SENSORY EVALUATION: MEASURING SENSITIVITY
Diagram
The smallest you can sense, and all that is above it
Threshold testing
Finds the smallest amount of a taste, smell or texture that a person can just detect. Below it, nothing is sensed.
↓
Above the threshold
Once the stimulus is above the threshold, it can be sensed, and stronger stimuli feel stronger.
↓
Suprathreshold testing
Measures how strong a stimulus feels above the threshold: a little strong, strong, very strong, and how the feeling grows.
Figure 1: Threshold testing finds the smallest detectable amount; suprathreshold testing measures intensity above it. Drawn in our own style from the measurement of sensory thresholds chapter (p. 173) of Lawless and Heymann, Sensory Evaluation of Food.
Introduction
Threshold testing and suprathreshold testing both measure how people sense a stimulus, but they work at different levels. Threshold testing finds the smallest detectable amount of a taste, smell or other stimulus, while suprathreshold testing measures the intensity of stimuli above that level. Lawless and Heymann give the measurement of sensory thresholds its own chapter (p. 173).
The core difference is the level: threshold testing asks whether anything is sensed at all, and suprathreshold testing asks how strong the sensation feels once it is clearly sensed.
Quick comparison
Table 1: Threshold testing compared with Suprathreshold testing
| Aspect | Threshold testing | Suprathreshold testing |
|---|---|---|
| What is measured | The smallest amount that can just be detected | How strong a stimulus feels, once it is clearly above the threshold |
| Stimulus level | At the edge of sensing, very weak | Clearly above the threshold, weak to strong |
| Question asked | Can you sense anything at all | How strong is it |
| Answer | Yes or no at each level, giving a threshold value | A number on an intensity scale |
| Samples used | A series of very weak samples, rising in small steps | A series of clearly sensed samples, from weak to strong |
| Who tastes | Trained or screened panellists, or consumers for a population threshold | Trained panellists, who can rate intensity reliably |
| Scale used | None: the result is a concentration or a threshold value | An intensity scale, such as a line scale or a category scale |
| What it shows | How sensitive a person is | How intensity grows as the stimulus grows |
| Use | Screening panellists, setting detection limits, finding the smallest change that matters | Building intensity scales, setting sweetness or saltiness levels, profiling products |
| Variability | High: thresholds vary a lot from person to person | Lower: trained panellists rate intensity more steadily |
Scientific explanation
Definition
Threshold testing measures the sensory threshold: the smallest amount of a stimulus, such as a taste in water or a smell in air, that a person can just detect. Below the threshold the stimulus is not sensed at all, and above it the stimulus is sensed. Lawless and Heymann cover the measurement of sensory thresholds in their own chapter (p. 173).
Suprathreshold testing measures sensation above the threshold: how strong a stimulus feels when it is clearly sensed, and how that feeling grows as the stimulus grows stronger.
Working principle
Threshold testing works by presenting a series of samples in rising steps, from so weak that nothing is sensed to just strong enough to be sensed. The threshold is the level where the panellist starts to detect the stimulus more often than guessing would give, and it is found by counting right answers across the series.
Suprathreshold testing works by presenting clearly sensed samples and asking the panellist to rate how strong each one feels. The ratings grow with the stimulus, but not in a straight line: doubling the stimulus does not always double the feeling, and the shape of that growth is part of what the test measures.
Apparatus
Threshold testing needs a series of samples at known, rising concentrations, coded cups, water for rinsing, and a ballot for yes or no answers. Suprathreshold testing needs samples at known levels above the threshold, and ballots with intensity scales. Both need a quiet test room and a trained panel leader.
Measurement procedure
For threshold testing, the panellist tastes a series of samples in rising order, or in a forced choice between a sample and a blank, and says at each step whether anything is sensed. The threshold is the lowest level sensed more often than guessing would give. For suprathreshold testing, the panellist tastes samples at several levels above the threshold and rates the intensity of each on a scale.
The books used here describe the threshold methods in full; they do not give one single standard procedure for both, because the two tests ask different questions and use different scales.
Factors affecting readings
Fatigue matters for both: tasting many weak samples is tiring, and a tired panellist misses the threshold. Adaptation matters too: sensing the same taste again and again dulls it, so the panellist must rinse and rest between samples. The person’s own sensitivity matters most for thresholds, and training matters most for intensity ratings.
Formulas and calculations
The books used for this article give no single formula for this comparison, so none is given here. Thresholds are found by counting right answers across a series of rising levels and comparing the count with guessing, and suprathreshold intensity is read from the ratings on the scale, not from a calculation.
Industrial applications
- Threshold testing is used to screen panellists, keeping only those who can sense the tastes and smells the panel needs.
- Threshold testing is used to find the smallest amount of an off flavour that buyers can detect, so a fault limit can be set.
- Threshold testing is used to set detection limits for flavours and taints in quality control.
- Suprathreshold testing is used to set the sweetness, saltiness or strength of a product at the level buyers like.
- Suprathreshold testing is used in descriptive analysis, where each attribute is rated for intensity above its threshold.
Advantages and limitations
- Threshold testing, advantage: gives a clear, simple value for sensitivity, and it is good for screening panellists.
- Threshold testing, limitation: thresholds vary a lot from person to person, so many panellists are needed for a steady value.
- Suprathreshold testing, advantage: measures what the buyer actually feels, which is intensity, not the edge of sensing.
- Suprathreshold testing, limitation: needs trained panellists who can rate intensity reliably, and ratings are harder than yes or no answers.
FAQ
What is a sensory threshold?
The smallest amount of a taste, smell or other stimulus that a person can just detect. Below it, nothing is sensed; above it, the stimulus is sensed.
Does everyone have the same threshold?
No: thresholds vary a lot from person to person, and with age, illness and smoking. That is why thresholds are measured across a panel, not in one person.
Can a food be below threshold and still matter?
Yes, in a way: a flavour below threshold cannot be sensed on its own, but several weak flavours together can add up to a sensed flavour. That is why suprathreshold testing matters too.
Why does intensity not double when the stimulus doubles?
Because the senses do not work in a straight line: the feeling grows more slowly than the stimulus. Doubling the sugar does not double the sweetness.
Which test is used to screen panellists?
Threshold testing: people who cannot sense the tastes or smells the panel needs are not taken on, and people who can are trained further.
