to confer properties totally different from the native starch. A simple modification process is represented by washing, air classification, centrifugation, and pre-gelatinization.