SOP: Duo-Trio Test
GIFSQ — Global Institute for Food Safety & Quality
Principle
Three samples — two identical and one different — are used to determine whether a perceptible difference exists between two products. The panelist first receives one of the identical pair as a known reference sample (R), then the other two samples successively in random order, and must indicate which of the two coded samples matches the reference. An answer is required in every case, even when the panelist is only guessing, so the chance probability is one-half. The test is essentially a paired comparison discrimination method presented with three samples; it is useful for detecting differences when carryover or after-taste may be present, and for training and testing panels.
Apparatus required
- Individual judging booths (identical, uniformly illuminated), each fitted with drinking water, a glass, clean towels and a basin or receptacle for expectoration
- Coloured lighting (where needed to mask small colour differences between samples)
- Stainless steel, glass or china serving dishes and cups; plain serving trays; small stainless steel forks and spoons
- Prepared evaluation cards for the duo-trio test
Sample preparation
Prepare the two products in exactly the same manner. Code the samples with three- or five-digit numbers. Prepare a reference sample (R) — one of the identical pair — and two test samples (the second of the identical pair plus the different sample), to be presented in random order.
Procedure
Present the reference sample (R) to the panelist first and ask the panelist to taste it carefully
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Present the pair of coded samples successively, in random order
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Ask the panelist to judge which of the two coded samples is the same as the reference R (a positive answer is required even if it is a guess)
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Repeat for the required number of sets (trained panel: 5–12 judges; 3 samples per test — 2 identical and 1 different)
Interpretation of results
Sum the total number of comparisons made and the total number of correct responses, and refer to the tabulated data for the paired/duo-trio difference test (one-tailed test) in Table 19-4 of the book. The test is a one-tailed situation, since only the coded control should be correctly matched to the known reference sample; the probability of either sample being matched to the reference is 1:1. Note that the test does not provide information on the nature of the difference, even if one exists, nor on the intensity of the difference.
Worked illustration from the book: to test whether 25% substitution of mandarin orange juice with sweet orange juice in orange squash manufacture could be detected, a duo-trio test was run on two successive days with six panelists (12 judgements in total). Eight matches to the reference were correct. For 12 judgements in a two-sample test, Table 19-4 requires 10 correct judgements for significance even at the 5% level — the substitution could not be detected.
Specimen evaluation card fields: set number; code numbers of the pair; which sample is the same as R; signature.
Precautions
- 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.
- Judge quickly but not hurriedly; make odour observations by sniffing before tasting.
- Allow retesting, but avoid prolonged testing; tasters may swallow or spit out samples, and should rinse between samples (or use bread or puffed rice) to remove a strong flavour.
- Present the reference and test samples in random order to every panelist.
- 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.