Recent advancements in cross-linked starches for food applications- a review

ABSTRACT

The industrial food applications of native starches are limited due to their limited resistance to shear, susceptibility to thermal decomposition, and high tendency for retrogradation. Cross-linking of starches adds intra and inter-molecular bonds at various locations in the starch molecule that result in the stability of the granule and enhance functional attributes. This process involves the use of various cross-linking agents such as sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), epichlorohydrin (EPI), phosphorus chloride (POCl₃), and citric acid, etc., to introduce covalent or non-covalent linkages between starch molecules. Cross-linked starch exhibits improved resistance to retrogradation, enhanced freeze-thaw stability, and increased stability during cooking, shearing, and processing, such as high or low temperatures and pH. This review paper offers an overview of cross-linking modifications, emphasizing their importance in addressing native starch limitations. It underscores the significance of selecting appropriate cross-linking agents to customize starch properties for specific applications. Furthermore, the paper discusses the properties and applications of cross-linked starches and delves into regulatory considerations regarding their use. Regulatory consideration for cross-linked starches has also been discussed. Cross-linking modifications offer a promising avenue to unlock the full potential of starch and expand its utility across a wide range of applications.

KEYWORDS:

• Modifications
• cross-linking
• epichlorohydrin
• STMP
• STPP
• pasting
• rheology

Introduction

Starch is a versatile and sustainable biopolymer with extensive food and packaging applications^[1,2]. The abundance of hydroxyl (OH) groups in starch is a characteristic that renders it highly sensitive to water and similar liquids.^[3] This innate hydrophilic behavior of native starch has presented challenges in the development of starch-based materials. Therefore, researchers have directed their efforts toward employing different chemical modifications to transform the numerous OH groups present in starch into different molecular structures. Chemical modifications, including oxidization, esterification, grafting, and cross-linking of starches, have gained prominence and play a pivotal role in tailoring starch to meet the specific requirements of these diverse fields. These methods offer valuable means of tailoring starch to suit diverse applications by altering its fundamental chemical structure.^[4] Cross-linking of starches represents the most frequently used and transformative approach in food science and technology, unlocking a myriad of properties and applications that have revolutionized the food industry. Starch crosslinking introduces a covalent interconnection via hydrogen bonds in starch granules, providing strong bonding between the molecules and restricting the movement of the polymer chains. Cross-linking of starch granules increases its resistance to high temperatures, acidity, and shear forces, rendering it exceptionally useful in processed foods. Cross-linked starch bestows enhanced thickening capabilities, contributing to the desired consistency in products like sauces, gravies, and desserts.^[5] The textural improvements facilitated by cross-linking open up avenues for the creation of smoother, more appealing food items. Furthermore, cross-linked starch is categorized as a type 4 resistant starch (RS-4) due to the covalent bonding during the crosslinking process, which strengthens granule structure and maintains the starch granules integrity during cooking, leading to increased resistance to enzyme hydrolysis.^[6,7]

So far, phosphorus oxychloride (POCl₃), sodium tripolyphosphate (STPP), sodium trimetaphosphate (STMP), sodium hexametaphosphate (SHMP), citric acid, and adipic acid are commonly employed as cross-linking agents for starch. Beyond these attributes, cross-linked starches are pivotal to extending the shelf life of various food products, providing a buffer against undesirable textural changes over time. As we explore the multifaceted world of cross-linked starches in the subsequent sections of this article, their applications will come to light, revealing their indispensable role in shaping the texture, stability, and sensory appeal of a diverse range of food products.

The article highlights the necessity for manufacturers to maintain stringent quality control measures and offers insights into how expert consultation and adherence to regional regulatory nuances are vital for navigating this multifaceted regulatory landscape. In essence, this review article aims to provide a comprehensive overview of the regulatory intricacies surrounding cross-linked starches in food applications, serving as an indispensable resource for industry professionals and researchers seeking to innovate while ensuring compliance with global food regulations.

Types and mechanisms of cross-linking agents

The chemical cross-linking of starch involves the use of chemical agents that can react with the hydroxyl groups (−OH) in starch molecules, forming covalent bonds between them.^[8] A starch molecule has two types of hydroxyl groups: the primary (6-OH) and the secondary (2-OH and 3-OH). The reaction of these hydroxyl groups with multifunctional reagents results in crosslinked starches (Figure 1). Within a starch granule, numerous chains are in close proximity, and as a result, these reactions occur not only within individual chains but also connect adjacent chains. The crosslinking agents develop either ether or ester intermolecular bonds between the hydroxyl groups on starch molecules, as detailed in studies by Yao et al.^[9] Remarkably, only a small quantity of multifunctional reagent is sufficient to interconnect starch molecules through crosslinking reactions.

Figure 1. Mechanism of starch crosslinking.

Phosphorylation cross-linking

Phosphoryl chloride (POCl₃), sodium trimetaphosphate (STMP), and sodium tripolyphosphate (STPP), and a mixture of STMP – STPP have been used to prepare cross-linked phosphorylated starches.^[1] Starch phosphates can be categorized into two groups: monostarch phosphate and distarch phosphate (cross-linked starch). Monostarch phosphate can be obtained by esterification of native starch using STPP or mixtures of sodium dihydrogen phosphate dihydrate and disodium hydrogen phosphate twehydrate. Distarch phosphate can be obtained using POCl₃ and STMP. Distarch phosphate starches are used as a thickener and stabilizer, offering stability against gelling and retrogradation and high resistance to syneresis during storage. Monostarch phosphate displays improved paste clarity, viscosity, and water-binding capacity. Conversely, the formation of distarch phosphates contributes to preserving granule integrity and imparting greater resistance to retrogradation, high temperatures, and low pH compared to native starch. A mechanism of cross-linking of starches is shown in Figure 2.

Figure 2. Mechanism of different cross-linking agents for starch cross-linking.

Cross-linking with phosphorous oxychloride (POCl₃)

Phosphorous oxychloride (POCl₃) exhibits a rapid reactivity with starch, resulting in the formation of a distarch phosphate. Crosslinking with POCl₃ is notably influenced by the pH of the reaction environment, and the most effective crosslinking reactions occur at pH levels exceeding 11. Moreover, the presence of sodium sulfate, typically at a concentration of 2% w/w based on dry starch weight, serves to enhance further both the rate and efficiency of the crosslinking reaction, as reported by Gao et al.^[10] POCl₃ contains three chloride ions and plays a pivotal role in this process. Upon introduction of POCl₃ into the starch solution, the initial chloride ion rapidly reacts with water, forming a phosphorous dichloride with a very short half-life of approximately .01 s, as documented by Delley et al.^[11] This phosphorous dichloride is likely the crucial crosslinking agent. Simultaneously, the remaining two chloride ions in the phosphoryl chloride react with water over a longer half-life period of about 4 min at 25°C. It is imperative to note that phosphoryl chloride must be added to the starch solution as swiftly as possible in order to generate the phosphorous dichloride in situ. This ensures that the phosphorous dichloride has sufficient time to permeate the starch granules, where it facilitates the cross-linking process.

Cross-linking with sodium trimetaphosphate (STMP)

Sodium trimetaphosphate (STMP) serves as a traditional cross-linking agent for starch, inducing both intra and intermolecular bonds through esterification reactions within the starch. This esterification process is pivotal in creating crosslinks within the starch molecule, consequently reducing its affinity for water and reinforcing the structural integrity of the hydrogel through network formation. The alkaline environment in the crosslinking stage triggers the esterification of hydroxyl groups on starch molecules by STMP’s phosphate group, forming starch phosphate diesters.^[12] Crosslinking occurs between C6 and either C2 or C3 in the process. The crosslinking of starch with STMP follows a chemical mechanism in which STMP initially dissolves in water, resulting in the generation of phosphate ions. It’s important to highlight that each phosphate linkage, whether mono or distarch phosphate, results in a negatively charged group (−PO4). Consequently, the overall charge of the crosslinked starch molecule is contingent upon the extent of the reaction. The presence of mono and distarch phosphate groups in the crosslinked starches is evident from the peaks observed between 5.0–4.2 and 4.0–3.0 ppm in the pNMR spectrum. The distarch phosphates signify the presence of crosslinks in the molecule. Notably, the intensity of the distarch phosphates peak is higher when using a higher STMP concentration, indicating a more extensive crosslinking reaction.^[13]

Cross-linking with sodium tripolyphosphate (STPP)

STPP has two different reagents that react with starch through different mechanisms. Below a pH of 9.0, the terminal phosphate groups of STPP become protonated, generating monometaphosphates. These can swiftly react with starch hydroxyl groups, yielding monostarch phosphates.^[14] Conversely, at a reaction pH exceeding 10, ionized hydroxyls can initiate an attack on the central phosphate of STPP, resulting in the formation of starch pyrophosphates. Starch hydroxyl groups can subsequently engage in further attacks, leading to the production of distarch phosphate. At high pH (>10), all acidic sites on STPP become negatively charged, forming a dense shield that likely impedes the nucleophilic attack on STPP. The substantial repulsion from electronegative hydroxyls on starch, caused by tripolyphosphate, results in a relatively low degree of phosphorylation at alkaline pH. In conditions of high temperature, starch hydroxyls undergo slight ionization by alkali, leading them to attack the central phosphate carrying only one ionized hydroxyl rather than the fully ionized terminal phosphates. This bimolecular reaction forms a starch pyrophosphate with the release of orthophosphate. Subsequently, a second starch hydroxyl group can engage in an attack on the starch pyrophosphate, giving rise to distarch phosphate.

Cross-linking with epichlorohydrin

The reaction between epichlorohydrin (1-chloro-2,3-epoxypropane) and starch is a multistep process, as detailed by Zhang et al.^[15] Epichlorohydrin, a multifunctional compound, initiates the crosslinking of starch during this reaction. As the process unfolds, a single crosslink is formed, consuming either one or two epichlorohydrin molecules in the process. This leads to the creation of diester and diglycerol molecules within the crosslinked starch structures. To perform this crosslinking reaction, a starch solution is combined with a lower concentration of epichlorohydrin. The reaction mixture is stirred, and the temperature is maintained at 40°C for 17 h. To halt the reaction, a 3% v/v hydrochloric acid solution is introduced to adjust the pH to 5.25. The resulting mixture is then filtered, washed, and subsequently dried, as described by Zhang et al. in their^[15]2014 study.

Cross-linking with citric acid

One compelling reason for employing citric acid as a cross-linking agent lies in the fact that unreacted citric acid (CA) is regarded as nutritionally safe and may also function as a starch plasticizer.^[16] Cross-linking with citric acid operates through the formation of a cyclic anhydride intermediate. The cycle opens when starch -OH functional groups catalyze esterification, resulting in a new carboxylic acid unit in citric acid (CA). This unit possesses the ability to create a new intra-molecular anhydride moiety with the adjacent carboxylic acid unit. Esterification likely occurs predominantly at primary -OH groups, recognized as the most reactive positions when compared to secondary -OH groups within the structural unity of starch.^[17]

Crosslinking with adipate

Adipic acid is a commonly employed reagent for starch modification. Due to its possession of two carboxylic groups, it can generate both cross-linked starch and monosubstituted derivatives. The reaction is carried out in a water suspension under alkali conditions, involving the gradual addition of an acetic anhydride/adipic acid mixture. The combination of acetic anhydride and adipic acid forms a mixed acetic adipic anhydride, which reacts with starch to produce acetylated distarch-adipate. The reaction proceeds rapidly at pH 8.0, but the reagent must be added slowly while maintaining a pH close to 8.0.^[18]

Properties of cross-linked starches

Functional properties

Diverse functional properties of starches, like swelling power (SP), solubility, water absorption capacity (WAC) and oil absorption capacity (OAC), pasting attributes, etc. are affected by crosslinking. A brief overview of the effect of different crosslinking agents on starch functional attributes is given in Table 1. Crosslinking decreased the SP of different starches, such as barnyard starch,^[19] sorghum starch,^[36] tiger nut starch,^[20] corn starch,^[24] talipot starch,^[27] etc., whereas the SP increased for Cyperus Starch,^[5] Cyrtosperma senegalense starch,^[21] etc, Similarly, the solubility also was found to be significantly changed by cross-linking. The solubility of starches like corn,^[24] cassava,^[25] lotus seed,^[23] Litchi Kernel,^[19] maize,^[37] etc, were decreased.

Table 1. A brief overview of the effect of different crosslinking agents on starch functional attributes.

Crosslinking agent	Starch	Major findings	Reference
STMP	Barnyard millet	Crosslinking with different concentrations (1, 3, and 5%) considerably reduced the SP, solubility, and pasting profile and exhibited shear thinning behavior. It increased the gelatinization temperature and reduced the ∆H value.	Sharma et al.^[Citation19]
STMP	Tigernut starch	Crosslinking at a concentration of 1.5% and a pH of 10.2 improved starch crystallinity and reduced swelling, solubility, syneresis, and freeze-thaw stability. It also improved the viscosity profile.	Xiaofan et al.^[Citation20]
STMP	Icacina senegalensis	Crosslinking at a concentration of 3% and a pH of 10.5 augmented the SP and gelatinization temperature.	Iweajunwa et al.^[Citation21]
STMP	Faba bean	Crosslinking at a concentration of 1, 3, and 5% and a pH of 10.5 decreased the swelling ability, solubility, and peak viscosity, increasing the pasting temperature. Gel exhibited pseudoplastic and shear-thinning behavior.	Sharma et al.^[Citation22]
STMP	Lotus seed starch	Crosslinking at a concentration of 1, 3, and 5% and a pH of 10.5 decreased the solubility, amylose content, and swelling power. Increased pasting temperature and decreased peak viscosity.	Chandak et al.^[Citation23]
STMP/STPP	Corn	Crosslinking at a concentration of 1, 5, and 10% and a pH of 11 potentially reduced the hydration ability and pasting properties. Increased the gelatinization temperature.	Ge et al.^[Citation24]
STMP/STPP	Cassava starch	A lower concentration (1%) at a pH of 11.5 increased solubility, and PV decreased swelling and paste clarity. Increased onset temperature and ∆H value decreased the peak and conclusion temperatures. It potentially reduced the digestibility.	Sriprablom et al.^[Citation25]
STPP/STMP	Tiger nut	Crosslinking at a concentration of 10% and a pH of 10.5 decreased the amylose content, water absorption ability, swelling and solubility, light transmittance, and pasting properties.	Amorim et al.^[Citation26]
Citric acid	Cyperus esculentus	Crosslinking improved the flow behavior and augmented the water-holding capacity and thermal stability.	Olayemi et al.^[Citation5]
Citric acid	Talipot starch	Treating with different levels (5–40%) of citric acid potentially reduced the pasting and thermal attributes. It decreased the digestibility and increased resistant starch content.	Navaf and Sunooj,^[Citation27]
Citric acid	Wheat starch	A 40% citric acid crosslinking decreased the granule crystallinity and digestibility.	Li et al.^[Citation28]
Citric acid	Potato starch	A 20–60% citric acid decreased the pasting profile and starch digestibility. SP was increased at 20% and then decreased at higher concentrations.	Remya, Jyothi, and Sreekumar^[Citation29]
Epichlorohydrin	Mandua Starch	Crosslinking at 1% concentration and pH of 10.5 remarkably decreased starch digestibility. Light transmittance, swelling capacity, and solubility were decreased.	Malik et al.^[Citation8]
POCl3	Kithul starch	Crosslinking (2 g/kg on dry weight of starch) restricted the swelling ability, solubility, and light transmittance. Increased the pasting and rheological attributes and reduced the digestibility.	Sudheesh et al.^[Citation30]
POCl3	Talipot starch	Thermal pretreatment potentially augmented the degree of crosslinking (.1% at a pH of 11), reduced swelling ability, and retrogradation properties.	Aaliya et al.^[Citation31]
POCl3	Bana starch	Crosslinking using .5 and 1% concentration at a pH of 11.5 decreased the SP, PV, retrogradation, and freeze-thaw stability.	Surendra Babu et al.^[Citation32]
POCl3	Maize starch	Crosslinking with .1% POCl3 at a pH of 11 for 1 h decreased the solubility, WBC, and To values.	Kou and Gao^[Citation33]
Adipic acid	Corn starch	Adipic acid crosslinking significantly improved the solubility and swelling power of starch aerogel. It increased the digestibility.	Wang et al.^[Citation34]
Adipic acid	Potato starch	Crosslinking of retrograded starch with adipic acid decreased the PT, SP, and solubility and increased the hydrolysis.	Kapelko et al.^[Citation35]

The extent of the impact on SP and solubility of crosslinked starches varies with the starch and crosslinking method. For example, The STMP/STPP crosslinking of cassava starch decreased the SP (16.92 to 10.76 g/g) with successive increases in reagent concentration. However, the solubility was increased at a lower concentration (1%) and decreased by a further increase in concentration.^[25] Likewise, the STMP/STPP cross-linked corn starch exhibited a reduction in SP and solubility from 16.85 to 5.70 g/g and .46% to .18%, respectively.^[24] Tesfay et al.^[38] observed that the SP and solubility of enset starch were significantly reduced with an increase in epichlorohydrin concentration. Similarly, a reduction in SP was noted in epichlorohydrin crosslinked mandua starch at a concentration of 1.0% w/w.^[8]

The restricted SP and solubility of STMP/STPP crosslinked starches could be due to the presence of a phosphate group, which facilitates the development of additional covalent bonds between starch molecules. This could be beneficial for retaining the starch granule integrity and strengthening the intergranular bonds. In addition, crosslinking hinders the movement of amorphous regions and limits solubility and SP.^[24] Further, the crosslinking of starch augments the hydrogen bonding between the starch chains and contributes to the restricted swelling and solubility of crosslinked starches.^[8]

The crosslinking of starch with different crosslinking agents significantly impacts their WAC and OAC. The citric acid crosslinking of sago starch increased its WAC and reduced the OAC.^[39] The presence of a higher number of hydrophilic sites in citric acid crosslinked starch contributes to its higher WAC. Similarly, the STMP/STPP crosslinking of breadfruit starch significantly enhanced its WAC, whereas it did not show any significant difference in OAC.^[26] The researchers also stated that the presence of hydrophilic phosphorus may be responsible for increased WAC of STMP/STPP crosslinked starch, and it also contributes to the electrostatic repulsion between starch chains, thereby facilitating higher water uptake and swelling. The crosslinking of banana starch with sodium hexametaphosphate considerably increased the WAC and OAC.^[40] The researchers stated that the increased space between starch chains arising from the steric hindrance may contribute to increased OAC. On the contrary, the crosslinking of potato starch with phosphorous oxychloride exhibited a significantly lower WAC than its native counterpart.^[41] The formation of covalent bonding strengthens the granules and thus resists the mobility of the amorphous region, reducing WAC.

Crosslinking with different reagents significantly altered the pasting attributes of starches. The pasting profile of epichlorohydrin crosslinked pearl millet starch from different cultivars is given in Figure 3. It was noticed that the impact of crosslinking varies among different cultivars. Sodium hexametaphosphate crosslinking decreased the pasting attributes of banana starch, including pasting temperature (PT), peak viscosity (PV), trough viscosity (TV), breakdown viscosity (BV), final viscosity (FV), and setback viscosity (SV).^[40] Likewise, similar changes in pasting behavior were observed in STMP/STPP crosslinked breadfruit starch^[26] and STMP crosslinked maize starch.^[37] Likewise, the crosslinking of litchi^[43] and lotus^[23] starches with STMP (1–5%) reduced the PV, BV and augmented the PT, TV, FV, and SV with an increase in STMP concentration.

Figure 3. Pasting properties of native starch and cross-linked starches analyzed using a Micro ViscoAmylo-Graph. Where: NM= normal maize, WM= Waxy maize NP=normal potato, and waxy^[42].

Nevertheless, Sriprablom et al.^[25] observed that the impact of STMP/STPP crosslinking on pasting attributes of cassava starch varies with the concentration. All the concentrations (1, 1.5, and 2%) increased the PT and decreased the BV. However, the PV was increased at 1% and decreased at higher concentrations (1.5 and 2%). The TV, FV, and SV were increased at a concentration of 1 and 1.5 and decreased at 2% as compared to their native counterpart. Similarly, an increase in STMP concentration from 1–5% decreased the PV and BV and augmented the PT of faba bean starch. However, the TV value was higher for crosslinked starches and decreased with increased reagent concentration. The SV and FV were found to be higher at 1% STMP concentration and decreased at higher concentrations than the native counterparts.^[22] Similarly, STMP crosslinked barnyard starch also exhibited a concentration-dependent change in its pasting behavior.^[19] The cross-linking of sorghum starch with epichlorohydrin also exhibited concentration-dependent changes in pasting behavior. A lower concentration (.1 and .3%) increased the PV, TV, FV, and PT of crosslinked sorghum starch, whereas it decreased at a higher concentration (.5 and 1%).^[36] A Figure 3 is showing pasting properties of native starch and cross-linked starches of normal maize, waxy maize and normal potato.^[42]

The cross-linking of talipot starch with 10% STMP/STPP mixture potentially increased its pasting behavior. The authors also stated that the crosslinking enhanced the starch granules’ intra and intermolecular interaction, which resists granule degradation during heating and leads to a rise in viscosity.^[44] Similarly, the crosslinking of kithul starch with phosphorus oxychloride also increased the PV, TV, SV, and FV.^[30] Crosslinking affects the pasting behavior of starch differently depending on the starch source, crosslinking agent, its concentration, and reaction conditions. A lower reagent concentration and a lower degree of crosslinking can augment the swelling and peak viscosity while heating starch with water, whereas a higher degree of crosslinking increases the granule rigidity and reduces the swelling capacity.^[45]

Similar to the pasting attributes, the rheological attributes of the crosslinked starches depend on the starch, crosslinking agent concentration, and the reaction conditions. The cross-linking of sorghum starch with .1% of epichlorohydrin increased its storage modulus (G′) and loss modulus (G′′) and decreased the Tan δ value. However, a higher concentration of epichlorohydrin (.5 and 1%) exhibited a decrease in G′ and G′′, and an increase in tan δ value.^[36] The cross-linking of different cultivars of pearl millet with epichlorohydrin significantly decreased their G′ value. It also noted a significant reduction in yield stress and consistency index after crosslinking.^[46] Likewise, the crosslinking of litchi starch with various levels of STMP reduced the magnitude of yield stress and consistency index as the concentration of STMP increased from 1 to 5%.^[19] The reduction in the G′ value of crosslinked starch could be due to their restricted swelling, which resists the formation of intergranular interaction.^[46] Contrary to the above, the crosslinking of kithul starch^[30] and talipot starch^[31] with POCl₃ exhibited higher G′ and G′′ values as compared to their native counterparts. The authors stated that the increased G′ and G′′ of crosslinked starch could be due to the lower degree of deformation and improved granule rigidity.

Thermal properties

The thermal attributes of crosslinked starch (onset (T_o), peak (T_p), and conclusion (T_c) gelatinization temperature and change in the enthalpy (∆H)) have been studied by differential scanning calorimetry (DSC). The introduction of the phosphate group to the starch molecules by crosslinking strengthens the intermolecular interaction, thus inhibiting the gelatinization and increasing gelatinization temperatures.^[47] The thermal attributes of citric acid crosslinked tigernut starch were studied using DSC. The T_o, T_p, T_c, and ∆H values increased with an increase in citric acid concentrations.^[5] Likewise, the crosslinking of maize starch with different levels of POCl₃ and STMP slightly increased the T_o value (Figure 4). The cross-linking of barnyard millet starch with varying levels of sodium trimetaphosphate (STMP) increased the T_o and T_p values, whereas it decreased the T_c and ∆H values. The increased T_o and T_p are due to the introduction of the phosphate group into the starch molecules, which strengthened the molecular interaction. Besides, the starch granules undergo hydrogen and covalent bonding after crosslinking, thus resulting in an internal structure that is highly amorphous and complex, which results in decreased ∆H values.^[19]

Figure 4. DSC thermogram of native and crosslinked maize starches. Source: Kou and Gao^[33].

The cross-linking of corn starch with different proportions of STMP/STPP significantly augmented the T_o, T_p, and T_c values and reduced the ∆H values.^[24] Similarly, Gonenc and Us^[48] studied the different pH and reaction times on thermal attributes of glutaraldehyde crosslinked corn starch. The crosslinking under both acidic and alkaline mediums broadened the gelatinization temperature of corn starch. A crosslinking in an acidic medium induced an augment in T_o, T_p, and T_c, whereas a crosslinking in an alkaline medium caused a reduction in T_o and T_p. The ∆H value is also decreased in both starches crosslinked under both acidic and alkaline mediums. The cross-linking of cassava starch using STMP/STPP exhibited a concentration-dependent increase in the T_o and ∆H values.^[25] Similarly, the crosslinking of litchi kernel starch with STMP potentially increased its T_o, T_p, T_c, and ∆H with an increase in STMP concentration.^[19] The cross-linking of maize starch with STMP exhibited a shift in T_o, T_p, and T_c to a higher temperature, whereas it did not exhibit any significant changes in ∆H value.^[37] Likewise, Falsafi et al.^[49] also reported that crosslinking of normal maize starch with conventional and ultrasound-assisted methods elevated the T_o, T_p, and T_c values and reduced the ∆H value. The researchers concluded that the introduction of covalent bonds into starch molecules tightens their lattice structure and increases their stability, thus increasing the gelatinization temperature. At the same time, the reduction in ΔH might be due to the breakdown of ordered segments of the granules as a result of acute crosslinking conditions. The cross-linking of banana starch using sodium hexametabisulpates significantly increased the T_o value. However, it did not show any significant difference in T_p and T_c values and decreased the ∆H value.^[40]

The cross-linking of corn, pea, and faba bean starch with POCl₃ and STMP in aqueous and semidry forms was studied by.^[50] The crosslinked starches exhibited potential changes in their gelatinization attributes. The crosslinking with POCl₃ shifted the endothermic peak to a higher temperature for all three starch samples. Likewise, the STMP treatment in a semidry form significantly decreased the T_o value and increased the T_p and T_c value of all starches as compared to their native counterpart. There was a significant elevation in T_o, T_p, and T_c in corn starch, but only an increase in T_c was observed in pulse starch, crosslinked with STMP in aqueous form, indicating that crosslinking agents and crosslinking methods affect the crosslinking of starches differently.

The cross-linking of the waxy maize and waxy rice starches with phytic acid significantly decreased their thermal attributes as compared to control and chemically treated starches. The substitution of the phosphate group destabilizes the starch structure, thus facilitating the easy melting of starch crystals.^[51] The STMP/STPP crosslinked barley starch increased the T_o, T_p, and T_c values, whereas it decreased the ∆H values.^[52] The cross-linking of porous starch also potentially impacts its thermal attributes. Cyperus esculentus porous starch crosslinked using sodium phytate decreased its T_o and T_c and ∆H values, whereas it did not significantly impact the T_p value.^[53] Conversely, the crosslinking of porous corn starch with epichlorohydrin shifted its T_o, T_p, and T_c to a higher temperature.^[54] In conclusion, cross liked starches reduce granule rupture, loss of viscosity, and the formation of a stringy paste during heating, providing a starch suitable for canned foods and products.

Morphological properties

Scanning electron microscopy is one of the most effective tools for analyzing and interpreting surface morphologies of crosslinked starches. The changes in the granule morphology of crosslinked starches vary with the degree of crosslinking. The cross-linking of faba bean, pea, and corn starches with different crosslinking agents, i.e., POCl₃ and STMP (semidry and aqueous form), did not cause remarkable changes in granule morphology, except for a few fissures and smooth indentations on some starch granules (Figure 5a).^[50] Similarly, STMP crosslinking of corn starch with a lower concentration (1 and 5%) did not cause noticeable changes in granule morphology; however, a higher concentration (10%) led to the agglomeration of starch granules.^[24]

Figure 5. (a) Micrograph of native and crosslinked (1) corn, (2) faba bean, and (3) field pea starches using different crosslinking agents. Native (A), POCl₃-aqueous (B), STMP-semidry (C), and STMP-aqueous (D).

Source:^[50]

The cross-linking of talipot starch using STMP/STPP (99:1) caused surface roughness and slight surface erosion on granule surfaces.^[44] Besides, the STMP crosslinking of maize starch maintained its intact shape with the formation of cracks on granule surfaces.^[37] The STMP/STPP crosslinking of cassava starch did not cause any remarkable changes in the granule surfaces.^[55] Meanwhile, the STMP/STPP crosslinking of barley starch made its granule surface rough, and smooth dents were formed.^[52] The cross-linking of maize starch with STMP/STPP mixtures did not cause any remarkable changes in granule shape and size.^[49]

The cross-linking of sorghum starch with varying levels of epichlorohydrin showed a rough surface, cavities, and cracks on their granules, and the effectiveness increased with an increase in concentration from .1 to 1% Figure 5b.^[36] Similarly, the epichlorohydrin crosslinking of mandua starch made its granule surface slightly rough, and the formation of pores or grooves was observed on some of the granules.^[8] Crosslinking of kithul starch with POCl₃ caused the formation of deep grooves on granule surfaces.^[30] Similarly, the crosslinking of talipot starch using POCl₃ showed roughness and erosion on the granule surface.^[31]

Figure 5b. Micrograph of native and epichlorohydrin (EPI) crosslinked sorghum starch: A- 0% EPI, .1% EPI, C- .3% EPI, .5% EPI, and E- 1.0% EPI. Source: Sandhu et al.^[36].

The confocal electron microscopy of STMP/STPP cross-linked corn starch showed that the granular area of modified starch became dark and increased with an increase in reagent concentration.^[24] Alterations in starch granule structure due to increased treatment concentration may affect light absorption and cause darkening. Likewise, the photomicrograph of citric acid crosslinked Cyperus starch revealed the formation of granule aggregates, and it increased with an increase in citric acid concentration.^[5]

Retrogradation

The retrogradation characteristics of crosslinked starch are generally studied in terms of paste clarity, light transmittance, and syneresis. The crosslinking of starch significantly alters its retrogradation characteristics. The STMP/STPP crosslinked barley starch exhibited a higher light transmittance and reduced retrogradation tendency than the native counterpart.^[56] Similarly, restricted syneresis and retrogradation were observed in STMP crosslinked tigernut starch.^[20] The cross-linking of cassava starch with a lower concentration of STMP/STPP considerably reduced its rate of retrogradation. However, a higher concentration of STMP/STPP did not show significant changes from the native starch. Low-concentration crosslinking limits the mobility of starch chains and could delay retrogradation. However, a higher concentration brings a more closely packed structure, enhancing the ability of hydrogen bonding between starch chains and increasing the rate of retrogradation.^[25] The cross-linking of lotus seed starch with different concentrations of STMP exhibited a higher light transmittance and a lower rate of retrogradation.^[23] Similarly, the retrogradation ability of maize starch was also decreased by crosslinking with STMP.^[37] Park and Lim^[51] reported that crosslinking of waxy rice and waxy wheat starch decreased their enthalpy of retrogradation. It could be due to the suppression of retrogradation by cross-linking.

The cross-linking of sorghum starch with epichlorohydrin enhanced its stability and diminished the retrogradation.^[36] Similarly, the crosslinking of Cyperus esculentus porous starch with sodium phytate exhibited higher stability during retrogradation than the native and porous starches.^[53] On the contrary, the POCL₃ crosslinked kithul starch significantly increased the rate of retrogradation.^[30] It was claimed that the realignment of starch chains leached through the surface damages that occurred during modification enhanced its retrogradation.

Freeze-thaw stability

The stability of starch-containing food products during the freeze-thaw process is an important trait affecting their textural properties. During freezing, the water in the starch changes its phase to ice, and during thawing, the resultant water is easily released from the matrix, known as syneresis. The phase separation leads to the reassociation and recrystallization of starch chains. Hence, the repetitive freeze-thaw causes phase separation, causing syneresis and retrogradation.^[57] The crosslinking of starch with different crosslinking agents potentially affects its freeze-thaw stability. The cross-linking of turmeric starch with STMP significantly increased the freeze-thaw stability and decreased the syneresis value considerably.^[58] Similarly, the crosslinking of sago starch with STMP,^[59] tigernut starch with STMP,^[20] rice starch with POCl₃,^[60] sago starch with citric acid^[39] breadfruit starch with STMP/STPP,^[26] sago starch with STMP^[59] also improved their freeze-thaw stability.

The treatment of chestnut starch with two hydrated sodium dihydrogen phosphate significantly increased its freeze-thaw stability.^[61] The introduction of the phosphate group into the starch molecule led to the formation of a three-dimensional network, which offered steric hindrance to the starch molecule and made the system more stable.^[20] On the contrary, the crosslinking of chestnut starch with STMP/STPP reduced its freeze-thaw stability. The higher syneresis of crosslinked starch could be due to the restricted swelling of crosslinked starch, leading to the release of a relatively large quantity of water^[62]

Applications of cross-linked starches

Bakery products

The crosslinked starch has gained much popularity in baked food products like biscuits, pies, bread, cakes, etc, for various reasons. It may enhance the texture, shelf life, and overall quality of the baked products. Crosslinked starches are more resistant to oven temperature; hence, it is predominantly used as ingredients in baked products^[63]. In the bread manufacturing process, starch can absorb ≈ 64% water, and in dough development, starch acts as an inert filler in a continuous protein matrix. It also performs as a bi-continuous network of protein and starch in dough development. Further, the rheological attributes of dough depend on the interaction of hydrated gluten and aggregated granular starch. In addition, the granule shapes play a potential part in the retention of the gas bubbles. A small granule could stabilize the gas bubbles, whereas a granule with a size larger than the gas cell wall destabilizes it. Moreover, the texture and quality of bread are affected by retrogradation during storage.^[64]

The substitution of flour and native starch with crosslinked starches potentially improves the quality of bread. A substitution of 10% flour with STMP/STPP crosslinked rice and corn starch potentially increased the dough stability, whereas it reduced the WAC and dough development time. Further, it exhibited a higher softness and reduced hardness. In addition, the substitution of crosslinked rice starch augmented the bread volume, evenness, and overall textural attributes, whereas crosslinked corn starch bread exhibited reduced evenness and overall textural attributes.^[65] Similarly, Abdul Shukri et al.^[66] studied the impact of the addition of crosslinked rice starch on dough rheology and characteristics of bun. It was noted that the substitution of crosslinked starch reduced the water absorption capacity, dough stability, and breakdown time. However, it enhanced the dough development time (at 15% crosslinked starch) and mixing tolerance time. Further, the substitution of crosslinked starch increased the dietary fiber content and reduced the hardness, whereas it did not affect the sensory characteristics of the bun.

The substitution of crosslinked and dual crosslinked maize starch significantly reduced the water binding capacity of control flour, whereas it did not impact the rheological attributes of the dough. Further, a 10% substitution of crosslinked and dual crosslinked maize starch potentially improved the specific volume of bread, whereas it decreased at a 20% substitution. In addition, it diminished the crumb hardness and augmented the cohesiveness. As compared to the control and crosslinked starch, dual crosslinked maize starch significantly reduced the retrogradation ability and rate of bread staling.^[67] Like bread, the substitution of crosslinked starches significantly impacts the properties of biscuits. Azaripour and Abbasi^[68] reported that the crosslinking reduced the viscosity and improved the thermal stability during baking. Further crosslinked starches reduced the density and increased stiffness of the biscuit produced. In muffins, crosslinked starch with a high swelling ability overwhelms the insoluble structural network development during the cooling process, preventing starch retrogradation and keeping muffins soft^[69]. The cross-linked starch can also be explored as a fat replacer in bakery products. Rodriguez-Sandoval et al.^[70] explored crosslinked cassava starch as a fat replacer in muffins. The study reported that the substitution of different proportions of crosslinked starch adversely affected dough development and stability. However, the substitution of crosslinked starch (8%) did not cause a significant impact on baking properties, i.e., weight loss, specific volume, crumb moisture content, or color parameters of the muffins. Further, it had a textural characteristic comparable to control and good overall acceptability. Fasuan et al.^[71] reported that the crosslinking of amaranth starch with sodium hexametaphosphate significantly improved its alkenyl group, and it indicates that the crosslinked amaranth starch is explored as a substituent of fat in food systems.

Milk and milk products

Starch plays a significant role in dairy-based products; it gives a creamy texture and flavor and acts as a stabilizer. It is widely explored as a thickener in yogurt manufacturing to impart a pleasing texture and reduce defects and cracks in the surface of curd milk.^[72] Moreover, starch reduces wheying-off during storage and transportation and boosts the number of solids in yogurt, which is crucial for its viscosity and sensory properties.^[73] Pang et al.^[74] successfully incorporated the acetylated distarch phosphate into the yogurt and reported that crosslinked starch incorporated yogurt possessed a well-organized and homogenized microstructure as compared to the control and yogurt incorporated with unmodified starch. In addition, it improved the apparent viscosity, thixotropy, pseudo-plasticity, firmness, and adhesiveness of yogurt.

The addition of crosslinked acetylated cassava starch successfully improved the structure and stability of yogurt. It was noticed that cassava starch incorporated yogurt exhibited a higher elastic and viscous modulus, conductivity, and particle size as the concentration of starch increased. Furthermore, it contributes to the structural stability of the yogurt.^[75] The improved rheological attributes of the yogurt with added crosslinked starches could be due to the development of starch gel and further strengthening of the starch gel network by crosslinking. It will contribute to the viscoelastic characteristic of the yogurt. Furthermore, the interaction of protein and starch will also contribute to increased rheological attributes of starch-incorporated yogurt, and this network is formed by hydrogen bonding.^[75]

Ice cream is a dairy-based product that contains a high proportion of fat. Surendra Babuet al.^[32] reported the impact of the addition of different levels of citric acid crosslinked starch (1 and 2%) to ice cream containing different levels of fats (6% and 1%). The researchers noted that the addition of 1% starch exhibited dominant sensory and quality attributes, and it improved the texture of the ice cream, which is similar to the control ice cream (containing 11% fat). However, a 2% starch adversely affected the overrun and melting resistance of the ice creams. Similarly, The substitution of different levels of POCl₃ crosslinked low and high amylose rice starch (5, 10, and 15%) to the cream potentially altered its freeze-thaw stability and textural attributes. An increase in starch concentration enhanced the firmness and the freeze-thaw stability of the cream, and a 5% fat substitution maintained its creaminess.

Soup and sauce

Starch is a natural thickening agent commonly used to give body and consistency to the products; it is explored in soups and sauces. Corn and tapioca starches are the most commonly used as the thickening agent in soup and sauce. Its thickening ability is enhanced by gelatinization, and it has a number of influences, like granule shape, size, amylose content, etc.^[76] Nevertheless, native starch exhibits certain limits like high retrogradation, low shearing stability, etc., which cannot meet particular food processing requirements. Hence, the starch is modified to obtain the desirable properties of starch as a thickening agent.^[77] Similarly, the interaction or blending of starch with non-starch hydrocarbons like gums improves the rheological attributes.^[76] The addition of potato starch/xanthan gum in tomato ketchup exhibited dominant sensory properties, textural attributes, and viscosities when compared with the ketchup with added native and hydroxy propylated potato starch (Cai et al.^[78]

The crosslinking of starch enhances its rheological attributes, thermal stability, resistance to shear, paste clarity, and freeze-thaw stability, and all these collectively contribute to the thickening property of crosslinked starch in different food products like soup and sauce.^[79] The cross-linking of tapioca starch with different concentrations of STMP/STPP potentially altered its paste clarity, rheological properties, and thermal stability. It was noticed that the addition of 1% crosslinked cassava starch significantly enhanced the gel properties and shear stability. Further, the textural and sensory attributes were augmented by the incorporation of crosslinked cassava starch into the soup.^[80] Likewise, the addition of 8% di-starch phosphate barley starch enhanced the consistency of the soup with higher linear viscoelasticity, yield stress, complex viscosity, and lower tan δ value.^[56]

Processed meat

In meat industries, starch serves as multifunctional ingredients with diverse functionalities, including binding agent, emulsifying agent, fat replacer, gelling agent, water retention agent, etc.^[81] In meat emulsions, heat-induced swelling of starch granules in the protein gel matrix may be beneficial. Furthermore, starch granules will absorb water during gelatinization and augment the water retention ability of the meat products.^[82] Wu et al.^[83] observed that while heating myofibrillar protein with starch, the starch granules undergo gelatinization and expand and fill the protein network of myofibrillar protein. It leads to the binding of water and the development of the texture of the myofibrillar protein gel.

Starch can be used to increase the water-binding ability of meat patties. Lee and Chin^[84] studied the effect of corn starch as a water-binding agent in pork patties and noted that adding corn starch increased the water-binding ability and shear value, decreasing the cooking and thawing loss. Adding modified pea and corn starch as binding agents in low-fat bologna formulation reduced the cooking loss, expressible moisture, and purge loss. Further, it increased the bologna’s hardness, chewiness, and cohesiveness.^[82][85] reported that crosslinked and acetylated tapioca starch enhanced the texture and gelation properties of Kung-wan and the rheological attributes of the meat batter. The modified starches were performed as a filler in a gel matrix. The gel made from crosslinked starch is denser and compact due to its restricted swelling. Similar observations were reported when the kung-wan was incorporated by acetylated distarch phosphate tapioca and corn starches.^[86] Likewise, The addition of crosslinked esterified potato and tapioca starch significantly increased the strength and water-holding capacity (WHC) of the myofibrillar protein starch gel.^[87]

It has been noted that the starch granules, having a size similar to the size of fat emulsions, could be explored as a fat substitute in low-fat food products to reduce the risk of high-fat content. Hence, meat and meat-based products can be substituted by starch and its modified forms as fat substituents.^[81] Garcia-Santos et al.^[88] successfully substituted the resistant starch as a fat substituent in beef sausage and reported that the fat substitution did not impact the centesimal composition, texture, and sensory attributes of the beef sausage. Similarly, Jairath et al.^[89] also successfully used corn starch as a fat substitute in buffalo calf meat sausage. It was noted that successive replacement of fat with corn starch ameliorated the sausage’s WHC, emulsion stability, texture, juiciness, etc., and 6% fat substitution exhibited optimal properties.

Regulatory considerations

Cross-linking, on the other hand, is regarded as an effective route of starch modification. It can incorporate covalent bonds and augment the naturally occurring intermolecular hydrogen bonds, enhancing the industrial applicability and versatility of starch by enhancing its mechanical properties. However, some of the chemicals used for alteration have been seen to have a detrimental impact on human health when used in an unregulated amount, i.e., in excess or beyond the limit. The United States Food and Drug Administration (US FDA) plays a crucial role in ensuring the safety and quality of food-grade starch, particularly when it comes to the application of cross-linked starch in various food products. To uphold consumer health and the transparency of food products, the US FDA framed 21 CFR 172.892 to ensure the use of chemically modified starch in foods in a controlled manner. These regulatory considerations are essential to maintaining product quality, ensuring that additives used in the cross-linking process are safe, and transparently disclosing their presence on product labels. One key aspect involves obtaining approval for the additives used in the cross-linking process, ensuring they are Generally Recognized as Safe (GRAS), particularly in the United States under FDA oversight. The USFDA specifies that not more than .1, 1, and .12% Degree of substitution (DS) (w/w of starch) of phosphoryl chloride, sodium trimetaphosphate, and adipic acetic mixed anhydride, respectively, should be used for food-grade starch. Further, the permitted maximum level of phosphorus (P) content of .4% in cross-linked starch using a mixture of STMP and STPP.^[90] Quality and performance standards are often in place, ensuring that cross-linked starch doesn’t compromise the texture, viscosity, or overall stability of food products. These regulatory measures collectively serve to maintain the integrity and safety of food products in which cross-linked starch is used, benefiting both manufacturers and consumers alike.

Additionally, accurate labeling is critical, requiring manufacturers to transparently disclose the presence of cross-linked starch on product labels, providing vital information to consumers, especially those with dietary restrictions or allergies.

Conclusion

In recent years, the modified starch industry has experienced substantial growth and innovation, driven by the increasing demand for its diverse applications in both the food and non-food sectors. One of the key advancements in this industry is the practice of cross-linking starch, which plays a pivotal role in enhancing its functionality and versatility. Cross-linking starch is a process that involves the introduction of covalent bonds within the starch structure, typically achieved through the use of various cross-linking agents. Cross-linking of starch with various cross-linking agents in the starch structure makes it resistant to high temperatures and shear forces, allowing it to withstand rigorous processing conditions in food manufacturing. and more suitable for applications like salad dressings, canned foods, etc. In applications like deep-fried foods, pudding, fruit filling, gravies, soups, sauces, baby foods, etc., crosslinked starches are widely applied. Further, cross-linked starches can be used to create heat-stable gels in convenience foods or as a fat replacer in low-fat products. The process of cross-linking starch represents a pivotal innovation within the modified starch industry, empowering starch to excel in demanding processing conditions while expanding its applications. Numerous food applications have solidified cross-linked starches as a cornerstone of modern food manufacturing, contributing to the development of higher quality, healthier, and more convenient food products.

Disclosure statement

No potential conflict of interest was reported by the author(s).