Aquaculture Magazine

April / May 2016

Why Fish Quality Deteriorates During Storage – and What You Can Do About It

Deleterious changes in the quality of the lipid (fat) in marine fish during frozen storage is also caused by a combination of dehydration, ice crystal formation, and increased extracellular salt concentrations in fish fat (lipid).

By: Michael Jahncke*

Freezing does not destroy all microbes present, but temperatures below about +15°F (-9.4°C) preclude microbial growth, as the water activity (Aw) at this temperature is 0.90. However, enzymatic and non-enzymatic chemical reactions occur even at 0°F (-18°C).

Shelf-life

Shelf life is the time between food manufacture and attainment of unacceptability to customers. Regardless of the type of preservation employed, all foods eventually undergo change in flavor, color, texture, and nutritional status.

Intrinsic factors that affect shelf life include: The amount and type of lipid (fat), type of pigments, amount of water (i.e., water activity), pH value and acidity, etc.

Extrinsic factors include: Package, film permeability, seal integrity, chemical reactivity, light transmission potential, internal gas atmosphere, storage temperature and temperature fluctuations, relative humidity, etc.

Classification of Fish

Examples of high fat fish include:

• Salmon

• Tuna

• Trout

• Herring

• Sardines

• Mackerel

Examples of low fat fish include:

• Tilapia

• Cod

• Haddock

• Pollock

• Flounder

Composition of Fish

Lean fish, such as flounder, cod, Pollock, tilapia, are approximately 78-83% water, 15-20% protein, 1-4% lipid (fat), and 1-1.3% ash. Fatty fish, tuna, mackerel, blue fish, mahi-mahi, on the other hand, contain anywhere between 4 and 25% fat, Usually fat and water (moisture) content total approximately 80%. Fish composition, however, will vary between and within fish species depending on sex, age, season, water temperatures, and type and abundance of available food. Major compositional changes within species can be due to changes in their nutritional status. There is one report in the literature of a fish captured post spawning being 95% water.

Muscle structure (Protein) 

Similar to other vertebrate species, fish muscle is comprised of both red and white muscle fibers. Red muscle is located primarily along the lateral line and may comprise, depending upon species, up to 30% of the fish muscle. Red muscles are used by the fish primarily during sustained swimming activity. White muscle, on the other hand, is used for short bursts of swimming activity. Red muscle fibers are also higher in fat (lipid) content compared with white muscle fibers. In some species, there can be 4.5 times more fat in red muscle compared with white muscle fibers.

Lipids (Fat)

Fat in fish is found primarily under the skin, in the belly flap region and in the dark red muscle. The amount of fat in fish can vary from 0.6% in cod to over 25% in mackerel. Fish fat is extremely polyunsaturated, which means that it is a very perishable product. Marine fish contain many long chain omega-3 fatty acids, in contrast to mammals which contain a higher percentage of omega-6 fatty acids compared with the omega-3 fatty acids found in marine species.

The omega-3 fatty acids are good for cardiovascular health, but they are also much more susceptible to oxidizing and turning rancid during refrigerated and frozen storage.

Refrigerated Storage:

Microorganisms are found on all surfaces (skin and gills) and in the intestines of live fish or fresh fish. The total numbers of microorganisms vary greatly. There are several different types of spoilage bacteria, but one of the main types of spoilage bacteria belong to the genus Pseudomonas and can be found on meat, poultry, seafood and milk and dairy products. Spoilage bacteria are generally harmless, but can cause changes in the color, flavor, odor, and texture of food. In contrast, pathogenic bacteria are illness-causing bacteria that can produce toxins or cause infections.

Problem

Unpreserved raw fish and unpasteurized cooked fish and fishery products spoil rapidly during refrigerated storage.

Solution

Maintaining low constant storage temperatures. First-in First-out (FIFO) management schemes to shorten the time fish are kept in refrigerated storage. A trained workforce that understands the importance of following Good Manufacturing Practices (GMPs) and having and following Sanitation Standard Operating Procedures (SSOPs) and implementing following, and monitoring the 8 Key Sanitation Conditions and Practices mandated by the USFDA. (NOTE: Packaged products such as refrigerated unpreserved raw fish and refrigerated unpasteurized cooked fishery products require a film with a minimum Oxygen Transmission Rate [OTR] of 10,000 cc/m2/24hr or 3,000 cc/m2/24hr, or higher to prevent formation of toxin from C. botulinum).

Effects of Freezing Method and Frozen Storage on Quality of Fish

Storage at 0°F (-18°C) is satisfactory for maintaining adequate shelf life for most frozen foods. At that temperature, chemical changes that degrade food quality occur at a slower rate which eventually terminates shelf life. High barrier packaging films for frozen product and lower constant storage temperatures will help to lengthen the shelf life of these type products.

Freezing does not destroy all microbes present, but temperatures below about +15°F (-9.4°C) preclude microbial growth, as the water activity (Aw) at this temperature is 0.90. However, enzymatic and non-enzymatic chemical reactions occur even at 0°F (-18°C).

Q10 Effect

The effect of temperature on the biochemical reaction rate is called the temperature accelerating effect. (i.e., Temperature Accelerating Factor: Q10). The biochemical reaction rate is increased 2-3 fold when temperature is increased by 10°C (18°F). Conversely, the reaction rate is reduced 2-3 fold when temperature is decreased by 10°C (18°F).

Problem

Changes in fish texture, lipid (fat) oxidation, loss of pigment color, etc., are caused by a combination of dehydration, ice crystal formation, increased extracellular salt concentrations, etc. Dehydration can be caused by the removal of water during a slow freezing process, along with poor packaging, and low humidity levels in the room. Large ice crystals form during a slow freezing process and fluctuating storage temperatures by diffusion of water from the surrounding muscle fibers. These large ice crystals distort cells and muscle fibers, and create spaces in between the muscle fibers. The large ice crystals are formed during slow freezing, because the cell exteriors cool faster than the cell interiors. During this slow freezing of extracellular water, extracellular salt concentrations may increase as much as tenfold in the unfrozen solute. The higher extracellular salt concentrations will osmotically draw intracellular water into extracellular spaces causing extracellular space expansion.

Solutions

A high barrier packaging film to prevent oxygen transmission and dehydration (freezer burn) to help maintain texture, color and lipid (fat) quality. Quick freezing rates and low constant storage temperatures, to ensure lower extracellular salt concentrations, resulting in smaller ice crystals, less disruption of muscle fibers, and protection of pigments and lipid (fat).

Texture Changes in Gadoid 

Species

Problem

The Gadidae family of fish includes some of the most important commercial species of fish such as haddock, pollock, cod, whiting, cusk and hakes. The textural deterioration in these species during frozen storage is due primarily to the breakdown of trimethylamine oxide (TMAO), which is a compound found naturally in many marine species, into dimethyl-amine (DMA) and formaldehyde (FA), by the enzyme TMAO-dimethylase (TMAO-ase). The FA then cross-links the protein fibers resulting in a dry, fibrous fillet.

Solution

Quick freeze these type fish and optimize the First In First Out (FIFO) policy to maintain good quality. If these type products will be stored for more than 6 months, keep them at temperatures less than 0°F (-18°C).

Undesirable Lipid (Fat) Changes in Fish

Problem

Deleterious changes in the quality of the lipid (fat) in marine fish during frozen storage is also caused by a combination of dehydration, ice crystal formation, and increased extracellular salt concentrations in fish fat (lipid). The fat (lipid) in higher fat fish will tend to oxidize or turn rancid, due to exposure to oxygen, high salt and mineral concentrations, iron in the blood of the fish, and fluctuating storage temperatures.

Solution

Use a high barrier film to prevent oxidation of the fat and dehydration which can lead to rancidity and hydrolysis of the lipids. Quick freeze the fish to minimize formation of large ice crystals, and to minimize high concentrations of salt and minerals, which can oxidize and accelerate rancidity. As stated above, these high concentrations of salt and minerals form in the muscle tissue during a slow freezing process. Minimize exposure to light, and minimize fluctuating storage temperatures, which can lead to partial thawing and refreezing resulting in large ice crystal formation.

Conclusions

Poor packaging, low relative humidity, and substantial temperature fluctuation cause deterioration of seafood products in both refrigerated and frozen storage. Rather modest temperature abuse can dramatically shorten acceptable shelf life in refrigerated and frozen foods as can excessive cycling of storage temperature. In addition, the 3 Ps also have a profound effect on the shelf life of chilled and frozen food (i.e., Nature of the Product; Type of Chilling/Freezing Process Used; Type of Packaging Used).

Thus, it is important to recognize that adequate chilled and frozen food shelf life is linked not only to the chilled and frozen food storage environment, but also to the 3 Ps.


*Dr. Michael Jahncke is a Professor at Virginia Tech and the Director of the Virginia Seafood Agricultural Research and Extension Center. He earned his Ph.D. in Food Science at Cornell University. His research interests include safety and quality of wild catch and aquaculture species, sensory evaluation of seafood products, handling and processing of fish and fishery products, public, environmental, and animal health issues associated with aquaculture systems.


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