By coating paper trays with a mixture of silver nanoparticles and binders, scientists at the Chennai-based Tamil Nadu Veterinary and Animal Sciences University (TANUVAS) have been able to increase the shelf life of eggs by at least 14 days. This was possible by using silver nanoparticle-coated paper trays that killed both Gram-positive and Gram-negative bacteria found on egg shells. The results were published on June 7 in the journal Bulletin of Materials Science.
Egg shells get contaminated with bacteria when they come in contact with bird droppings at poultry farms. Also, the unhygienic condition of trays used for storing eggs at retail outlets leads to high incidence of bacterial contamination. E. coli, Staphylococcus aureus, Streptococcus species and Salmonella species are found on egg shells.
Silver nanoparticles have proven anti-bacterial properties.“As much as half the number of eggs gets spoiled due to microbes present on egg shells. These microbes enter the egg through the pores present in the shell. In India, E. coli is the most commonly found bacteria on egg shells,” says Dr. Kaliyaperumal Viswanathan, the first author of the paper and a research scientist at TANUVAS. Since bacterial contamination of eggs causes disease outbreaks, reducing or preventing contamination would lead to reduced disease outbreaks and possibly increased shelf life.
Since the antibacterial property of silver nanoparticles is well established, the team led by Dr. Dhinakar G. Raj, Project Director of Translational Research Platform for Veterinary Biologicals, TANUVAS, mixed silver nanoparticles with chitosan (an antibacterial drug) and a binder to coat the paper trays. About 21 mg of silver was used for coating 100 grams of egg trays.
Fresh eggs collected from farms were used for the studies. The eggs were completely covered by sandwiching them between two silver nanoparticle-coated paper trays.
While the physical property such as the shape of the egg remained the same in eggs kept in silver nanoparticle-coated trays and control trays, the difference in specific gravity was marked. While the specific gravity of eggs kept in control trays reduced by 38 per cent at the end of 27 days, the reduction was just 8 per cent in the case of eggs stored in coated trays.
Similarly, the air cell depth, which increases in size with the age of egg, was also studied. At the end of 30 days, the air cell depth increased up to 10.6 mm in control samples, while the increase was only 7 mm in the case of eggs kept in silver nanoparticle-coated trays.
The albumin index (viscosity and clarity) which reflects the quality of eggs was measured. At the end of 12 days, the albumin index value reduced up to 57 per cent in the case of control samples, while it was only 12 per cent in the case of eggs kept in coated trays.
Finally, the yolk index value (an indirect measure of internal egg quality) reduced by 73 per cent at the end of 18 days in control samples while it reduced by 37 per cent in test samples. Even at the end of 27 days, the yolk index value reduced by only 40 per cent in test samples.
“We tested the eggs for silver nanoparticles but did not find any inside the eggs,” says Dr. Raj. “Silver nanoparticle-coated paper trays remain effective for nine months.” It is currently not possible to coat silver nanoparticles on plastic trays. It would cost Rs.2 for coating a paper tray for storing six eggs if the tray is used only once. But the cost will come down drastically if the trays are reused, he says.
Using the coated paper trays for storing eggs is “highly suitable for large-scale commercial outlets”. Dr. Raj says the rural households too would benefit as they get only a few eggs a day. “So they can use the coated trays to accumulate eggs till they have more eggs to sell,” he says.