Effect of a developed bilayer film of acorn starch and zein electrospun enriched with cinnamon oil on microbiological, chemical, and sensory characteristics of rainbow trout fillet

Fresh fish is a delicate treasure. It offers a rich source of protein and healthy fats, but its high water content and weak structure make it prone to rapid decay. Once a fish is caught, a silent battle begins between the food and the microscopic world. Bacteria, yeasts, and molds, along with natural enzymes inside the fish, start to break down proteins and fats. This process creates unpleasant smells, changes the texture from firm to slimy, and alters the color from a bright, ruddy shine to a dull brown. For consumers and the food industry, the challenge is simple yet difficult: how to keep this perishable food fresh for as long as possible without relying on the heavy, non-biodegradable plastics that dominate modern packaging.

Scientists have been turning to nature for answers, looking at biodegradable films made from plant and animal proteins. One promising approach involves layering different materials to create a shield that not only blocks air and moisture but also actively fights spoilage. In this context, researchers in Iran explored a specific combination of ingredients to protect rainbow trout, a popular and widely farmed fish. They focused on two main components: starch extracted from acorns, which are abundant in their region, and zein, a protein found in corn that forms strong, flexible fibers when spun into a fine mesh. To this mixture, they added cinnamon essential oil, a natural substance known for its ability to kill bacteria and slow down oxidation. The goal was to see if wrapping the fish in a double-layered film made of these materials could extend its shelf life while keeping it safe and tasty.

The researchers set up a controlled experiment using fresh rainbow trout fillets. They divided the fish into six groups to test different packaging methods. One group was left unwrapped as a control. The others were wrapped in films made of just acorn starch, just electrospun zein fibers, zein fibers with cinnamon oil, a double layer of starch and zein, and finally, the double layer of starch and zein infused with cinnamon oil. All the fish were stored in a refrigerator at a cool four degrees Celsius, and the team checked their condition every three days for twelve days. They measured everything from the number of bacteria growing on the surface to the chemical changes inside the meat, and even asked a panel of people to taste and smell the fish to judge its quality.

The results showed a clear difference between the fish wrapped in the advanced film and the rest. The unwrapped control group began to spoil quickly. By the sixth day, the number of cold-loving bacteria on the control fish had risen to a level considered unsafe for consumption. In contrast, the fish wrapped in the double-layer film with cinnamon oil stayed fresh much longer. This specific treatment delayed the bacteria reaching that unsafe limit until the twelfth day. The film was particularly effective at holding back the growth of harmful bacteria, yeasts, and molds. While the control fish saw a massive increase in these microbes, the fish in the cinnamon-infused film had significantly lower counts of all three types of spoilage organisms by the end of the study.

Chemical tests confirmed what the bacteria counts suggested. As fish spoils, its pH level rises, and it produces volatile nitrogen compounds that create the characteristic "fishy" odor of decay. The control fish showed a sharp rise in these indicators, with the nitrogen levels more than doubling by the end of the storage period. The fish wrapped in the cinnamon film, however, maintained a much lower pH and produced far fewer of these foul-smelling compounds. The film acted as a barrier that slowed down the chemical breakdown of the proteins and fats inside the fish. This chemical stability translated directly to the sensory experience. When the panelists tasted and smelled the fish, they rated the control group as unacceptable after just six days, noting slimy textures and bad odors. The fish protected by the cinnamon film remained acceptable for nine days, retaining a better color, firmer texture, and a more pleasant smell.

The study concluded that the combination of acorn starch and electrospun zein, when loaded with cinnamon essential oil, created a highly effective active packaging material. This bilayer film did more than just cover the fish; it actively suppressed the growth of spoilage microbes and slowed the chemical reactions that lead to decay. By extending the safe storage time by about three days compared to the control, the film offered a sustainable alternative to plastic. The researchers noted that this natural, biodegradable solution could potentially be used for other perishable foods, though further testing would be needed to confirm its effectiveness on meats and cheeses. The work demonstrated that by layering simple, natural ingredients and harnessing the power of a common spice, it is possible to significantly improve the preservation of fresh fish without relying on synthetic chemicals or non-recyclable plastics.