Spoiled fish

Background

Spoilage

Spoilage is the degradation of food such that the food becomes unfit for human consumption. Food can be spoiled by a number of means, including physical and chemical means. However, the most prevalent cause of food spoilage is microbial growth and residence in the food, which results in numerous undesirable metabolites being produced in the food that cause unwanted flavors and odors. Approximately 25% of the world’s food produced post harvest or post slaughter is lost to microbial degradation of food alone.

The main culprits are microbial organisms known as spoilage specific organisms (SSOs). The concept of SSOs arises from the fact that not all bacteria cause food spoilage; indeed, the degree of food spoilage is not proportional to the amount of microbes present on the food. SSOs are solely responsible for spoilage of the food and the typical characteristics associated with that spoilage. They are typically present in very low numbers and comprise a low percentage of the microflora present on the food.

Identification of SSOs is done by comparisons of the physical and chemical features of the collective spoiled products with the individual products left behind by each organism in the spoilage microflora. In particular, the qualitative ability of each organism to produce off-odors (spoilage potential) and the quantitative ability of each organism to produce spoilage metabolites (spoilage activity) are examined. This simple phenotypic identification scheme, along with a 16S rDNA gene sequencing to confirm results, allows scientists to discover which organism or organisms in the spoilage microflora are directly responsible for the spoilage.

Each unique environment has its own unique SSOs, because each different environment selects for particular organisms to thrive. The spoilage domain for an SSO is identified based upon the conditions (pH, temperature, water activity, and atmosphere) under which that SSO can grow and produce the metabolites that cause spoilage.

Spoiled Food vs. Harmful Food

It is important to note that spoilage bacteria normally outgrow pathogenic bacteria during storage. Thus some foods may spoil before they become toxic. Spoilage bacteria and pathogenic species in spoiled food have different effects. Pathogens are responsible for the symptoms that result from eating spoiled food; SSOs may or may not have a direct harmful effect to the consumers.

Spoilage in Fish

Fish spoilage manifests itself physically in numerous ways. In terms of smell, spoiled fish will generally have a fishy, sour, or ammonia-like stench. Appearance-wise, spoiled fish may appear to be dry or mushy in certain areas, and the gills may have slime. Spoiled fish will also have flesh that is soft, or does not spring back when pressed upon. Typically, spoiled fish will also have a green or yellowish discoloration; however, this arises not from spoilage metabolites, but rather the oxidation during frozen storage of the oxygen transporters in fish blood (myoglobin to metamyoglobin).

Compared to other foods, fish is unique as a substrate for microbial growth. This uniqueness stems from several important factors: the poikilotherm nature of fish, a high post mortem pH in the flesh (typically greater than 6.0), the presence of non-protein-nitrogen (NPN) in large quantities, and the presence of trimethylamine oxide (TMAO).

The poikilotherm nature of fish selects for bacteria that can thrive in a wide range of temperatures. For example, the microflora of temperate water fish is dominated by psychrotrophic Gram-negative, rod-shaped bacteria such as those found in the genera Pseudomonos and Moraxella, with only varying proportions of Gram-positive organisms such as Bacillus.

The high post mortem pH of fish flesh is caused by the fact that fish flesh is low in carbohydrates (less than 0.5%) in the muscle tissue and that only small amounts of lactic acid are produced after death. This allows pH sensitive organisms such as Shewanella putrefaciens to grow in seafood but not in other meats.

The NPN fraction of the fish flesh consists of low-molecular-weight water-soluble nitrogen contains compounds, particularly free amino acids and nucleotides, that allow it to serve as a readily available bacterial growth substrate. Decomposition of these compounds is responsible for many of the off-odors and off-flavors typically found in spoilage. For example, the breakdown of cysteine and methionine by certain microbes, both sulfur-containing amino acids, forms hydrogen sulfides and methylmercaptane respectively which causes undesirable odors to emanate from spoiled fish.

The presence of TMAO in fish is well-established, and it is known to cause a high redox potential in the fish flesh, although the significance of this is not clear. The spoilage of fish is influenced most by the presence of TMAO in conditions where oxygen is not present. Some anaerobic bacteria are able to utilize TMAO as the terminal electron acceptor in an anaerobic respiration process with trimethylamine (TMA) as the primary product; TMA contributes to the characteristic ammonia-like and fishy off-flavours in spoiled fish.

Which microbes are present?

You may refer to organisms by genus or by genus and species, depending upon how detailed the your information might be. If there is already a microbewiki page describing that organism, make a link to it.

Do the microbes that are present interact with each other?

Describe any negative (competition) or positive (symbiosis) behavior

Do the microbes change their environment?

Do they alter pH, attach to surfaces, secrete anything, etc. etc.

Do the microbes carry out any metabolism that affects their environment?

Do they ferment sugars to produce acid, break down large molecules, fix nitrogen, etc. etc.

Current Research

Enter summaries of the most recent research. You may find it more appropriate to include this as a subsection under several of your other sections rather than separately here at the end. You should include at least FOUR topics of research and summarize each in terms of the question being asked, the results so far, and the topics for future study. (more will be expected from larger groups than from smaller groups)

References

[Sample reference] [http://ijs.sgmjournals.org/cgi/reprint/50/2/489 Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Gram and Dalgaard, 2002 L. Gram and P. Dalgaard, Fish spoilage bacteria-problems and solutions, Current Opinion in Biotechnology 13 (2002), pp. 262–266. Article

Gram and Huss, 1996 L. Gram and H.H. Huss, Microbiological spoilage of fish and fish products, International Journal of Food Microbiology 33 (1) (1996), pp. 121–137. Article]

Edited by Tal Do, Phillip Lai, Duy Nguyen, and Tania Yaser, students of Rachel Larsen