SOP: Threshold Test

GIFSQ — Global Institute for Food Safety & Quality

Principle

Sensitivity tests measure the ability of an individual to smell, taste or feel specific characteristics in foods, beverages or pure substances. They are used frequently for selecting panelists for evaluation work in product research and development, and for establishing the intensity of sensory response to a food or food component. The threshold is defined as a statistically determined point on the stimulus scale at which a transition in a series of sensations or judgements occurs.

There are three types of threshold:

a. Stimulus/detection threshold — the magnitude of stimulus at which a transition occurs from no sensation to sensation.

b. Recognition/identification threshold — the minimum concentration at which a stimulus is correctly identified.

c. Terminal/saturation threshold — the magnitude of a stimulus above which there is no increase in the perceived intensity of the stimulus.

Recognition threshold tests with basic tastes or odours are most frequently employed for panel selection; with materials such as spices they are used for assessing the intensity of odour or flavour as the mean threshold value by a trained panel. The threshold value is given as a mere number, which is the denominator of the dilution where the odour or flavour is recognized. The tests are also used where the minimum detectable difference of an additive or of an off-flavour is to be established.

Apparatus required

• Individual judging booths (identical, uniformly illuminated), each fitted with drinking water, a glass, clean towels and a basin or receptacle for expectoration
• Beakers containing the series of threshold test solutions, in increasing concentration
• Prepared evaluation cards for the sensitivity-threshold test

Materials

Stock solutions for the four taste qualities (prepare exactly as given):

a. Salty stock solution — 5.845 g of sodium chloride per litre.

b. Sweet stock solution — 34.23 g of sucrose per litre.

c. Sour stock solution — 2.1015 g of citric acid per litre.

d. Bitter stock solution — 19.41 g of caffeine per litre.

Threshold test solutions (dilute the indicated volume of stock solution to 1 litre with water):

• Solution 1: salty 2 ml; sweet 2 ml; sour 0.5 ml; bitter 0.5 ml
• Solution 2: salty 4 ml; sweet 4 ml; sour 1 ml; bitter 1 ml
• Solution 3: salty 8 ml; sweet 8 ml; sour 2 ml; bitter 2 ml
• Solution 4: salty 16 ml; sweet 16 ml; sour 4 ml; bitter 4 ml
• Solution 5: salty 32 ml; sweet 32 ml; sour 6 ml; bitter 6 ml
• Solution 6: salty 64 ml; sweet 64 ml; sour 8 ml; bitter 8 ml
• Solution 7: salty 128 ml; sweet 128 ml; sour 10 ml; bitter 10 ml
• Solution 8: salty 256 ml; sweet 256 ml; sour 12 ml; bitter 12 ml
• Solution 9: salty 2.994 g/litre (prepared directly); sweet 17.526 g/litre; sour 14 ml of stock; bitter 14 ml of stock
• Solution 10: salty 5.988 g/litre; sweet 35.052 g/litre; sour 16 ml of stock; bitter 16 ml of stock
• Solution 11: salty 11.976 g/litre; sweet 70.103 g/litre; sour 32 ml of stock; bitter 32 ml of stock
• Solution 12: salty 23.953 g/litre; sweet 140.206 g/litre; sour 64 ml of stock; bitter 64 ml of stock

The book (Table 19-3, after Jellinek, J. Nutri. Dist., 1, 219 (1964)) also tabulates a concentration column alongside the dilution volumes (salty: 0.0002, 0.0004, 0.0008, 0.0016, 0.0032, 0.0064, 0.0128, 0.0256, 0.0512, 0.1024, 0.2048, 0.4096; sour: 0.00005, 0.0001, 0.0002, 0.0004, 0.0006, 0.0008, 0.0010, 0.0012, 0.0014, 0.0016, 0.0032, 0.0064). The unit of that column is not legible in the available copy and the sour column does not agree with the dilution volumes as molarity — the column is reproduced as printed and should be checked against the printed book. The number of solutions used for the test itself is 5–10.

Sample preparation

Prepare the four taste-quality series as above. The panelist receives the series of beakers with increasing concentrations of one of the four taste qualities (sweet, salty, sour, bitter).

Procedure

The panelist receives the series of beakers, starting with beaker No. 1 (untrained panel; 5–10 solutions)

↓

The panelist tastes beaker No. 1, then No. 2, No. 3, and so on in increasing concentration; re-tasting of already tasted solutions is not allowed

↓

For each beaker the panelist describes the taste or gives an intensity score, using the scale: 0 = None, or the taste of pure water; ? = Different from water, but the taste quality not identifiable; the threshold mark = Threshold, very weak (taste identifiable); 1 = Weak; 2 = Medium; 3 = Strong; 4 = Very strong; 5 = Extremely strong (the threshold symbol is garbled in the available copy; the scale is carried as printed)

↓

Record the description of the taste and feeling factors for each set number on the evaluation card

Interpretation of results

The recognition threshold is the minimum concentration at which the stimulus is correctly identified; the detection threshold is the concentration at which the transition from no sensation to sensation occurs. For assessing the intensity of odour or flavour (e.g., of spices) by a trained panel, report the mean threshold value as the denominator of the dilution at which the odour or flavour is recognized.

Specimen evaluation card fields: set number; description of taste and feeling factors; signature.

Precautions

• Do not allow re-tasting of already tasted solutions.
• Conduct judging in individual booths so that judgements are independent; allow no communication between panel members except consultation with the panel leader on any point of doubt.
• Test in the hour after a normal meal (e.g., 10.00 a.m. to 12 noon, or 3 to 5 p.m.).
• Panel members should not smoke or chew pan or supari for at least half an hour before the test.
• Keep the evaluation card simple, clearly typed or printed, with unambiguous terms and directions in the desired sequence of action.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Jellinek, B., J. Nutri. Dist., 1, 219 (1964) — source of the threshold solution preparation procedure cited in the book.