Additives as external plasticizers in starch-based bioplastics - towards improving the properties of food packaging: A review

The environmental burden of petroleum-based plastics, especially in single-use packaging, has driven the search for sustainable, bio-based alternatives. Starch-based materials are promising due to their abundance, low cost, and biodegradability. However, native starch lacks the mechanical and barrier properties required for packaging applications, necessitating the use of plasticizers and additives. This review critically evaluates starch as the primary biopolymer and its interactions with various external plasticizers, aiming to enhance its physicochemical properties aimed for potential applications in food packaging. Despite extensive research on starch modification and composite formulation, the molecular mechanisms by which starch interacts with plasticizers, and how this influences the material properties remain poorly understood. Addressing this gap, the review categorises plasticizers into distinct chemical classes: polyols, saccharides, acids, esters, salts, amines/amides and oils, and examines how each group affects the starch polymer at the molecular level. The plasticising effects vary depending on the nature of hydrogen bonding with the polar groups of starch, even among highly compatible plasticizers including polyols, saccharides and amines/amides. Essential oils, though limited in direct interaction, enhance water resistance and antimicrobial properties. Citric acid functions as a multifunctional additive with notable plasticising effects. Salts, particularly deep eutectic solvents (DES) and ionic liquids, present novel solutions for developing starch-based materials. Key challenges such as plasticizer migration, water sensitivity, and limited mechanical strength are discussed, alongside considerations of toxicity, cost, and scalability. By mapping structure-function relationship in starch-plasticizer systems, this review supports the advancement of fully bio-based, biodegradable packaging materials with optimised performance.