Spoilage of Specific Food Groups ◾ 259
mucopeptide), conalbumin (chelates iron), antivitamin proteins (avidin binds riboflavin), and protease inhibitors in eggs inhibit microbial growth. The most predominant spoilage of shell eggs
is caused by Gram-negative motile rods from several genera that include Pseudomonas, Proteus,
Alcaligenes, Aeromonas, and the coliform group. The different types of spoilage are designated as
rot. Some examples are green rot, causing greening of albumen because of growth of Pseudomonas
fluorescens; black rot, causing muddy discoloration of yolk because of H 2 S production by Proteus
vulgaris; and red rot by Ser. mercescens, caused by production of red pigment. On some occasions,
molds from genera Penicillium, Alternaria, and Mucor can grow inside eggs, especially when the
eggs are spoiled, and produce different types of fungal rots. 1
Egg Products
Liquid eggs consisting of whole eggs or yolks or whites only are generally pasteurized or frozen, or
both, to prevent microbial growth. If the liquid products are held at room temperature following
breaking and prior to pasteurization, spoilage bacteria can grow and cause off-flavor (putrid), sourness, or fish flavor (resulting from formation of trimethylamine). Pasteurized eggs at refrigerated
temperatures have a limited shelf life unless additional preservatives are added. The predominant
bacteria in pasteurized products are some Gram-positive bacteria that survive pasteurization, but
spoilage is mainly caused by psychrotrophic Gram-negative bacteria getting into products after
heat treatment. Dried eggs are not susceptible to microbial spoilage because of low A W .
Fish, Crustaceans, and Mollusks
Fish
Fish harvested from both fresh and saltwater are susceptible to spoilage through autolytic enzyme
actions, oxidation of unsaturated fatty acids, and microbial growth. Protein hydrolysis by autolytic
enzymes (proteinases) is predominant if the fish are not gutted following catch. Oxidation of unsaturated fatty acids is also high in fatty fish. Microbial spoilage is determined by the microbial types,
their level, fish environment, fish types, methods used for harvest, and subsequent handling. These
aspects have been discussed in Chapter 4. Fish tissues have high levels of NPN compounds (free
amino acids, trimethylamine oxide, and creatinine), peptides, and proteins, but almost no carbohydrates; the pH is generally above 6.0. Gram-negative aerobic rods, such as Pseudomonas spp.,
Acinetobacter, Moraxella, and Flavobacterium, and facultative anaerobic rods, such as Shewanella,
Alcaligenes, Vibrio, and coliforms, are the major spoilage bacteria. However, because of the relatively shorter generation time, spoilage by psychrotrophic Pseudomonas spp. predominates under
aerobic storage at both refrigerated and slightly higher temperatures. In fish stored under vacuum
or CO 2 , lactic acid bacteria (including Enterococcus) can become predominant. 4
Gram-negative rods initially metabolize the NPN compounds by decay (oxidation), followed by putrefaction to produce different types of volatile compounds, such as NH 3 , trimethylamine (N:CH 3 from reduction of trimethylamine oxide), histamine (from histidine, the cause
of Scombroid poisoning), putrescine, cadaverine, indoles, H 2 S, mercaptans, dimethyl sulfide
(especially by She. putrefaciens), and volatile fatty acids (acetic, isobutyric, and isovaleric acids).
Proteolytic bacterial species also produce extracellular proteinases that hydrolyze fish proteins and
supply peptides and amino acids for further metabolism by spoilage bacteria. The volatile compounds produce different types of off-odors, namely, stale, fishy (because of trimethylamine), and
mucopeptide), conalbumin (chelates iron), antivitamin proteins (avidin binds riboflavin), and protease inhibitors in eggs inhibit microbial growth. The most predominant spoilage of shell eggs
is caused by Gram-negative motile rods from several genera that include Pseudomonas, Proteus,
Alcaligenes, Aeromonas, and the coliform group. The different types of spoilage are designated as
rot. Some examples are green rot, causing greening of albumen because of growth of Pseudomonas
fluorescens; black rot, causing muddy discoloration of yolk because of H 2 S production by Proteus
vulgaris; and red rot by Ser. mercescens, caused by production of red pigment. On some occasions,
molds from genera Penicillium, Alternaria, and Mucor can grow inside eggs, especially when the
eggs are spoiled, and produce different types of fungal rots. 1
Egg Products
Liquid eggs consisting of whole eggs or yolks or whites only are generally pasteurized or frozen, or
both, to prevent microbial growth. If the liquid products are held at room temperature following
breaking and prior to pasteurization, spoilage bacteria can grow and cause off-flavor (putrid), sourness, or fish flavor (resulting from formation of trimethylamine). Pasteurized eggs at refrigerated
temperatures have a limited shelf life unless additional preservatives are added. The predominant
bacteria in pasteurized products are some Gram-positive bacteria that survive pasteurization, but
spoilage is mainly caused by psychrotrophic Gram-negative bacteria getting into products after
heat treatment. Dried eggs are not susceptible to microbial spoilage because of low A W .
Fish, Crustaceans, and Mollusks
Fish
Fish harvested from both fresh and saltwater are susceptible to spoilage through autolytic enzyme
actions, oxidation of unsaturated fatty acids, and microbial growth. Protein hydrolysis by autolytic
enzymes (proteinases) is predominant if the fish are not gutted following catch. Oxidation of unsaturated fatty acids is also high in fatty fish. Microbial spoilage is determined by the microbial types,
their level, fish environment, fish types, methods used for harvest, and subsequent handling. These
aspects have been discussed in Chapter 4. Fish tissues have high levels of NPN compounds (free
amino acids, trimethylamine oxide, and creatinine), peptides, and proteins, but almost no carbohydrates; the pH is generally above 6.0. Gram-negative aerobic rods, such as Pseudomonas spp.,
Acinetobacter, Moraxella, and Flavobacterium, and facultative anaerobic rods, such as Shewanella,
Alcaligenes, Vibrio, and coliforms, are the major spoilage bacteria. However, because of the relatively shorter generation time, spoilage by psychrotrophic Pseudomonas spp. predominates under
aerobic storage at both refrigerated and slightly higher temperatures. In fish stored under vacuum
or CO 2 , lactic acid bacteria (including Enterococcus) can become predominant. 4
Gram-negative rods initially metabolize the NPN compounds by decay (oxidation), followed by putrefaction to produce different types of volatile compounds, such as NH 3 , trimethylamine (N:CH 3 from reduction of trimethylamine oxide), histamine (from histidine, the cause
of Scombroid poisoning), putrescine, cadaverine, indoles, H 2 S, mercaptans, dimethyl sulfide
(especially by She. putrefaciens), and volatile fatty acids (acetic, isobutyric, and isovaleric acids).
Proteolytic bacterial species also produce extracellular proteinases that hydrolyze fish proteins and
supply peptides and amino acids for further metabolism by spoilage bacteria. The volatile compounds produce different types of off-odors, namely, stale, fishy (because of trimethylamine), and