Page 34 - FoodFocusThailand No.160 July 2019

SPECIAL FOCUS
SPECIAL FOCUS

How to Reduce

Fishy and Muddy Odors

of Fish for Processing

Fish is a good source of dietary protein that is easy to digest, helps
growing and repairing tissues and organ. Moreover, essential
unsaturated fatty acids for health benefits such as omega-3 fatty
acids are also found in fish. Fish products have shown variable
consumption trends in health-conscious consumers today.
However, the fishy odor, found in both of freshwater and marine fish
species, is a key factor that is unacceptable for consumers. Mainly,
the fishy odor is caused by the following reasons:

Endogenous enzyme activity and microorganisms 1. To avoid fishy odor problems by lipid oxidation reactions, the
1in the fish cause volatile substances, developing a fishy manufacturers should select only fresh and clean fish for further processing
odor called Trimethylamine or TMA in short. In general, the TMA substance and should not store in the ice for too long.
is found in marine fish. It does not occur right after the fish die, but it is 2. The manufacturers should rinse the flesh of fish in clean cold water
gradually increased during chilling storage of fish for example; covering before cooking in order to 1) to remove fat, a precursor to lipid oxidation,
by ice flakes and keeping at a refrigerator for a long time. Since, fresh 2) to remove fish slime and TMA substances, and 3) to eliminate blood
marine fish contains trimethylamine N-oxide or TMAO that will be changed containing heme compound. Washing with cold water also reduces the
to TMA and energy by trimethylamine oxidase enzyme of bacteria in fish initial microbial counts on the fish skin. Therefore, cold water is a great
meat such as Shewanella putrefaciens for bacterial growth. Therefore, choice to eliminate the fishy odor. After cleaning the fish with cold water,
an increase in TMA amount causes a fishy odor and is directly related to the skin and fatty portions, where unsaturated fatty acids are tightly packed,
the number of bacteria, whereas the sensory quality and freshness of fish should be trimmed away. Here, more of the fishy odor can be lessened.
are declined. 3. To reduce muddy odor before processing, the manufacturers should
soak raw fish in a solution containing polyphenol extract from tea or
Lipid oxidation contributes to the development of a mixture of polyphenol extract and acetic acid. Liu et al. (2017) reported
2 fishy odor, especially in fatty fish such as salmon and tuna. that soaking the channel catfish holding the muddy odor in a solution
The reaction generates smelly volatile substances, including aldehyde, containing 0.4 g of polyphenol extract of tea per 1 liter with 0.5% acetic
ketone, alkane, alkenes, such as (Z)-4-heptenal, 2,3-pentanedione. acid for 8-25 minutes at room temperature, then rinsing with water again
Abundant unsaturated fatty acids, found in fatty fish, are the most can wipe out the MIB in flesh and cut down the amount of geosmin by
substrates to promote the lipid oxidation. Furthermore, heme substance more than 50%. Use of combination between polyphenol extract from the
that is an important component in red blood cells and fish tissues can tea and acetic acid to remove muddy odor is more effective than single
catalyze the lipid oxidation, since it is composed of iron element. The use of the polyphenol extract. As acetic acid has acidic property, thereby
most parts of fish flesh occurring fishy odor from the lipid oxidation are enhancing the efficiency of removing muddy odor in fish.
dark muscles that lie mostly along the lateral line of the fish, since these 4. The manufacturers should use various herbs and spices in cooking
portions are rich in unsaturated fatty acid and heme. As a result, the lipid as they can help in “Masking” the fishy odor. Some studies found that
oxidation in the dark muscle areas occurs greater than in the other parts. natural antioxidants, especially Thai herbs such as galangal, ginger, black
As well, unpleasant odor can be found, especially muddy odor from pepper, and long pepper (Piper retrofractum Vahl), can simply mask fishy
freshwater fish like catfish, Nile tilapia, striped catfish, and channel catfish. odor. Such herbs are popularly used as an ingredient to prepare fish
Mainly, the substances that caused undesirable odor are geosmin and dishes to neutralize the fishy odor in several cooking methods such as
2-methylisoborneol or in short, MIB. Both substances, produced by blue- stir-fried curry, roasted curry paste, red curry, Tom Yum soup, and coconut
green algae and actinomycetes bacteria such as Streptomyces sp. in soup. You can smell the fresh flavor of the herbs which can drown out
freshwater resources, are saturated cyclic tertiary alcohols with good fat the fishy odor while eating. It also benefits from antioxidant in herbs,
solubility. Both compounds are absorbed from water resources via gill inhibits free radicals in unsaturated fats, which causes the fishy odor from
and tissue, and accumulated in lipid tissue of fish and other aquatic lipid oxidation.
animals, for instance, shrimp. However, these undesirable compounds 5. Fish processing can help reduce muddy odor. For example,
are not toxic to living organisms. Actually, farm-raised freshwater fish will marinating with acetic acid and salt can reduce the muddy odor in tilapia
be managed to reduce muddy odor by various methods such as a proper due to the decomposition reaction of geosmin breaking down into an
feeding management, sufficient oxygen supply, live aquatic animals in odorless substance called argosmin. In addition, fermentation with salt
clean water ponds for the good taste of fish flesh. Nevertheless, the strong following with deep frying is another way to reduce the fishy odor, since
odor of mud still exists, resulting in a key factor to the consumer the geosmin, an oil-soluble substance, can be removed from tilapia meat
unacceptance. during frying. However, some kinds of processing such as salting, sun
Fishy and muddy odors for all fish products can be eliminated by drying, or microwave heating cannot reduce the unpleasant odor of tilapia.
suitable preparing and processing of fish in various methods, including;

34 FOOD FOCUS THAILAND JUL 2019

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