SOP: Refractive Index of Oils and Fats

GIFSQ — Global Institute for Food Safety & Quality

Principle

The refractive index of an oil or fat, measured at a specified temperature, is an identity characteristic used to distinguish one oil from another and to detect admixture. It is read on an Abbe or Butyro refractometer against a monochromatic light source.

Apparatus required

• Abbe or Butyro refractometer, with monochromatic light (tungsten lamp with compensator, or sodium vapour lamp)
• Constant-temperature water bath, to hold the refractometer at 40°C ± 1°C
• Filter paper

Procedure

Check that the instrument gives the correct reading at the specified temperature as shown in the instrument’s calibration table; check the readings given by two standards whose refractive indices are within the sample test range, and if the readings differ from the stated values, correct them using the instrument key

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Adjust the temperature of the refractometer to 40°C ± 1°C by circulating water from a constant-temperature water bath

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Melt the sample, if it is not already a liquid, and filter it through filter paper

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Smear the sample on the cleaned prism and read the refractive index

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After taking the reading, clean the prism using lukewarm water and wipe dry

Calculation — Temperature correction

Readings taken at a temperature other than the specified temperature are corrected with the approximate equation:

R = R' + K (T' − T)

Where,

• R = refractometer reading adjusted to the specified temperature
• R’ = the reading taken at T’°C
• T’ = the temperature at which the reading R’ was taken
• T = the specified temperature (generally 40°C)
• K = a constant: 0.00365 for fats and 0.00385 for oils (Abbe refractometer); 0.55 for fats and 0.58 for oils (Butyro refractometer — convert the reading to refractive index using standard tables)

References

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