MICROBIOLOGY Textbook - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.3. MICROBIOLOGY OF EGGS AND EGG PRODUCTS

Egg microflora. Microbial contamination of eggs can occur through both endogenous and exogenous pathways. In infected birds, pathogens (such as those causing tuberculosis and salmonellosis) penetrate the interior of the egg while it is still in the Ovary or oviduct. Waterfowl eggs are particularly hazardous regarding salmonellosis, as they frequently harbor species such as S. pullorum, S. gallinarium, S. enteritidis, S. dublin, S. newport, S. typhimurium, and others. Consequently, the commercial sale of duck and goose eggs, as well as their use in public catering establishments, is prohibited.

Exogenous contamination of the eggshell occurs after laying, with droppings, soil, and feathers serving as the primary sources of microorganisms. Clean eggs contain no more than 10¹—10² microorganisms per 1 cm² of surface area, whereas contaminated eggs may harbor up to 10⁵—10⁶. The surface microflora of eggs is highly diverse. The most commonly detected organisms include coliforms (coliform Bacteria), Proteus species, pseudomonads, aerobic spore-forming rods, micrococci, and mold spores. Eggs with contaminated surfaces are unfit for sale and must be washed. However, washed eggs lose their storage stability; therefore, to prevent microbial spoilage, it is recommended to treat their surfaces with protective agents (such as mineral oil, polyvinyl alcohol, or methylcellulose).

The contents of freshly laid eggs from healthy birds are initially free of microorganisms and remain sterile for a considerable period. This is attributed to the presence of Lysozyme—a bactericidal substance in the egg albumen that destroys bacterial Cell walls (see Section 3.3). The eggshell, cuticle (epiorganic membrane), and shell membranes protect the egg against microbial penetration. If the cuticle is damaged, microbes can penetrate the interior through shell pores and multiply into colonies that are clearly visible during candling (ovoscopy). The rate of microbial penetration depends on egg freshness, ambient Temperature and humidity, and the specific types of microorganisms. Eggs are considered fresh if their storage duration under appropriate conditions does not exceed 25 days. They should be stored at a temperature of (0 ± 2) °C and a relative humidity of 85—88 %. Storage life can be extended using modified atmosphere packaging (MAP), specifically in an atmosphere of nitrogen and carbon dioxide or in an ozone-enriched environment.

Due to their chemical composition, eggs provide an excellent substrate for microbial proliferation and are classified as perishable foods. The primary forms of egg spoilage are putrefaction (rotting) and mold growth.

Putrefaction. As a result of pseudomonad proliferation, the egg white becomes turbid and liquefied, eventually acquiring a greenish hue that deepens to dark green (green rot). When bacteria of the genera Proteus and Pseudomonas multiply, the albumen liquefies and turns brown, then black (black rot). Due to the accumulation of gases generated by putrefactive bacteria, the shell may rupture, releasing the contents with a foul odor. The proliferation of putrefactive aerobic bacilli destroys the yolk membrane, causing the yolk to mix with the albumen and form a turbid, liquid mass. During candling, such eggs are opaque; this defect is known as a "bacterial rot" or "tumak". Bacteria that produce red or pink pigments (Serratia marcescens, Micrococcus roseus), along with certain Yeasts, impart a pink coloration to the egg contents (pink rot). Mixed putrefaction is caused by coliforms, aerobic bacilli, Proteus species, pseudomonads, and other microorganisms possessing active proteases. In such cases, the egg white liquefies, turns gray, and develops a putrid odor.

Mold spoilage. Mold Fungi typically proliferate on the inner shell membrane and around the air cell. The initial stage of mold growth is characterized by The formation of a dark spot visible during candling. Over time, the fungal mycelium spreads, the spot enlarges, the albumen and yolk mix, and the egg completely loses its translucency. This type of spoilage is referred to as "moldy rot" (or "moldy tumak").

Frozen egg products. These products include melange, which is a shelled mixture of egg white and yolk in their natural proportions, preserved by freezing. Only Grade I eggs are used for making melange. Because melange is highly perishable, strict Sanitary and hygienic conditions must be maintained during its production. Its microflora consists of micrococci, sarcinae, Proteus species, pseudomonads, coliform bacteria, aerobic bacilli, and mold spores. Pathogens such as Salmonella and Staphylococcus aureus are occasionally present. To minimize the risk of pathogen Introduction and reduce overall bacterial contamination, it is recommended to disinfect eggs prior to breaking them. A brief pasteurization of melange at 65—70 °C for 1—2 minutes is also sometimes performed.

The egg mass is frozen at —18...—20 °C. During freezing and storage, the microorganisms present in the melange gradually die off. However, not all microbes are eliminated; thawing triggers their reactivation, which is why thawed melange should be stored at 4—5 °C for no more than 3—5 hours.

Dry egg products. For long-term storage, the egg mass is preserved by dehydration. The liquid egg is fed onto a rotating atomizer disk where it is sprayed into micro-droplets 50—100 µm in size and dried at temperatures not exceeding 60 °C. Microbiological parameters of the finished product depend on raw material quality, production hygiene, and spray-drying parameters. The Use of fresh egg mass, followed by filtration, homogenization, and pasteurization with immediate drying, yields egg powder that complies with sanitary regulations (SanPiN), which mandate that total viable microbial count (TVMC) does not exceed 5 × 10⁴ CFU/g; coliforms must be absent in 0.1 g; Staphylococcus aureus in 1 g; Proteus species in 1 g; and Salmonella in 25 g.

Egg powder retains bacterial spores and some vegetative Cells. During storage, microorganisms gradually die off due to the low moisture content of the product (6–8 %). Dry egg products can be stored for up to 6 months at temperatures not exceeding 20 °C and relative humidity no higher than 75 %, or for up to 2 years from the date of manufacture at temperatures below 2 °C and relative humidity of 60—70 %.