Unraveling the Effect of Citric Acid on Microstructure, Rheology, and Structural Recovery of Thermoplastic Potato Starch

Abstract

The main objective of the present work is to explore the role of citric acid (CA) on microstructural changes and their impact on the melt rheological behavior, mechanical performance, and shape recovery of thermoplastic starch (TPS). The results of frequency sweep test show a pronounced nonterminal storage modulus (G’) at lower frequencies (high melt elasticity) for TPS, which is found to be greatly reduced in favor of improving the processability through the addition of CA. This could be explained in terms of reduced disentanglements of amylopectin molecules, as a crucial parameter responsible for the 3D‐type interconnected microstructure of TPS, through catalyzing effect of CA on hydrolysis. The results of the temperature sweep test (cooling mode) show a pronounced liquid‐to‐solid transition at ∼136℃ for CA‐modified TPS. This transition could be attributed to the temperature‐driven significant strengthening of the formation of single helical amylose crystalline structure and/or the hydrogen bonds between hydrolyzed amylopectin chains at temperatures below 136℃. The results are found to be consistent with the results obtained from the tensile and DMTA experiments. As expected, the shape recovery of the CA‐modified TPS sample is lower than that of unmodified TPS because of the lower elastic stored energy (stiffness) in the presence of CA, which can be largely compensated by incorporating appropriate nanoparticles.

Thermoplastic starch hybrid biocomposite films with improved strength and flexibility produced through crosslinking via carboxylic acid

“…The hydrolysed amylopectin chain behaves like the linear amylose chain, promoting stable single helix crystalline formation during drying. The same observation was also found in the study of Javidi et al 21 where they found that the TPCS chain liberated from the amylopectin has a high potential to recrystallize into a single helix helical structure during the post-processing stage. Besides that, they also concluded that adding the carboxylic acids could effectively reduce the retrogradation in the corn starch films due to the cross-linking reaction between the TPCS chains.…”

“…Coplasticization between glycerol and carboxylic acid has higher plasticization effect than single plasticizer system (glycerol) in reducing the intermolecular interaction of the TPCS chains and promoting higher film's flexibility. Co-plasticization has higher efficiency to disrupt the intra and inter-molecular structure of TPCS and promote greater dynamic motions of the molecules 21 Generally, the CA-TPCS films showed marginally higher tensile strength and Young's modulus than the TA-TPCS films in their respective pH values. The different mechanical properties between the CA-TPCS and TA-TPCS may be attributed to the different carboxyl groups in their chemical structure.…”

“…Carboxylic acid is expected to interact with the TPCS as internal plasticizer or external plasticizer by forming physical or chemical bonding between the TPCS chain, plasticizer, and nanocellulose. 21,42,45 As reported in the literature, the carboxylic acid can act as internal plasticizer form covalent bonding with the TPCS chain through mono-esterification without cross-linking the intermolecular TPCS chains. The mono-esterification formed starch citrate in the TPCS chain, which acts as the internal plasticizer and interrupts the inter and intramolecular hydrogen bond of the TPCS chain, as shown in Figure 2.…”

“…19 The incorporation of the carboxylic acid can enhance the TPS film’s mechanical and hydration properties by forming strong hydrogen bonding and ester chemical bonding with the TPS chains. 20,21 The carboxylic acid can function as an external plasticizer and as an internal plasticizer in the TPS matrix. The dual function carboxylic acid is due to the partial esterification ability of carboxylic acid with the TPS chains, which interrupts the intra and inter TPS molecular chains by chemically attaching to them.…”

Effect of Citric Acid and Sodium Trimetaphosphate Induced Crosslinking on Functional, Rheological, Structural, and Thermal Properties of Buckwheat Starch

“…The crosslinking of starches with crosslinkers such as CA, [ 6‐9 ] STMP, [ 10‐13 ] and epichlorohydrin [ 14 ] have reported to introduce intra‐ and inter‐molecular bonds at selected hydroxyl (─OH) groups of two adjoining intact starch molecule. It mainly occurrs in the amorphous region of starch, and induces amylopectin–amylopectin and amylose–amylopectin crosslinking.…”