Pasteurization of Shell Eggs

U.S. Dept. of Agriculture risk assessment studies report that one in 20,000 fresh eggs may contain low numbers of Salmonella enteritidis, which can cause foodborne illness unless the eggs and foods that contain them are cooked sufficiently to kill the bacteria. The Food and Drug Administration requires that eggs which have not been treated to destroy Salmonella be refrigerated at 45°F or below in retail stores and bear a safe handling statement on the label telling the consumer to thoroughly cook the eggs and foods containing them.

FDA says that foods prepared with raw shell eggs that are not broken for immediate preparation and service should be cooked to heat all parts of the food to 155°F for 15 seconds. The agency recommends that pasteurized eggs be substituted for raw shell eggs in preparing such foods as Caesar salad, hollandaise or Béarnaise sauce, mayonnaise, eggnog, ice cream, and egg-fortified beverages that are not thoroughly cooked.

Pasteurized eggs are available on the market in two forms—liquid eggs and pasteurized shell eggs. Liquid eggs are prepared by breaking the eggs and pasteurizing the contents at ultrahigh-temperatures and aseptically packaging in gabletop cartons.

The first pasteurized shell eggs for the retail and foodservice markets were introduced in May 1996 by Michael Foods Egg Products Co., Minneapolis, Minn. The retail products are marketed under the brand name Crystal Farms in the upper Midwest, primarily the Minneapolis area, and the pasteurized eggs in bulk packs (15 dozen eggs) for foodservice use under the M.G. Waldbaum brand name. The pasteurized eggs are produced at the company’s facility in Wakefield, Neb., using a hot-water-water immersion process that controls the temperature and time to give a 5-log reduction in Salmonella while maintaining the desirable characteristics of the egg. The company is licensing the hot-water system from a patent by the University of Missouri.

Hershell Ball (phone 800-308-6269), Michael Foods’ Vice President, Research & Development, said that they are also looking at using high-moisture hot air as the heat-transfer medium and have applied for a patent for this method. Studies have shown that the lethality is the same as in hot-water pasteurization. The advantages of the hot-air system, he said, are that it avoids handling large amounts of water and the associated problems of treatment and cleanup, and it provides a little more control, so that the integrity of the process is better. The company has done a lot of testing with pilot-scale equipment, he said, using outside engineering resources to confirm the company’s model of the process.

Pasteurized Eggs Corp., Meredith, N.H., has been marketing its eggs under the brand name Davidson’s Pasteurized Eggs™ since 1998. They are sold in the retail market on the East Coast and in the Midwest and nationally for the foodservice market. A processing facility was recently opened in the Chicago area, and more facilities are planned. According to a company representative, the company is fully commercialized with its technology and able to ship the product on a national basis.

According to Mike Wagner (phone 800-410-7619), Pasteurized Eggs Corp.’s Director of Plant Operations, the company uses a continuous hot-water-immersion process patented in 1998 by John Davidson (U.S. patent 5,843,505) to fully pasteurize the shell eggs. The eggs, on flats, are conveyed through a hot-water bath until the center of the yolk reaches the desired temperature for the desired length of time. This consistently achieves a 5-log reduction of Salmonella while still maintaining the functionality (e.g., whippability) of the albumen.

As the eggs exit the tank, they are unloaded, graded, destacked, and sanitized. Each egg is then sealed to prevent subsequent contamination, such as might occur if an egg breaks, then dried, stamped with a “D” (for Davidson’s), and packed into 6-, 12-, or 18-egg cartons for retail sale or 2.5-dozen pulp flats for foodservice use.

Wagner said that the process gives a “sell-by” shelf life of up to 6 months at 41ºF. He said that the company has a strict HACCP plan and goes to great pains to ensure that the packing and loading rooms and equipment are thoroughly cleaned and sanitized.

The company has experimented with several other methods for pasteurizing the eggs, he added, but the hot-water method they use is the most natural and economic way they came up with.

Although hot water immersion is the only method currently being used commercially for pasteurizing shell eggs, several other methods, besides the hot-air method mentioned above, are being studied and experimented with.

• Ionizing Radiation. On July 20, 2000, FDA approved use of up to 3 kGy of ionizing radiation on eggs in the shell to reduce the level of Salmonella in the egg. There are two types of ionizing radiation being considered, electron-beam and gamma.

Charles Beard (phone 770-493-9401), Vice President, Research & Technical Programs, U.S. Poultry & Egg Association, Tucker, Ga., said that research conducted 10 years ago has demonstrated that irradiation of eggs is effective but that the white becomes slightly opaque and does not whip as well. He said that e-beam irradiation has advantages over gamma irradiation, since it does not require the same extent of concrete shielding and the equipment can be inserted into an egg processing facility quite readily. The disadvantage is that electron beams don’t penetrate as deeply as gamma rays, which can be used to irradiate pallets of cases of eggs.

Irradiation appears to be an alternative to hot water pasteurization, Beard said, so there needs to be a consideration of how much benefit there is to gain. For flocks that are known to be shedding S. enteritidis, irradiation can be an alternative to sending all the eggs to a breaking and pasteurization plant for production of liquid eggs, since eggs destined for breaking are sold for less. The improved economic recovery might justify and compensate for the cost of irradiation.

Gary Waters (phone 406-295-5094), Veterinary Consultant, Sanders County, Mont., said that pasteurization of shell eggs costs more than 20¢/doz, which is prohibitive, and he is especially interested in seeing e-beam irradiation applied to eggs. The technology is very rapid, he said, taking only a fraction of second to sterilize a dozen eggs, and it would cost only 2¢/dozen. However, the eggs must be transported to the e-beam facility, unless the egg company decides to buy a very expensive machine and put it into its plant.

Since the electrons disrupt the DNA chain in anything living, the bacteria are destroyed, Waters said. The eggs would be sterilized and have less need for refrigeration to maintain quality. However, he added, the process might affect the freshness of the egg and its whippability.

Dennis G. Olson (phone 402-390-0141), Vice President for Product Applications, SureBeam Corp., Omaha, Neb., said that the company has done some testing on use of e-beam irradiation for pasteurization of shell eggs and found it effective, but no one is using it commercially for that purpose. One drawback they have found, he said, is that some of the egg white viscosity can be lost. Also, if the dose is too high it can even weaken the yolk membrane. For industrial use, such as in bakery products, there would be no problem, since the functional properties of the egg yolk and white are not affected, but there might be some physical changes that retail customers might not like when they, for example, fry an egg.

Ball said that Michael Foods has considered irradiation but he questions whether the FDA-approved absorbed dose is equivalent to the 5-log reduction obtained with the hot-water system. Also, he believes irradiation causes thinning of egg whites, so that instead of looking like very fresh AA quality eggs, they look like B quality or older. He also found objectionable flavor and odors. He said that Michael Foods patented a procedure that will prevent a decline in the quality of eggs treated by irradiation, but the eggs still have objectionable flavors and aromas.

• Microwaves. Gregory J. Fleischman (phone 708-728-4122), Research Chemical Engineer, Food and Drug Administration, National Center for Food Safety and Technology, Summit-Argo, Ill., is studying microwave irradiation of shell eggs to obtain background about the process so that FDA can critically review proposed new processes.

His three-year project, which began in October 2000, consists of a mathematical part and a microbiogical part. The math part looks at microwave-associated phenomena, such as heating, dielectric characteristics, and power distribution within the egg. There’s already a lot in the literature about growth of Salmonella in eggs, he said, but he’s doing his own work to determine where it is growing. His studies have indicated that Salmonella need only indirect contact with the yolk through the yolk membrane to grow well in eggs, although the membrane appears to be a barrier to movement of the bacteria into the yolk itself.

A microwave process would be perfect for eggs, he said. In thermal-conduction-based processes, the eggs are immersed in hot water or hot air, and the outer portion, the albumen, may be overprocessed during the time it takes to heat the interior portion, the yolk, potentially adversely affecting the albumen. The quality of the egg depends a lot on the albumen. In contrast, microwaves penetrate both the white and the yolk quickly.

He is using simple geometries—slabs of yolk vs albumen—in his mathematical studies to determine the effect of dielectric properties and egg curvature on the power distribution. Results have indicated that although albumen absorbs microwaves better than yolk, the focussing effect caused by egg curvature offsets this, leading to a suitable power distribution. Furthermore, he believes any cold spots could be easily overcome. If ever microwave processing needed a specific type of product it could do better than any other process, he said, it’s this. Microwaves are ideally suited for pasteurization of shell eggs.

Ball at Michael Foods said that microwave technology is developing rapidly but questioned whether it is able to control the microwaves to achieve uniform heating. A disadvantage is the cost of the equipment, which must also provide the environment that will maintain the temperature over the holding period needed to maintain the 5-log reduction. Microwaves can heat the eggs faster than hot water or hot air, but the cost of microwave pasteurization is currently prohibitive compared to hot-water and hot-air processes. We have to assess the incremental value of heating faster to the overall cost of the process, he said.

by NEIL H. MERMELSTEIN
Editor