Physical and Chemical Properties 5th Grade Reading
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Chemic Properties of Starch and Its Application in the Food Industry
Submitted: Apr fifth, 2019 Reviewed: June 6th, 2019 Published: Baronial 5th, 2019
DOI: 10.5772/intechopen.87777
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Abstract
Starch is an important nutrient product and a versatile biomaterial used globe-wide for different purposes in many industrial sectors including foods, health, cloth, chemical and engineering sector. Starch versatility in industrial applications is largely defined by its physicochemical properties and functionality. Starch in its native form has limited functionality and application. Only advancements in biotechnology and chemical technological have led to wide-range modification of starch for dissimilar purposes. The objective of this affiliate is to examine the different chemical reactions of starch and expose the nutrient applications of the modification products. Several literatures on starch and reaction chemistry including online journals and books were analyzed, harmonized and rationalized. The reactions and mechanisms presented are explained based on the principles of reaction chemical science. Chemic modification of starch is based on the chemical reactivity of the constituent glucose monomers which are polyhydroxyl and can undergo several reactions. Starch can undergo reactions such as hydrolysis, esterification, etherification and oxidation. These reactions give modified starches which can be used in baked foods, confectionaries, soups and salad dressings. This affiliate discusses the unlike chemical reactions of starch, the associated changes in functionality, likewise as the applications of chemically modified starches in the nutrient industry.
Keywords
- reactions of starch
- hydrolysis
- esterification
- etherification
- baked products
- confectioneries
- gravies
- soups and sauces
- mayonnaises and salad dressing
*Address all correspondence to: omoregieegharevba@yahoo.com
1. Introduction
Starch as well known as amylum, is an important nutrient production and biomaterial used earth-wide for different purposes. Though traditionally used in the food industry, technological advancement has led to its steady relevance in many other sectors such equally health and medicine, material, paper, fine chemicals, petroleum engineering science, agronomics, and construction engineering [1]. It is used in the nutrient manufacture either as food products or additives for thickening, preservation and quality enhancer in broiled foods, confectioneries, pastas, soups and sauces, and mayonnaises. Starch is a polysaccharide of glucose made of two types of α-d-glucan chains, amylose and amylopectin. Starch molecules produced by each plant species have specific structures and compositions (such as length of glucose chains or the amylose/amylopectin ratio), and the protein and fatty content of the storage organs may vary significantly. Therefore, starch differs depending on the source. This inherent functional diversity due to the unlike biological sources enlarges its range of industrial uses [2, three].
The structural and compositional differences in starches from different sources make up one's mind its properties and mode of interactions with other constituents of foods that gives the terminal product the desired taste and texture. In the food manufacture, starch can be used every bit a food additive to control the uniformity, stability and texture of soups and sauces, to resist the gel breakdown during processing and to raise the shelf life of products [ii]. Starch is relatively hands extractable and does not crave complicated purification processes. It is considered to exist bachelor in big quantities in major plant sources such every bit cereal grains and tubers. These sources are generally considered inexpensive and affordable and serve every bit raw materials for commercial production [four].
Starch from
Effigy 1.
Corn (A) and potato tuber (B) [
The stability of native starch nether different pH values and temperatures varies unfavorably. For instance, native starch granule is insoluble in water at room temperature and extremely resistant to hydrolysis past amylase. Hence native starch has limited functionality. In order to enhance properties and functionality such as solubility, texture, viscosity and thermal stability, which are necessary for the desired product or role in the industry, native starches are modified. The widening vista of awarding possibilities of starches with different properties has made research in non-conventional starches and other native starches more imperative [2, half-dozen, seven]. Recent studies on the relationship betwixt the structural characteristics and functional properties of starches from different sources have continued to provide important information for optimizing industrial applications.
Modification has been achieved by and large by concrete and chemic ways. Enzymic and genetic modifications are biotechnological processes which are increasingly existence explored [viii]. While physical modification methods seemed unproblematic and cheap, such as superheating, dry out heating, osmotic pressure treatment, multiple deep freezing and thawing, instantaneous controlled pressure-drop process, stirring ball milling, vacuum ball milling, pulsed electric fields treatment, corona electrical discharges, etc., chemical modification involves the introduction of new functional moieties into the starch molecule via its hydroxyl groups, resulting in marked alter in its physicochemical characteristic. The functional characteristics of chemically modified starch depends on a number of factors including the botanic origin of the native starch, reagent used, concentration of reagent, pH, reaction time, the presence of goad, type of substituent, degree of substitution, and the distribution of the substituents in the modified starch molecule. Modification is more often than not accomplished through chemical derivatization, such equally etherification, esterification, acetylation, cationization, oxidation, hydrolysis, and cross-linking [vii]. This affiliate discusses the chemic properties of starch and how they determine its application in the food manufacture.
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2. Amylose and amylopectin
The chemical behaviour of starch is dependent on the nature of its constituent compounds. Starch is a homopolysaccharides made up of glucose units. Still, the homopolysaccharide are of two types namely: amylose, which is a linear concatenation consisting of well-nigh 500–2000 glucose units, and amylopectin, which is highly branched and consist of over 1,000,000 glucose units. The two types of homopolysaccharides constitute approximately 98–99% of the dry weight of starch [two]. The ratio of the two polysaccharides usually varies depending on the botanical origin of the starch. Botanic source reports that starch chain generally consist of 20% amylose and up to 80% amylopectin by mass. It is believed that starch with up to fourscore% amylose can exist [7]. Some classification categorize starch containing <fifteen% amylose as 'waxy', 20–35% equally 'normal' and greater ≥forty% every bit 'loftier' amylose starches [ix].
Amylose and amylopectin have different physiochemical properties which impact on the overall properties of the starch. Hence it is often important to determine the concentration of each individual component of the starch, as well as the overall starch concentration [10]. The physicochemical (e.g., gelatinization and retrogradation) and functional (e.thou., solubility, swelling, water absorption, syneresis and rheological behaviour of gels) properties determine the potential uses of starches in the food industry. These backdrop depend on the molecular and structural limerick of amylose and amylopectin, percent composition and arrangement of these 2 homopolysaccharides in starch granules which often determine the granule size and shape depending on other genetic factors every bit a event of the particular species of constitute [2].
In nutrient products, the functional roles of starch could be as a thickener, binding agent, emulsifier, clouding amanuensis or gelling agent. In the food industry, native starch is usually reprocessed and modified through chemical processes to improve its functionality for the desired purpose. Chemical modification involves the introduction of new functional groups into the starch molecule which produces in a modified starch with markedly altered physicochemical backdrop. Such modified starch shows profound alter in functionality such as solubility, gelatinization, pasting and retrogradation [11].
The chemical reactivity of starch is dependent on the reactivity of the constituent glucose units [eleven]. The chemical and functional properties accomplished post-obit such modification depends largely on the reaction conditions such every bit modifying reagent(s), concentration of the reactants, reaction time, type of catalyst used, pH, and temperature. The type of substituents, degree of substitution and distribution of substituents in the starch molecule affects the functional properties.
2.ane Amylose
Amylose is a linear polymer of α-d-glucose units linked by α-one,iv glycosidic bonds (Figure 2). The linear nature of amylose chain and its percentage content in starch, and the relative molecular arrangement with amylopectin affect the overall functionality of the starch. Hence starch varies greatly in grade and functionality between and within botanical species and even from the same found cultivar grown nether different conditions. This variability provides starches of different properties, which can create challenges of raw materials inconsistency during processing [12].
Effigy two.
Chemic structure of amylopectin chain and amylose chain.
2.2 Amylopectin
Amylopectin is a branched polymer of α-d-glucose units linked by α-1,iv and α-1,6 glycosidic bonds (Effigy two). The α-one,6 glycosidic linkages occurs at the branching point while the linear portions inside a branch are linked by α-i,4 glycosidic bonds. In comparison to amylose, amylopectin is a much larger molecule with a college molecular weight and a heavily branched construction built from most 95% (α-1,4) and 5% (α-1,6) linkages. Amylopectin unit chains are relatively short with a wide distribution profile, compared to amylose molecules. They are typically, 18–25 units long on average [xiii, 14].
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3. Physicochemical properties of starch
Physical properties are those backdrop exhibited without whatsoever modify in chemical characteristics of starch and practise not involve the breaking and creation of chemical bonds such as solubility, gelatinization, retrogradation, glass transition, etc. On the other hand, chemic properties changes due to chemical reactions and commonly involve the breakage and cosmos 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 tin can potentially be used for predicting some of its functionality such as texture, pasting, retrogradation, etc. [12, 15]. Amylose has more than proportional relationships with pasting and gel textural properties, while amylopectin which are predominant in regular and waxy corn starches, has higher proportional relationship with compactness.
3.1 Solubility and gelatinization
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 sticky solution. This two-phase structure, chosen starch paste, is desirable for many food applications where candy starches are used every bit thickeners or binders [ii, sixteen].
3.2 Retrogradation and shear
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 [xv]. 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 ways, such as, hydrolysis and esterification are mostly used to produce starches that can withstand retrogradation. Preventing retrogradation of starch is of import for starch used in frozen foods because information technology is accelerated at common cold temperatures, producing an opaque, crystallized, coarse texture as a consequence 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].
Amylose linear chain dissolves in water at 120–150°C and is characterized past loftier thermostability, resistance to amylase, high crystallinity and high susceptibility to retrogradation. Amylopectin, which is the branched chain is all the same, wearisome to retrogradation, with crystalline forms actualization only on the exterior of the globule and characterized by a significantly lower re-pasting temperature of 40–70°C and an increased susceptibility to amylases activeness than amylose. Retrogradation of starch is affected botanical origin of the starch, amylose content, length of the amylopectin chains, density of the paste, paste storage atmospheric condition, physical or chemical modifications and the presence of other compounds. Recrystallization of starch applies but to amylose chains, and it occurs most readily at temperatures effectually 0°C, and likewise at temperatures in a higher place 100°C [8]. Concrete 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 equally an culling food source for diabetic patients and equally a charge per unit decision-making polymer coat in controlled drug commitment systems [8].
Starch granules swollen with water are predisposed to fragmentation if exposed to physical severe force per unit area change. This becomes of major business where the integrity of the granules is required to maintain viscosity. Shear is the disintegration phenomenon of swollen starch granules or gel. Starch shear arises from the shear stress which builds up during the procedure of retrogradation and/or gel drying of the gelatinized starch [nineteen]. The stress acting in reverse directions creates a fault-line that causes the cloth to open or tear apart. Shearing mostly 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 earlier cooking. But once cooked or gelatinized, starch granules becomes susceptible to shear, resulting in loss of viscosity and textural stability [19].
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4. Chemical properties of starch
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-two, C-iii and C-6 which are costless from the glycosidic bail linkages and pyranose band formation, are usually free for exchange reactions involving either the attached hydrogen or the entire hydroxyl group. While the ▬OH at C-6 is a master alcoholic hydroxyl group, those at C-two and C-3 are secondary alcoholic hydroxyl grouping. Hence starch can undergo hydrolytic cleavage of its chains at the glycosidic bonds; oxidative reaction with the ▬OH or C▬C bond creating carbonyl groups; and other reactions with various functional and multifunctional reagents to produce esterified and etherified starches. Most of the reactions crave activation of the hydroxyl of glucose units in acidic or basic media [seven].
four.1 Reactions of starch
The reactivity of starch is dependent on the hydroxyl functions of the constituent α-D-glucan polymers (Figure two). Thus starch is able to undergo the following reactions.
4.1.1 Hydrolysis
Hydrolysis is an addition reaction and simply involves the improver of a water molecule across a bond resulting in the cleavage of that bond and formation of the cleavage products, ordinarily with hydroxyl group or alcohol functionality. Hydrolysis of starch can be achieved past chemic or enzymatic procedure. Chemical process of hydrolysis commonly employs heating starch in the presence of water or dilute muriatic acid (Figure 3). Hydrolysis is also used to remove fat substances associated with native starches. Hydrolysis under acidic condition is chosen roasting, resulting in acid modified starch. Treatment of starch with sodium or potassium hydroxide results in alkaline modified starch. Hot aqueous alkaline solutions tin can be used, and this improves the reducing value of that starch [21, 22, 23].
Figure three.
Hydrolysis of α(one → 4) glycosidic bond.
The products of starch hydrolysis include dextrin or maltodextrin, maltose and glucose. Dextrins are mixtures of polymers of d-glucose units linked by α-(ane → iv) or α-(1 → half dozen) glycosidic bonds. The percentage of products obtained depends on the conditions used for the reaction such as duration and strength/amount of reagents used. Enzymic hydrolysis uses the enzyme malto-amylase to attain hydrolysis and this is the process that ordinarily occurs in starch digestion in the alimentary canal [nine]. Dextrins are white, xanthous, or dark-brown h2o-soluble pulverization which yield optically active solutions of low viscosity. Most of them tin be detected with iodine solution, giving a cherry coloration. White and yellowish dextrins from starch roasted with footling 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, common cold water solubility, viscosity, color and stability.
Hydrolytic processes have been used in the nutrient industry to produce starch derivatives with better functional properties and processing applications [2]. Acrid and alkali steeping are the two well-nigh widely used methods for starch isolation in the nutrient industry, with numerous modifications. Thermo-brine isolation method known as nixtamalization has been used in Central America since pre-Hispanic times. Acrid and alkali isolation processes bear upon the amylose/amylopectin, protein and lipid content besides every bit the granule size and shape of the final production [23].
4.1.2 Esterification reaction
The condensation of an alcohol and carboxylic acid usually nether acidic condition, to produce an ester and h2o, is chosen esterification [24]. Basically, the reaction is between the carboxylic acid group and the alcohol grouping with the elimination of a water molecule (Effigy 4). When the acid anhydride is used, an alkaline condition is preferred in the reaction.
Figure 4.
Esterification reaction of carboxylic acids and alcohols.
The reaction is usually reversible and the frontward reaction is favoured under depression pH and excess of alcohol while the contrary is favoured under loftier pH. Remover of one of the product during the reaction will also favour the forrard reaction.
For starch, the reaction is between the carboxylic acid group (▬COOH) of fatty acids or ▬COCl of fatty acid chlorides and the alcohol grouping (▬OH) of the glucose units. Esterification is by and large 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 too every bit other functional properties of the starch. The degree of substitution (DS) is dependent on the concentration of reagent used, the blazon of reagent used, the catalyst and the duration of reaction [25].
4.1.two.1 Acetylation of starch
Starch tin 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 acerb anhydride to requite starch with glucose units with acetate function. The DS of the hydroxyl group with acetate group is dependent on the reaction atmospheric condition. Acetylated corn starch of DS 0.05, 0.07 and 0.08 have been obtained using 4, half-dozen and 8% (starch d.west.) acerb anhydride respectively and aqueous sodium hydroxide equally goad [25].
Effigy 5.
Acetylation of starch with acerb anhydride.
The introduction of the more than beefy acetyl grouping compares with hydroxyl group causes steric hindrance to the alignment of the linear bondage. This allows for easy h2o percolation between chains thus increasing the granule swelling ability and solubility resulting in lower gelatinization temperature [25]. The steric hindrance of less polar acetyl group too reduces the amount of inter-molecular hydrogen bail formation, and weakens the granule structure, preventing molecular re-association and realignment required for retrogradation. However, depending on the DS and the interplay betwixt the a weakened granular structure every bit result of intermission of the inter- and intra-molecular bonds, and reduced bonding with water molecules every bit a effect 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 depression DS are commonly used in the food manufacture for quality consistency, and as texture and stability enhancers. The Food and Drug Administration (FDA) maximum DS of acetylated starches for food application is 0.1 [19]. 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 not-food applications [25]. At 0.0275 DS, corn starch showroom lower paste gelling, which is practically lost at 0.05 DS. Almost commercial starch acetates have <0.05 DS [19].
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 1950s 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 equally 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 [xix].
Figure half-dozen.
Esterification of starch with acetic anhydride and adipic anhydride.
4.1.ii.2 Succinylation of starch
When starch granule is esterified with succinic anhydride, information technology produces succinyl starch, and the procedure is unremarkably referred to equally succinylation of starch. Succinylation of starch was earlier achieved in the presence of aqueous pyridine and nether reflux at 115°C (Figure 7). Nonetheless, environmental concerns have led to the development of more green synthetic routes. Thus succinic ester of starch accept been prepared past mixing starch with succinic anhydride solution in acetone and refluxing at 110°C for 4 h [25]. Sui et al. [26] was also able to induce a reaction by drop-wise addition of succinic anhydride to a water suspension of starch while maintaining pH at 8.5 past drop-wise addition of sodium hydroxide.
Effigy 7.
Succinylation reaction of starch.
Succinyl grouping weakens the inter-molecular bonding of starch polymeric bondage 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 confronting shear at high temperature and during cooling. Starch succinate is ionic and acts as polyelectrolytes. At depression degree of substitution (DS), the succinate makes the starch more hydrophilic and viscos in solution [8, 25]. For its viscosity enhancing consequence, succinylated starches could find application in production of not-gelling custard creams, and for its increased hydrophilicity, it could be used for enhancing the juicy/polish taste of meat and fried products. Starch succinates can besides be used in soups, snacks, and frozen/refrigerated food products as thickening or stabilizing agents.
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 grouping introduce a reasonable level of lipophilicity to the product making information technology accept dual functionality which tin be used in emulsification and flavours encapsulation. OSA treated starches are used to stabilize oil-in-water nutrient emulsions associated with drink concentrates containing flavor and clouding oils [xix]. It helps to protect emulsified and spray dried flavor oils against oxidation during storage. FDA allows a DS of 0.02.
Figure 8.
Esterification of starch with octenylsuccinic acid anhydride.
Commercial production of acetylated starch dodecyl succinate, di-substituted starch of low dodecyl succinate remainder employs acetic anhydride reagent at alkaline pH [15]. An alkali-starch circuitous forms offset, which so interacts with the carboxylic anhydride to course a starch ester with the elimination of carboxylate ion and one molecule of water [15]. Starch succinate offers freeze-thaw stability, high-thickening, depression-gelatinization temperature, clarity of paste, good film-forming properties and resistance to retrogradation.
4.i.ii.three Phosphorylation reaction
Inorganic esters also exist, for case, esters of phosphorous acid (HthreePO3) and phosphoric acid (H3PO4). When starch granules are reacted with phosphorylating agents such equally phosphoric acrid, mono- or di-starch phosphate is formed (Figure 9). The resulting starch has increased stability at high and low temperatures, more resistant against acidic status, and is applicable as a thickening agent. Orthophosphate and pyrophosphate has been used to attain phosphorylation of starch under slightly acidic and high temperature conditions [27].
Figure 9.
Phosphorylation reaction of starch.
Phosphoryl trichloride (Figure 10), sodium tripolyphosphate (Figure xi) and sodium trimetaphosphate (Figure 12) accept too 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. Notwithstanding this depends on the reagents and reaction conditions. Ordinarily, monoesters, rather than diesters, are produced with a higher caste of exchange [8]. Steric hindrance as a event of the introduced phosphate groups inhibits the linearity of amylose or the outer co-operative of the amylopectin chain where it reacted. This weakens the inter-molecular association and creates chains disaggregation, which leads to better paste clarity [8].
Figure 10.
Phosphorylation of starch with phosphoryl trichloride.
Figure 11.
Phosphorylation of starch with sodium tripolyphosphate.
Figure 12.
Phosphorylation of starch with sodium trimetaphosphate.
Distarch phosphate has the phosphate grouping esterified with ii hydroxyl groups of ii 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 feature. 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 be and anyone may exist responsible for the phosphorylation reaction depending on the reaction conditions. Phosphorylation has been demonstrated to generally occur at the C-iii and C-half dozen of the glucose units, and the degree of phosphorylation depends on distribution of the chain length of the starch polymers [30]. Blennow et al. [31] also demonstrated that phosphate groups may play important role in the size distribution of the amylopectin side chains of phosphorylated starches. Some researchers take reported that most 60–70% of total phosphorus of starch monophosphate is located at C-vi while the remainder is located at C-three of anhydroglucose units. Most phosphate groups (88%) are on chain β of amylopectin [9].
Landerito and Wang [32] reported that phosphorylated starch prepared by the slurry handling 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 out-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 than decumbent to phosphorylation, followed by mutual and high-amylose starches. Enzymic phosphorylation of starch has been reported [33]. Extrusion condition of 200°C, sodium tripolyphosphate concentration of ≥i.iv g/100 ml and pH 8.5 have been used to obtain starch phosphate with high caste of substitution [34].
4.i.3 Etherification
More often than not, alcohols (▬OH) groups condenses with i another at high temperatures under acidic conditions to form ethers (Figure 13). The reaction machinery is through a proton transfer from the catalyst to one of the molecule to form a cation, which loses the proton by extracting the ▬OH of the second molecule to form an ether and water.
Effigy 13.
Etherification reaction.
Etherification of starch is usually done by apply of epoxide reagents as depicted in Figures 14 and 15. The epoxides are outset reduced to diols through a nucleophilic ring opening of the epoxide (cleaving the C▬O bond under aqueous, acidic or alcoholic condition) before the eventual condensation of one of the ▬OH grouping with that of starch [24]. Some etherification reactions occur under alkaline condition. Like esterification, etherification helps to mostly introduce lipophilic alkyl groups into the starch chains thereby reducing the hydrophilicity and the degree of inter- and intra-molecular hydrogen bonding [8].
Figure 14.
Etherification of starch with propylene oxide.
Figure xv.
Etherification of starch with ethylene oxide.
4.1.3.i Hydroxypropylation of starch
This reaction process produces hydroxypropylated starch (HPS), which is a starch ether produce by reaction of starch with propylene epoxide in the presence of an alkaline catalyst (Figure 14). HPS is used for enhancing stability and viscosity of food products. The hydroxypropyl groups introduced into the starch chains affect the inter- and intra-molecular hydrogen bonds, thereby allowing for more ease of displacement of starch bondage in the amorphous regions [eight]. HPS is more stable to prolonged high temperatures than starch acetate especially at pH half dozen, and has improves freeze-thaw stability. It is by and large used in refrigerated or frozen foods and in the dairy industry. The FDA allowable DS for HPS is 0.2 [19].
iv.one.3.2 Hydroxyethylation of starch
Hydroxyethylation of starch is performed past reacting starch with epoxyethane or ethylene oxide to produce the starch ether, hydroxyethylated starch (HES) (Figure 15). The health concerns of hydroxyethylated starch are limiting its apply in the food manufacture. Nonetheless they are mostly used in medicine and pharmaceuticals as plasma volume expander and extracorporeal perfusion fluids [35].
4.1.iii.3 Carboxymethylation of starch
This is an etherification reaction process where starch is reacted with sodium chloroacetate or chloroacetic acrid under certain conditions to produce carboxymethylated starch (CMS) (Figure 16). The reaction involves refluxing chloroacetic acetic acid with dry starch (anhydroglucose units) in the presence of sodium hydroxide in a solvent mixture of ethanol/isopropanol (ratio 3:v). Anhydroglucose unit can be obtained from acid hydrolysed starch [36].
Figure sixteen.
Etherification of starch with sodium chloroacetate.
4.i.iii.3.1 Cationization of starch
Some other etherification reaction is cationization of starch in which starch react with electrophiles or electron-withdrawing reagents such every bit ammonium, amino, imino, sulfonium, or phosphonium groups to produce cationic starches (Figures 17–nineteen), which are of import industrial derivatives [15]. Cationic starches are ordinarily prepared under alkaline conditions, and they exhibit college dispersibility and solubility with better transparency and stability.
Figure 17.
Reaction of starch with aziridine to produce amino-ethylated starches [
Figure 18.
Reaction of starch and dialkyl cyanamides to produce aminoalkyl starches [
Effigy 19.
Etherification of starch with sulfonium salt to produce a sulfonium cationic starch.
Cationic starches containing tertiary amino or quaternary ammonium groups are the most of import commercial derivatives, however they are mostly used in the cloth and paper industry.
For the product of sulfonium starch, halogenoalkyl sulfonium salts (due east.grand., 2-chloroethyl-methyl-ethyl sulfonium iodide or whatsoever β-halogenoalkyl sulfonium salt), vinyl sulfonium salts and the epoxy alkyl sulfonium tin exist used (Effigy xix). Commonly R1 is unsaturated group like alkylene, hydroxyalkylene, aralkylene, cycloalkylene, and phenylene grouping, while each of R2 and Riii can be alkyl, aryl, aralkyl, cycloalkyl and alkylene sulfonium groups and may also incorporate ether oxygen linkages and amino groups [37]. Factors such as reagent used and temperature, affect the reaction menses which normally takes about sixteen–20 h.
Sulfonium starch display positive accuse and can be used as thickeners in the course of aqueous dispersions or pastes. These dispersions are fabricated by heating the suitable amount of sulfonium starch and h2o to a temperature of approximately 93°C. Upon cooling, the resulting dispersion becomes considerably clearer and more resistant to viscosity change compared to the untreated starch. Starch succinate and starch citrates which are obtained through esterification reactions have also been observed to exhibit high cationic properties [8].
4.ane.4 Oxidation
Oxidation of starch with strong oxidizing agents mimics reaction of primary alcohols and diols. Primary alcohol ▬OH functions are oxidized (Effigy 20) to its respective carbonyls (aldehydes and carboxylic acrid), while vicinal diols (Figure 21) are cleaved past strong oxidants like periodic acrid into its corresponding carbonyl compounds (aldehyde and/or ketones) [24]. Oxidation of secondary alcohol ▬OH produces ketones (Figure 22). Oxidation may event in breakage of some intra- and inter-molecular bonds and partial depolymerization of the starch chains [38].
Figure twenty.
Oxidation reaction of primary hydroxyl groups of alcohols.
Figure 21.
Oxidation reaction of vicinal hydroxyl groups of alcohols.
Figure 22.
Oxidation reaction of secondary hydroxyl groups of alcohols.
Starches treated with oxidants fall into 2 broad classes: oxidized and bleached.
Oxidized starches are starches treated with oxidizing agents similar sodium hypochlorite (NaOCl). The oxidizing agent can set on the glycosidic bonds hydrolysing them to alcohol (▬OH) functions or/and C▬C bonds of the glucose unit, oxidizing them to carbonyl functions of aldehydes, ketones and carboxylates (Figure 23). Higher pH favors formation of carboxylate groups over aldehydes and ketones. Some depolymerization usually occurs in the process. Introduction of carboxylate groups provides both steric hindrance and electrostatic repulsion. Oxidation is usually carried out on whole granules and it causes the granule to dissolve, rather than corking and thicken [nineteen]. The reaction can innovate upwards to 1.1% of carboxyl groups in the granule [39]. Oxidation with chlorine or sodium hypochlorite reduces the trend of amylose to acquaintance or retrograde. The reaction rate of starch with hypochlorite is remarkably affected by pH, which tend to exist college at well-nigh pH seven simply becomes very slow at pH ten [40]. Oxidized starches are used where intermediate viscosity and soft gels are desired, and where the instability of acid-converted starches is unacceptable [41]. Hence, pastes of oxidized starches have a lower trend to gel compared to those of thin-humid (or acid hydrolized) starches of comparable viscosity.
Figure 23.
Oxidation reaction of starch to produce oxidized starch.
Other oxidants such as chlorine, hydrogen peroxide and potassium permanganate, dichromates and chlorochromates, etc. are less commonly used. Oxidized starches are reported to give batters improved adhesion to meat products and are widely used in dough and baked foods [41].
Bleached starch is obtained from oxidation of starch with lower concentrations of oxidizing agents like hydrogen peroxide, sodium hypochlorite, potassium permanganate or other oxidants used to remove color from naturally occurring pigments. Bleaching is done to improve the whiteness and/or eliminate microbial contamination. Reagent levels of most 0.5% are unremarkably used, and loss of some starch viscosity due to hydrolysis usually occurs.
four.i.v Cross-linking of starch
Cantankerous-linking of the starch polymer chains with reagents that could form bonds with more ane hydroxyl group of molecule results in cross-linked starch. Such reactions randomly add together inter- and intra-molecular bonds at dissimilar locations in the starch granule which helps to strengthen and stabilize the polymers in the granule. Such processes may utilize hydrolysis, oxidation, esterification, etherification, phosphorylation or combinations of these methods in a sequential or one-mix procedure to achieve the desired product that meets the required physicochemical characteristic of gelatinization, viscosity, retrogradation, and textural backdrop for nutrient applications. In some instances, multifunctional reagents capable of forming either ether or ester inter-molecular linkages betwixt hydroxyl groups on starch molecules are used. Reactions usually take place at the primary ▬OH group of C-vi and secondary ▬OH of C-2 and C-three of the glucose units. Epichlorohydrin monosodium phosphate, phosphoryl trichloride, sodium trimetaphosphate, sodium tripolyphosphate, a mixture of adipic and acetic anhydride, and vinyl chloride are the main agents used to cross-link food form starches [15]. Di-starch phosphate (Figure 12) which is a phosphorylated starch is an example of a crosslinked starch. Acetylated distarch adipate (Figure 6), hydroxypropyl distarch phosphate, hydroxypropyl distarch glycerol are other examples of crosslinked starch [8]. The FDA specify that not more than 0.ane, 1 and 0.12% DS (w/w of starch) of phosphoryl chloride, sodium trimetaphosphate and adipic-acetic mixed anhydride, respectively, should exist used for food grade starch [19].
Cantankerous-linked starch exhibit increased resistance to processing weather condition such equally loftier or low temperatures and pH. Cross-linking reduces granule rupture, loss of viscosity and the formation of a stringy paste during cooking, providing a starch suitable for canned foods and products. Cantankerous-linked starch shows smaller swelling volume, lower solubility and lower transmittance than native starch [xv]. While oxidation may increment retrogradation, crosslinking reduces it. Hence a combination of the two chemical modification methods can exist used to go the starch with desired counterbalanced characteristics.
4.i.6 Approaches to modification of starch
As mentioned in the introductory section, native starches are modified to better their physicochemical properties due to dissimilar reasons. Different approaches have been reported including physical, chemical, enzymatic and genetic approach. But the almost widely used is the chemical approach. For instance, since starch must be gelatinized for it to be digestible in human being diet and nutrition, and the process of gelatinizing native starches usually takes appreciable amount of time for granule to swell and grade paste of gel as obtained in cooking rice and corn flour porridge, it can be modified to reduce gelatinization fourth dimension past concrete methods such every bit extrusion, spray-drier and drum dryer, which promote fast starch gelatinization to produce pregelatinized starch [42, 43, 44]. Pre-gelatinized starch exhibit reduced gelatinization temperature and time. The modified starches are unremarkably dries to obtain flours and/or pre-gelatinized starches of long-term stability and quick preparation [nine]. Pregelatinized starches are partially or totally soluble in cold water and readily course pastes [45]. It absorbs more than water and disperses readily in water than the untreated starch, forming gel at room temperature and less decumbent to deposit [46]. Using gelatinized starch in nutrient products affects the food qualities and backdrop, such every bit, bread volume and crumb [47]; pastas elasticity and softness, lusciousness and digestibility, tolerance in the backdrop of beating and cake mixtures, ice creams, doughnuts, growth of carbohydrate crystals in food products [48]; texture, volume, shelf-live and stability during thawing of cakes and breads [49]. Liquefaction, partial hydrolysis and dextrinization may occur during pregelatinization depending on the processing weather condition [42, 43, 44].
The process of physical modification does non involved any chemic reaction of starch with a modifying reagent and is referred to as concrete modification of starch and the products are known as physically modified starches. However, about modifications of starches are performed through chemical processes. The chemic reactions of starch (hydrolysis, esterification, etherification, oxidation and cationization) are generally exploited in the industry to produce converted or modified starches fit for different purposes in the industry.
According to the Nutrient and Nutrition Program (FNP) of the FAO [fifty], a modified starch is a nutrient starch which has 1 or more of its original physicochemical characteristics contradistinct by handling in accordance with expert manufacturing practice by 1 of the reaction procedures such equally hydrolysis, esterification, etherification, oxidation and cross-linking. For starches subjected to heating in the presence of acid or with alkali, the alteration (mainly hydrolysis) is considered a minor fragmentation. Bleaching is also essentially a procedure resulting in the color change only. However, oxidation involves the deliberate creation of carboxyl groups. Treatment of starch with substituting reagents such as orthophosphoric acid etc., results in fractional substitution in the 2-, 3- or half-dozen-position of the anhydroglucose unit (AGU) unless the 6-position is occupied for branching in amylopectin chain. For cross-linked starch, where polyfunctional substituting agent, such equally phosphorus oxychloride, connects ii chains, the structure can be represented by Starch▬O▬R▬O▬Starch, where R is the cantankerous-linking group and Starch refers to the linear and/or branched structure [50].
Evolving biotechnological innovations are progressing with enzymatic and genetic modification of starch equally a greener culling to chemical modification due to environmental concerns. Enzymatic modifications basically employ hydrolytic enzymes found in certain leaner. For instance amylomaltases or α-ane,4-α-ane,iv-glucosyl transferases from
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five. Starch functionality and its applications in nutrient
The reactions of starch explained above are exploited to create dissimilar types of modified or converted starched to obtain starches with appropriate physicochemical characteristics such as gelatinization, retrogradation, heat stability, solubility, transmittance, color, texture, etc., for dissimilar industrial applications. The food industry is very mindful of safe of chemical residues hence non all types of modified starched are used in foods. Mostly, modified starches are used for adhesion and every bit binder in dilapidated and breaded foods, formed meat and snack seasonings; as dustings for chewing glue and products produced in the baker; as crisping cover for fried snacks; fat replacer and juiciness enhancement in ice foam and salad dressings; flavor encapsulating agents in beverage clouds; emulsion stabilizers in beverages, creamers and canned foods; foam stabilizer in marshmallows; gelling agents in mucilage drops and jelly gum; and as expanders in broiled snacks and cereal meals [19]. Table 1 gives a summary of the chemic modification processes and their food application.
| Chemical process | Specific treatment | Products | Function | Food application | References |
|---|---|---|---|---|---|
| Hydrolysis | Acrid treatment | Acid-hydrolized starch, acid thinned or thin-humid and fluidity starches | Reduced hot-paste viscosity, improved gelling or gel force. Enhanced textural backdrop | Gum, pastilles, jellies | [19] |
| Acid treatment | Dextrinized starch | Increased solubility and gel stability, reduced viscosity and improved emulsification backdrop. Encapsulate volatiles aromatic compound such every bit limonene, isoamyl acetate, ethyl hexanoate and β-ionones | Fat replacer in baker and dairy products, bakery glazes, protective coating in confectionery. Flavour encapsulator in seasonings | [1, 19] | |
| NaOH or KOH treatment | Alkaline hydrolysed starch | Increase viscosity | [22] | ||
| Oxidation | Sodium hypochlorite oxidized starch | Oxidized starch | Lower viscosity, improved whitening of granules, high paste clarity, low temperature stability, and increased adhesion. Reduces retrogradation of cooked starch pastes | As folder in battered meat and breading, flick former and binder in confectionery, crispy coating in diverse fried nutrient stuffs, texturizer in dairy products | [15] |
| Esterification | Monosubstituted starch (starch acetates, starch hydroxypropyl ethers, starch monophosphate esters) | Freeze-thaw stability, improved emulsification properties | As emulsion stabilizers and for flavor encapsulation in refrigerated and frozen foods | [15, 19] | |
| Acetylation with acetic acid anhydride | Starch acetate | Increased lipophilicity emulsion stabilizer. Improves quality of whatsoever fatty/oil-containing products. Reduces rancidity by preventing oxidation. Increase viscosity | Bulking amanuensis in snack foods, stabilizer and thickener in most foods, improves smoothness and sheen of soups and sauces. Cholesterol-free salad dressings, and season encapsulating agents in clouding agents, creamer and potable. Substitute to mucilage arabic, egg yolk and caseinates | [1, 15, xix] | |
| Succinylation with succinic acrid anhydride | Starch succinate | Improved viscosity and juice taste. Freeze-thaw stability | Soups, snacks, and frozen/refrigerated food products. Every bit thickener and in non-gelling custard creams. Meat and fried products to improve juicy or smooth taste and retain season | [25] | |
| Succinylation with OSA | OSA starch | Increased paste viscosity, emulsion stabilizer and lower gelatinization temperature. Reduces glycemic response after consumption of beverages | Beverage emulsion stabilizers, and mayonnaises. Flavour encapsulating agent for battered meat and meat products | [nineteen, 25, 53, 54] | |
| Treatment with adipic anhydride | Starch adipate | Higher paste viscosity, clarity and stability | Thickening agent in foods | [25] | |
| Phosphorylation | Starch phosphate | Amend paste clarity, lower gelatinization temperature, higher viscosity, reduced retrogradation, and improved freeze-thaw stability | Frozen foods | [8] | |
| Distarch phosphate | Stability against high temperature, depression pH and shear, and improved firmness of the swollen starch granule as well as improved viscosity and textural feature, resistance to syneresis during storage | Equally a thickener and stabilizer in foods such equally soups and sauces | [8] | ||
| Etherification | Etherified starches | Improved clarity of starch paste, greater viscosity, reduced syneresis and freeze-thaw stability | As stabilizer in broad range of food applications such every bit gravies, dips, sauces, fruit pie fillings and puddings. Season encapsulating agent in beverages clouds | [15] | |
| Carboxymethylation | CMS | Common cold-h2o solubility | Candy foods, sweets | [1] | |
| Hydroxypropylation | HPS | Improves freeze-thaw stability, water-holding properties, lowers the swelling/pasting temperature, increases paste clarity and reduces gel formation. More than stable to prolong high temperatures. Increment solubility | Salad dressing, water ice creams, refrigerated and frozen foods, and dairy products | [19] | |
| Cationization | Sulfonium starch | Higher dispersibility and solubility with improve paste clarity and stability | [nineteen] | ||
| Crosslinking | Crosslinked starches | College stability to granules swelling, high temperature, high shear and depression pH. Amend viscosity and freeze-thaw stability. Volume expander. Delays retrogradation and reduce paste clarity | Equally thickener and texturizers in soups, sauces, gravies, bakery and dairy products. Filling in fruit pies and canned foods. In breadstuff and dough products every bit expander and to improve rheological properties | [9, 15, 53, 55] | |
| Crosslinked-hydroxypropylated starch | A smooth, viscous, articulate thickener and freeze-thaw stability | Gravies, dips, sauces, fruit fillings and puddings | [15] | ||
| Pre-gelatinized starch | Cold-water solubility and thickening | Instant soups, sauces, dressing, desserts and bakery mixes. Thickener in food that receive minimal oestrus processing such as pastas | [xv, xix] |
Table i.
Awarding of chemically modification starches in foods.
5.i Baked products (bread, pies, samosas, wafers, biscuits and sausages)
Baked products similar biscuits, pies, bread, cakes wafers and sausages are high density products requiring heat resistant starches. Hence crosslinked starches are used since they are more resistant to oven blistering temperatures of 120 ≥ 230°C. Gelatinized starches are also used in ready-to-eat cereal meals such as corn-flakes, etc. The temperature, humidity and caste of stirring decide the texture and quality of the product.
5.2 Confectionery (candy, sweets and sweetmeat)
Oxidized starches have high clarity or transmittance, low viscosity and low temperature stability. It is oft used in confectioneries for coating candies and sweets since they easily cook.
5.3 Gravies, soups and sauces (soups, sauces, tomato paste or ketchup)
Etherified and crosslinked starches are mostly used. Crosslinked starched have college stability for granules-swelling, high temperature resistant, high shear stability and acidic atmospheric condition stability. They are used every bit viscosifiers and texturizers in soups, sauces, gravies, bakery and dairy products. Etherified starches accept improved clarity of starch paste, greater viscosity, reduced syneresis and freeze-thaw stability. Crosslinked starches are used in wide range of food applications such equally gravies, dips, sauces, fruit pie fillings and puddings.
5.iv Mayonnaises, salad dressing, ice cream, spreads and beverages
Hydrolyzed and esterified starches are mostly used in salad dressing and beverages. Hydrolyzed starch (acid-modified starches) has lower paste viscosity nether cold and hot conditions. Hence they are used in mayonnaises and salad dressing [19]. Esterified starches have lower gelatinization temperature and retrogradation, lower tendency to course gels and higher paste clarity, and are used in refrigerated and frozen foods, equally emulsion stabilizers and for encapsulation of beverage clouds. OSA starch is used every bit emulsifiers in mayonnaises and salad dressings.
5.5 Pasta (spaghettis, macaroni, others)
Pregelatinized and crosslinked starches are mostly used in pastas. Gelatinized starch affects pastas elasticity and softness, delectableness and digestibility. Crosslinking gives the needed structural firmness to the pasta.
5.6 Puddings (custard, pap, others)
Pregelatinized starches are used in puddings, instant lactic mixtures and breakfast foods to achieve thickening or water retentivity without employing heat. They are too used in ready-to-use bread mixtures. They are used where little or no heat is required and the increased absorption and retention of water improves the quality of the product; as an agglutinant in the meat industry; and equally a filling for fruit pies [9, 49].
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6. Conclusion
The importance of starch as a biopolymer continues to exist on the upward trend due to its versatility. It has transformed from its traditional utilise every bit free energy-source food to more sophisticated food and not-food applications. Its growing relevance in mod technological application is as a result of its susceptibility to modification, which transforms the native properties into more than desirable and malleable characteristics fit for different purposes. These modifications are just possible due to the chemic reactivity of the constituent glucose monomers of the starch chains. Though the starch granule is inherently nearly unreactive, information technology is still easily activated for reaction past certain atmospheric condition such as loftier or low pH, college temperature, presence of a goad, etc. Under the correct condition, starch molecules tin can undergo hydrolysis, oxidation, esterification and etherification reactions to produced products of improved organoleptic, textural, mechanical and thermoplastic properties of desirable foods and not-foods awarding. Modified starches like starch acetate, starch phosphate, HPS, CMS, sulfonium starches and their crosslinked derivatives are used for diverse applications in the food industry. However, concerns for chemic residues in these products and environmental considerations for hazardous chemicals used in some of the process, accept led to more studies for greener modification processes. Though biotechnology has evolved enzymic and genetic modification processes for production of some modified starches, they are still highly limited and sometimes uneconomical, hence chemical modification remains the about versatile and mostly used.
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Disharmonize of interest
The author declares no disharmonize of interest.
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Submitted: April 5th, 2019 Reviewed: June 6th, 2019 Published: Baronial 5th, 2019
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