This modification also involves heating of starch granules above the glass transition temperature for varied lengths of time, but sets itself apart from annealing processes in that it is generally conducted at reduced levels of moisture (<35%) at relatively higher processing temperatures (80–140°C). An in-depth review of this topic is provided by Jacobs and Delcour (1998). Similar to annealing, traditional HMT starches retain their granular form and characteristic polarization cross (Stute, 1992; Abraham, 1993; Hoover and Vasanthan, 1994; Vasanthan, Sosulski, and Hoover, 1995; Gunaratne and Hoover, 2002; Adebowale and Lawal, 2003), although some subtle increases in granule size, surface cracking, and hollowing at granule centers have been reported (Kawabata et al., 1994; Vasanthan, Sosulski, and Hoover, 1995; Vermeylen, Goderis, and Delcour, 2006). Changes within both the crystalline and amorphous regions are thought to occur during HMT (Kawabata et al., 1994; Hoover and Manuel, 1996a, 1996b; Jacobs and Delcour, 1998; Miyoshi, 2002).
In regard to granule crystalline regions, HMT starches consistently exhibit both an upward shift in gelatinization temperature (To, Tp, and often Tc) and a broadened gelatinization temperature range (Jacobs et al., 1998; Adebowale and Lawal, 2003; Shin et al., 2005; Vermeylen, Goderis, and Delcour, 2006), whereas gelatinization enthalpy is reported to either decrease (Vasanthan, Sosulski, and Hoover, 1995; Perera, Hoover, and Martin, 1997; Shin et al., 2005; Vermeylen, Goderis, and Delcour, 2006) or to remain unchanged (Hoover, Swamidas, and Vasanthan, 1993; Hoover and Manuel, 1996a,b). For potato starch, Vermeylen, Goderis, and Delcour (2006) observed a decrease in both gelatinization enthalpy and total granule crystallinity for HMT temperatures up to 120°C, above which temperature (130°C) total crystallinity increased.
It is well established that native starches exhibiting a B- or C-type (mixed) packing arrangement can undergo a gradual transition to the A polymorphic form upon HMT (Stute, 1992; Perera, Hoover, and Martin, 1997; Gunaratne and Hoover, 2002). This transition is favored by a high temperature during HMT and a gradual cooling thereafter (Vermeylen, Goderis, and Delcour, 2006). For potato starch, the gradual disappearance of the B polymorph with increasing treatment temperatures (90–120°C) coincided with an observed decrease in total crystallinity, the decrease of which was offset and eventually overcome by simultaneous formation of the A polymorph (at a treatment temperature of 130°C), accounting for the final increase in total crystallinity (Vermeylen, Goderis, and Delcour, 2006). This polymorphic transition was hypothesized to be enhanced by a release of linear segments of AP branch chains arising from thermal degradation (Vermeylen, Goderis, and Delcour, 2006), which has been previously reported to occur during HMT (Lu, Chen, and Lii, 1996). Thus, relative crystallinity can either increase or decrease with HMT, depending on the starch source and conditions employed (Jacobs et al., 1998). Possible changes within granule crystalline regions associated with HMT have been attributed to disruption of low-level crystallites and strengthening of higher melting crystallites (Luo et al., 2006), alteration of the starch packing arrangement (Stute, 1992; Hoover, Swamidas, and Vasanthan, 1993; Hoover and Vasanthan, 1994; Kawabata et al., 1994; Jacobs et al., 1998), growth or perfection of existing crystallites (Hoover and Vasanthan, 1994; Jacobs et al., 1998), disruption of stacked lamellae (Hoover and Vasanthan, 1994; Vermeylen, Goderis, and Delcour, 2006), and recrystallization of starch chains (Vermeylen, Goderis, and Delcour, 2006).
Aside from the noted impact on crystalline regions, the greatest changes associated with HMT have been proposed to occur within granule amorphous regions (Hoover and Manuel, 1996a; Miyoshi, 2002). Reported alterations within amorphous regions include development of new order and/or crystallites (AM–AM, AM–AP interactions) (Hoover and Vasanthan, 1994; Hoover and Manuel, 1996a), formation or enhancement of AM–lipid complexes (Hoover and Vasanthan, 1994; Kawabata et al., 1994; Hoover and Manuel, 1996a; Miyoshi, 2002), and conversion of AM from an unordered to a partial helical form (Lorenz and Kulp, 1982; Hoover and Vasanthan, 1994). Alterations within granule amorphous regions affect both the gelatinization (melting) and physical properties of starch.
Due to the diverse combinations of experimental conditions employed for HMT (moisture content, temperature, length of treatment, heat source, starch source, etc.), the characteristics and properties of HMT starches vary and are not easily defined in a consistent and unified fashion. In almost all cases, HMT results in decreased levels of starch solubility, swelling power, and AM leaching (Abraham, 1993; Hoover, Swamidas, and Vasanthan, 1993; Hoover and Vasanthan, 1994; Kawabata et al., 1994; Hoover and Manuel, 1996b; Kurakake et al., 1997; Lewandowicz, Jankowski, and Fornal, 1997; Perera, Hoover, and Martin, 1997; Gunaratne and Hoover, 2002; Lawal and Adebowale, 2005; Luo et al., 2006). In addition, HMT starches generally display a decreased peak viscosity and an increased pasting temperature (Hoover, Swamidas, and Vasanthan, 1993; Hoover and Vansanthan, 1994; Hoover and Manuel, 1996a; Eerlingen et al., 1997; Kurakake et al., 1997; Jacobs and Delcour; 1998; Adebowale and Lawal, 2003; Singh et al., 2005; Stevenson, Biswas, and Inglett, 2005; Luo et al., 2006), although effects for other pasting attributes are not always consistent among literature reports. Concentrated HMT starch gels exhibit increased gel hardness relative to those of native starches (Eerlingen et al., 1997; Gunaratne and Corke, 2007a), whereas the opposite effect is observed for dilute HMT starch gels. Reports on the susceptibility of HMT starches to retrogradation (Takaya, Sano, and Nishinari, 2000; Miyoshi, 2002; Gunaratne and Hoover, 2002; Lawal and Adebowale, 2005), acid/enzyme-catalyzed hydrolysis (Jacobs and Delcour, 1998), and chemical modification (Vasanthan, Sosulski, and Hoover, 1995; Perera, Hoover, and Martin, 1997; Liu, Corke, and Ramsden, 2000; Gunaratne and Corke, 2007a) are mixed, and vary according to botanical source and treatment parameters.
To facilitate more rapid processing of HMT starches, alternative methods utilizing reduced pressure (Maruta et al., 1994) and microwave heating (Stevenson, Biswas, and Inglett, 2005; Anderson and Guraya, 2006; Luo et al., 2006) have been reported. HMT is commonly employed in conjunction with treatment with an acid (Brumovsky and Thompson, 2001; Shin et al., 2004a), miscellaneous processing techniques (Kurakake et al., 1997; Pukkahuta, Shobsngob, and Varavinit, 2007), and chemical modification (Vasanthan, Sosulski, and Hoover, 1995; Perera, Hoover, and Martin, 1997; Perera and Hoover, 1998; Liu, Corke, and Ramsden, 2000; Sang and Seib, 2006; Gunaratne and Corke, 2007a) to create starch products with novel properties.