What the eye cannot measure
Fish may look normal while changes are already happening inside its flesh. Laboratory tests helpmeasure these hidden changes. Their names may sound complicated, but the ideas behind themare quite simple.
K-value: measuring early biochemical change
When a fish is alive, its muscles contain an energy substance called ATP. After harvest, ATP
naturally breaks down. The K-value shows how far this process has progressed—rather like
reading the fish’s internal energy clock. In simplified terms:
A lower K-value generally indicates less ATP breakdown, while a higher K-value
indicates greater post-harvest change. K-value can be useful for measuring early freshness, but the rate of change varies by species, temperature and handling. A numerical limit must therefore be validated for the particular fish and
storage system.
TVB-N: measuring volatile spoilage compounds
Total Volatile Basic Nitrogen, or TVB-N, measures compounds that increase as enzymes and microorganisms break down parts of the fish. Think of it as a warning signal that becomes stronger as spoilage develops.
TVB-N is widely used as a spoilage indicator. However, it may remain comparatively low during the earliest stages of quality loss and then rise more rapidly later. It is therefore better suited to supporting a spoilage decision than proving exceptional early freshness.
ICAR–Central Institute of Fisheries Technology research on fish sold in supermarkets reported variation in TVB-N among samples and found that some exceeded 30 mg nitrogen per 100 g. This reinforces the need for actual measurement rather than assumptions based on appearance or selling format.
TMA-N and the characteristic “fishy” smell
Trimethylamine nitrogen, or TMA-N, is associated particularly with bacterial spoilage in many marine species. It contributes to the familiar strong fishy odour that develops during deterioration. Its usefulness is species-dependent because freshwater and marine fish differ in their natural chemistry.
pH: useful, but not decisive The pH of fish muscle changes after harvest. It may initially fall and later rise as alkaline spoilage compounds accumulate. Because initial pH and its pattern vary by species, stress, season and storage condition, pH is normally a supporting measurement rather than a stand-alone freshness test.
Oxidation measurements
Fatty fish can develop rancidity even when microbial spoilage is limited. Tests such as peroxide value and TBARS help assess primary and secondary fat oxidation. These are especially relevant to fatty species, cut products, products exposed to oxygen and fish held in frozen storage for longer periods. Physical measurements complete the picture
Laboratory or production-floor instruments can also measure:
Texture or firmness
Colour using L, a and b* values
Drip or thaw loss
Water-holding capacity
Electrical properties associated with tissue changes
These results should be compared with a validated baseline for the same species, cut and
process.
Why universal cut-offs are risky
Published TVB-N rejection criteria often differ by species. European controls, for example, use specified TVB-N limits for defined species categories and use laboratory testing when organoleptic examination raises doubt. A figure taken from one fish cannot automatically be applied to another fish, shrimp, fillet, marinated item or frozen product.
REMEMBER
Laboratory tests are like different parts of a health check. Each measures something
different, and the clearest answer comes from reading the results together.