Introduction: One of the methods for increasing shelf life of food is the use of edible coatings, since films and coatings have an important role in the enhance of shelf life and food product’s marketing. Various edible films are nowadays used for food packaging. Using of these biodegradable compounds is a new method of replacing polymeric materials and of increasing shelf life of food. Film and coating can control the diffusion of water, oxygen and carbon dioxide. Edible coatings also have the ability to enhance color, acid, sugar and taste during storage. Starch as a natural polymer be a lot in the nature and has a low cost and due to its structure containing amylose and amylopectin it is a suitable option for its use as film and edible coating because starch is barrier for oxygen, semi-permeable to CO2, and has good mechanical properties, therefore suitable for packaging films and edible coatings. However, due to the hydrophilic properties of starch, the starch based films are permeable to moisture and water vapor. One of the ways to reduce water vapor permeability for films and edible coatings is to use hydrophobic compounds such as oleic acid that formed in two layers. There is starch in the lower part of layer. Hydrophobic compound place in the upper layer and it is the barrier for water vapor. The main purpose of this research was to produce edible films based on starch and oleic acid and to investigate their characteristics (thickness, solubility, water vapor permeability, and tensile strength). One of the suitable treatments was then selected and tested at three concentrations as a film coating for greengage, and cases of pH drop and loss of weight, color, and hardness of the samples were recorded for four weeks.
Material and methods: First, the film of starch and oleic acid were prepared and then the edible film characteristics such as thickness, solubility, water vapor permeability and tensile strength were measured. After evaluating the film parameters, the optimal amount of oleic acid was selected, then the starch and oleic acid solution of 8, 10, 12 g / l were prepared based on film formulation including starch, glycerol, oleic acid and tween 80. Then the characteristics of Greengage such as weight loss, pH, color and rigidity were measured.
Result & discussion: The addition of oleic acid resulted increases in thickness of the edible films because of the increase in the material during solvent drying and variation in the relative humidity of the environment during drying of the material.
Solubility: By increasing the amount of oleic acid, the amount of solubility decreased, which could be due to the change in the polarity of the film compounds, which increases the hydrophobicity of the film by increasing the oleic acid that is non-polar, and keep the polar parts away from water. Oleic acid, due to good hydrophilic properties, significantly reduces the vapor flow rate. According to the results, the lowest water vapor permeability is for 10% of oleic acid in the film. According to the results, the amount of tensile strength decreased with increasing oleic acid content. On the other hand, elongation has increased with the increase of oleic acid, and this again decreased in the film with 15% oleic acid. Oleic acid is a short-chain fatty acid, so it can be located between the amylose and amylopectin strands as a plasticizer and because of this, the amount of hydrogen cross link in the film has decreased and its resistance has decreased. Subsequently tensile stress of film increased due to the good and proper distribution of oleic acid in the film. But at the level of 15%, this amount of oleic acid was high, increasing the gap between the amylose and amylopectin strands, followed by weakened films, and both the tensile strength and long elongation were reduced. Regarding the parameters obtained in the film production (mechanical strength and transparency and water vapor permeability) the film formulation with 10% oleic acid was selected and the solutions with total solid of 8%, 10% and 12% for Greengage coating are selected. It is observed that weight loss for coated samples is less than the control sample. These results confirm the starch barrier against the passage of oxygen and carbon dioxide. In all treatments pH has increased over time. The increase in pH in the control sample is than the coated samples because of different respiration rates. Coated sample is a suitable gass barrier followed by reduce the amount of respiration rate, resulting in a slower rate of pH change. In the third and fourth weeks, changes were significant, and in the control samples, the amount of light was higher and the green’s index in them decreased. Color changes in fruits can take place due to loss of moisture and chemical and enzymes reactions. Proteolytic enzymes such as polyphenol oxidase can destroy phenolic compounds that affect color. According to the results, the highest weight loss and moisture loss associated with the control sample showed the highest rigidity over time, which significantly reduced these changes with coating the sample. It seems that the concentration of 10 and 12% of oxidized starch and oleic acid emulsion can be used for proper coating with the least loss of weight and color. Due to the fact that the oxidized starch is very transparent and has a low viscosity, its appearance is not negatively affected and can be used in food coatings.