The Role of Odour in Sensory Evaluation: Detecting Spoilage in Foods

Every time you open a container of milk and instantly know something is off – without tasting it – your olfactory system is doing exactly what it was designed to do. Odour is one of the most immediate and reliable indicators of food quality. In sensory evaluation, it is assessed before taste, before texture, and sometimes even before appearance. Understanding how odour works, what specific smells signal, and why people perceive smells differently is essential knowledge in food quality assurance.

How the human nose detects odour
Why odour is critical in food quality assessment
Common odour descriptors in sensory evaluation
Fragrant
Rancid
Oily
Metallic
Why odour assessment is inherently subjective
Genetic differences
Age-related decline
Cultural background and personal preferences
Standardising odour evaluation in practice
Emerging technology in odour detection

How the human nose detects odour

Odour perception begins the moment volatile compounds released by food enter the nasal cavity. According to the American Physiological Society, these volatile molecules reach the olfactory epithelium inside the nasal cavity, where they bind to olfactory receptor neurons and trigger an electrical signal that travels to the brain. The brain then interprets this signal as a specific smell.

StatPearls (NCBI) explains that the process involves G-protein coupled receptors (GPCRs) on olfactory sensory neurons. When an odorant binds to these receptors, a biochemical cascade produces cyclic AMP (cAMP), which opens ion channels and generates the nerve impulse sent to the olfactory bulb and then to higher brain centres. The olfactory bulb, in turn, connects closely with the limbic system – the emotional and memory centre of the brain – which is why smells so often trigger strong, immediate reactions.

The University of Oregon’s Introduction to Neurobiology notes that the typical human can distinguish up to 10,000 distinct odours. Furthermore, research published in PMC confirms that the olfactory system can detect odorants at concentrations as low as a few parts per trillion – a sensitivity level that makes smell one of the most powerful early-warning systems available for detecting food spoilage, often well before visible changes appear.

Why odour is critical in food quality assessment

Odour is not simply a pleasantness check. It reflects the underlying chemistry of food. A study published in PMC on spoilage detection explains that off-odours in meat and fish are largely produced by bacterial enzymes acting on amino acids – a process called decarboxylation. This breakdown of free amino acids produces amines such as cadaverine and isopentylamine, which generate the characteristic rancid, fishy smells associated with spoiled protein-rich foods. Similarly, lipid oxidation in meat and dairy products generates aliphatic acids and aldehydes that produce fatty and oily off-notes.

Research in PMC on food spoilage indicators further notes that microbial metabolism – particularly the degradation of glucose by bacteria – is a major driver of off-odour development in perishable foods during storage. This means the odour change is not random; it is a direct chemical marker of microbial activity. In practical terms, a trained evaluator smelling spoilage is detecting real biochemical evidence of food degradation.

As Lawless (1991) in the Journal of Food Quality points out, the human sense of smell remains the ultimate discriminator of food aroma and flavour quality, with instrumental analyses being a poor substitute because chemical components must always be cross-referenced against human sensory thresholds to determine their actual impact.

Common odour descriptors in sensory evaluation

To make odour assessment more consistent and communicable, sensory professionals use standardised descriptors. St. Croix Sensory Inc. identifies eight broad odour descriptor categories used globally: Vegetable, Fruity, Floral, Medicinal, Chemical, Fishy, Offensive, and Earthy – each with specific sub-descriptors that allow evaluators to communicate precisely. In food quality work, the most practically significant descriptors include:

Fragrant

A fragrant odour – the clean, sweet, or characteristic smell of a food in its prime – signals that the food’s volatile compounds are present in their intended concentrations. Fresh herbs, ripe fruit, and properly aged cheese all carry fragrant notes that confirm quality and freshness. The presence of this odour type indicates that beneficial biochemical processes are still intact.

Rancid

Rancidity is one of the most telling signs of spoilage, particularly in fats and oils. The DLG Expert Report on sensory panel training explains that certain carbonyl compounds produced during lipid degradation are described using off-aroma descriptors including rancid, fishy, and cardboard-like. The rancid smell – often likened to old paint or stale nuts – develops when fats are exposed to air, heat, or light, triggering oxidative breakdown. This is a common quality defect in nuts, oils, butter, and fatty processed foods.

Oily

An oily odour in a food that should not contain significant fat may suggest contamination or improper processing. In oil-containing products, an oily smell that deviates from the expected aroma profile can indicate that fat degradation has begun or that an inappropriate oil was used. This descriptor is particularly relevant in evaluating processed and packaged foods.

Metallic

A metallic odour is often associated with interaction between food and metal packaging, enzymatic browning reactions, or excess mineral content. DLG’s sensory analysis report identifies trans-4,5-epoxy-2-(E)-decenal as a specific compound responsible for metallic off-aromas – a compound that can form during lipid oxidation. This odour is especially noticeable in improperly stored canned goods or foods processed with worn metal equipment.

Why odour assessment is inherently subjective

Despite the biochemical precision behind smell, odour evaluation remains qualitative and subjective in practice. Several factors drive this variability.

Genetic differences

Olfactory sensitivity varies significantly from person to person due to genetics. Research documented on the sense of smell shows that specific odorant receptors and their genetic variants determine whether an individual can detect certain aroma compounds at all. For example, the receptor OR5A1 governs the ability to detect β-ionone, a key aroma compound in many foods, while OR6A2 is linked to sensitivity to the characteristic smell of coriander. Some individuals are “super-smellers” capable of detecting trace concentrations, while others have specific anosmias – the complete inability to detect particular odorants.

According to StatPearls, the estimated prevalence of olfactory dysfunction in adults under 65 is around 2%, but this figure rises sharply to approximately 75% in populations over 80 years old. This age-related decline is partly attributed to the gradual ossification of the cribriform plate and the cumulative damage to olfactory receptors over a lifetime. For food quality panels, this means that assessor age is a relevant variable – younger adults generally exhibit more acute and sensitive smell detection.

Cultural background and personal preferences

Odour perception is also shaped by cultural exposure and personal history. Research published in Science Advances confirms that the character and hedonic quality of an odour depend at least in part on the experiences and background of the individual perceiving it. A fermented food aroma regarded as appealing in one culture may be perceived as offensive in another. This cultural dimension makes it especially important for sensory panels to be diverse and for evaluators to be trained against standardised references rather than personal preferences.

Standardising odour evaluation in practice

Given its subjectivity, odour evaluation requires structured protocols to be useful as a quality control tool. FlavorSum’s sensory analysis guidelines highlight the importance of controlling external conditions – evaluators should avoid strong foods, perfumes, and smoking for at least an hour before testing, and panels should be conducted in neutrally scented, well-ventilated environments. Sample presentation must be standardised in terms of temperature and container type to avoid introducing confounding odours.

Panel composition matters too. St. Croix Sensory Inc. recommends that each odour panel include six to eight assessors selected to represent a cross-section of age and gender, reducing individual bias. Evaluators use standardised descriptor vocabularies and intensity scales – typically rated from 0 (none) to 5 (strong) – so that results can be compared and documented reliably across sessions.

The DLG Expert Report on panel training describes the use of aroma wheels – structured, multi-ring diagrams that organise odour families, sub-families, and specific descriptors – as a core training tool. These wheels provide evaluators with a common language, helping them move from a vague impression like “off-smelling” to a precise description like “rancid-fatty” or “metallic,” which carries actionable quality information.

Emerging technology in odour detection

While human evaluators remain the gold standard for nuanced odour assessment, technology is playing an expanding supporting role. A 2025 systematic review in MDPI highlights the electronic nose (e-nose) – a device that uses arrays of chemical sensors to detect volatile organic compounds and generate an odour “fingerprint” of food samples. These systems are particularly effective for freshness evaluation, spoilage detection, and adulteration identification in meat, dairy, and seafood.

Research from Skoltech, published in 2024, demonstrated that an e-nose can detect the microbial transition point between fresh and spoiled meat earlier than computer vision systems – identifying shifts in bacterial composition before they become visually apparent. However, as Springer’s Current Food Science and Technology Reports notes, these electronic systems currently lack the sensitivity and contextual discrimination of trained human evaluators, particularly for subtle or complex odour changes. The most effective approach in modern food quality assurance is a hybrid one: e-nose technology for routine, high-throughput screening, and trained human panels for final quality decisions and complex assessments.

What do you think? Given that odour sensitivity declines with age and varies significantly between individuals, how should food manufacturers design sensory panels to ensure consistent and reliable results? And as electronic nose technology continues to improve, at what point – if ever – do you think it could fully replace human evaluators in odour-based quality assessment?

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Table of Contents

How the human nose detects odour 🔗

Why odour is critical in food quality assessment 🔗

Common odour descriptors in sensory evaluation 🔗

Fragrant 🔗

Rancid 🔗

Oily 🔗

Metallic 🔗

Why odour assessment is inherently subjective 🔗

Genetic differences 🔗

Age-related decline 🔗

Cultural background and personal preferences 🔗

Standardising odour evaluation in practice 🔗

Emerging technology in odour detection 🔗