Chapter XVII: The Chemistry of the Coffee Bean (2)

Ether extract, fixed 1.06% Total nitrogen 3.40% Caffein 5.42% Crude fiber 0.25% Total ash 17.43% Reducing sugar 2.70% Caffetannic acid 15.33% Protein 7.71%

It is difficult to make the trade terms, such as acidity, astringency, etc., used in describing a cup of coffee, conform with the chemical meanings of the same terms. However, a fair explanation of the cause of some of these qualities can be made. Careful work by Warnier[181] showed the actual acidities of some East India coffees to be:

TABLE V--ACIDITY OF SOME EAST INDIA COFFEES

Coffee from Acid Content
Sindjai 0.033%
Timor 0.028%
Bauthain 0.019%
Boengei 0.016%
Loewae 0.021%
Waloe Pengenten 0.018%
Kawi Redjo 0.015%
Palman Tjiasem 0.022%
Malang 0.013%

These figures may be taken as reliable examples of the true acid content of coffee; and though they seem very low, it is not at all incomprehensible that the acids which they indicate produce the acidity in a cup of coffee. They probably are mainly volatile organic acids, together with other acidic-natured products of roasting. We know that very small quantities of acids are readily detected in fruit juices and beer, and that variation in their percentage is quickly noticed, while the neutralization of this small amount of acidity leaves an insipid drink. Hence, it seems quite likely that this small acid content gives to the coffee brew its essential acidity. A few minor experiments on neutralization have proven that a very insipid beverage is produced by thus treating a coffee infusion.

The body, or what might be called the licorice-like character, of coffee, is due conceivably to the presence of bodies of a glucosidic nature and to caramel. Astringency, or bitterness, is dependent upon the decomposition products of crude fiber and chlorogenic acid, and upon the soluble mineral content of the bean. The degree to which a coffee is sweet-tasting or not is, of course, dependent upon its other characteristics, but probably varies with the reducing sugar content. Aside from the effects of these constituents upon cup quality, the influence of volatile aromatic and flavoring constituents is always evident in the cup valuation, and introduces a controlling factor in the production of an individualistic drink.

_Coffee Extracts_

The uncertainty of the quality of coffee brews as made from day to day, the inconvenience to the housewife of conducting the extraction, and the inevitable trend of the human race toward labor-saving devices, have combined their influences to produce a demand for a substance which will give a good cup of coffee when added to water. This gave rise to a number of concentrated liquid and solid "extracts of coffee," which, because of their general poor quality, soon brought this type of product into disrepute. This is not surprising; for these preparations were mainly mixtures of caramel and carelessly prepared extracts of chicory, roasted cereals, and cheap coffee.

Liquid extracts of coffee galore have appeared on the market only soon to disappear. Difficulty is experienced in having them maintain their quality over a protracted period of time, primarily due to the hydrolyzing action of water on the dissolved substances. They also ferment readily, although a small percentage of preservative, such as benzoate of soda, will halt spoilage.[182]

So much trouble is not encountered with coffee-extract powders--the so-called "soluble" or "instant" coffees. The majority of these powdered dry extracts do, however, show great affinity for atmospheric moisture. Their hygroscopicity necessitates packing and keeping them in air-tight containers to prevent them running into a solid, slowly soluble mass.

The general method of procedure employed in the preparation of these powders is to extract ground roasted coffee with water, and to evaporate the aqueous solution to dryness with great care. The major difficulty which seems to arise is that the heat needed to effect evaporation changes the character of the soluble material, at the same time driving off some volatile constituents which are essential to a natural flavor. Many complex and clever processes have been developed for avoiding these difficulties, and quite a number of patents on processes, and several on the resultant product, have been allowed; but the commercial production of a soluble coffee of freshly-brewed-coffee-duplicating-power is yet to be accomplished. However, there are now on the market several coffee-extract powders which dissolve readily in water, giving quite a fair approximation of freshly brewed coffee. The improvement shown since they first appeared augurs well for the eventual attainment of their ultimate goal.

_Adulterants and Substitutes_

There would appear to be three reasons why substitutes for coffee are sought--the high cost, or absence, of the real product; the acquiring of a preferential taste, by the consumer, for the substitute; and the injurious effects of coffee when used to excess. Makers of coffee substitutes usually emphasize the latter reason; but many substitutes, which are, or have been, on the market, seem to depend for their existence on the other two. Properly speaking, there are scarcely any real substitutes for coffee. The substances used to replace it are mostly like it only in appearance, and barely simulate it in taste. Besides, many of them are not used alone, but are mixed with real coffee as adulterants.

The two main coffee substitutes are chicory and cereals. Chicory, succory, _Cichorium Intybus_, is a perennial plant, growing to a height of about three feet, bearing blue flowers, having a long tap root, and possessing a foliage which is sometimes used as cattle food. The plant is cultivated generally for the sake of its root, which is cut into slices, kiln-dried, and then roasted in the same manner as coffee, usually with the addition of a small proportion of some kind of fat. The preparation and use of roasted chicory originated in Holland, about 1750. Fresh chicory[183] contains about 77 percent water, 7.5 gummy matter, 1.1 of glucose, 4.0 of bitter extractive, 0.6 fat, 9.0 cellulose, inulin and fiber, and 0.8 ash. Pure roasted chicory[184] contains 74.2 percent water-soluble material, comprised of 16.3 percent water, 26.1 glucose, 9.6 dextrin and inulin, 3.2 protein, 16.4 coloring matter, and 2.6 ash; and 25.8 percent insoluble substances, namely, 3.2 percent protein, 5.7 fat, 12.3 cellulose, and 4.6 ash. The effect of roasting upon chicory is to drive off a large percentage of water, increasing the reducing sugars, changing a large proportion of the bitter extractives and inulin, and forming dextrin and caramel as well as the characteristic chicory flavor.

The cereal substitutes contain almost every type of grain, mainly wheat, rye, oats, buckwheat, and bran. They are prepared in two general ways, by roasting the grains, or the mixtures of grains, with or without the addition of such substances as sugar, molasses, tannin, citric acid, etc., or by first making the floured grains into a dough, and then baking, grinding, and roasting. Prior to these treatments, the grains may be subjected to a variety of other treatments, such as impregnation with various compounds, or germination. The effect of roasting on these grains and other substitutes is the production of a destructive distillation, as in the case of coffee; the crude fiber, starches, and other carbohydrates, etc., being decomposed, with the production of a flavor and an aroma faintly suggesting coffee.

The number, of other substitutes and imitations which have been employed are too numerous to warrant their complete description; but it will prove interesting to enumerate a few of the more important ones, such as malt, starch, acorns, soya beans, beet roots, figs, prunes, date stones, ivory nuts, sweet potatoes, beets, carrots, peas, and other vegetables, bananas, dried pears, grape seeds, dandelion roots, rinds of citrus fruits, lupine seeds, whey, peanuts, juniper berries, rice, the fruit of the wax palm, cola nuts, chick peas, cassia seeds, and the seeds of any trees and plants indigenous to the country in which the substitute is produced.

Aside from adulteration by mixing substitutes with ground coffee, and an occasional case of factitious molded berries, the main sophistications of coffee comprise coating and coloring the whole beans. Coloring of green and roasted coffees is practised to conceal damaged and inferior beans. Lead and zinc chromates, Prussian blue, ferric oxid, coal-tar colors, and other substances of a harmful nature, have been employed for this purpose, being made to adhere to the beans with adhesives. As glazes and coatings, a variety of substances have been employed, such as butter, margarin, vegetable oils, paraffin, vaseline, gums, dextrin, gelatin, resins, glue, milk, glycerin, salt, sodium bicarbonate, vinegar, Irish moss, isinglass, albumen, etc. It is usually claimed that coating is applied to retain aroma and to act as a clarifying agent; but the real reasons are usually to increase weight through absorption of water, to render low-grade coffees more attractive, to eliminate by-products, and to assist in advertising.

METHODS OF ANALYSIS OF COFFEES[185]

(_Official and Tentative_)

(Sole responsibility for any errors in compilation or printing of
these methods is assumed by the author.)

GREEN COFFEE

1. _Macroscopic Examination--Tentative_

A macroscopic examination is usually sufficient to show the presence of excessive amounts of black and blighted coffee beans, coffee hulls, stones, and other foreign matter. These can be separated by hand-picking and determined gravi-metrically.

2. _Coloring Matters--Tentative_

Shake vigorously 100 grams or more of the sample with cold water or 70 percent alcohol by volume. Strain through a coarse sieve and allow to settle. Identify soluble colors in the solution and insoluble pigments in the sediment.

ROASTED COFFEE

3. _Macroscopic Examination--Tentative_

Artificial coffee beans are apparent from their exact regularity of form. Roasted legumes and lumps of chicory, when present in whole roasted coffee, can be picked out and identified microscopically. In the case of ground coffee, sprinkle some of the sample on cold water and stir lightly. Fragments of pure coffee, if not over-roasted, will float; while fragments of chicory, legumes, cereals, etc., will sink immediately, chicory coloring the water a decided brown. In all cases identify the particles that sink by microscopical examination.

4. _Preparation of Sample--Official_

Grind the sample to pass through a sieve having holes 0.5 mm. in diameter and preserve in a tightly stoppered bottle.

5. _Moisture--Tentative_

Dry 5 grams of the sample at 105°--110°C. for 5 hours and subsequent periods of an hour each until constant weight is obtained. The same procedure may be used, drying _in vacuo_ at the temperature of boiling water. In the case of whole coffee, grind rapidly to a coarse powder and weigh at once portions for the determination without sifting and without unnecessary exposure to the air.

6. _Soluble Solids--Tentative_

Place 4 grams of the sample in a 200-cc. flask, add water to the mark, and allow the mass to infuse for eight hours, with occasional shaking; let stand 16 hours longer without shaking, filter, evaporate 50 cc. of filtrate to dryness in a flat-bottomed dish, dry at 100° C., cool and weigh.

7. _Ash--Official_

Char a quantity of the substance, representing about 2 grams of the dry material, and burn until free of carbon at a low heat, not to exceed dull redness. If a carbon-free ash can not be obtained in this manner, exhaust the charred mass with hot water, collect the insoluble residue on a filter, burn till the ash is white or nearly so, and then add the filtrate to the ash and evaporate to dryness. Heat to low redness, until ash is white or grayish white, and weigh.

8. _Ash Insoluble in Acid--Official_

Boil the water-insoluble residue, obtained as directed under 9, or the total ash obtained as directed under 7, with 25 cc. of 10-percent hydrochloric acid (sp. gr. 1.050) for 5 minutes, collect the insoluble matter on a Gooch crucible or an ashless filter, wash with hot water, ignite and weigh.

9. _Soluble and Insoluble Ash--Official_

Heat 5 to 10 grams of the sample in a platinum dish of from 50 to 100 cc. capacity at 100° C. until the water is expelled, and add a few drops of pure olive oil and heat slowly over a flame until swelling ceases. Then place the dish in a muffle and heat at low redness until a white ash is obtained. Add water to the ash, in the platinum dish, heat nearly to boiling, filter through ash-free filter paper, and wash with hot water until the combined filtrate and washings measure to about 60 cc. Return the filter and contents to the platinum dish, carefully ignite, cool and weigh. Compute percentages of water-insoluble ash and water-soluble ash.

10. _Alkalinity of the Soluble Ash--Official_

Cool the filtrate from 9 and titrate with N/10 hydrochloric acid, using methyl orange as an indicator.

Express the alkalinity in terms of the number of cc. of N/10 acid per 1 gram of the sample.

11. _Soluble Phosphoric Acid in the Ash--Official_

Acidify the solution of soluble ash, obtained in 9, with dilute nitric acid and determine phosphoric acid (P_2_O_5). For percentages up to 5 use an aliquot corresponding to 0.4 gram of substance, for percentages between 5 and 20 use an aliquot corresponding to 0.2 gram of substance, and for percentages above 20 use an aliquot corresponding to 0.1 gram of substance. Dilute to 75-100 cc., heat in a water-bath to 60°-65° C., and for percentages below 5 add 20-25 cc. of freshly filtered molybdate solution. For percentages between 5 and 20 add 30-35 cc. of molybdate solution. For percentages greater than 20 add sufficient molybdate solution to insure complete precipitation. Stir, let stand in the bath for about 15 minutes, filter _at once_, wash once or twice with water by decantation, using 25-30 cc. each time, agitate the precipitate thoroughly and allow to settle; transfer to the filter and wash with cold water until the filtrate from two fillings of the filter yields a pink color upon the addition of phenolphthalein and one drop of the standard alkali. Transfer the precipitate and filter to the beaker, or precipitating vessel, dissolve the precipitate in a small excess of the standard alkali, add a few drops of phenolphthalein solution, and titrate with the standard acid.

12. _Insoluble Phosphoric Acid in the Ash--Official_

Determine phosphoric acid (P_2_O_5) in the Insoluble ash by the foregoing method.

13. _Chlorides--Official_

Moisten 5 grams of the substance in a platinum dish with 20 cc. of a 5-percent solution of sodium carbonate, evaporate to dryness and ignite as thoroughly as possible at a temperature not exceeding dull redness. Extract with hot water, filter and wash. Return the residue to the platinum dish and ignite to an ash; dissolve in nitric acid, and add this solution to the water extract. Add a known volume of N/10 silver nitrate in slight excess to the combined solutions. Stir well, filter and wash the silver chloride precipitate thoroughly. To the filtrate and washings add 5 cc. of a saturated solution of ferric alum and a few cc. of nitric acid. Titrate the excess silver with N/10 ammonium or potassium thiocyanate until a permanent light brown color appears. Calculate the amount of chlorin.

14. _Caffein--The Fendler and Stüber Method--Tentative_

Pulverize the coffee to pass without residue through a sieve having circular openings 1 mm. in diameter. Treat a 10-gram sample with 10 grams of 10-percent ammonium hydroxid and 200 grams of chloroform in a glass-stoppered bottle and shake continuously by machine or hand for one-half hour. Pour the entire contents of the bottle on a 12.5-cm. folded filter, covering with a watch glass. Weigh 150 grams of the filtrate into a 250-cc. flask and evaporate on the steam bath, removing the last chloroform with a blast of air. Digest the residue with 80 cc. of hot water for ten minutes on a steam bath with frequent shaking, and let cool. Treat the solution with 20 cc. (for roasted coffee) or 10 cc. (for unroasted coffee) of 1-percent potassium permanganate and let stand for 15 minutes at room temperature. Add 2 cc. of 3-percent hydrogen peroxid (containing 1 cc. of glacial acetic acid in 100 cc.). If the liquid is still red or reddish, add hydrogen peroxid, 1 cc. at a time, until the excess of potassium permanganate is destroyed. Place the flask on the steam bath for 15 minutes, adding hydrogen peroxid in 0.5-cc. portions until the liquid becomes no lighter in color. Cool and filter into a separatory funnel, washing with cold water. Extract four times with 25 cc. of chloroform. Evaporate the chloroform extract from a weighed flask with aid of an air blast and dry at 100° C. to constant weight (one-half hour is usually sufficient). Weigh the residue as caffein and calculate on 7.5 grams of coffee. Test the purity of the residue by determining nitrogen and multiplying by 3.464 to obtain caffein.

15. _Caffein--Power-Chestnut Method--Official_

Moisten 10 grams of the finely powdered sample with alcohol, transfer to a Soxhlet, or similar extraction apparatus, and extract with alcohol for 8 hours. (Care should be exercised to assure complete extraction.) Transfer the extract with the aid of hot water to a porcelain dish containing 10 grams of heavy magnesium oxid in suspension in 100 cc. of water. (This reagent should meet the U.S.P. requirements.) Evaporate slowly on the steam bath with frequent stirring to a dry, powdery mass. Rub the residue with a pestle into a paste with boiling water. Transfer with hot water to a smooth filter, cleaning the dish with a rubber-tipped glass rod. Collect the filtrate in a liter flask marked at 250 cc. and wash with boiling water until the filtrate reaches the mark. Add 10 cc. of 10-percent sulphuric acid and boil gently for 30 minutes with a funnel in the neck of the flask. Cool and filter through a moistened double paper into a separatory funnel and wash with small portions of 0.5-percent sulphuric acid. Extract with six successive 25-cc. portions of chloroform. Wash the combined chloroform extracts in a separatory funnel with 5 cc. of 1-percent potassium hydroxid solution. Filter the chloroform into an Erlenmeyer flask. Wash the potassium hydroxid with 2 portions of chloroform of 10 cc. each, adding them to the flask together with the chloroform washings of the filter paper. Evaporate or distil on the steam bath to a small volume (10-15 cc.), transfer with chloroform to a tared beaker, evaporate carefully, dry for 30 minutes in a water oven, and weigh. The purity of the residue can be tested by determining nitrogen and multiplying by the factor 3.464.

16. _Crude Fiber--Official_

Prepare solutions of sulphuric acid and sodium hydroxid of exactly 1.25-percent strength, determined by titration. Extract a quantity of the substance representing about 2 grams of the dry material with ordinary ether, or use residue from the determination of the ether extract. To this residue in a 500-cc. flask add 200 cc. of boiling 1.25-percent sulphuric acid; connect the flask with a reflux condenser, the tube of which passes only a short distance beyond the rubber stopper into the flask, or simply cover a tall conical flask, which is well suited for this determination, with a watch glass or short stemmed funnel. Boil at once and continue boiling gently for thirty minutes. A blast of air conducted into the flask may serve to reduce the frothing of the liquid. Filter through linen, and wash with boiling water until the washings are no longer acid; rinse the substance back into the flask with 200 cc. of the boiling 1.25-percent solution of sodium hydroxid free, or nearly so, of sodium carbonate; boil at once and continue boiling gently for thirty minutes in the same manner as directed above for the treatment with acid. Filter at once rapidly, wash with boiling water until the washings are neutral. The last filtration may be performed upon a Gooch crucible, a linen filter, or a tared filter paper. If a linen filter is used, rinse the crude fiber, after washing is completed, into a flat-bottomed platinum dish by means of a jet of water; evaporate to dryness on a steam bath, dry to constant weight at 110° C., weigh, incinerate completely, and weigh again. The loss in weight is considered to be crude fiber. If a tared filter paper is used, weigh in a weighing bottle. In any case, the crude fiber after drying to constant weight at 110° C., must be incinerated and the amount of the ash deducted from the original weight.

17. _Starch--Tentative_

Extract 5 grams of the finely pulverized sample on a hardened filter with five successive portions (10 cc. each) of ether, wash with small portions of 95-percent alcohol by volume until a total of 200 cc. have passed through, place the residue in a beaker with 50 cc. of water, immerse the beaker in boiling water and stir constantly for 15 minutes or until all the starch is gelatinized; cool to 55° C., add 20 cc. of malt extract and maintain at this temperature for an hour. Heat again to boiling for a few minutes, cool to 55° C., add 20 cc. of malt extract and maintain at this temperature for an hour or until the residue treated with iodin shows no blue color upon microscopic examination. Cool, make up directly to 250 cc., and filter. Place 200 cc. of the filtrate in a flask with 20 cc. of hydrochloric acid (sp. gr. 1.125); connect with a reflux condenser and heat in a boiling water bath for 2.5 hours. Cool, nearly neutralize with sodium hydroxid solution, and make up to 500 cc. Mix the solution well, pour through a dry filter and determine the dextrose in an aliquot. Conduct a blank determination upon the same volume of the malt extract as used upon the sample, and correct the weight of reduced copper accordingly. The weight of the dextrose obtained multiplied by 0.90 gives the weight of starch.

18. _Sugars--Tentative_

See original.[186]

19. _Petroleum Ether Extract--Official_

Dry 2 grams of coffee at 100° C., extract with petroleum ether (boiling point 35° to 50° C.) for 16 hours, evaporate the solvent, dry the residue at 100° C., cool, and weigh.

20. _Total Acidity--Tentative_

Treat 10 grams of the sample, prepared as directed under 4, with 75 cc. of 80-percent alcohol by volume in an Erlenmeyer flask, stopper, and allow to stand 16 hours, shaking occasionally. Filter and transfer an aliquot of the filtrate (25 cc. in the case of green coffee, 10 cc. in the case of roasted coffee) to a beaker, dilute to about 100 cc. with water and titrate with N/10 alkali, using phenolphthalein as an indicator. Express the result as the number of cc. of N/10 alkali required to neutralize the acidity of 100 grams of the sample.

21. _Volatile Acidity--Tentative_

Into a volatile acid apparatus introduce a few glass beads, and over these place 20 grams of the unground sample. Add 100 cc. of recently boiled water to the sample, place a sufficient quantity of recently boiled water in the outer flask and distil until the distillate is no longer acid to litmus paper. Usually 100 cc. of distillate will be collected. Titrate the distillate with N/10 alkali, using phenolphthalein as an indicator. Express the result as the number of cc. of N/10 alkali required to neutralize the acidity of 100 grams of the sample.

UNOFFICIAL METHODS

22. _Protein_

Determine nitrogen in 3 grams of the sample by the Kjeldahl or Gunning method. This gives the total nitrogen due to both the proteids and the caffein. To obtain the protein nitrogen, subtract from the total nitrogen the nitrogen due to caffein, obtained by direct determination on the separated caffein or by calculation (caffein divided by 3.464 gives nitrogen). Multiply by 6.25 to obtain the amount of protein.

23. _Ten Percent Extract--McGill Method_

Weigh into a tared flask the equivalent of 10 grains of the dried substance, add water until the contents of the flask weigh 110 grams, connect with a reflux condenser and heat, beginning the boiling in 10 to 15 minutes. Boil for 1 hour, cool for 15 minutes, weigh again, making up any loss by the addition of water, filter, and take the specific gravity of the filtrate at 15° C.

According to McGill, a 10-percent extract of pure coffee has a specific gravity of 1.00986 at 15° C., and under the same treatment chicory gives an extract with a specific gravity of 1.02821. In mixtures of coffee and chicory the approximate percentage of chicory may be calculated by the following formula:

(1.02821 - sp. gr.)
Percent of chicory = 100 ------------------
0.01835

The index of refraction of the above solution may be taken with the Zeiss immersion refractometer or with the Abbe refractometer.

With a 10-percent coffee extract, n_d 20° = 1.3377.

With a 10-percent chicory extract, n_d 20° = 1.3448.

Determinations of the solids, ash, sugar, nitrogen, etc., may be made in the 10-percent extract, if desired.

24. _Caffetannic Acid--Krug's Method_[187]

Treat 2 grains of the coffee with 10 cc. of water and digest for 36 hours; add 25 cc. of 90-percent alcohol and digest 24 hours more, filter, and wash with 90-percent alcohol. The filtrate contains tannin, caffein, color, and fat. Heat the filtrate to the boiling point and add a saturated solution of lead acetate. If this is carefully done, a caffetannate of lead will be precipitated containing 49 percent of lead. As soon as the precipitate has become flocculent, collect on a tared filter, wash with 90-percent alcohol until free from lead, wash with ether, dry and weigh. The precipitate multiplied by 0.51597 gives the weight of the caffetannic acid.