Microbial Spoilage

When laid, the contents of the shell egg are generally free from bacteria. The shell egg contents are protected by the shell and associated membranes and chemical inhibitors in the egg albumen. Some of the components in the albumen that provide an unfavorable medium for microbial growth include lysozyme, conalbumin, ovomucoid, avidin, riboflavin, and others. (Although shell eggs have built-in natural protection as just described, Salmonella has been found in freshly laid eggs. Therefore, it must be assumed that all shell eggs are contaminated with this bacterium.) Deteriorative conditions due to microbial growth that cause shell eggs to become inedible follow.

a. Black Rot. When viewed with a candling light, shell eggs with black rot are virtually opaque. When broken out, the egg content has either a gelatinous yolk, blackened throughout with a grey, watery albumen, or a dark brown, mealy yolk with a dark brown albumen. The bacteria associated with this type of spoilage are species of Proteus and Aeromonas.

b. White Rot. With white rot, threadlike shadows may be seen in the thin white.

In later stages, the yolk appears severely blemished when the shell egg is viewed with the candling light. Upon opening, the egg yolk shows a crusted appearance and frequently has a fruity odor. Various organisms have been associated with this rot, including Citrobacter, Salmonella, and Alcaligenes.

c Sour Eggs. On candling, sour eggs show a weak white and murky shadow around an off-center, swollen yolk. These eggs are also called fluorescent or fluorescent green rot. A fluorescent green pigment throughout the albumen is produced by Pseudomonas species.

d. Green Whites. Green whites of broken out shell eggs exhibit fluorescence when observed with UV light. Shell eggs with green white may or may not have a sour odor. This defect is caused mainly by Pseudomonas fluorescens.

e. Musty Eggs. These shell eggs appear clear and free from foreign material when candled. The musty odor may be caused by odors in the atmosphere being absorbed by the egg contents. Also, some microorganisms occasionally invade shell eggs and produce a musty odor.

f. Moldy Eggs. Mold growth is visible as spots on the shell, in checked areas of the shell, or inside the egg. The mold contamination of shell eggs seem to be due to the reuse of moldy packing materials. Several molds, such as Penicillium, Alternaria, and Rhizopus, can grow on shell eggs.

g. Red Rot. With red rot, the egg whites are stained red throughout and the yolks are surrounded by custard-like material. An ammoniacal to putrid odor may occur. Serratia marcescens is the usual cause of red rot.

h. Custard Rot. In this rot, the yolk is encrusted with custard-like material and occasionally flecked with olive green pigment. The albumen becomes thin with an orange tint. These shell eggs may have a slightly putrid to putrid odor. Citrobacter and Proteus vulgaris have been associated with this type of spoilage.

i. Other Rots. Alcaligenes has been the organism causing both yellow rots and green rots. These rots are similar in odor and in the appearance of albumen. However, the yolk is dark yellow in yellow rot and dark green to black in green rots.

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