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SOP: Dilution Test

GIFSQ — Global Institute for Food Safety & Quality

Principle

Dilution tests establish the smallest amount of an unknown material, developed as a substitute for a standard product, that can be detected when it is mixed with the standard product — e.g., margarine in butter, dried whole milk in fresh milk, or synthetic orange flavour ingredients in natural flavour. The quality of the test material is represented by the dilution number, which is the per cent of the test material in the mixture of the standard product at which there exists a just identifiable difference in odour and taste between them. A higher dilution number indicates a better-quality test material.

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 (questionnaire as in the threshold and difference tests)

Sample preparation

Carry out a preliminary test to establish the higher and the lower per cent of the test material in the mixture which are likely to be respectively definitely detectable by all and not detectable by all. Define a series of mixtures including those which represent the upper and lower limits — usually six mixtures are enough, although eight may be used if greater precision is desired. Code the mixtures with three- or five-digit numbers.

Procedure

Prepare the series of mixtures of the test material with the standard product, covering the established upper and lower limits (trained panel: 12–24 judges; 5–10 mixtures per session)

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Give a simple paired difference test of this test series against the standard, starting with the mixture containing the lower per cent of the test substance

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Obtain 15 to 20 judgements for each sample of the series

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Determine the mixture at which the frequency of the difference judgement just reaches the 5% significance level

Interpretation of results

The per cent of the test material in that mixture is the dilution number — the per cent at which a just identifiable difference in odour and taste exists between the mixture and the standard. A bigger dilution number indicates a better quality of the test material. The questionnaire and the statistical analyses are similar to those in the threshold and difference tests.

A useful variation is the dilution flavour profile test: the product is analysed by stepwise dilution, and the detectable flavour components are recorded in their sequence of appearance and intensity. This is useful in analysing complex processed foods, helps in studying the flavour secrets of successful products, and in studying the influence of variety, processing, ageing, storage and other factors.

Precautions

  • Establish the upper and lower limits by a preliminary test before defining the mixture series.
  • Start the paired difference testing with the mixture containing the lowest per cent of the test substance.
  • Obtain 15 to 20 judgements for each sample of the series.
  • 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.
  • 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.