RU2404428C1 - Method for definition of caffeine content in tea and coffee - Google Patents

Abstract

FIELD: food industry.

SUBSTANCE: method for definition of caffeine content includes caffeine preliminarily removal from the extraction base by chloroform with subsequent extract evaporation to produce a dry residue, the dry residue dissolution in hydrochloric acid, quantitative definition of analysable substance in the prepared sample. Quantitative definition of analysable substance is performed by way of placing the prepared sample into a cell containing photogenerated iodine in the presence of a sensibilisator mixture, registering iodine quantity reduction which is performed amperometrically by variation of intensity of current in the cell with subsequent absorbing solution light radiation till replenishment of iodine loss and definition of caffeine content by variation of intensity of current in the cell and time of iodine generation in the cell.

EFFECT: definition selectivity and sensibility improvement.

2 dwg, 3 tbl, 2 ex

Description

The invention relates to the food industry, in particular the determination of caffeine in coffee and tea.

There is a method of determining caffeine in tea by weighing ash after burning a suspended tea sample and calculating its content in relation to the weight of ash and the weight of the sample; weighing the dried extract obtained from a weighted sample of tea and calculating the content of extractives in relation to the weight of the dried extract and the weight of the sample; extracting a tea sample with an aqueous solution of ammonia in chloroform, drying the extract, treating with hydrochloric acid, adding an iodine solution, filtering and titrating with sodium thiosulfate, based on which the caffeine content is calculated (GOST 1938).

However, in this way, the determination of caffeine in coffee is impossible due to the low sensitivity of the method and the low completeness of caffeine excretion, since it is extracted at temperatures of 185-200 ° C.

A known method of controlling tea, which consists in extracting tea with hot water, the allocation of aromatizing substances with a mixture of pentane with diethyl ether, obtaining a concentrate by distillation of the solvent, followed by separation and determination of terpene compounds by gas chromatography. RU 8220185, G01N 33/02

However, in this way, the determination of caffeine in coffee is difficult due to the formation of a colloidal solution and contamination of chromatographic columns.

A known method for determining the quality of coffee by weighing ash after burning a suspended coffee sample and calculating its content in relation to the weight of the coffee sample and the weight of ash; weighing the dried extract obtained from the suspended coffee sample and calculating the content of extractives in relation to the weight of the dried extract and the weight of the sample; extracting a weighed coffee sample with boiling distilled water, extracting the aqueous extract with an aqueous solution of chloroform in potassium hydroxide, drying the chloroform extract, treating it with hydrochloric acid and hydrogen peroxide to oxidize caffeine in tetramethyl purple acid, drying, dissolving in distilled water and measuring its optical density using a colorimeter, according to the results of which the caffeine content is calculated [GOST 29148-97 Natural instant coffee. Minsk, 1.01.1999, p.14].

The disadvantages of the method are its complexity, duration and unsatisfactory accuracy of determination.

There is a method of determining the quality of coffee by the content of extractives, ash and caffeine, which includes grinding coffee, extracting it with boiling distilled water, preparing a sample corresponding to 10 mg of coffee per 200 ml of distilled water, measuring light transmission in the wavelength range of 200-360 nm and calculating quality indicators by equations. RU 22561747, G01N 33/02

A known method for the determination of caffeine, including processing the sample with an ammonia solution, separating it from the base by adsorption of caffeine with diatomaceous earth, further stripping it with chloroform and photometric analysis of the obtained sample at 276 nm [GOST 291548-97 ISO 4052-83 (E) Coffee. Determination of caffeine content (reference method)].

The method is characterized by the breadth of its scope, reproducibility of results, specificity, ease of use, speed of obtaining results, however, it is very sensitive to the conditions of its use, which reduces its selectivity and is expensive due to the use of diatomaceous earth as an adsorbent.

An object of the present invention is to provide an iodometric method for determining caffeine in coffee and tea.

The technical result of the claimed invention is to increase the selectivity and sensitivity of the determination.

This is achieved by the fact that the method for determining caffeine, including preliminary separation of it from the base by extraction with chloroform, followed by evaporation of the extract to a dry residue, dissolution of the dry residue in hydrochloric acid, quantification of the analyte in the prepared sample, according to the invention, the analyte is quantified by placing the prepared samples in a cell containing photogenerated iodine in the presence of a mixture of sensitizers: eosin, sodium fluorescein, Auramine in a molar ratio of 1: 1: 1, recording the decrease in the amount of iodine, carried out amperometrically by changing the current strength in the cell, followed by irradiation of the absorption solution with light until the iodine loss is compensated, and determining the amount of caffeine by changing the current strength and time of iodine generation in the cell.

The essence of the claimed invention lies in the fact that the caffeine in the sample is oxidized by iodine to tetramethylpuric acid, resulting in a change in the amount of iodine, which leads to a change in the current strength in the cell. After reaching a constant current strength, the cell was irradiated with light from a stabilized source and the generation time necessary to make up for the loss of iodine in the absorption cell was measured. The amount of caffeine in the samples is determined by calibration graphs.

The rate of iodine generation depends on the sensitizer used, which allows you to adjust the sensitivity of the determination.

When choosing a mixture of sensitizers for the generation of iodine, the following dyes were used, are shown in table 1.

Table 1 Sensitizer Characteristics Sensitizer Structural formula Molecular mass The weight of the sample, g λ _max , nm [3] Eosin

648 0.3240 518 Fluorescein sodium

354 0.1770 680 Auramine

268 0.1340 440

The kinetics of the formation of photogenerated iodine were examined in the presence of these sensitizers. The determination results are shown in table 2.

Example 1

The mass of the weighed sensitizer shown in Table 1 was quantitatively transferred to a 50 ml volumetric flask. The volume in the volumetric flask was adjusted to the mark with water.

40 ml of a 0.5 M solution of potassium iodide, 20 ml of an acetate buffer solution with a pH of 5.6, 1 ml of a 0.01 M solution of a dye (or a mixture of dyes) were placed in a glass. The volume was adjusted to 250 ml with distilled water. To the resulting solution was added 1 ml of 0.01 n iodine solution and heated to boiling. The solution was boiled for 1-2 minutes. After boiling, the solution was cooled and, in the case of evaporation, it was again brought to 250 ml with distilled water. The solution was placed in a glass of an amperometric cell and irradiated with light from a stabilized source until the readings on the microammeter scale changed; the time of action of light was measured by a stopwatch. When performing analyzes in solution, a certain amount of iodine is generated and then 0.1-2.0 ml of a 0.001 N sodium thiosulfate solution is added to transfer iodine to iodide ion. The determination results are shown in table 2.

The composition of the selected mixture of eosin / sodium fluorescein / auramine sensitizers, the iodine generation rate in this case is 5.4 · 10 ^-7 mM / s, which is 2 times higher than the generation rate when using eosin as a sensitizer. An increase in the generation rate occurs due to absorption of a wider range of wavelengths by a mixture of dyes, which makes it possible to more fully use the power of the light source and reduce the intensity of its irradiation, and therefore, the danger of overheating of the system.

Example 2. A sample of instant coffee weighing 0.2 g and 0.3 g is placed in a glass, pour 50 ml of boiling distilled water. The resulting solution is cooled to 18 ÷ 20 ° C, transferred to a volumetric flask with a capacity of 100 ml, bring the volume of distilled water to the mark. 10 ml of chloroform, 2 ml of sample and 0.5 ml of potassium hydroxide solution are successively added to a 25 ml separatory funnel. The sample is extracted for 1 min. After delamination of the mixture, the lower chloroform layer is transferred to an evaporation cup. Chloroform is distilled off in a water bath. 1.0 ml of hydrochloric acid solution (ρ = 1.19 g / ml) was added to the dry residue, washing off the residue to the bottom of the cup and quantitatively transferring to a 25 ml volumetric flask. The volume in the volumetric flask was adjusted to the mark with distilled water.

In an amperometric cell, 40 ml of a 0.5 M potassium iodide solution, 20 ml of an acetate buffer solution with a pH of 5.6, 10 ml of a solution of a mixture of sensitizers: eosin / sodium fluorescein / auramine taken in a molar ratio of 1: 1: 1, respectively, are placed. 2 platinum microelectrodes are placed in the solution, to which a potential difference of 0.04 V is applied. At a constant mixing speed, the solution is exposed to light from a stabilized source and iodine is generated to its content 2.9 · 10 ^-4 mM, with a generation rate of 2.278 · 10 ^-7 mM / s. Iodine concentration is judged by the change in current in the circuit on a galvanometer scale.

After generating a certain amount of iodine, the light source is turned off and 0.5-1.5 ml of the analyzed solution is introduced into the cell, while recording a change in the readings of the galvanometer. After achieving a constant current strength, the cell is purged with air for 1-2 minutes, irradiated with light, and the generation time is measured, which went to make up for the decrease in iodine.

For subsequent determinations, the solution in the amperometric cell is again irradiated with light as described above, generating a certain amount of iodine in it. The same system is used for determinations 8-12 times. The caffeine content is determined by calibration graphs obtained using standard solutions (Fig. 1, 2). The reliability of the results was monitored by the standard method and the additive method. The results of determining the component in coffee and tea are shown in table 3.

Thus, we proposed a method for the determination of caffeine in tea and coffee, based on the extraction of aqueous extract of caffeine with chloroform, subsequent removal of chloroform and oxidation of caffeine to tetramethyl purple acid with a solution of photogenerated iodine obtained by irradiating a solution of potassium iodide in an acetate buffer medium in the presence of a mixture of sensitizers with a stabilized light source . The lower limit for the determination of caffeine by this method is 1.17 μg according to the change in current strength and 0.022 μg according to the change in the time of iodine generation in the absorption cell. The determination method does not require expensive reagents and equipment; it is simple to implement.

For the analysis we used the following reagents: potassium hydroxide GOST-24363-80; acetic acid GOST 61-75; sodium hydroxide GOST-4328-77; potassium iodide GOST-4232-74; hydrochloric acid GOST-14261-77; anhydrous caffeine FS 42-0249-07; sodium eosinate TU-6-09-183-73; Auramine About TU 6-09-2247-75; fluorescein sodium TU 609-2464-82; chloroform GOST 20015-88.

Claims (1)

A method for determining caffeine, including preliminary separation of it from the base by extraction with chloroform, followed by evaporation of the extract to a dry residue, dissolving the dry residue in hydrochloric acid, quantitative determination of the analyte in the prepared sample, characterized in that the quantitative determination of the analyte is carried out by placing the prepared sample in the cell containing photogenerated iodine in the presence of a mixture of sensitizers: eosin, sodium fluorescein, auramine in molar co ratio 1: 1: 1, reducing the amount of fixing iodine, carried out amperometrically the change of current in the cell, followed by irradiation with light of the absorption solution to compensate loss of iodine and determining the amount of caffeine to change the current strength and the time of generation of iodine in the cell.

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