11.1.11 Rheological Properties
Rheological properties of a material reflect its structure. During gelatinization, starch granules swell to several times their initial volume. Swelling is accompanied by leaching of granule constituents, predominantly amylose, and the formation of a three-dimensional network (Steeneken, 1989; Tester and Morrison, 1990). The storage dynamic modulus (G') is a measure of the energy stored in the material and recovered from it per cycle, while the loss modulus (G'') is a measure of the energy dissipated or lost per cycle of sinusoidal deformation (Ferry, 1980).
The ratio of the energy lost to the energy stored for each cycle can be defined by tan 5, which is another parameter indicating the physical behavior of a system. The G' of starch progressively increases at a certain temperature (TG') to a maximum (peak G') and then drops with continued heating in a dynamic rheometer. The initial increase in G' could be attributed to the degree of granular swelling to fill the entire available volume of the system (Eliasson, 1986) and intergranular contact might form a three-dimensional network of the swollen granules (Wong and Lelievre, 1981). Compared with cereal starches, information on the rheological behavior of tuber and root starches under well-defined flow regimes is limited (Hoover, 2001). The rheological properties of sweet potato starch is shown in Table 11.1.10.
Table 11.1.10 Rheological properties of sweet potato starch
Rheological properties | Value
T G′ (°C) | 70
Peak G′ | 819
Peak G′′ | 70
Breakdown in G′ | 274
Peak tan δ | 0.08
G′: storage dynamic modulus, G′′: loss modulus
The rheological properties of sweet potato cultivars in Peru were also examined by Garcia and Walter (1998). Starch exhibits unique viscosity behavior with change of temperature, concentration and shear rate (Nurul et al., 1999). This can be measured in terms of rheological/pasting curves (plots of viscosity versus temperature) obtained with the Brabender Viscoamylograph or the Rapid Viscosity Analyzer (RVA) and rheometer. Information obtained from rheology/pasting curves is vital when considering a starch as a possible component of a food product (Adebowale and Lawal, 2003). Chemical modification leads to a considerable change in the rheological and pasting properties of starches. Starch paste viscosity can be increased or reduced by applying a suitable chemical modification (Agboola et al, 1991).
11.1.12 Morphological Properties
The morphological characteristics of starches from various botanical sources vary with the genotype. The variation in the morphology such as size and shape of starch granules is attributed to the biological origin (Singh et al., 2007). The granule size is variable and ranges from 1-110 pm (Hoover, 2001). The shapes of the various SPS granules varied from polygonal, round to cupoliform/bell shapes (Chen et al., 2003; Zhu et al., 2011). Hoover (2001) reported SPS granules as round, oval and polygonal, with sizes ranging from 2 to 42 pm. Thao and Noomhorm, (2011) reported that the mean length of SPS granules ranges from 14–17 pm. The ratio of large particles to small ones for all sweet potato starches was nearly equal. Granule size and particle size distribution influences water binding capacity, swelling power and paste clarity, as well as applicable ability of starches in food processing (Chen, 2003).
Two types of angular granules were present in SPS granules with axial diameters of 5-12 and 15–25 pm. Physico-chemical properties, such as percent light transmittance, amylose content, swelling power and water-binding capacity were significantly correlated with the average granule size of the starches separated from different plant sources (Singh and Singh, 2001; Zhou et al., 1998). The differences in the granule size of the starches are presumably attributed to cultivar differences, growing conditions and plant physiology. Moreover, starch granule size plays a significant role in influencing the pasting parameters of starches (Noda et al., 2004; Zaidul et al., 2007). Fine starch granules could be used as fat substitutes in high fat foods (Ma et al., 2006). Based on optical micrographic observations of gelatinization, a pomegranate structure for sweet potato granular structure was described. The outer layer of granule is equivalent to the skin of pomegranate, blocklets are the same as the garnet of pomegranate, and the amylopectin cluster with one reducing end at hilum is equivalent to the primary body of pomegranate (Lian et al., 2012).
11.1.13 Modified Starches
Starches have many useful properties as a food ingredient, but the native starches have limitations that reduce their use at the industrial level. The diversity of the modern food industry and the enormous variety of food products require that starch would be able to tolerate a wide range of processing techniques as well as various distribution, storage and final preparations (Sodhi and Singh, 2005). Depending on the end use, any of these aforementioned properties can be altered by suitably modifying the starch to provide starch products with properties to meet the needs for specific uses. Starch modification involves the alteration of the physico-chemical characteristics of the native starch to improve its functional characteristics, which can be used to tailor starch to specific food applications (Hermansson and Svegmark, 1996; Light, 1989).
Starch modification can be accomplished through physical alteration, chemical degradation, enzymatic modifications or genetic transformation (Yiu et al., 2008). The main source of starch is corn, but the availability of corn to the Indian starch industry is decreasing day by day, because of increased demand by industries involved in the production of breakfast cereals and snacks. For this reason it has becoming necessary to utilize non-conventional starches such as tuber starch to fill this gap. Sweet potato is an important crop in many developing countries, which is seasonal, cheaper, available in abundance, and can be used in the production of modified starch (Singh et al, 2005). A few studies have been reported on native and modified sweet potato starch (Oladebeye et al., 2009; Takeda et al, 1986).
Microscopy (light and SEM) has played an important role in increasing the understanding of granular structure of modified starches. It has been used to detect structural changes caused by the modifications and the most substituted regions in starch granules (Kaur et al., 2004).
11.1.13.1 Hydrothermal Treatment
Hydrothermal treatment of starch has proved to increase gelatinization temperature (Lim et al., 2001), restrict swelling and increase starch paste stability (Hoover and Vasanthan, 1994; Jacobs, et al., 1998). Annealing (ANN) and heat-moisture treatment (HMT) are two hydrothermal methods that have been used to modify starch digestibility. ANN of starch is a physical treatment of starch granules in the presence of heat and water. During ANN, starch granules in excess (>60 % w/w) or at intermediate water content (40 % w/w) are held at a temperature above the glass transition temperature (Tg), but below the onset (To) temperature of gelatinization for a set period of time (Hoover and Vasanthan, 1994; Tester et al., 2000). The following changes have been shown to occur in all starches on ANN: