SOP: Triangle Test

GIFSQ — Global Institute for Food Safety & Quality

Principle

Three samples — two identical and one different — are presented simultaneously, all as unknowns, and the judge must identify the odd sample. The panelist must give an answer in every case, even when only guessing, so the chance probability is one-third. The test is a one-tailed test used primarily to detect flavour differences; it is applied when intersample effects such as after-taste are minimal, and for training and testing panels. It may be extended to measure the degree of difference and preference: after identifying the odd sample, the panelist states whether the difference is slight, moderate or large, and which sample is preferred — only the results of panelists who correctly identified the odd sample are used for the preference judgement.

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 triangle test

Sample preparation

Prepare the two products in exactly the same manner. Code the samples with three- or five-digit numbers (not A/B letter codes on the bench — the book’s specimen card uses letter-coded sampling sequences only for design). Keep all samples as uniform as possible with respect to characteristics other than the one under test — if the odd sample differs in colour from the paired samples, the panelist can identify it without tasting. Randomize the sampling sequence (e.g., AAB, ABA, BAA) across panelists.

Procedure

Present the three coded samples to the panelist simultaneously as unknowns

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Ask the panelist to determine which of the three is the odd sample (an answer is required in every case, even when the panelist is only guessing)

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Record whether the choice was a guess (Yes/No)

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Ask the panelist to indicate the degree of difference between the like and odd samples: None, Slight, Moderate, Much

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Ask for acceptability: is the odd sample preferred, or the like sample preferred (Yes/No for each)?

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Record any comments

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Repeat for the required number of panelists (trained panel: 5–12 judges; 3 samples per test — 2 identical and 1 different)

Interpretation of results

Refer the number of correct identifications to the tabulated data for the triangle difference test in Table 19-4 of the book to decide statistical significance. Evaluate degrees of difference and preference only from the panelists who correctly identified the odd sample. If the number of correct identifications only just reaches the level of significance and the panel is divided in its preference, carry out a separate preference test (e.g., paired comparison) with a larger panel.

Worked illustration from the book: to test whether marmalade containing 40% Seville orange pulp differed from marmalade containing 25% Seville orange pulp plus 15% Tenali orange pulp, ten panelists judged on two consecutive days (16 judgements). The odd sample was correctly identified 11 times — significant at the 1% level by Table 19-4. Degrees of difference among the correct identifications: Moderate 7, Much 4. Of the 11 correct identifiers, 9 preferred the 25% Seville + 15% Tenali sample — not significant at the 5% level for paired preference (Table 19-4), so no significant preference was established.

Specimen evaluation card fields: odd sample check-boxes for each sampling sequence (AAB, ABA, BAA); guess (Yes/No); degree of difference (None / Slight / Moderate / Much); acceptability (odd sample preferred Yes/No; like sample preferred Yes/No); comments; signature.

Precautions

• Make all samples uniform in every characteristic except the one under test; use coloured lighting if colour differences would betray the odd sample.
• 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.
• Do not treat questions on degree of difference and preference as afterthoughts — they must be answered from critical tasting at the time of 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.