Dr. Maribel Ovando-Martinez is an associate professor in the Biotechnology of Natural Resources Laboratory at the University of Sonora. Marcos Leon-Bejarano obtained his MSc degree at the University of Sonora. Dr. Senay Simsek is a professor at North Dakota State University.
Starch represents a great polymeric matrix because it is biocompatible and biodegradable. However, starch application as a food packing material is limited mainly by their poor water-barrier properties. Chemical modification with octenyl succinate anhydride (OSA) is an option to solve this problem, as well as to increase the encapsulation efficiency of antioxidants. The nut industry generates a large amount of byproducts because of the high demand and consumption of nuts. Byproducts such as shells, skins, leaves, husks, and others are a great source of antioxidants, such as phenolic compounds, which are characterized by their antioxidant activity. Therefore, the addition of antioxidants from nut-byproducts to modified starch-based films could improve their properties and made them suitable as food packing materials.
Preparation of Materials to Elaborate the Modified Starch-based Films
Native potato starch was modified to obtain octenyl succinate starch. Pecan nutshell and hazelnut skin were ground, mixed with aqueous ethanol solution (60%) to extract the antioxidants, and finally freeze-dried to determine the phenols and flavonoids content. Once all materials were ready, the film-forming solution was prepared by mixing octenyl succinate starch, glycerol, and water at 95 ºC. The solution was cooled down to 60 ºC and nut-byproduct antioxidants dissolved in 60% ethanol solution (0, 25, 50, 75, 100 mg) were added to the forming-solution and mixed. The film-forming solution was cast in square plastic plates and dried at 60 ºC. Dried films were removed to measure thickness, water-barrier properties, color, mechanical characteristics, surface topography, and biodegradability.
Octenyl Succinate Starch-based Films Properties
The film thickness increased slightly, and water vapor permeability did not vary. However, when nut-byproduct antioxidants concentration increased, its solubility in water decreased due to an increase in the hydrophobicity of the surface of the film. The decrease in water solubility indicates an improvement in the water-barrier property. Starch-based films with hazelnut skin were darker in color than pecan nutshell films because the phenolics and flavonoids extracted from pecan shells and hazelnut skin have a reddish-brown color. Although films showed different colors and transparency, they absorbed the UV light. The addition of nut-byproduct antioxidants decreased the rigidity of the films because the antioxidants acted as plasticizing agents when interacting with the modified starch. Also, puncture and tear resistance slightly increased, meaning films have more plastic behavior and mechanical resistance than films with octenyl succinate starch. The increase of antioxidants in the films decreased its biodegradability in soil (Figure 1A-B). Albeit biodegradability values are around 70%, films are considered biodegradable materials. The slight increase in roughness is related to antioxidants interaction with the polymeric matrix, and also with the water-barrier and biodegradability changes.
Figure 1. Starch based-films with nut-byproducts. A) Surface and roughness of films. B) Biodegradability of films in soil.
In conclusion, such biodegradability values and the improvement of film properties indicated these films could be an option for the replacement of petroleum-based plastics and add value to nut-byproducts