## 3 Edible Coatings
### Active Edible Packaging
#### 3.4.3 Phenolic Acids as Active Compounds
Organic phenolic acids, characterised by the presence of phenolic rings, arouse great interest among researchers due to their antioxidant and antibacterial properties. It has been proven that adding ferulic acid or gallic acid makes it possible to achieve a beneficial effect on the water vapour permeability of edible films and improve mechanical strength, antioxidant, and antibacterial activity. When food is exposed to light, oxidative degradation accelerates, which leads to loss of nutrients, discolouration, and a change in taste. [PERSON] et al. [142] observed that chitosan films with the addition of phenolic acid used for coating shrimps had higher opacity than films without the addition of acid, i.e., they were characterised by a greater ability to absorb light. This is probably due to aromatic groups found in phenolic acids. In addition, the addition of phenolic acid caused the obtained films to have a lower moisture content, and the lowest value was obtained in the case of chitosan films with the addition of gallic acid, which could be caused by the presence of three phenolic hydroxyl groups present in this acid. Low moisture content affecting the solubility of the film in water is an important factor determining the biodegradability of the films used as food packaging materials. Current research development is also focused on combining ferulic acid and edible coatings to improve food performance. The distinguishing feature of ferulic acid is its strong antioxidant and antimicrobial effect, increasing the durability and safety of food. The study proved the positive effect of the combination of sodium alginate and ferulic acid in apple coating on reducing the _Listeria monocytogenes_ population after 7 days of storage. In addition, no effect was observed on the fungal species _Saccharomyces cerevisiae_[143]. Caffeic acid, synthetised by plants as a secondary metabolite, as well as its natural derivatives (mainly esters), are referred to as strong antioxidants because they can protect cells from oxidative stress even in low concentrations. Oxidative changes can occur in both fats and proteins, affecting the quality of many food products. In food production, synthetic antioxidants are increasingly replaced by natural compounds, and phenolic acids are increasingly attracting interest. Caffeic acid is found in many foods, such as legumes, tomatoes, nuts, cereal grains, sunflower seeds, and various types of fruit [144]. Caffeic acid was used to produce active composite films consisting of N, O-carboxymethyl chitosan and methyl cellulose in various concentrations. During the agar test, the effect of the presence of caffeic acid on the occurrence of pathogenic bacteria was examined. It was proved that the inclusion of acid in composite films resulted in a 6-fold increase in the effectiveness of reducing the development of _Escherichia coli_ strains, from 16.32% for the control film to 97.73% for the film containing caffeic acid. In addition, antimicrobial efficacy was also doubled when _Staphylococcus aureus_ growth was reduced from 49.41 to 94.59%. These results show that the addition of caffeic acid to composite films has a positive effect on reducing the occurrence of food-borne pathogens [126]. The presence of caffeic and ferulic acids enables the formation of cross-linking bonds. Moreover, ferulic acid can react with some amino acids present in proteins, e.g., tyrosine, lysine, and cysteine, resulting in the formation of a cross-link. The research shows that caffeic acid is more effective than ferulic acid and, therefore, can increase the safety of biodegradable packaging by improving its barrier and physicochemical properties [145]. [PERSON] et al. [146] studied the properties of chitosan films by adding various concentrations of gallic acid. The authors observed a beneficial antimicrobial effect against _Escherichia coli_, _Salmonella typhimurium_, _Listeria imocua_, and _Bacillus subtilis_.