**3.2 Chemical modification of starches**

Starch may be modified through physical or chemical methods and its use relates to improved quality and decreased cost of the products.

According to Light (1990), modified starch is used in foods, for 3 main reasons:


Some researchers used simultaneously a chemical reagent and the extrusion process to obtain modified starch for various purposes, such as production of expanded extruded products (Lai et al*.,* 1989); starch phosphate production (Chang & Lii, 1992); alcohol production (Chang, 1989), extruded rice flour (Clerici & El-Dash, 2006) and acidic extruded rice flour (Clerici et al., 2009) for production gluten-free bread, and lactic beverage

Physical and/or Chemical Modifications of Starch by Thermoplastic Extrusion 47

Chang & Lii (1992) compared the conventional process to that of extrusion for phosphatation of cassava and corn starches and they verified that, in order to prepare starch phosphate with a similar degree of substitution, the extrusion process requires less reagent than the conventional method, and the latter requires an excessive amount of reagents and

Salay & Ciacco (1990) also found that it is possible to obtain starch phosphate with a low degree of substitution (DS) value through the extrusion process and observed that the extrusion temperature of 200oC, concentration ≥ 1.4 g/100 mL of sodium tripolyphasphate

Nabeshima & Grossmann (2001) obtained cassava cross-linked starch with different DS and degree of gelatinization for use in food by using cassava starch with sodium trimetaphosphate (STMP) processed on a Cerealtec single-screw extruder at different

Seker et al. (2003, 2004, 2005a, 2005b) mixed starch with sodium hydroxide and sodium trimetaphosphate, the mixture was then extruded in both single- and twin-screw extruders and it was verified that the mixing elements did not change the amount of phosphorus incorporated into the starch in both processes. They developed works showing the phosphatation process through starch extrusion and its changes in rheological properties.

Acid-modified starch suffers hydrolysis of some glucosidic bonds, which occurs first in the amorphous regions of the starch containing branch points and -D bonds (16), reducing the molecular size and diminishing the viscosity of the paste. Depending on the treatment intensity, there is formation of dextrins (Wurzburg,1986). Kerr, quoted by Wurzburg (1986) showed that during acid modification, the amount of starch amylose increases, indicating

Acid-modified starches are normally made out of a starch paste (about 36% to 40% of solids) heated at a temperature below the starch gelatinization temperature (about 40º-60ºC) and the addition of mineral acid, agitation for a varied period (about one to several hours). When viscosity or degree of conversion desired is reached, the acid is neutralized and the starch is retrieved through filtration or centrifugation, washing and drying. The type of mineral acid, its concentration, temperature, starch concentration and reaction time

Acid-modified starches differ from granular starch in lower viscosity of the paste (under cold and hot conditions) and other properties. However, they have the same physical form,

The literature indicates, most of the times, starch modification through mineral acid. However, Mehltretter (1967) used organic acids to modify starch and found that some carboxylic acids such as formylic acid react with starch at room temperature and in the presence of water; whereas other acids, such as acetic acid and citric acid do not react in an

insolubility in cold water and similar birefringence (Shildneck & Smith, 1967).

causes water pollution, increasing the cost of production.

and pH 8.5 were the conditions that resulted in a higher value of DS.

extrusion temperatures and concentrations of STMP and NaOH.

**3.2.2 Acid-modified starch** 

that acid preferably hydrolyses amylopectin.

influence starch properties (Wurzburg, 1986).

aqueous medium and require heating to force the reaction.


production (Lee et al., 1992) . The simultaneous use of these modifications poses advantages such as saving reagent, and absence of effluent formation, low reaction time, processing at lower moisture content, and the elimination of drying the starch dispersion.

Table 2. News source extruded starches

#### **3.2.1 Starch phosphates**

Native starches usually contain small amounts of phosphorus (0.1%). In tubers and roots, phosphorus is covalently bound to starch (Hodge et al., 1948), while in cereal starches, it occurs mainly as a phospholipid contaminant (Lim et al., 1994).

Starch phosphates are esters derived from phosphoric acid. When only a hydroxyl is involved in the starch phosphate binding, the product is a monoester. The other starch phosphate class is the *cross-linked* type which contains mono-, di- and triester starch phosphate (Hamilton & Paschall, 1967). Approximately 60%-70% of total phosphorus of starch monophosphate is located at C-6 while the rest is located at C-3 of anhydroglucose units (Tobata & Hizukuri, *apud* Wurzburg, 1986). Most phosphate groups (88%) are on chain β of amylopectin (Wurzburg, 1986)

Cross-linked starch is obtained by introducing an agent capable of reacting with the hydroxyl groups of two different molecules within the granule. These synthetic bridges reinforce the natural hydrogen bonds, delaying the speed of granule swelling and reducing the rupture of the swollen granule (Wurzburg, 1986). Its main use is as filling in fruit pies and canned goods.

Chang & Lii (1992) compared the conventional process to that of extrusion for phosphatation of cassava and corn starches and they verified that, in order to prepare starch phosphate with a similar degree of substitution, the extrusion process requires less reagent than the conventional method, and the latter requires an excessive amount of reagents and causes water pollution, increasing the cost of production.

Salay & Ciacco (1990) also found that it is possible to obtain starch phosphate with a low degree of substitution (DS) value through the extrusion process and observed that the extrusion temperature of 200oC, concentration ≥ 1.4 g/100 mL of sodium tripolyphasphate and pH 8.5 were the conditions that resulted in a higher value of DS.

Nabeshima & Grossmann (2001) obtained cassava cross-linked starch with different DS and degree of gelatinization for use in food by using cassava starch with sodium trimetaphosphate (STMP) processed on a Cerealtec single-screw extruder at different extrusion temperatures and concentrations of STMP and NaOH.

Seker et al. (2003, 2004, 2005a, 2005b) mixed starch with sodium hydroxide and sodium trimetaphosphate, the mixture was then extruded in both single- and twin-screw extruders and it was verified that the mixing elements did not change the amount of phosphorus incorporated into the starch in both processes. They developed works showing the phosphatation process through starch extrusion and its changes in rheological properties.
