## 3 Edible Coatings
### Active Edible Packaging
#### 3.4.3 Phenolic Acids as Active Compounds  
Phenolic compounds are powerful antioxidants that can \"scavenge\" and recombine radicals and chelate metal ions. They act as electron transfers, resulting in stable products. For this reason, plant extracts containing phenolic compounds are increasingly used in developing active food packaging, and recent studies have shown the effectiveness of incorporating such extracts into products. Phenolic acids are found in a variety of plant sources such as fruits, vegetables, spices, cereals, and beverages. A promising source of phenolic compounds is also extracted from grain milling, which are by-products, e.g., bran and husks. Essential oils, mainly used as flavouring agents, include phenolic and terpenoid compounds with powerful antioxidant properties. The research shows that the films containing tannic acid had a more rigid structure and were more durable, and the addition of gallic acid affects, among others, the flexibility of the film because it acts as a plasticiser [137]. Phenolic acids, belonging to phenols, contain hydroxyl and carboxyl groups in their structure and are distinguished by their bioactive properties. Other proper ties of phenolic acids are shown in Figure 2. One of the classes of secondary metabolites includes phenolic compounds that are involved in the basic biochemical processes of the plants from which they are obtained, affecting the regulation of growth, development, and reproduction. Ferulic acid, a derivative of cinnamic acid, is obtained mainly from the leaves, seeds, and bark of coniferous trees, but another source confirming the high content of phenolic acids is the outer layer of cereal grains. It was observed that the differences in the antioxidant activity of phenolic acids, such as ferulic acid, caffe acid, or p-coumaric acid, result from the number of hydroxyl groups present in the molecule. A higher level of antioxidant activity occurs when they are esterified [138]. A frequent phenomenon is the division of phenolic acids, classifying them as two subgroups: an acid based on the C\({}_{6}\)-C\({}_{1}\) structure, which includes acids such as pro catechin, gallic, and syringe, and also as aromatic compounds with a 3-carbon side chain (C\({}_{6}\)-C\({}_{3}\)), which includes coumaric, caffe, ferulic, and sinapic acids [139]. [PERSON] et al. [140] developed nanocoatings based on candeillia wax and tarbush (_Florensia cernua_) extract to delay the ripening of \"Fuji\" apples during 56 days of storage. After developing appropriate values of wettability of the nano-coating on the surface of apples and covering the fruit with them, the physicochemical changes, water vapour permeability, and microbiological stability of apples were evaluated. It was observed that the coating reduced the loss of firmness and reduced the occurrence of physicochemical and microbiological changes (decreased growth rate of fungi and yeast) compared to apples not subjected to coating. Phenolic acids can be extracted from fresh, frozen, or dried plant biomass. However, plant biomass is usually air-dried or lyophilised before phenolic acid extraction and then ground into a uniform powder. The extraction method from dry biomass is more advantageous because enzymes found in fresh cells can cause degradation of phenolic acids. Scientific reports indicate that the lyophilisation process allows for the preservation of a higher level of phenolic content in plant biomass compared to air-drying. The extraction process is influenced by many factors, including the chemical properties of the acid, polarity, concentration levels in plant tissue and the desired level of purity of the extract. The first step of the process is the crude extraction of phenolic compounds using 20-50%  
Figure 2: Properties of phenolic acids. ethanol, methanol, acetone, or a combination of these solvents. The next important step is alkaline hydrolysis using sodium hydroxide (1-4 M), the main purpose of which is to release insoluble phenolic acids into their soluble forms [141].