The Refractive Index (RI) of oil or fat is a means for identification of the nature of a particular oil due to the difference in saturation, conjugation, presence of hydroxyl substituted, and chain length of fatty acids.
Measurement of refractive index is convenient for the following the progress of hydrogenation of oils and fats.
The refractive index is measured under different temperature conditions applied such as 20°C for oils, 40°C for solid fats which are fully molten at that temperature, 60°C for hydrogenated fats, and 80°C for waxes.
The Refractive index is very specific for a particular oil or fat.
RI is expressed as the ratio between the sine of the angle of incidence to the sine of the angle of refraction when a ray of light of a known wavelength (mean of the D lines of sodium) passes from air into the oil or fat..
Refractometer – Abbe or Butyro refractometer.
The temperature of the refractometer should be controlled to within ±0.1°C and for this purpose, it should be provided with a thermostatically controlled water bath and a motor-driven pump to circulate water through the instrument.
The instrument should be standardized, following the manufacturer’s instructions, with a liquid of known purity and refractive index or with a glass prism of known refractive index.
Distilled water, which has a refractive index of 1.3330 at 20.0°C, is a satisfactory liquid for standardization.
Light Source – If the refractometer is equipped with a compensator, a tungsten lamp or a daylight bulb may be used. Otherwise, a monochromatic light, such as an electric sodium vapor lamp, should be used.
Melt the sample, if it is not already liquid, and filter through a filter paper to remove any impurities and the last traces of moisture.
Make sure that the sample is completely dry. Adjust the temperature of the refractometer to 40.0 ± 0.1°C or to any other desired temperature.
Ensure that the prisms are clean and completely dry, and then place a few drops of the sample on the lower prism.
Close the prisms, tighten firmly with the screwhead, and allow to stand for 1-2 min.
Adjust the instrument and light to obtain the most distinct reading possible, and determine the refractive index.
Temperature Corrections Unless the correction factors are specified in the detailed specification, approximate corrections shall be made using the following equation:
R = R’ + K (T’ – T )
R = the reading of the refractometer reduced to the specified temperature, T°C;
R’ = the reading at T’°C;
K = constant, 0.000365 for fats, and 0.000385 for oils (if Abbe refractometer is
used ), or = 0.55 for fats and 0.58 for oils (if Butyro refractometer is used );
T’ = the temperature at which the reading R’ is taken; and
T = the specified temperature (~40.0°C).
RESULTS AND INFERENCE
The mean of the results of two determinations should be taken and the temperature maintained should be reported along with the refractive index.
The difference between the results of two determinations carried out simultaneously or in rapid succession by the same analyst (repeatability) shall not exceed 0.1% by mass.
The refractive index of commonly used edible oils is given as below.
• Before the measurement, the prism should be properly cleaned with acetone or with suitable solvent.
• Sample should be free from moisture and any other impurities melted at the same temperature at which reading has to be taken.
• Temperature of the prism should be accurately maintained.