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2.2.1.4.2 Weakly hydrated
In weakly hydrated (<70% water wet basis, wb), the transformation mechanisms are more complex. It has been shown by a second DTA endotherm appears at intermediate water content(Donovan, 1979). Figure 2.15 shows the variation of classical transitions of the potato starch as a function of water content. The appearance of both endothermic G and M1 has been observed for many starch (potato, wheat, oats, corn normal, waxy maize, cassava, peas smooth and wrinkled peas ...) but their origin has been interpreted differently as two transitions gelatinization in excess water and melting in low hydrated environment (Donovan, 1979; Galliard, 1987) or a glass transition associated with a merger process off balance (Biliaderis, Page, Maurice & Juliano, 1986). The presence of lipids in cereal starches may lead to a third transition (95-100 °C) attributed to the melting of amylose-lipid complex formed during heating(Colonna & Buleon, 2009; Sablani, 2009). Figure 2. 15: Variation of classical transitions of the potato starch as a function of water content (Donovan, 1979) 2.2.1.4.3 Retrogradation The Retrogradation is the term for the structural reorganization of the starch during the lowering of temperature. During cooling, when the concentration is sufficiently high (> 1%), amylose reorganizes and forms an opaque gel not thermally reversible. Amylose gels quickly (Miles, Morris & Ring, 1985) while the gelation of amylopectin is slower and occurs after several weeks (Gudmundsson & Eliasson, 1990; Ring et al., 1987). The gelation of amylopectin is limited due to its structure plugged. The formation of starch gels occurs in two steps characterized by a transition type of ball/helix at segments of polymer chains followed by crystallization by stacking chains. The starch gels are formed from a matrix of amylose trapping ghosts of grains rich in amylopectin (Miles, Morris & Ring, 1985). However, there may be a phase reversal when the ratio amylose/amylopectin is less than 30/70 (Leloup, Colonna & Buleon, 1991). The gels of amylose and amylopectin crystallinity generally have type B. The retrogradation of dilute solutions of amylose and amylopectin was observed by transmission electron microscopy (TEM) and cryo-TEM (Putaux, Buleon & Chanzy, 2000). For amylose, the authors observe at first an aggregation of several molecules that form vermicular, in a second time, combine to form a branched extended network. The arms of this network can be described as an alternation of type B crystals and amorphous regions. Over time, this network is condensed by syneresis and form aggregates of semi-crystalline. For amylopectin, the aggregation mechanisms are similar to those of amylose, but the units are semi-crystalline organization of the side chains of amylopectin. For breeding rice with improved quality, the thermal and retrogradation properties of starch may be routinely measured. Since direct measurement is time-consuming and expensive, rapid predictive methods based on near-infrared spectroscopy (NIRS) can be applied to measure these quality parameters(Bao, Shen & Jin, 2007).
2.2.1.5 Starch modification
Starch is a basic material which is widely used in industry both in its native form and after having undergone physical and/or chemicals. These improve the functional properties of starch allowing it to expand its fields of application in many industrial sectors (BeMiller, 1967; Bergthaller & Hollmann, 2007; Hermansson, Eriksson & Jordansson, 1991; Singh, Kaur & McCarthy, 2007; Wu, Chen, Li & Wang, 2010). Some examples of modifications are presented in Table 2.1. The reactions of acid hydrolysis and etherification (hydroxypropylation) are described in more detail in the next, as they relate to chemical changes made to the study of starches.
Table 2. 1: Properties and applications of modified starches(Singh, Kaur & McCarthy, 2007).
Type of modifications
Properties
Applications
Pregelatinized
Soluble in water at room temperature
Instant food preparations
Acid or enzymatic hydrolysis
Molecular weight reduction: reduction of viscosity, and increased retrogradation
confectionery, gum
Oxidation
Viscosity reduction, transparency, and low temperature stabilization
Textural agents in desserts, Confectionery
Dextrinisation
Lowering viscosity, reduced the amount of sugar
Forming agent, confectionery
Etherification
Improves the transparency of starch, reducing syneresis and stability
Sauces, soups, fruit in sugar, puddings
Esterification
Lower gelatinization temperature, low tendency to form gels, transparent paste
Refrigerated foods, encapsulation, emulsion stabilizer
Crosslinking
Stability of grain swelling at high temperatures, shear and acidic conditions
Sauces, soups (UHT, canned, frozen), cream desserts,
yogurt, fruit in sugar
2.2.1.5.1 Hydrolysis
Acid hydrolysis is a chemical modification of starch that improves its solubility at low temperatures. It leads to a reduction in molecular weight and reduces the viscosity of hot starch for use in higher concentrations. This treatment weakens the grain structure and makes the grains of starch soluble in cold water for the most advanced treatments.
The reaction mechanism of acid hydrolysis of a polysaccharide is the mechanism of “ion ring”, in which occurs the division of a glycosidic bond following protonation of the oxygen of the glycosidic bond. Generally, acid hydrolysis is carried out at high concentrations of starch (30-36% starch) at temperatures of 40-60 °C with 8% hydrochloric acid or 15% sulfuric acid over a period ranging 30 min to several hours (Stephen, 1995). When the viscosity (or fluid) or desired degree of conversion is reached, the solution is neutralized and the starch grains are washed.
Acid hydrolysis does not occur randomly. It would be preferentially at the terminal links of chains, especially those adjacent to units of nonreducing ends (BeMiller, 1967). The kinetics of hydrolysis can be decomposed into two distinct phases that characterize the action of acid on two different fractions. The first phase corresponds to the rapid hydrolysis of the fraction of the more amorphous starch. The second, much slower, correspond to hydrolysis of the crystalline part and an amorphous denser located near the “crystallite” starch (Robin, Mercier, Charbonniere & Guilbot, 1974). The degradation of starch granules depends on many factors. It depends firstly on the botanical origin of starch, that is to say the type crystalline morphology of starch grains (shape, size, surface) and the ratio amylose/amylopectin . It also depends on the specific parameters to acid hydrolysis, namely the type of acid, acid concentration, starch concentration, temperature, time of hydrolysis and agitation (Robin, Mercier, Charbonniere & Guilbot, 1974).
2.2.1.5.2 Etherification by hydroxypropylation
This chemical modification is primarily intended to improve the texture of the starch, transparency and stability (Choi & Kerr, 2003; Frazier, Donald & Richmond, 1998; Kim, Hermansson & Eriksson, 1992). Hydroxypropylation characterized by the grafting of functional groups on the hydroxyl functions of macromolecules of starch. Hydroxypropylated starches are generally prepared by an etherification reaction in the presence of propylene oxide in alkaline medium at temperatures between 30 and 50 ° C to preserve the granular structure (Figure 2.16).
Figure 2. 16: Hydroxypropylation reaction
Hydroxypropylated starches are defined by the degree of substitution which represents the average number of hydroxypropyl groups per glucose unit. The hydroxypropyl groups introduced on the starch chains are capable of breaking hydrogen bonds inter-and intramolecular, weakening the granular structure of starch (Wootton & Manatsathit, 1983). The hydroxypropyl groups are introduced mainly on the starch chains located in the amorphous region is mainly composed of amylose (Galliard, 1987). It was confirmed on corn and potato hydroxypropylated starch that amylose is more substituted than amylopectin, the modified amylopectin being located near branch points (Kavitha & BeMiller, 1998; Shi & BeMiller, 2000). Some authors have shown potato starch that hydroxypropyl groups are distributed evenly on amylopectin (Richardson, Cohen & Gorton, 2001). However, these authors have shown that the reaction conditions and the botanical origin determine the distribution of groups on the starch macromolecules. The nature of the hydrophilic hydroxypropyl groups can reduce the phenomenon of syneresis during a cycle of freezing / thawing. Choi and Kerr (2003) have shown that pulsed NMR absorption capacity of water in hydroxypropylated wheat starches increases with increasing degree of substitution within a specific range of water activity. Increasing the degree of substitution decreases the gelatinization temperature, increases the swelling of the grains and improves the solubility (Kaur, Singh & Singh, 2004). The decrease in domestic bonds weakens the grain structure and increases the degree of mobility of starch chains in amorphous areas.
The microscopic analysis performed on hydroxypropylated potato starch showed morphological modifications localized on the starch grains. Most of the structural changes occurring in the central region, relatively less organized starch grains, while the peripheral regions seem to be less affected (Huber & BeMiller, 2001; Kim, Hermansson & Eriksson, 1992). However it has been shown for pea starch and waxy corn as hydroxypropylation is distributed evenly over the grain (Biliaderis, 1982). The morphological modification of grains are more obvious for large degrees of substitution (Kim, Hermansson & Eriksson, 1992).
Some authors have observed that hydroxypropylation causes an increase in viscosity due to significant swelling of starch grains during heating (Kim, Hermansson & Eriksson, 1992; Liu, Ramsden & Corke, 1999; Shi & BeMiller, 2000). Hydroxypropylation can also retard the retrogradation phenomena (Butler, Christianson, Scheerens & Berry, 1986; Hoover, Hannouz & Sosulski, 1988; Perera & Hoover, 1999). This has been attributed to steric effects imposed by hydroxypropyl groups that disrupt the formation of ordered structures during aggregation of chains and crystallization. In theory, Hydroxypropyl starch does not gel and keep a viscous character. However, Kaur et al (2004) have shown the emergence of many micro-fibrils in the form of sticks on hydroxypropylated potato starch gels stored 30 days at 4 ° C. These authors suggest that during this long storage period, a phase separation may be responsible for the formation of these structures.
2.2.1.5.3 Dual modification
Dual modification, a combination of substitution and

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