**3.2.2 Acid-modified starch**

46 Thermoplastic Elastomers

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

> Decrease of retrogradation tendency, high cold thickening capacity, high gel strength

> No effect in the resistant starch

No effect in the resistant starch

Potential for use in instant meal

Hydrolylisis or dextrinization Govindasamy

Limited degradation Govindasamy

Ready-to-eat , popped millet Ushakumari et

(1997)

(1996)

Alves et al. (1999)

al. (2004)

Bello-Pérez et al. (2006)

Bello-Pérez et al. (2006)

Menegassi et al.

( 2011)

Starch or flour Process Application Reference

formation

formation

products

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

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

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

lower moisture content, and the elimination of drying the starch dispersion.

Single screw extruder

twin-screw extruder

Single screw extruder

Single-screw equipment

Single-screw equipment

Single-screw equipment

occurs mainly as a phospholipid contaminant (Lim et al., 1994).

Table 2. News source extruded starches

β of amylopectin (Wurzburg, 1986)

and canned goods.

Flaked, extrusion cooked and roller-dried products

Sago +Alfaamylase

Yam (*Dioscorea* 

Foxtail millet grains

Unripe banana (*Manguifera indica L.)*

Mango (*Musa paradisiaca L.*)

Amaranth flour *Amaranthus cruentus* L

**3.2.1 Starch phosphates** 

*alata*)

Sago Co-rotating

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 that acid preferably hydrolyses amylopectin.

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 influence starch properties (Wurzburg, 1986).

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

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 aqueous medium and require heating to force the reaction.

Physical and/or Chemical Modifications of Starch by Thermoplastic Extrusion 49

Flores et al. (2010) used a mixture of experimental design to study the physical and microbiological properties of tapioca starch-based glycerol edible films with the addition of xanthan gum (XG) and potassium sorbate (PS) and obtained through extrusion technology. The results showed that PS presence decreased the ultimate tensile strength and elastic modulus and increased strain at break. XG produced a reinforcing effect on the films and

Guan & Hanna (2006) have extruded biodegradable composite foams based on starch acetate and poly (tetraethylene adipate-co-terephthalate) (EBC). It was reported that low EBC contents in the blends favored the miscibility of the two polymers, as characterized by an increase of the glass transition temperature of starch acetate, a decrease in the melting point temperature of starch and EBC in a differential scanning calorimetry (SEM) analysis and the formation of a homogeneous morphology observed with SEM. Large amounts of

Multifunctional epoxy-based copolymers can be used as chain-extender (CE) to increase the molecular weight and create branching in polylactides (PLA). Li & Huneault (2011) studied the effect of a multifunctional epoxy-acrylic-styrene copolymer on the properties of PLA/Thermoplastic Starch (PLA/TPS) blends that were prepared by twin-screw extrusion. The plasticizers were mixed together in the first half of the extruder to complete starch gelatinization. Water was removed by devolatilization at midex-truder and the PLA matrix was mixed with the water-free TPS in the latter portion of the compounding process. The standard blends comprised 27% TPS in the PLA matrix. The TPS phase itself comprised 36% plasticizer in the form of glycerol or sorbitol. The blends were injection molded into standard test bars and their tensile properties were measured. It was found that the combination of interfacial modification and chain-extension strategies led to greatly improved ductility. The viscosity of the PLA/TPS blends was also dramatically increased by adding a small amount of epoxy-based chain extender. This is of great interest for polymer processing techniques (such as foaming or film blowing) that require high melt strength.

Nabar et al. (2006) produced the cylindrical starch foam shapes on a small scale (∼11-12 kg/hr) Werner Pfleiderer ZSK-30 twin-screw extrusion (TSE) process using water, which functions as a plasticizer as well as a blowing agent. The properties of the starch foams depend on the type of starch used (hydroxypropylated high amylose corn starch, 70% amylose), the amount of water and additives (poly(hydroxyamino ether)) (PHAE) used, and

Process Authors

Twin-screw extrusion Li & Huneault

(2011)

(2011)

Rahman (2011)

extrusion conditions such as temperature and the screw configuration.

Starch+ Polymers or another composite

Starch + polylactides (PLA)/ Thermoplastic starch (TPS)

Table 3 summarizes some other works with biopolymers using starch as base.

Potato starch + polypropylene Corotating twin-screw extruder Roy et al.

Tapioca starch+ glycerol Twin screw extrusion at 150°C Yunos &

also enhanced solubility in water and decreased moisture content.

EBC decreased the miscibility of these two polymers.
