Evaluating Food Quality Through Colour: An Essential Sensory Characteristic

When you pick up a piece of fish at a market, colour is the first thing you notice – and for good reason. A fish’s colour tells a detailed story about its freshness, handling history, and safety for consumption. In seafood quality assurance, colour is not merely an aesthetic attribute; it is a primary functional indicator that trained evaluators rely on to make rapid, reliable decisions about whether a product is fit for sale or should be rejected. Understanding how and why colour changes occur – and how to assess them systematically – is fundamental to quality control in the fish industry.

Why colour matters in food quality assessment 🔗

Colour is one of the most immediate sensory signals available during quality inspection. Before any smell is detected or texture assessed, the eye registers colour – and in fish, that first visual impression carries significant quality information. According to the Food and Agriculture Organization (FAO), sensory assessment using sight is one of the primary tools for evaluating fish quality in commercial and port-market settings, where grading for freshness, colour, and degree of deterioration follows the same fundamental principles.

Colour changes in food, particularly seafood, are among the earliest and most visible signs of quality loss. They can signal enzymatic activity, microbial proliferation, chemical oxidation, or pigment degradation – all processes that directly affect safety and marketability. This makes colour evaluation an indispensable first step in any structured sensory assessment protocol.

How colour changes occur in seafood 🔗

Colour changes in seafood do not happen randomly – they are the result of specific biochemical processes that begin the moment a fish dies. These processes fall into two broad categories: enzymatic actions and non-enzymatic actions.

Enzymatic colour changes 🔗

Once a fish dies, its natural defence systems shut down and endogenous enzymes begin breaking down tissues – a process called autolysis. One of the most visible enzymatic colour changes is black spot formation (melanosis) in shrimp and certain fish species. Research published in Food Control confirms that lipid oxidation and protein decomposition driven by endogenous enzymatic activity directly reduce the visual quality of fish during storage.

In shrimp, melanosis occurs due to the action of the enzyme polyphenol oxidase (PPO), which catalyses an oxidative reaction on the amino acid tyrosine, producing melanin pigment. This causes dark discolouration on the surface. While the eating quality may not always be immediately compromised, the appearance is severely affected, reducing marketability. The reaction requires oxygen and is accelerated by the presence of heavy metals like copper and iron.

Enzymatic activity also affects the gill tissue and muscle of finfish. Proteinase enzymes degrade muscle proteins and gill tissue, contributing to colour dullness and structural breakdown that further affects the visual impression of the fish.

Non-enzymatic colour changes: fat oxidation 🔗

Non-enzymatic colour changes are primarily driven by lipid (fat) oxidation. Fish – especially fatty species like mackerel, sardines, herring, and salmon – contain high concentrations of polyunsaturated fatty acids (PUFAs), including omega-3s. These PUFAs are highly susceptible to oxidation due to their multiple double bonds, which react readily with atmospheric oxygen.

Lipid oxidation proceeds through a free-radical chain reaction in three stages: initiation (triggered by heat, UV light, or metal ions), propagation (formation of peroxides and hydroperoxides), and termination (breakdown into aldehydes, ketones, and other secondary products). These secondary oxidation products react with proteins and other compounds to form coloured substances that cause visible browning and discolouration. A review published in the Italian Journal of Animal Science highlights that the products of fat oxidation interact with protein decomposition compounds to produce coloured substances – a process particularly significant in oily fish. In salted and dried fish, this browning from oxidized fat is commonly referred to as “rust.”

Pigment decomposition 🔗

Colour in fish is also determined by specific pigments, and their breakdown is a direct indicator of quality loss. Carotenoid pigments are responsible for the characteristic hues of many fish species – the pink-orange of salmon, the reddish tones of trout. These pigments are degraded by oxidative reactions during storage, leading to a washed-out, dull appearance that signals declining quality.

In tuna and other dark-fleshed species, myoglobin is the primary pigment responsible for the deep red colour of fresh muscle. As fish deteriorates, myoglobin is oxidised to metmyoglobin, shifting the flesh colour from bright red to brownish-grey. Greening of the flesh – observed in swordfish and tuna – can result from the oxidation of hemochrome or from hydrogen sulphide produced by putrefactive bacteria reacting with muscle pigments. Academic course materials on fish quality defects note that proper evisceration and removal of blood immediately after the catch can significantly reduce the risk of such discolouration.

Sensory evaluation of colour in fish: what evaluators look for 🔗

The FAO’s guidelines on fish quality assessment establish that objective sensory evaluation – where personal bias is minimized and descriptions are dispassionate and accurate – is essential wherever quality standards need to be established or controlled. Colour assessment of fish follows this objective approach, examining specific anatomical features in a defined sequence.

Skin colour and surface appearance 🔗

The skin of a fresh fish has a bright, metallic sheen resulting from the intact structure of specialised cells called iridophores, which contain reflective crystals. The natural pigmentation should be vibrant and species-appropriate. As quality deteriorates, the skin loses its lustre, becomes dull and opaque, and may show discolouration. Certain bacteria also produce their own pigments on the skin surface – for example, Pseudomonas fluorescens can produce a greenish-yellow tint, while Micrococcus species produce yellow pigments – providing additional visual cues of microbial spoilage.

Gill colour 🔗

The gills are one of the most reliable and commonly assessed colour indicators of fish freshness. Fresh fish gills are bright red or pink, reflecting the oxygenated state of hemoglobin in the gill tissue. As spoilage progresses, this colour changes in a predictable sequence: bright red → dull red → brown → grey or green. These transitions reflect the breakdown of blood proteins and the activity of bacteria producing coloured metabolites. A review in the journal Sensors confirms that gill colour is among the key sensory attributes – alongside skin brightness, meat colour, and elasticity – used to classify fish freshness under structured evaluation systems.

Eye colour and clarity 🔗

The eyes offer another clear window into freshness. Fresh fish eyes are clear, bright, and slightly bulging, with a transparent cornea and a well-defined black pupil. As deterioration sets in, dehydration and bacterial action cause the eyes to become cloudy and sunken. The cornea loses transparency, and the pupil takes on a greyish or milky appearance. Research published in Food Research International has explored the use of computer vision techniques to objectively characterise changes in the chromatic and morphological properties of fish eyes during storage, demonstrating just how reliably eye colour tracks freshness over time.

Flesh colour 🔗

The colour of the flesh itself varies by species and should be evaluated against species-specific standards. White fish like cod should appear translucent and pearly when fresh. Oily fish like salmon should maintain a characteristic pink or orange hue. Any browning, yellowing around the edges, or development of an opaque chalky appearance indicates progressive quality loss. Yellowing of flesh beneath the skin in frozen cod, for instance, occurs when the freezing process disrupts chromatophores and releases carotenoids that migrate into subcutaneous fat layers.

Standardised methods for colour evaluation 🔗

To ensure consistency and reliability, colour evaluation in seafood quality assurance follows standardised protocols. The Quality Index Method (QIM), originally developed by the Tasmanian Food Research Unit and now widely used across Europe, is one of the most established frameworks. As documented by the FAO, QIM assigns demerit scores – from 0 for very fresh fish to progressively higher totals as deterioration advances – based on specific sensory parameters that include skin appearance, gill colour, eye condition, and overall colour of the flesh. A score of zero indicates optimal freshness; increasing scores directly correlate with declining quality.

Evaluations must be conducted under controlled conditions: standardised lighting (daylight-equivalent fluorescent bulbs or natural daylight), consistent background colour, controlled observation angle, and defined distance from the sample. These controls are critical because colour perception is highly sensitive to lighting conditions. The FAO notes that individual differences in colour perception – including colour blindness – must be screened for when selecting and training sensory panel members, as such variation can undermine the objectivity of assessments.

A comprehensive review in Food and Bioprocess Technology covering freshness evaluation methods confirms that sensory evaluation, including colour assessment, remains among the most important methods used alongside chemical measurements of lipid oxidation, volatile compounds, and microbial indicators for a complete picture of fish quality.

Colour as part of a broader quality picture 🔗

Colour evaluation does not operate in isolation. It is always interpreted alongside other sensory attributes – odour, texture, and taste – to arrive at a complete quality assessment. However, colour carries particular weight because it is the first attribute assessed and often the most decisive in consumer acceptance decisions. A fish with dull skin, brown gills, and cloudy eyes will rarely pass a quality inspection regardless of other attributes, because the visual evidence of deterioration is already conclusive.

Research on intelligent packaging systems for fish freshness monitoring has shown that colour-based indicators – such as pH-sensitive dyes that change colour as volatile amines accumulate during spoilage – are being developed to complement traditional sensory evaluation in real-time monitoring applications. These technologies essentially replicate, in a sensor format, the same principle that trained evaluators apply during visual assessment: that colour change is a direct, reliable proxy for the biochemical state of the fish.

The underlying message is consistent: colour change in seafood is not cosmetic. It reflects real, measurable biochemical deterioration – enzymatic breakdown, oxidative rancidity, microbial activity, and pigment decomposition – all happening simultaneously and visibly. Mastering colour evaluation is therefore not just a skill; it is a core competency for anyone working in seafood quality assurance.

What do you think? If colour is so reliable as a freshness indicator, should it carry more formal weight in regulatory grading schemes for seafood – or does its subjectivity under different lighting conditions limit its role? And given that enzymatic and oxidative colour changes begin immediately after a fish is caught, how critical is the time between harvest and the first quality inspection?

References

1. https://www.fao.org/4/x5989e/X5989e01.htm
2. https://www.sciencedirect.com/science/article/abs/pii/S0956713521009439
3. https://www.tandfonline.com/doi/full/10.1080/1828051X.2015.1128687
4. https://courseware.cutm.ac.in/wp-content/uploads/2020/06/L-8-Quality-defects-in-fish-and-fishery-products_2.pdf
5. https://www.fao.org/4/v7180e/v7180e09.htm
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC7919655/
7. https://pubmed.ncbi.nlm.nih.gov/30599937/
8. https://pubmed.ncbi.nlm.nih.gov/24915394/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC9901331/