Effect of degree of esterification and amidation of pectin on ice recrystallisation

Understanding the inhibitory effect of pectin's structure on ice recrystallization is essential for the formulation design of frozen foods, as texture is a critical quality attribute that directly influences consumer acceptance. Controlling ice crystal growth is an effective strategy for regulating the texture of frozen materials, which is particularly important for maintaining the quality of frozen products during storage and thawing. Pectin, a natural polysaccharide commonly found in plant cell walls, has demonstrated significant ice crystal inhibition effects; however, the structure-activity relationship underlying this phenomenon remains unclear. This study aimed to investigate the influence of esterification and amidation on the ice crystal inhibition capacity of pectin. Pectin with varying degrees of esterification and amidation were constructed through enzymatic de-esterification and amidation modifications. Splat tests and molecular dynamic (MD) simulations were employed to delineate the potential mechanisms by which methyl esterification and amidation of pectin influence ice crystal growth. The findings indicated that lower esterified pectin demonstrated stronger ice recrystallization inhibition (IRI), whereas amidation diminished the IRI capability. Specifically, the average diameter of the largest ice crystals in 51% esterified Pectin was 29.19 μm, approaching that of the largest ice crystals in a sucrose solution (mean largest grain size, MLGS, 19.4% larger). Molecular dynamics simulations revealed that the number of ice molecules in different amidated pectin systems reached equilibrium around 52 ns, while the ice crystal growth in Pectin with varying degrees of esterification concluded later, reaching completion at 62 ns. This indicates a more robust ability of pectin with lower esterification degrees to inhibit ice crystal growth. Observations during the ice crystal growth process showed that amidated pectin chains adopted a concave shape when spliced with adjacent periodic images. Ice crystals tended to grow preferentially along the sides of amidated pectin, a phenomenon not observed in Pectin with different degrees of esterification. This suggests that amidated pectin presents a reduced contact surface with ice crystals, increasing the likelihood of being enveloped by the growing ice interface. Analysis of the interaction energy revealed that a higher interaction energy between pectin and water molecules correlates with an increased number of hydrogen bonds, leading to a tighter association and enhanced inhibition of ice crystal growth. Consequently, pectin with a lower degree of esterification exhibits stronger interactions with water molecules, hindering their binding to ice crystal growth interfaces and thus facilitating ice crystal inhibition. This study elucidates the impact of esterification and amidation degrees on the mechanisms by which pectin inhibits ice crystal growth, including: 1) hydrophilic groups, such as hydroxyl and carboxyl, of pectin can bind to the ice crystal growth interface, while hydrophobic groups, such as methyl, inhibit water molecule attachment; 2) enhanced hydrophilicity of pectin increases its interaction with water molecules, thereby inhibiting their migration; 3) alterations in pectin hydrophilicity result in conformational changes at the ice crystal growth interface, affecting the contact area with ice crystals. These findings have significant implications for the design and formulation of more effective frozen food products, potentially leading to improved texture, stability, and quality. The insights gained from this research not only contribute to a better understanding of the mechanisms behind pectin's IRI activity but also pave the way for future studies to explore other polysaccharides and their derivatives. Such research could lead to the development of novel cryoprotectants for various industrial applications, enhancing the quality and shelf life of frozen foods while addressing consumer preferences for healthier, natural ingredients.