SOP: Estimation of Gallates

GIFSQ — Global Institute for Food Safety & Quality

Principle

The gallates — propyl (PG), octyl (OG) and dodecyl (DG) — in 95% methanol, when shaken with solid ammonium ferrous sulphate, form a clear and stable blue solution with an absorption maximum at 580 nm. Such solutions obey Beer’s law over the range 0.0 to 0.8 mg of gallate. The absorption spectrum of PG, OG or DG is similar, but the optical density at any given concentration depends on the particular ester present.

Apparatus required

• Extraction vessels: boiling tubes, each with a bulb of 10 to 12 ml capacity blown in its side at the bottom (to separate an upper layer by tilting the tube)
• 50-ml calibrated flask
• Spectrophotometer

Reagents

a. Liquid paraffin (BP grade).

b. 95% Methanol (AR grade).

c. Ammonium ferrous sulphate (AR grade).

d. Calcium carbonate (AR grade).

e. Acetone (AR grade): shake with calcium carbonate for 1 min and filter.

Procedure

Shake vigorously 10 g of warm liquid sample or melted fat with 25 ml of 95% methanol for 1 min in the extraction vessel (for solid samples, use a 5-g portion plus 5 ml of liquid — separation into clear layers is unlikely and unnecessary)

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Pour the upper layer into a 50-ml calibrated flask; repeat the extraction with 20 ml of 95% methanol; again transfer the upper layer to the flask and dilute to the mark

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Add 1 g of calcium carbonate to the contents of the flask, shake for 30 sec, filter through Whatman No. 1 filter paper, and reject the first few ml of filtrate; the calcium carbonate added must be sufficient to ensure a clear filtrate at this stage

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To 10 ml of the filtrate, add exactly 1 ml of acetone and about 10 mg of powdered ammonium ferrous sulphate, and shake for 1 min

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Set the solution aside for 30 min to attain full colour development, and measure the colour at 589 nm; for extremely dilute solutions use larger cells

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Prepare standard solutions using different concentrations of gallates and develop the colour similarly; draw a standard curve, plotting OD as ordinate and concentration as abscissa

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From the standard curve note the amount of gallate present in the sample

Calculation

Gallate present (mg) = d × K

Where,

• d = the OD measured in 1-cm cells
• K = 0.622 for PG, 0.785 for OG, or 0.952 for DG — these factors should preferably be determined for each batch of reagents

The amount of gallate present per 11 ml of final solution is calculated from this expression.

Note: the book states the absorption maximum as 580 nm but the procedure measures the colour at 589 nm — carried as printed, flagged; consult the printed book.

References

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

Cassidy, W. & A.J. Fisher, Analyst, 85, 295 (1960) — as cited in the book.