Judging of Dairy Products

Module 2. Requirement of sensory evaluation and physiology of human senses

Lesson 9
MEASUREMENT OF SENSATION INTENSITY

9.1 Measurement of Sensation Intensity

9.1.1 Threshold value

It can be defined as "statistically" determined point on the "stimulus scale" at which occurs a transition in a series of sensations. Several types of threshold have served as important tool in developing better understandings of the relationships between magnitude of stimulus and perceived sensation.

9.1.2 Absolute threshold

The least energy capable of producing a sensation. Sensation changes in lawful or predictable manner as the stimulus energy is increased, but changes in different way to each sense.

9.1.3 Detection/ Stimulus threshold

That magnitude of stimulus at which transition occurs from no perceived sensation to a perceived sensation (RL), similar to absolute threshold.

9.1.4 Difference threshold (DL)

The least amount of change of a given stimulus necessary to produce a change in sensation, and the interval/unit of differences is designated as "JND" just noticeable difference.

9.1.5 Recognition threshold (Identification)

It is the minimum concentration at which a substance is correctly identified.

9.1.7 Terminal threshold

It is that magnitude of stimulus above, which there is no increase in perceived intensity of quality for stimulus. Above this point, pain occurs to the subject.

9.2 Relation Between Stimulus and Perception

Functional Relationship between above two is dealt by "Psychophysics".

The three parameters:

• Task undertaken by the observer.

• The manner of presenting the stimulus, and

• Statistical measure used to describe the data.

9.3 Psychological Errors in Judgment

• The error of habituation: results from a tendency to continue to give the same response when a series of slowly increasing/decreasing stimulus is presented.

• The error of expectation: include the overly anxious observer to find a difference when none exists.

• A stimulus error: results when a subject knows that the test is being given in a certain way or when containers used or the procedure followed suggest differences and therefore cause him to find them when they do not exist.

• The Logical error: occurs when two characteristics of a food, which are logically associated in the minds of the observer, are rated the same.

• The error of Leniency: applies to ratings where bias in favour of some person or some object causes the observer to rate them higher than they should.

• Error of Central Tendency: arising in the tests involving judgment, rather hesitant to use the extreme values on a scale. This error probably also applies in sensory evaluation of unfamiliar foods.

• The Contrast error: may be noted, where expected or preferred method of evaluation is not followed. Foods may the be rated lower than with expected method; also, when a poor sample follows a good sample, the contrast error appears greater than when they re judged separately.

• The Proximity error: associated with judging scales, is attributed to the fact that adjacent traits tend to be rated similarly. Simultaneous scoring of color, texture, odor, taste & acceptability on the same set of samples can give different scores from those obtained when each trait is judged individually.

• A Time error or positional bias: i.e. over selection of one sample on the basis of its order of presentation, has been demonstrated in paired tests.

• The Association error: is tendency to repeat previous impressions -a form of conditioned response.

Error of the First and Second kind :
Failure to detect a stimulus that is actually present is called and error of the First kind.
Reporting a signal when no stimulus is present is called an error of the Second kind.
These errors may be caused by expectation and can be influence by motivation.
The most effective method of improving ratings, and thereby reducing psychological errors, is to train judges, carefully. Training that includes practice, followed by group discussion, has been recommended as being most effective.

Adaptation

When an exposure to a stimulus is prolonged, sensory response declines, i.e. adaptation occurs. This applies to direct sensory response as well as electrical activity. Complete Adaptation, i.e. no response, is possible but is certainly of little importance in the sensory examination of foods. Adaptation appears to be due to some special inhibition of the cell receptor membrane in the case of taste, rather than exhaustion of some receptive substance in the cell. Adaptation is relatively slow at higher concentration. Recovery from adaptation is rapid at first and slows thereafter.

1. The task may be:

Classification: whether the stimulus is present/absent.

Order: the stimulus is greater/lesser

Intervals: the apparent difference between two/more perceptions.

Ratio: report the ratio of magnitude of perceptions.

Magnitude: the magnitude of a perception.

2. Two types of the stimuli arrangement are commonly used.

(I) Fixed Stimuli: not varied during the observation.

(2) Adjustable Stimuli: may be altered during experiment.

Statistical Measures

Usually involve measures of central tendency -median and measures of variability.
Discrimination: relates how two stimuli differ.
Scaling considers how much of stimulus present.