Investigation into the shelf life extending of chicken eggs using UV-C irradiated starch-oleic acid coating solutions

The extension of the shelf life of chicken egg by the coating during the storage is the main purpose of this study. Chicken egg is one of the most important foods because of the high protein, essential amino acids and fat-soluble vitamins content. The freshness of chicken egg maintain by water and carbon dioxide. They emit to surrounding atmosphere through shell-pores during the shelf life of the chicken egg. It leads to reducing the quality of chicken egg. The shelf life of chicken egg can be extended by preventing the emission of the gases using biopolymer coating. Starch is one of the best options to coating chicken egg. Starch is biobased, easy access and low price material. The presence of hydroxyl groups in the starch chains create hydrogen bonds between starch and water. As well as the considerable free space between the starch chains facilitate the movement of the water molecules through this space. The high permeability of starch to water vapor is an obstacle to achieving this aim. Nevertheless, virgin starch can be decreased water vapor permeability by increasing water transition distance. Furthermore, water vapor barrier properties might be intensified by hydrophobic materials i.e. lipids, into the formulation of solution coating. Oleic acid (OA) is a fatty acid that can reduce water vapor transition rate of the hydrophilic biobased materials. OA is a liquid fatty acid at room temperature. Hence, it is miscible with biobased material easier than saturated fatty acid without further heating treatment. The hydrophilicity and the permeability of starch to water vapor can be resolved using mixing with fatty acids. On the other hand UV irradiation as an inexpensive, easy to operate and environmental-friendly (green) technology, to modify the biopolymers, has received increasing attention during recent years. However, other ionizing beams i.e. Gamma can improve some packaging properties of biopolymers. But there is a serious concern about its nuclear wastes. UV radiation induces the production of free radicals in aquatic solutions. The free radicals attack to starch chains. As a result, the injured chains possess a tremendous potential to produce cross-links. Accordingly, it seems that the exposure of the aqueous film solution to UV ray can be used as a green process to modify the packaging properties of biopolymers.

An aqueous dispersions of starch (5 wt%) was prepared and heated until its gelatinization (85°C for 90 min). Glycerol as plasticizer (40 wt% of dry base) was also added. Then the solution was stirred for 15 min. Oleic acid (OA) (1 wt% of dry base) mixed with Tween 80 as emulsifier (10 wt% of the OA). This solution was mixed and heated )50 ˚C for 10 min (Then, 10 ml of distilled water was added gradually to the solution, and homogenized by ultrasonic homogenizer (Dr. Hielscher, Teltow, Germany) for 7 min. The OA based emulsion was added to starch solution gradually and mixed for 10 min. after that the solution was homogenized again by ultrasonic homogenizer for 7 min. The solution mounted under three UV-C lamps (8w, Phillips, Holland) at a distance of 5 cm. The solution was stirred during this time with using a magnetic stirrer. After 0, 30, 60, and 90 minutes, the UV-C exposed solutions were applied to chicken egg coating.

Weight loss of eggs during storage is mainly caused by evaporation of water from the albumen through the porous shells. The weight loss increased during 7 weeks storage. The greatest water loss (11.83%) was observed in control (uncoated eggs). Coating the shell egg led to minimum weight loss. The results of Haugh index (HU) and Yolk index (YI) showed, however in all treatments HU and YI decreased. But this reduction was less than the control. Starch-oleic acid, and modified starch-oleic acid for 90 min by UV-C showed the most HU and YI. The pH of albumen is an important factor in the quality of chicken egg. Moisture and carbon dioxide of the white evaporate through the pores and replace by air. Decreasing the CO2 content lead to increasing pH of the eggs. The properties of the albumen were affected by this phenomenon. The pH of albumen was increased in control, while in the coated eggs it was remained almost constant. The albumen of the coated eggs by starch-oleic acid and modified starch-oleic acid for 90 min by UV-C have the least pH.

Regard to the results starch-oleic acid and modified starch-oleic acid for 90 min by UV-C were the best options to coating the egg shell and increasing the shelf life of the egg. It seems these coating can extend the shelf life of the egg more than two times.