Commercial starch is a white powder. Starch is a mixture of two polymers: amylose and amylopectin. Experimental evidence indicates that amylose is not a straight chain of glucose units but instead is coiled like a spring, with six glucose monomers per turn part b of Figure 5.
When coiled in this fashion, amylose has just enough room in its core to accommodate an iodine molecule. The characteristic blue-violet color that appears when starch is treated with iodine is due to the formation of the amylose-iodine complex. This color test is sensitive enough to detect even minute amounts of starch in solution.
Figure 5. A molecule of amylopectin may contain many thousands of glucose units with branch points occurring about every 25—30 units Figure 5. The helical structure of amylopectin is disrupted by the branching of the chain, so instead of the deep blue-violet color amylose gives with iodine, amylopectin produces a less intense reddish brown. These branch points occur more often in glycogen. Dextrins are glucose polysaccharides of intermediate size. The shine and stiffness imparted to clothing by starch are due to the presence of dextrins formed when clothing is ironed.
Because of their characteristic stickiness with wetting, dextrins are used as adhesives on stamps, envelopes, and labels; as binders to hold pills and tablets together; and as pastes. Dextrins are more easily digested than starch and are therefore used extensively in the commercial preparation of infant foods.
The complete hydrolysis of starch yields, in successive stages, glucose:. This variability provides starches of different properties, which can create challenges of raw materials inconsistency during processing [ 12 ]. Chemical structure of amylopectin chain and amylose chain. Amylopectin unit chains are relatively short with a broad distribution profile, compared to amylose molecules.
They are typically, 18—25 units long on average [ 13 , 14 ]. Physical properties are those properties exhibited without any change in chemical characteristics of starch and do not involve the breaking and creation of chemical bonds such as solubility, gelatinization, retrogradation, glass transition, etc. On the other hand, chemical properties changes due to chemical reactions and usually involve the breakage and creation of new bonds.
Examples of such chemical processes in starch include hydrolysis, oxidation, esterification and etherification. Research strongly indicates that the molecular weight and branching attributes of starch which play important roles in the shape and size of granules can potentially be used for predicting some of its functionality such as texture, pasting, retrogradation, etc.
Amylose has more proportional relationships with pasting and gel textural properties, while amylopectin which are predominant in regular and waxy corn starches, has higher proportional relationship with firmness. When unprocessed or native starch granules which are relatively inert are heated in the presence of adequate water, usually during industrial processes, swelling of the granules occur and the amylose dissolves and diffuses out of the swollen granules which upon cooling forms a homogenous gel phase of amylose-amylopectin.
The swollen amylopectin-enriched granules aggregate into gel particles, generating a viscous solution. This two-phase structure, called starch paste, is desirable for many food applications where processed starches are used as thickeners or binders [ 2 , 16 ].
Retrogradation of starch is a phenomenon that occurs when the disordered arrangement of the polymer molecules of gelatinized starch begins to re-align into an ordered structure in the food product [ 15 ]. Preventing retrogradation affects the freeze-thaw stability and textural characteristics and helps to elongate the shelf life of the food product.
Starch modification through chemical means, such as, hydrolysis and esterification are generally used to produce starches that can withstand retrogradation. Preventing retrogradation of starch is important for starch used in frozen foods because it is accelerated at cold temperatures, producing an opaque, crystallized, coarse texture as a result of the separation of the liquid from the gel or syneresis [ 17 , 18 ].
Crosslinked oxidized starches have been reported be more stable against retrogradation [ 15 ]. Retrogradation of starch is affected botanical origin of the starch, amylose content, length of the amylopectin chains, density of the paste, paste storage conditions, physical or chemical modifications and the presence of other compounds.
Physical modification process such as repeated freezing and thawing of the starch paste aggravate retrogradation. The resulting starch thus produced is resistant starch that exhibit resistance to digestibility by amylase enzymes and can be used as an alternative nutrient source for diabetic patients and as a rate controlling polymer coat in controlled drug delivery systems [ 8 ].
Starch granules swollen with water are predisposed to fragmentation if exposed to physical severe pressure change. This becomes of major concern where the integrity of the granules is required to maintain viscosity.
Shear is the disintegration phenomenon of swollen starch granules or gel. The stress acting in opposite directions creates a fault-line that causes the material to open up or tear apart. Shearing generally depends on the fluid gel viscosity and flow velocity [ 20 ]. Starch granules in their raw unswollen forms are not susceptible to damage by shear even in the slurry before cooking.
But once cooked or gelatinized, starch granules becomes susceptible to shear, resulting in loss of viscosity and textural stability [ 19 ]. The chemical properties of starch are dependent on the reactivity of starch which is a function of the polyhydroxyl functional groups in the constituent glucose monomers. The hydroxyl groups at position C-2, C-3 and C-6 which are free from the glycosidic bond linkages and pyranose ring formation, are usually free for substitution reactions involving either the attached hydrogen or the entire hydroxyl group.
Most of the reactions require activation of the hydroxyl of glucose units in acidic or basic media [ 7 ]. Thus starch is able to undergo the following reactions. Hydrolysis is an addition reaction and simply involves the addition of a water molecule across a bond resulting in the cleavage of that bond and formation of the cleavage products, usually with hydroxyl group or alcohol functionality. Hydrolysis of starch can be achieved by chemical or enzymatic process.
Chemical process of hydrolysis usually employs heating starch in the presence of water or dilute hydrochloric acid Figure 3. Hydrolysis is also used to remove fatty substances associated with native starches.
Hydrolysis under acidic condition is called roasting, resulting in acid modified starch. Treatment of starch with sodium or potassium hydroxide results in alkaline modified starch. Hot aqueous alkaline solutions can be used, and this improves the reducing value of that starch [ 21 , 22 , 23 ].
The products of starch hydrolysis include dextrin or maltodextrin, maltose and glucose. Enzymic hydrolysis uses the enzyme malto-amylase to achieve hydrolysis and this is the process that usually occurs in starch digestion in the gastrointestinal tract [ 9 ]. Dextrins are white, yellow, or brown water-soluble powder which yield optically active solutions of low viscosity.
Most of them can be detected with iodine solution, giving a red coloration. White and yellow dextrins from starch roasted with little or no acid are called British gum.
The properties of dextrinized starch is dependent upon the reaction conditions moisture, temperature, pH, reaction time and the products characteristics vary in its content of reducing sugar, cold water solubility, viscosity, color and stability.
Hydrolytic processes have been used in the food industry to produce starch derivatives with better functional properties and processing applications [ 2 ]. Acid and alkali steeping are the two most widely used methods for starch isolation in the food industry, with numerous modifications. Thermo-alkali isolation method known as nixtamalization has been used in Central America since pre-Hispanic times.
The condensation of an alcohol and carboxylic acid usually under acidic condition, to produce an ester and water, is called esterification [ 24 ]. Basically, the reaction is between the carboxylic acid group and the alcohol group with the elimination of a water molecule Figure 4.
When the acid anhydride is used, an alkaline condition is preferred in the reaction. Esterification reaction of carboxylic acids and alcohols. The reaction is usually reversible and the forward reaction is favoured under low pH and excess of alcohol while the reverse is favoured under high pH. Remover of one of the product during the reaction will also favour the forward reaction. Esterification is generally used to introduce more lipophilic groups into the starch molecule making it more lipophilic and for producing crosslink starch when polyfunctional compounds or multifunctional or reagents capable of esterification or etherification are used [ 15 ].
Esterification weakens the inter-molecular bonding that holds the granules together and hence alter the granule shape and sizes as well as other functional properties of the starch. The degree of substitution DS is dependent on the concentration of reagent used, the type of reagent used, the catalyst and the duration of reaction [ 25 ].
Starch can be acetylated by reacting it with acetic anhydride to produce acetylated starch Figure 5. The hydroxyl group of the glucose units are esterified with the acetyl groups from the acetic anhydride to give starch with glucose units with acetate function.
The DS of the hydroxyl group with acetate group is dependent on the reaction conditions. Acetylated corn starch of DS 0. Acetylation of starch with acetic anhydride. The introduction of the more bulky acetyl group compares with hydroxyl group causes steric hindrance to the alignment of the linear chains.
This allows for easy water percolation between chains thus increasing the granule swelling power and solubility resulting in lower gelatinization temperature [ 25 ]. The steric hindrance of less polar acetyl group also reduces the amount of inter-molecular hydrogen bond formation, and weakens the granule structure, preventing molecular re-association and realignment required for retrogradation.
However, depending on the DS and the interplay between the a weakened granular structure as result of interruption of the inter- and intra-molecular bonds, and reduced bonding with water molecules as a result of the hydrophobicity of the acetyl groups, the viscosity of the final product can be enhanced. Acetylation improves paste clarity and freeze-thaw stability of starch. Starch acetates of low DS are commonly used in the food industry for quality consistency, and as texture and stability enhancers.
Starch acetate of high DS exhibit high degree of hydrophobicity and thermoplasticity and are soluble in organic solvents like chloroform and acetone, and are mostly used in non-food applications [ 25 ].
Acetylated distarch adipate, is a monosubstituted starch obtained by treating starch with acetic anhydride and adipic anhydride Figure 6. It has been used since the s due to desire for improved stability of product in cold and freezing weather conditions.
It is a good temperature change resistant agent used in foods as a bulking agent, stabilizer and thickener. It improves smoothness and sheen of soups and sauces [ 19 ]. The improved freeze-thaw stability of acetylated cross-linked waxy maize starch has led to its use in frozen sauces in vegetables, appetizers and pastries. Hydroxypropylation of cross-linked starch also dramatically improves the stability quality of puddings and frozen sauces [ 19 ].
Esterification of starch with acetic anhydride and adipic anhydride. When starch granule is esterified with succinic anhydride, it produces succinyl starch, and the process is commonly referred to as succinylation of starch.
However, environmental concerns have led to the development of more green synthetic routes. Sui et al. Succinylation reaction of starch. Succinyl group weakens the inter-molecular bonding of starch polymeric chains in the granules, facilitating swelling, solubilisation and gelatinization at lower temperatures.
Paste clarity is enhanced and retrogradation is reduced. However, there may be reduced stability against shear at high temperature and during cooling.
Starch succinate is ionic and acts as polyelectrolytes. At low degree of substitution DS , the succinate makes the starch more hydrophilic and viscos in solution [ 8 , 25 ]. Esterification of starch with octenylsuccinic anhydride OSA or octenylsuccinic acid in the presence of an alkali yields starch octenylsuccinate Figure 8 , while esterification with dodecyl succinic acid yield starch dodecyl succinate.
The octenyl or dodecyl group introduce a reasonable level of lipophilicity to the product making it have dual functionality which can be used in emulsification and flavours encapsulation. OSA treated starches are used to stabilize oil-in-water food emulsions associated with beverage concentrates containing flavor and clouding oils [ 19 ].
It helps to protect emulsified and spray dried flavour oils against oxidation during storage. FDA allows a DS of 0.
Esterification of starch with octenylsuccinic acid anhydride. Commercial production of acetylated starch dodecyl succinate, di-substituted starch of low dodecyl succinate residue employs acetic anhydride reagent at alkaline pH [ 15 ]. An alkali-starch complex forms first, which then interacts with the carboxylic anhydride to form a starch ester with the elimination of carboxylate ion and one molecule of water [ 15 ]. Starch succinate offers freeze-thaw stability, high-thickening, low-gelatinization temperature, clarity of paste, good film-forming properties and resistance to retrogradation.
When starch granules are reacted with phosphorylating agents such as phosphoric acid, mono- or di-starch phosphate is formed Figure 9. The resulting starch has increased stability at high and low temperatures, more resistant against acidic condition, and is applicable as a thickening agent.
Orthophosphate and pyrophosphate has been used to achieve phosphorylation of starch under slightly acidic and high temperature conditions [ 27 ]. Phosphorylation reaction of starch. Phosphoryl trichloride Figure 10 , sodium tripolyphosphate Figure 11 and sodium trimetaphosphate Figure 12 have also been used under higher pH to obtain monostarch phosphate and di-starch phosphate [ 15 , 28 ].
Phosphorylation reactions produce either monostarch phosphate or distarch phosphate which is a cross-linked derivative. However this depends on the reagents and reaction conditions. Usually, monoesters, rather than diesters, are produced with a higher degree of substitution [ 8 ]. Steric hindrance as a result of the introduced phosphate groups inhibits the linearity of amylose or the outer branch of the amylopectin chain where it reacted. This weakens the inter-molecular association and creates chains disaggregation, which leads to better paste clarity [ 8 ].
Phosphorylation of starch with phosphoryl trichloride. Phosphorylation of starch with sodium tripolyphosphate. Phosphorylation of starch with sodium trimetaphosphate. Distarch phosphate has the phosphate group esterified with two hydroxyl groups of two neighbouring starch polymer chains [ 29 ]. The phosphate bridge or cross-linking strengthens the mechanical structure of the starch granules.
Phosphate cross-linked starches exhibit stability against high temperature, low pH and shear, and improved firmness of the swollen starch granule as well as improved viscosity and textural characteristic. Distarch phosphate is used as thickener and stabilizer and provides stability against gelling and retrogradation and high resistance to syneresis during storage [ 8 ].
In solution, several specie of the phosphate ion can exist and anyone may be responsible for the phosphorylation reaction depending on the reaction conditions. Phosphorylation has been demonstrated to mostly occur at the C-3 and C-6 of the glucose units, and the degree of phosphorylation depends on distribution of the chain length of the starch polymers [ 30 ].
Blennow et al. Landerito and Wang [ 32 ] reported that phosphorylated starch prepared by the slurry treatment exhibited a lower gelatinization temperature, a higher peak viscosity, a lesser degree of retrogradation, and improved freeze-thaw stability compared with those prepared by the dry-mixing treatment.
They believed that phosphorylation probably occurred in both amylose and amylopectin chains, and the amount and location of incorporated phosphate groups varied with starch types, which may be due to their different amylose and amylopectin contents. Waxy starch was more prone to phosphorylation, followed by common and high-amylose starches. Enzymic phosphorylation of starch has been reported [ 33 ]. Etherification reaction. Etherification of starch is usually done by use of epoxide reagents as depicted in Figures 14 and The properties, isolation, fractionation, enzymatic degradation, biosynthesis, chemical modification, and specific methods of analysis of starch are presented.
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