SOP: Reichert-Meissl Value of Fats

GIFSQ — Global Institute for Food Safety & Quality

Principle

The Reichert-Meissl (R.M.) value is the number of millilitres of 0.1 N NaOH solution required to neutralise the steam-volatile, water-soluble fatty acids distilled from 5 g of an oil or fat under the precise conditions specified in the method. It is a measure of the water-soluble steam-volatile fatty acids, chiefly butyric and caproic acids. The R.M. fatty acids can be further split into fatty acids forming water-soluble silver salts and those forming water-insoluble silver salts.

Apparatus required

• Standard R.M.-Polenske-Kirschner apparatus: flat-bottom boiling flask, still head (10.7 cm wide and 18 cm high), condenser (52 cm long with 30 cm cooling length and 7 cm entry tube), and receiver (with graduations at 100 ml and 110 ml)
• Watch glass
• Water bath at 15°C
• 9 cm Whatman No. 4 filter paper
• 250 ml conical flask (dry)

Reagents

a. Analytical grade glycerol.

b. 50% w/w NaOH solution in water: protect from CO₂; use the clear portion.

c. Pumice powder, 1.4–2.0 mm in diameter.

d. Dilute H₂SO₄: 25 ml of conc H₂SO₄ made to 1000 ml with water.

e. Boiling distilled water, free of CO₂.

f. 0.1 N NaOH (not to be used if the Kirschner value is to be determined).

g. Phenolphthalein indicator.

Procedure

Heat the sample until the fat separates, but do not allow the temperature to rise above 50°C; filter through a filter paper until the filtrate is clear

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Weigh 5 g ± 0.01 g of the fat sample into the Polenske flask; add 20 g of glycerol and 2 ml of 50% NaOH solution from a burette which is protected from CO₂ pick-up; wet the tip of the burette before adding the alkali to free it of carbonate deposit, and reject the first 0.5 ml of NaOH

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Heat the mixture over a low flame with wire gauze until the liquid becomes clear and the fat has saponified; do not overheat at this stage, which causes discolouration

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When all the fat has saponified, cover the flask with a watch glass and allow to cool; add 93 ml of boiling CO₂-free distilled water and mix — the solution must be completely clear at this stage and pale yellow in colour (if it is not clear, indicating incomplete saponification, or if it is darker, indicating overheating, repeat with a fresh sample; an old sample of oil or fat may behave similarly)

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Add 0.1 g of pumice powder and 50 ml of dilute H₂SO₄; connect to the distillation apparatus; warm the mixture until any insoluble material present melts

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Increase the heat and distil 110 ml of solution in 19 to 21 min; the distillation is considered to begin when the first drop forms in the still-head

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Stop heating soon after 110 ml has distilled over; replace the graduated flask by a measuring cylinder to collect the drippings from the condenser; close the graduated flask with the stopper; do not mix

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Place the flask in a water bath at 15°C for 10 min, ensuring that the 110 ml graduation mark is below the water level; mix and filter through a 9 cm Whatman No. 4 filter paper, rejecting the first 2 or 3 ml of the filtrate and collecting the rest in a dry flask

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Wash the condenser, still head and the 110 ml graduated flask with three 15 ml portions of distilled water, passing each washing through the measuring cylinder, the 110 ml flask and the stopper; filter through the same filter paper, ensuring that all the insoluble matter is transferred to the paper; discard this filtrate (do not mix it with the filtrate of the distillate) — the filtrate for the R.M. value must be free from water-insoluble fatty acids

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Run a blank determination without the sample

Calculation — R.M. value

Pipette 100 ml of the filtrate into a dry 250 ml conical flask and titrate with 0.1 N NaOH using phenolphthalein as indicator:

R.M. value = (Sample titre − Blank titre) × N of NaOH × 11

Correction at high altitudes: R.M. and Polenske values are affected by the low barometric pressures which occur at high altitudes; under such conditions use:

                                (Observed R.M. − 10) × log 760
Corrected R.M. value = ───────────────────────────────────────── + 10
                                            log p

Where,

• p = barometric pressure in mm of mercury at the place of determination

Note: the source prints “og p” in the denominator; it is carried here as log p.

References

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