1. Introduction

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1. Introduction

Starch is one of the most ubiquitous storage substances in nature. In its native form, starch has limited applications due to properties that are not always desirable for certain types of processing. Examples of these shortcomings include its inability to withstand the high temperature, pressure, and some strong chemical reagents use in most industrial food and pharmaceutical processes. Starch modification, which implies the alteration of the physical and chemical characteristics of native starch, is used to improve its properties. The chemical modification of starch is directly related to the reactions of the hydroxyl groups of the starch polymer. Almost all glucose residues in starch chains possess three free-hydroxyl groups capable of oxidation, etherification, and esterification. Starch modification through esterification involves starch chain reactions with inorganic and organic acids, salts of inorganic acids, and anhydrides or chlorides of organic acids. A commonly used modified starch in the food industry is acetylated distarch adipate, obtained by the esterification of starch with adipic acid and acetic anhydride [1]. This preparation is characterized by a higher thermal stability, shear resistance, and acid resistance compared to native starch and forms strong and stable gels that are resistant to retrogradation [2]. For this reason, these starch modifications are widely used in food products as a thickener, stabilizer, and binder [3]. However, the acetylated distarch adipate has applications that are regulated by the degree of substitution (DS) or the percentage of acetyl groups (% Ac). These characteristics determine the use of starch acetate, for example, for food application; the United States Food and Drug Administration (FDA) recommends a percentage of acetyl groups of less than 2.5 g/100 g [4].

Recently, scientists have focused a lot of attention on the preparation and testing of the properties of starch modified with citric acid. Citric acid (2-hydroxypropane-1,2,3 tricarboxylic acid, C

₆

H

₈

O

₇) itself is recognized as a safe food additive (GRAS) and is used widely in the food, cosmetic, and pharmaceutical industries ^[5]. The use of citric acid is considered safe for both the natural environment and human health ^[6]. The JECFA (Joint FAO/WHO Expert Committee on Food Additives) monograph on modified starches does not set any particular limitations on the use of starch citrate ^[7]. Citric acid has been used for starch modifications since the mid-20th century. A method for the production of its starch esters was patented in 1949 ^[8], whereas a preparation of starch crosslinked with this acid was patented in 1960 ^[9]. Citric acid is relatively often used in current investigations addressing starch modifications ^{[6][10][11][12][13][14][15][16][17]}. Mainly it serves as an acidity regulator. Citric acid is widely used as a reagent in reactions of esterification and crosslinking of starch conducted under various conditions and types of starch. It is also used for starch treatment at temperatures lower than its pasting temperatures ^{[18][19][20][21][22][23]}and in the heat moisture treatment of starch ^[24][25]. In the polymeric materials produced based on starch, it is used as a compatibilizer ^[26], a plasticizer ^[9], and a crosslinking agent ^{[11][12][13][14][27][28]}. In addition, it serves the role of a catalyst in carbonspheres production from starch that are used, i.e., lithium batteries ^[15]. In the production of starch citrate, citric acid is converted to its anhydride, which forms ester bonds with the hydroxyl groups in starch ^[29]. However, the superiority of citrates over other modifications can be explained by the cross-linking process itself. Crosslinking is a type of chemical modification that offers greater stabilization of the starch structure. It can be performed with polycarboxyl organic acids, which form bridges by reacting with hydroxyl groups of anhydroglucoses from the neighboring chains, thereby stabilizing the polymeric backbone. Citric acid was chosen for this overview because its heating with starch ensures high crosslinking of the latter. Its additional advantage to starch modification is the possibility of modelling starch resistance to amylolysis in a whole variability range (0–100%) because it is correlated with the process temperature and acid dose ^[30].

) itself is recognized as a safe food additive (GRAS) and is used widely in the food, cosmetic, and pharmaceutical industries [5]. The use of citric acid is considered safe for both the natural environment and human health [6]. The JECFA (Joint FAO/WHO Expert Committee on Food Additives) monograph on modified starches does not set any particular limitations on the use of starch citrate [7]. Citric acid has been used for starch modifications since the mid-20th century. A method for the production of its starch esters was patented in 1949 [8], whereas a preparation of starch crosslinked with this acid was patented in 1960 [9]. Citric acid is relatively often used in current investigations addressing starch modifications [6,10,11,12,13,14,15,16,17]. Mainly it serves as an acidity regulator. Citric acid is widely used as a reagent in reactions of esterification and crosslinking of starch conducted under various conditions and types of starch. It is also used for starch treatment at temperatures lower than its pasting temperatures [18,19,20,21,22,23] and in the heat moisture treatment of starch [24,25]. In the polymeric materials produced based on starch, it is used as a compatibilizer [26], a plasticizer [9], and a crosslinking agent [11,12,13,14,27,28]. In addition, it serves the role of a catalyst in carbonspheres production from starch that are used, i.e., lithium batteries [15]. In the production of starch citrate, citric acid is converted to its anhydride, which forms ester bonds with the hydroxyl groups in starch [29]. However, the superiority of citrates over other modifications can be explained by the cross-linking process itself. Crosslinking is a type of chemical modification that offers greater stabilization of the starch structure. It can be performed with polycarboxyl organic acids, which form bridges by reacting with hydroxyl groups of anhydroglucoses from the neighboring chains, thereby stabilizing the polymeric backbone. Citric acid was chosen for this overview because its heating with starch ensures high crosslinking of the latter. Its additional advantage to starch modification is the possibility of modelling starch resistance to amylolysis in a whole variability range (0–100%) because it is correlated with the process temperature and acid dose [30].

2. The Methods of Starch Citrate Preparation

In recent studies, processes of esterification/crosslinking with citric acid have been conducted with various starch types, including both these produced on the large industrial scale, i.e., “normal” and waxy maize starch ^{[27][31][32][33][34][35][36]}, wheat starch and its A and B fractions ^[22][37], potato starch ^{[38][39][40][41][42][43]}, rice starch ^[44][45], and cassava starch ^{[14][43][46][47]}, as well as with starch extracted from local plants, i.e., from banana ^[43][48], taro ^[49], yam ^[50][51], sweet potato ^[43][52], acha and iburu ^[53], kidney bean ^[54], lentil ^[43], and barley ^[16]. Investigations have been carried out with native starch (non-modified) and with starch modified using physical and chemical methods—porous starch ^[11], retrograded starch ^[41][42], starch nanocrystals ^[11], carboxymethyl starch ^[55], starch coated with nanoparticles ^[56], and alkaline starch suspension ^[57].

In recent studies, processes of esterification/crosslinking with citric acid have been conducted with various starch types, including both these produced on the large industrial scale, i.e., “normal” and waxy maize starch [27,31,32,33,34,35,36], wheat starch and its A and B fractions [22,37], potato starch [38,39,40,41,42,43], rice starch [44,45], and cassava starch [14,43,46,47], as well as with starch extracted from local plants, i.e., from banana [43,48], taro [49], yam [50,51], sweet potato [43,52], acha and iburu [53], kidney bean [54], lentil [43], and barley [16]. Investigations have been carried out with native starch (non-modified) and with starch modified using physical and chemical methods—porous starch [11], retrograded starch [41,42], starch nanocrystals [11], carboxymethyl starch [55], starch coated with nanoparticles [56], and alkaline starch suspension [57].

The process of starch esterification has been performed with various methods, which affected the degree of substitution and properties of the modified preparations (

Table 1

). The most frequent has been the “dry” esterification/crosslinking described by Klaushofer et al. [7], in which starch is first mixed with an aqueous solution of citric acid and then dried and roasted. The preparation obtained is then rinsed with water or/and an ethanol solution, dried again, and disintegrated.

This method has been modified by many researchers and these modifications concerned the use of various citric acid to starch ratios (from 1 g to 60 g of citric acid per 100 g of starch) and performing this reaction at various pH values (3.5–5.5), roasting temperatures (110–140 °C), and roasting times (1–5 h).

Different conditions producrf modified preparations with a substitution degree ranging from 0.01 to 0.42, which was found to depend on the type of starch material and its earlier modifications ^{[37][41][42][43]}, citric-acid-to-starch ratio ^{[11][37][38][39][41][42][43][46][47][52][58]}, pH value of the reaction ^[35][58], and the time and the temperature of roasting ^[35][42][47].

Different conditions producrf modified preparations with a substitution degree ranging from 0.01 to 0.42, which was found to depend on the type of starch material and its earlier modifications [37,41,42,43], citric-acid-to-starch ratio [11,37,38,39,41,42,43,46,47,52,58], pH value of the reaction [35,58], and the time and the temperature of roasting [35,42,47].

“Dried” esterification was carried out not only in roasters but also in extruders. A study conducted by indicates that the molar substitution degree of the starch citrates obtained in the process of reactive extrusion ranges from 0.01 to 0.03 depending on the amount of citric acid, the temperature of extrusion, and the number of passages. The effect of the amount of citric acid added to extruded starch on the substitution degree of the modified preparation was also observed in the work by Ye et al. [45]. The degree of substitution and the properties of starch citrates produced with the extrusion method were also affected by the addition of galactomannans (guar gum, tara gum, and locust bean gum) [31].

Another procedure of “dry” esterification was described by Kim and Min [36]. The reaction was induced by microwave-discharged cold plasma. The molecular degree of substitution of the modified preparation reached 0.012–0.015 and was similar to that of starch modified with the “classic” method, i.e., through roasting at a temperature of 135 °C for 1 h.

Starch esterification and crosslinking may also be induced by exposing the reaction mixture to ultraviolet irradiation and applying sodium benzoate as a photosensitizer. This method was used for film production from thermoplastic starch [10].

The method of “dry” esterification of the starch through its roasting with citric acid was also used to produce many modified preparations of starch intended not for the food industry but for pharmaceutical or technical purposes. Preparations of this type included crosslinked starch nanocrystals potentially used as drug delivery vehicles and reinforcements for nanocomposites with a hydrophobic polymer matrix ^[28], crosslinked porous starch used for the absorption of heavy metal ions ^[11], starch films with good barrier and mechanical properties^[13][28], novel starch foams that may replace petroleum-based foams ^[12], or hydrogels ensuring appropriate release of active substances of a drug ^[57].

The method of “dry” esterification of the starch through its roasting with citric acid was also used to produce many modified preparations of starch intended not for the food industry but for pharmaceutical or technical purposes. Preparations of this type included crosslinked starch nanocrystals potentially used as drug delivery vehicles and reinforcements for nanocomposites with a hydrophobic polymer matrix [28], crosslinked porous starch used for the absorption of heavy metal ions [11], starch films with good barrier and mechanical properties [13,28], novel starch foams that may replace petroleum-based foams [12], or hydrogels ensuring appropriate release of active substances of a drug [57].

Apart from the above-discussed “dry” esterification method described in a few works ^{[49][50][52][53]}, recent investigations on starch citrates have also addressed “wet” esterification performed according to Agboola et al. ^[59]. This method consists in the localization of an aqueous starch suspension, the addition of a solution of citric acid and a solution of sulfuric acid as a catalyst, and keeping this suspension at room temperature. The next steps include rinsing with water to remove residues of reagents, drying, and disintegration. Depending on the amount of citric acid, the degree of substitution of the modified preparations ranged from 0.002 to 0.023 ^[59]. The “wet” esterification conducted under slightly different conditions was also employed to produce hydrogels capable of encapsulating nutraceuticals ^[57].

Apart from the above-discussed “dry” esterification method described in a few works [49,50,52,53], recent investigations on starch citrates have also addressed “wet” esterification performed according to Agboola et al. [59]. This method consists in the localization of an aqueous starch suspension, the addition of a solution of citric acid and a solution of sulfuric acid as a catalyst, and keeping this suspension at room temperature. The next steps include rinsing with water to remove residues of reagents, drying, and disintegration. Depending on the amount of citric acid, the degree of substitution of the modified preparations ranged from 0.002 to 0.023 [59]. The “wet” esterification conducted under slightly different conditions was also employed to produce hydrogels capable of encapsulating nutraceuticals [57].

Hernandez-Jaimes et al. demonstrated the feasibility of producing starch citrates in the aqueous medium with the electrochemical method using a triangular-shaped potential cycle, with substitution degree of the obtained preparations ranging from 0.02 to 0.12 depending on the number of cycles [33]. The esterification/crosslinking of starch in an aqueous suspension was also used to produce “non-food” preparations, e.g., during production of curcumin-loaded starch coated with iron oxide nanoparticles [56], to be used in pharmacy (drug delivery systems) and during production of novel carboxymethyl starch-based biodegradable films with calcium montmorillonite that can be used for the production of seed tapes [55].

Heating a water suspension of starch, glycerol, and citric acid to the temperature of 75 °C and 85 °C was used to produce biodegradable and not-retrogradable eco films [14]. Authors of this work emphasized that, under these conditions, the reaction of esterification proceeds in two stages. In the first stage, citrate esters of glycerol are produced, and in the second stage, these esters react with starch, forming crosslinking bonds.

3. Summary

It may be concluded that the work on starch citrate esters represent a topical and meaningful direction of investigations on modified starch preparations. The possibility of modifying starch preparations by esterification/cross-linking with citric acid allows for the production of modified starch, which can be used in the pharmaceutical, packaging, or food industries. Especially significant may be investigations of resistant starch produced via esterification with citric acid because this method produces preparations exhibiting significant resistance and, simultaneously, good functional properties, which are potentially applicable as health-promoting additives that increase the dietary fiber content of food products.