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TANNIC ACID

Prepared at the 39th JECFA (1992), published in FNP Add 1 (1992) superseding specifications prepared at the 35th JECFA (1989), published in FNP 49 (1990) and in FNP 52 (1992)


SYNONYMS

Tannins (food grade), gallotannic acid, INS No. 181

DEFINITION

Consists of gallotannins obtained by solvent extraction from certain natural sources; the substance is not an acid in the chemical sense. The common name "Tannic acid" has been adopted to distinguish the commercial substance from other tannins, such as condensed tannins. These specifications relate only to hydrolysable gallotannins, i.e., those which yield gallic acid on hydrolysis. These specifications do not apply to many other kinds of tannins which occur in nature, including condensed (non-hydrolysable) tannins and hydrolysable ellagitannins. Hydrolysable gallotannins may be obtained from nutgalls, the excrescences which form on young twigs of various Quercus species, e.g., Q. infectoria; these include Chinese and Aleppo tannins. They may also be obtained from various Sumac species, e.g. Rhus corieria, R. galabra, R. thypia; these include Sicilian and American sumacs. All of these consist essentially of polydigalloyl esters of glucose. A further source of hydrolysable gallotannins is the seed pods of Tara (Caesalpinia spinosa); these tannins consist essentially of the polydigalloyl esters of quinic acid.

Assay

Not less than 96% on a dried basis

DESCRIPTION

Amorphous powder, glistening scales or spongy mass, varying in colour from yellowish white to light brown; odourless or with a faint, characteristic odour

FUNCTIONAL USES

Clarifying agent, flavouring agent, flvour adjunct

CHARACTERISTICS

IDENTIFICATION

Solubility

Soluble in water, acetone and ethyl alcohol; insoluble in benzene, chloroform and ether; 1 g dissolves in about 1 ml of warm glycerin

Colour reaction

To a 1 in 10 solution add a small quantity of ferric chloride TS. A bluish black colour or precipitate forms

Precipitate formation

A solution of the sample when added to a solution of either albumin or gelatin produces a precipitate

Test for gallic acid

Passes test after hydrolysis

See description under TESTS

PURITY

Loss on drying

Not more than 7% (105o, 2 h)

Sulfated ash

Not more than 1%

Test 1.0 g of the sample (Method I)

Gums or dextrin

Dissolve 1 g in 5 ml of water, filter, and to the filtrate add 10 ml of alcohol; no turbidity is produced within 15 min

Resinous substances

Dissolve 1 g in 5 ml of water, filter and dilute the filtrate to 15 ml; no turbidity is produced

Condensed tannins

Not more than 0.5%

See description under tests

Residual solvent

Not more than 25 mg/kg acetone or ethyl acetate, singly or in combination

See description under TESTS

Arsenic

Not more than 3 mg/kg (Method II)

Lead

Not more than 10 mg/kg

Heavy metals

Not more than 40 mg/kg

Test 0.5 g of the sample as directed in the Limit Test (Method II)

TESTS

IDENTIFICATION TESTS

Test for gallic acid

Proceed as directed under Chromatography (thin-layer chromatography)

Sample preparation:
Add 20 mg of tannase to 100 ml of 1 in 1000 solution of the sample. Incubate at 37o for 2 h.

Standard preparation:
1 in 1000 solution of gallic acid.

Developing solvent:
A mixture of 5 volumes of chloroform, 4 volumes of ethyl formate and 1 volume of formic acid.

Spraying reagent:
A mixture of 1 volume of 1 in 100, 10% ethanol solution of ferric chloride (FeCl3), and 1 volume of 1 in 100, 50% ethanol solution of potassium ferricyanide (K3FeCN)6).

Absorbent: Silica gel

Apply 10 µl of the sample preparation and 10 µl of the standard preparation. Stop the development when the solvent front has advanced about 15 cm, dry in air, and spray the reagent. The Rf value of the sample and that of the reference standard are identical.

PURITY TESTS

Condensed tannins

The following HPLC method can be used for the determination of condensed tannins in the sample.

Sample preparation:
Dissolve 200 mg of the sample in 100 ml of a mixture of 100 volumes of 10% methanol and 0.1 volume of phosphoric acid.

Standard preparation:
Dissolve 10 mg of rutin in 100 ml of methanol.

Conditions:

Column: Merck Hibar, lichrosolv RP 18 5 µ (Art. 50333), or equivalent.

Column teperature: Room temperature

Pump: gradient

Eluent: A: 0.2% H3PO4 in water; B: 0.2% H3PO4 in methanol

Gradient:

Time (min)         % A                %B

0                      90                    10

65                    25                    75

66                    0                    100

75                    0                    100

76                    90                    10

95                    90                    10

Elution rate: 1 ml/min.

Derection wavelength: 350 nm

Injection volume: 10 µl

Calculate the content of the condensed tannins as follows:

Where

A = peak area of the sample (condensed tannins)

Ao = peak area of the reference standard (rutin)

W = weight in mg of the sample taken (200)

Wo = weight in mg of the reference standard taken (10)

Residual solvent

Reagents

- Acetone

- Ethyl acetate

Standard solutions

Place 1 g of acetone and ethyl acetate in a volumetric flask and add water to total volume of 100 ml, and prepare 0.02 - 0.4 g/100 ml solutions by dilution of this solution.

Procedure

Place 1 g (1.0±0.1 g) of powdered sample in a sample vial. Add 5 µl of water to the sample vial and seal it quickly with a septum. Set the sample vial in a pre-conditioned gas chromatograph and start the analysis under the below-mentioned conditions.

Calibration

Take 1 g of tannic acid of solvent free or known residual solvent contents in a sample vial, add 5 µl of the standard solution and seal it quickly with a septum. Set the sample vial in a pre-conditioned gas chromatograph and start the anlysis under the following conditions and obtain calibration curves for each solvent.

Determine by Gas chromatography, using a head space sampler under the following conditions:

Column:

- length: 30 m

- diameter: 0.53 mm (i.d.)

- material: silica capillary

- Stationary phase: 100% methyl polysiloxane

- Column conditioning: Heat to 60o for 2-3 h with approximately 10 ml/min of nitrogen

Carrier gas: Nitrogen

Flow rate: 5 ml/min

Detector: Flame ionization

Temperatures

- injection port: 110o

- column: 40o

- detector: 110o

Head space sampler

Sample volume: 1.0±0.1 g + 5 µl

- Sample heating temp.: 80o

- Sample heating time: 40 min

- Syringe temperature: 85o

- Sample gas injection: 0.4 ml

Calculation

where

Ci = concentration of solvent-i (mg/kg)

Ai = peak area of solvent-i (Tv. sec.)

Cfi = conversion coefficient for solvent-i (slope of the calibration curve) (µg/Tv.sec)

METHOD OF ASSAY

Sample test:
Accurately weigh about 2.0 g of the sample, transfer to a 500-ml volumetric flask, add water to dissolve and make up to the volume with water. Transfer 100 ml of this solution into a 300 ml Erlenmeyer flask and add 7.2 g of Hide Powder (a suitable grade is available from L.H. Lincoln & Son, Inc., Tanning Materials, Coudersport, Pennsylvania, 16915 USA). Shake the flask for 20 min. Let stand for 10 min. and filter through a G4-filter. The filtrate shall be clear. Pipette 50 ml of the filtrate into a tared crystallizing dish. Evaporate to dryness on a steam bath and heat in an oven at 105o for 1 h. After cooling in a desiccator, weigh again and calculate the weight difference called A.

Blank test:
On each lot of Hide Powder (a suitable grade of Hide Powder may be obtained from L.H. Lincoln & Son, Inc., Tanning Materials, Coudersport, Pennsylvania, 16915 USA) a blank test has to be carried out. 7.2 g of Hide Powder EFT is weighed into a 300 ml erlenmeyer containing 100 ml water. Proceed as directed for the Sample test, beginning with "Shake the flask for 20 min...". The weight difference is called B.

Calculation

where

A = weight difference found in the sample test (g)

B = weight difference found in the blank test (g)

W = weight of the sample, on a dried basis (g)


Source: Joint FAO/WHO Expert Committee on Food Additives (JECFA)


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