Preparation of water-soluble chitosan via polyphenol conjugation and carboxymethylation: Characterization, antioxidant, and antibacterial properties

Food spoilage is a constant battle between nature's desire to decay and our desire to preserve. Two primary forces drive this deterioration: the growth of harmful bacteria and the slow oxidation that turns fats rancid and colors dull. For decades, scientists have looked to chitosan, a natural material derived from the shells of crustaceans, as a potential shield against these forces. Chitosan is biodegradable and possesses inherent properties that can inhibit microbial growth and slow down oxidation. However, in its natural state, this material has a significant limitation: it does not dissolve well in water, especially when the environment is neutral, which restricts its use in many liquid-based food applications or coatings. To make this promising material truly useful, researchers needed to find a way to make it soluble while simultaneously boosting its ability to fight spoilage.

A team of scientists at Prince of Songkhla University in Thailand set out to solve this problem by transforming chitosan derived from a different source: the pens of squid. Unlike the hard, mineral-rich shells of crabs and shrimp, squid pens are a soft, flexible byproduct of the seafood industry that are often discarded. The researchers first converted these pens into pure chitosan and then applied two distinct chemical strategies to improve the material. The first approach involved attaching molecules of epigallocatechin gallate, a powerful antioxidant found in green tea, directly to the chitosan chains. The second approach involved adding carboxymethyl groups, which are small chemical units that act like tiny handles to help the material interact with water. The goal was to see which method, or if a combination of both, could create a water-soluble material that was also a superior fighter against bacteria and oxidation.

The results showed that both methods successfully transformed the material, but they achieved this through different mechanisms. When the researchers attached the green tea molecules to the chitosan, the resulting material became highly soluble in water, with the version containing the highest amount of green tea molecules dissolving almost completely. More importantly, this modification dramatically boosted the material's ability to neutralize harmful free radicals and chelate metal ions that can trigger spoilage. The material with the most green tea attachments also became a significantly stronger antibacterial agent, particularly against a dangerous foodborne pathogen known as Listeria monocytogenes, reducing the amount of material needed to stop bacterial growth by a large margin. The researchers found that the green tea molecules did not just sit on the surface; they became part of the structure, providing extra chemical sites that could donate electrons and hydrogen atoms to stop oxidation in its tracks.

The second strategy, adding the carboxymethyl groups, also improved the material, though with a different focus. As the researchers increased the amount of these groups attached to the chitosan, the material's ability to dissolve in water increased steadily, reaching nearly ninety-eight percent solubility in the most heavily modified version. This modification also improved the material's ability to fight oxidation, but only in specific ways. It became better at neutralizing certain types of radicals and at binding metal ions, yet it did not improve the material's ability to reduce iron ions, a specific type of antioxidant activity that the green tea version excelled at. Furthermore, while the carboxymethyl material was more soluble, its ability to kill bacteria was actually weaker than that of the original, unmodified chitosan. This suggests that while adding these groups helps the material mix with water, it may slightly interfere with the natural electrical charge that helps chitosan attack bacterial cell walls.

By comparing the two approaches, the study highlights that there is no single "best" modification; rather, the ideal choice depends on the specific need. The version modified with green tea molecules emerged as the most potent all-around bioactive material, offering the strongest protection against both oxidation and bacterial growth. The carboxymethyl version, while less potent in killing bacteria, proved to be an excellent way to make the material dissolve easily in water, which is a critical requirement for many industrial applications. The researchers confirmed these structural changes using advanced imaging techniques that allowed them to see the new chemical bonds and the presence of the green tea molecules within the material. Ultimately, this work demonstrates that a waste product from the squid industry can be turned into a versatile, water-soluble tool for food preservation, offering a sustainable path to reducing food waste and improving food safety without relying on synthetic chemicals.