Fundamental Food Microbiology, Third Edition - Fuad Fathir

SPOILAGE OF SPECIFIC FOOD GROUPS 275
A. Fish
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IV. FISH, CRUSTACEANS, AND MOLLUSKS
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 temperature. In fish stored
under vacuum or CO 2, lactic acid bacteria (including Enterococcus) can become
predominant. 1,2
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; cause of Scombroid poisoning), putrescine, cadaverine,
indoles, H 2S, 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 (due to trimethylamine),
and putrid. Bacterial growth is also associated with slime production,
discoloration of gills and eyes (in whole fish), and loss of muscle texture (soft due
to proteolysis).
In fish stored by vacuum or by CO 2 packaging, growth of aerobic spoilage bacteria
is prevented. However, anaerobic and facultative anaerobic bacteria can grow, including
lactic acid bacteria. Under refrigeration, products have relatively long shelf life
due to slower growth of spoilage bacteria. Salted fish, especially lightly salted fish,
are susceptible to spoilage by halophilic bacteria such as Vibrio (at lower temperature)
and Micrococcus (at higher temperature). Smoked fish, especially with lower A w,
inhibit growth of most bacteria. However, molds can grow on the surface. Minced
fish flesh, surimi, and seafood analogs prepared from fish tissues generally have high
initial bacterial levels due to extensive processing (ca. 10 5–6 /g). The types include
those present in fish and those that get in during processing. These products, such as
fresh fish, can be spoiled rapidly by Gram-negative rods unless frozen quickly or
used soon after thawing. Canned fish (tuna, salmon, and sardines) are given heat
treatment to produce commercially sterile products. They can be spoiled by thermophilic
sporeformers unless proper preservation and storage conditions are used. 1–4