Cooling Eggs May Reduce Food-Borne Disease

by 5m Editor
23 June 2011, at 9:51am

US - Dr Kevin Keener's research at Purdue University has shown that cooling eggs after they are laid may increase the natural defences those eggs have against bacteria such as Salmonella.

Once eggs are laid, their natural resistance to pathogens begins to wear down but a Purdue University scientist believes he knows how to rearm those defences.

Kevin Keener, an associate professor of food science at Purdue University, created a process for rapidly cooling eggs that is designed to inhibit the growth of bacteria such as salmonella. The same cooling process would saturate the inside of an egg with carbon dioxide and alter pH levels, which he has found are connected to the activity of an enzyme called lysozyme, which defends egg whites from bacteria.

"This enzyme activity is directly related to the carbon dioxide and pH levels," said Dr Keener, whose results were published in the journal Poultry Science. "An increase in lysozyme would lead to increased safety in eggs."

Freshly laid eggs are saturated with carbon dioxide and have pH levels of about 7. Over time, the pH level rises to 9 and carbon dioxide escapes, Dr Keener said. As that happens, lysozyme becomes less active.

Dr Keener saturated purified egg white lysozymes with carbon dioxide and tested different pH levels. He found that at both high and low pH levels, the addition of carbon dioxide would increase lysozyme activity by as much as 50 per cent.

The cooling process would create the same conditions, he said.

Dr Keener continued: "When we cool the eggs, carbon dioxide is sucked inside the shell. We're able to resaturate the white of the egg with carbon dioxide, returning it to that original condition when the chicken laid it."

The additional lysozyme activity would give eggs more time to self-eliminate harmful bacteria.

Dr Keener's cooling technology uses carbon dioxide 'snow' to rapidly lower the eggs' temperature. Eggs are placed in a cooling chamber and carbon dioxide gas at about minus 110°F is generated. The cold gas is circulated around the eggs and forms a thin layer of ice inside the eggshell. After treatment, the ice layer melts and quickly lowers an egg's internal temperature to below 45°F. The eggshell does not crack during this process because it can resist expansion from a thin ice layer.

Dr Keener said Food and Drug Administration studies show that if eggs were cooled and stored at 45ªF or less within 12 hours of laying, there would be an estimated 100,000 fewer salmonella illnesses from eggs in the United States each year.

Dr Keener will continue to study the molecular changes that occur with egg cooling.

Abstract of Dr Keener's paper

The published paper in Poultry Science is by P. Banerjee, K.M. Keener and V.D. Lukito and is entitled 'Influence of Carbon Dioxide on the Activity of Chicken Egg White Lysozyme'.

Rapid cooling of shell eggs by using liquid carbon dioxide has shown increased bactericidal effects along with saturation of the egg albumen with carbon dioxide. Lysozyme is a bactericidal enzyme present in chicken eggs, and it lyses gram-positive bacteria. Newly laid chicken eggs have an initial pH of 7.6 to 8.5 and are saturated with carbon dioxide. During storage, the pH gradually increases to 9.7, accompanied by a loss of carbon dioxide.

It is hypothesised that the lysozyme activity is influenced by either carbon dioxide concentration or pH changes resulting from carbon dioxide loss.

The objective of this study was to determine the lytic activity of purified lysozyme and chicken egg white (unpurified lysozyme) under varying conditions of temperature, pH and carbon dioxide gas concentration.

Lytic activity was determined by a standard microbial assay using lyophilised Micrococcus lysodeikticus.

A 2 × 4 × 2 × 2 × 3 factorial design consisting of two temperatures (5 and 22ªC), four pH levels (4.5, 6.5, 8.0, and 9.5), two treatments (with and without carbon dioxide), two types of lysozyme (purified and unpurified egg white), and three replicates was used.

The highest lytic activity was found at pH 6.5 and 22°C. At pH 4.5 and 8.0, the addition of carbon dioxide increased lytic activity by more than 50 per cent at both temperatures. At pH 6.5, lytic activity was maintained with carbon dioxide addition at both temperatures. At pH 9.5, lytic activity without carbon dioxide addition was high; however, adding carbon dioxide reduced lytic activity to zero.

In conclusion, both pH and carbon dioxide treatment influence lysozyme activity.