• increase in granule stability
• crystalline perfection
• starch chain interactions within the amorphous and crystalline domains of the granule
• formation of double helices
• increase in gelatinization temperatures
• narrowing of the gelatinization temperature range
• decrease in granular swelling
• decrease in amylose leaching.
HMT of starches is defined as a physical modification in which starch granules are subjected to lower moisture levels than required for gelatinization for a certain period of time, at a temperature above the glass transition temperature but below the gelatinization temperature (Jacobs and Delcour, 1998). HMT modifies the physico-chemical properties of starches without destroying granule structure (Gunaratne and Hoover, 2002). HMT decreases the swelling power and solubility of starch compared to native starch, due to decrease in granular stability, resulting from uncoiling of double helices that may have been present in a crystalline array in the native granule. Starch gel structure was altered and gel hardness increased following HMT (Chung et al., 2000). Tuber starches have been shown to be more susceptible than legume or cereal starches towards heat-moisture treatment (Hoover and Vasanthan, 1994; Jacobs and Delcour, 1998). Sweet potato starch was also subjected to heat moisture treatment and it was discovered that the starch paste became short and shear-stable and the starch gel exhibited marked increases in hardness and adhesiveness (Collado and Corke, 1999).
11.1.13.2 Chemical Modification
Cross-linking treatment is intended to add chemical bonds at random locations in a granule. The cross-linking stabilizes the granules and strengthens the relatively tender starch (Acquarone and Rao, 2003). Pastes from cross-linked starches are more viscous and less likely to break down with extended cooking times, increased acid content or severe agitation (Wurzburg, 1986). Cross-linked starches offer acid-, heat-and shear-stability better than their parental native starches (Mason, 2009). Commonly used cross-linking agents for food application include phosphorus oxy-chloride (POCl3), sodium trimetaphosphate (STMP) and mixtures of adipic acid anhydride and acetic acid anhydride (Thomas and Atwell, 1999).
The rheological properties appeared much more affected by cross-linking than the thermal properties (Gunaratne and Corke, 2007; Kim and Yoo, 2010). For example, cross-linking hardly altered the melting enthalpy of sweet potato starch by DSC, but decreased the peak viscosity of pasting by more than four times (Gunaratne and Corke, 2007).
Hydroxypropylated starches are generally prepared by the etherification of native starch with propylene oxide in presence of an alkaline catalyst. The hydroxypropyl groups are hydrophilic in nature, and when introduced into starch chains, weaken or disrupt the internal bond structure that holds the granules together, and thus influence physico-chemical properties depending on the source of starch, reaction conditions, the type of substituent groups employed, and the extent of substitution (the molar substitution, MS) (Rutenberg and Solarek, 1984; Schmitz et al., 2006; Xie et al., 2005). Pastes of hydroxypropylated starch show improved clarity, higher viscosity, reduced syneresis and better freeze-thaw stability (Gunaratne and Corke, 2007; Mason, 2009).
Acetylated starch is obtained by esterification of native starch with acetic anhydride (Jarowenko, 1986). Acetylation depends upon factors such as reactant concentration, reaction time, pH and the presence of a catalyst, which finally determines the number of acetyl groups incorporated into the molecule (Betancur-Ancona et al, 1997). The properties of starch acetates are a function of the acetyl content, the type of starch, the size and shape of its molecular components, and the method of pre-treatment. Measurement of the acetyl content is a prime method for characterizing starch acetates. When a starch is acetylated, it can be obtained by a variable degree of substitution. In food applications, Food and Drug Administration (FDA) allows only a low degree of substitution (0.01-0.2).
Oxidized starches have attracted much attention and are widely used in food and industrial applications to provide surface sizing and coating properties (Lawal et al, 2005). The food products where oxidized starch is used are neutral tasting and low-viscosity, such as a lemon curd, salad cream and mayonnaise (Lawal, 2004). Oxidized starch is commonly prepared by reacting starch with a specified amount of oxidant under controlled temperature and pH. Periodate, chromic acid, permanganate, nitrogen dioxide and sodium hypochlorite have been commonly used to oxidize starches. The commercial production of oxidized starch generally employs sodium hypochlorite as the oxidizing agent. The factors affecting hypochlorite oxidation include pH, temperature, hypochlorite concentration, starch molecular structure and starch origin (Kantouch and Dokki, 1998; Wang and Wang, 2003).
11.1.13.3 Enzymatic Modification
Starch may be hydrolysed by amylolytic enzymes such as α-amylase. For example, when potato starch is the substrate, its hydrolysis yields mixtures of different saccharides, such as maltodextrins, and their precise compositions are of considerable commercial interest. The hydrolyzed products are widely used in the food, paper and textile industries (Marchal et al., 1999; Pandey et al., 2000). Amylases have many applications in the food, textile, paper and pharmaceutical industries (Gupta et al., 2003).
Generally, fungal glucoamylase, as well as bacterial or fungal α-amylase, may be used together to convert starch to simpler sugars. Zhang and Oates (1999) reported that the enzymatic susceptibility of starch is influenced by several factors, including the ratio of amylose to amylopectin, the crystalline structure and the size of the particles. Among these factors, the crystalline granular structure is perhaps the most important. A study reported the modification of sweet potato starch using 4-α-glucanotransferase, resulting in a more rigid and thermo reversible gel (Lee et al., 2008).
11.1.14 Utilization
Starches have wide applications in the food and related industries. Starch plays an important role in food systems by stabilizing it and creating the structure of the food. It also interacts with other components to deliver or maintain nutrient and flavor (Cui, 2005). Sweet potato starch (SPS) can be used in many products, including noodles, cakes, bread, biscuits, desserts, alcoholic and non-alcoholic drinks, puddings and confectionery products. Applications of starch in food systems are primarily governed by gelation, gelatinization, pasting, solubility, swelling, color and digestibility (Adebowale and Lawal, 2002). This starch is also used to manufacture starch syrup, glucose and isomerized glucose syrup, lactic acid beverages, bread and distilled spirits, called shochu in Japan. Noodles and isomerized saccharides as a sweetener for soft drinks are also made from sweet potato starch in China, Japan and Vietnam (Prain et al., 1997). The importance of sweet potato starch in some food applications is further described below.
11.1.14.1 Snack Foods
In snack foods, starch is frequently used to assist in achieving desired textural and sensory attributes by improving crispness, oil binding properties, expansions and overall eating quality (Cui, 2005). Properties of amylose and amylopectin as the main components of starch are important for the texture.