The demand for sustainable packaging continues to increase dramatically due to rising environmental awareness and the implementation of governmental regulations. Paper is characterised by its biodegradability, lightweight, and recyclability and is produced by renewable resources, making it an environmentally friendly alternative to synthetic packaging materials. To prevent the penetration of specific gases, aromas, vapour, water, oil, or grease that could compromise the sensory and hygienic integrity of the packed product, barrier coatings are applied to paper-based packaging. Conventional barrier coatings commonly consist of petroleum-based derivatives including polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), often criticised due to poor recyclability and very slow biodegradability. In contrast, starch is a highly promising biopolymer for the formation of fully bio-based barrier coatings with good tensile and gas barrier properties.
Surface coating with starch-based products is a well-established commercial practice to improve the properties and to achieve specific functionalities of paper as well as to prevent undesirable variation and contamination. However, starch exhibits several disadvantages like the inherent sensitivity to water, due to the hydrophilic and hygroscopic nature of starch, and comes along with poor mechanical properties when compared to synthetic polymers. Chemical crosslinking is a common approach to overcome these challenges, and to improve the water stability and mechanical properties of starch products. Since commonly used crosslinking agents including epichlorohydrin and glutaraldehyde, may pose a risk to human health and environment due to their toxicity, eco-friendly crosslinking agents, such as citric acid, have received more attention in recent years.
The addition of citric acid to renewable barrier materials has been the subject of recent publications that show, citric acid can enhance both the barrier and mechanical properties, which is attributed to its ability to crosslink polysaccharide materials, such as starch. Crosslinking describes the formation of bonds between polymer chains, creating a three-dimensional network structure. Based on its chemical structure, citric acid is able to crosslink starch chains, leading to improved barrier and mechanical properties of starch-based barrier coatings.
Several studies have shown that crosslinking starch with citric acid significantly decreases the moisture absorption, prevents swelling, and improves the resistance to dissolution of the starch films in liquid water or at high relative humidity. This is attributed to intermolecular crosslinking between different starch molecules, leading to an increased average molecular weight of the starch. Crosslinking of starch improves the barrier properties of starch films by minimising the molecular movement resulting in a reduced vapour permeability.
Another effect of citric acid crosslinking is the improvement of the mechanical properties of starch films. Citric acid crosslinked starch films have significantly higher tensile strength compared to non-crosslinked starch films. Crosslinking with citric acid interconnects the starch molecules in the film, which increases the molecular weight of the starch and provides better intermolecular interactions between molecules, finally leading to an increased tensile strength.
As producer of bio-based ingredients, Jungbunzlauer aims to manufacture ingredients that help our customers to improve products or processes to be healthier and safer for humans and the environment. Jungbunzlauer’s citric acid is manufactured by fermentation of renewable materials, such as carbohydrates from corn; it naturally appears as an intermediate in human, animal and plant metabolisms and is non-toxic. Once used citric acid can re-enter the cycle of nature since it is all fully biodegradable. Citric acid is listed as a generally permitted food additive (E 330) in Europe, and is affirmed as generally recognised as safe (GRAS) by the US Food and Drug Administration.
Citric acid is able to crosslink starch, resulting in reduced water sensitivity as well as in improved barrier and mechanical properties of starch films. Due to its non-hazardous handling and favourable ecological profile combined with its ability to cross-link polysaccharide materials, Jungbunzlauer’s citric acid is a safe and eco-friendly candidate to replace synthetic crosslinking agents in starch-based barrier coatings.