The Determination of Tannic Substances in Commercial Cocoa Powders

JOURNAL OF DAIRY SCIENCE VOLUME X X V I I

NOVEMBER, 1944

NUMBER 11

T H E D E T E R M I N A T I O N OF T A N N I C S U B S T A N C E S I N C O M M E R C I A L COCOA P O W D E R S W. S. M U E L L E R A~CDJ. W. K U Z M E S K I ~ Massachusetts State College, Amherst, Massach~tsetts INTRODUCTION

Cocoa and chocolate are extensively used in connection with milk and some of its products, viz., chocolate milk, chocolate ice cream, chocolate puddings, and milk chocolate candies. Recent data (7) indicated a close connection between the content of tannic substances in cocoa powder, as determined by the Ulrich method (8), and its toxicity when cocoa was fed with whole milk powder to white rats. Thus, the tannh~ substances a p p e a r to be of sufficient importance to w a r r a n t f u r t h e r investigation of their presence in commercial cocoa powders. Although the tannic substances of cocoa have not been defined too clearly in the literature, it m a y be assumed that they Inay include cacao-tannin, cacao-purple 2 and cacao-brown (1, 5). The amount of these various tamlic sul)stam~es present in commercial cocoa powder depends to a g r e a t extent upon treatments, such as fermentation, drying, roasting, and mhlition of alkali (Dutch process), and to a lesser extent upon the v a r i e t y of the cocoa bean used. Various methods have been worked out for the determination of the tannic acid content of plant materials, yet they do not seem a p p r o p r i a t e for cocoa products. The writers are not aware of a n y published methods which will determine accurately the tannic substances totally or separately in cocoa powder. Several methods have been suggested for the determination of cacao-purple. Heidusehka and Bienert (4) do not claim that their method is accurate enough for analytical work. Ulrich's (8) method for the determination of cacao-purple, which is a constituent of the cacao nib but thought to be absent in the shell, was proposed chiefly as a method for detecting Reccivcd for publication April 29, 1944. 1 Department of Dairy Industry and Control Service cooperating. Contribution No. 514 of the Massachusetts Agricultural Experiment Station. 2 In this paper cacao-purple is considered as being synonymous with cacao-red. Fincke (2) christened this tannic substance cacao-red, but K n a p p (5) considers cacaopurple preferable as this more accurate color description assists in distinguishing it from the oxidation products which are of red-brown color.

897 Copyrighted, 1944, by the A~IERICAN DAIRY SCIENCE ASSOCIATION.

898

w.

s.

MUELLER

AND J.

W.

KUZMESKI

cocoa shell in commercial powders. The Ulrich method became the generally accepted test for cacao-purple of cocoa powder. However, Fincke (3) reported this method unreliable for this purpose. Since the accuracy of Ulrich's method has been questioned, an attempt was made in this study to improve the method. The results obtained are given as a report of progress. Fifteen samples of commercial cocoa powder were analyzed by Ulrich's method before the writers were aware that the accuracy of this method had been seriously questioned. The results are included here, so that they may be compared with analyses of cocoa samples of less recent date, reported by other investigators. EXPERI~[ENTAL A n a l y s e s o f cocoa p o w d e r s b y U l r i c h ' s m e t h o d . Fifteen samples of commercial cocoa powder were analyzed by Ulrich's method. I n this method the material to be investigated is boiled with 50 per cent acetic acid, the filtrate is mixed with hydrochloric acid and precipitated by means of ferric chloride, and the precipitate is washed with hot water, filtered off, and weighed. The results of the analyses are summarized in table 1 as average percentages

TABLE 1 Average values for ferric chloride precgpitates of cocoa powders

Original samples Moisture-fat-free basis 6 6 9 4

Dutch and 9 unprocessed ................................ Dutch samples .......................................................... Unprocessed samp]es ......................................... Dutch samples when Dutch and unprocessed samples were made from similar base beans .......................................... 4 Unprocessed samples when Dutch and unprocessed samples were made from similar base beans .............................

per cent

per cent

11.01 8.44 12.72

14.13 lO.74 16.42

8.68

11.27

12.86

17.06

of ferric chloride precipitate calculated on moisture-fat-free basis and on the original sample. The average value of 14.13 per cent of ferric chloride precipitate, calculated on a moisture-fat-free basis, approximates the figures reported by other investigators, namely 13.81 per cent by Ulrich (8) and 15.17 per cent by Lythgoe (6). However, the figure for Dutch cocoa is lower than that reported by Lythgoe. A m o n g the Dutch and unprocessed cocoa samples analyzed, four samples of each were made from the same type of cocoa beans and such samples should give a more accurate comparison than samples of unknown history. Obviously, it would be desirable to have samples of Dutch and unprocessed cocoa which were made from the same lot of beans, but sufficient samples of this type were unavailable. The average percentage of ferric chloride precipitate was approximately one-third less for the

TANNIC SUBSTANCES IN COMMERCIAL COCOA POWDERS

899

Dutch cocoa than for the unprocessed cocoa. Moisture and f a t in the cocoa samples ranged f r o m 1.09 to 5.32 p e r cent and 10.10 to 23.71 per cent respectively: Modificatio~t of UlricJ~ 's method. Since Ulrich does not indicate the basis of his method or the n a t a r e of the material brought to precipitation, the precipitated material is usually r e f e r r e d to simply as " f e r r i c chloride prec i p i t a t e . " Some investigators, however, have reported the amount of ferric chloride precipitate as if it consisted entirely of tannic substances. Numerous ferric chloride precipitates were analyzed in this s t u d y in an a t t e m p t to secure some information on the nature of the material precipitated; also, the effect of altering the procedure for washing the precipitate was studied. The results are given in table 2. The ferric chloride precipitates, washed according to Ulrich's method and ashed, had an average ash content of 13.45 per cent, which varied f r o m 11.48 to 18.24 per cent. Analyses of the ash showed it to consist m a i n l y of iron and phosphorus, present either as separate oxides or in combination as ferric phosphate. The relatively high ash content of the ferric chloride precipitate indicated that the weight of the precipitate cannot be taken as a true measure of cacao-purple or other tannic substances, because the value is too high, at least to the extent of the weight of ~sh in the precipitate. I n order to determine whether the ash content could be reduced, thereby making the weight of the ferric chleride precipitate more nearly a true measure of the amount of tannic substances present in cocoa powder, it was deci(led to alter the portion of the Ulrich procedure which pertained to the washing of the precipitate. Qualitative tests made on a n u m b e r of precipitates, which had been washed with hot water according to the Ulri(.h method, showed the presence of small amounts of chloride, indicating that some of the iron a p p e a r i n g in the ash probably was due to incomplete removal of ferric chloride by washing. Two modifications in washing the precipitate were used. Some precipitates were washed first in cold acetic acid (1 + 1) and then in hot water. Other precipitates were washed with a cold mixture of acetic and hydrochloric acids of the same concentration as used in extracting the tannic substances, the final washing likewise being with hot water. I n general, washing the precipitate with either acid solution resulted in a lower percentage of ash in the precipitate and a lower percentagc of iron and a higher percentage of phosphorus in the ash. However, the mixture of acetic acid with hydrochloric acid was more effective than the acetic acid alone. W h a t m a n No. 42 filter p a p e r and a filter crucible were also compared as to their desirability for use in collecting and washing the precipitate. The washing was slightly more effective when the filter crucible was used, but the difference a p p e a r e d too small to be of a n y great significance. I t was also noted t h a t the proportion of iron to phosphorus approached the theoretical proportions existing in ferric phosphate, when the precipitates were washed with acid.

TABLE

2

An~ysesofferricchlorideprecipitate

Sample No.

9

B r a n d No. a n d kind of

9a

9b 10 10a

10b

T r e a t m e n t of p r e c i p i t a t e On dry fat-free basis

coeo~

II Dutch II Unprocessed III Dutch III Unprocessed IV Dutch IV Unprocessed XII Dutch Liquor XII Unprocessed Liquor XI Unprocessed

X Unprocessed

Fer

P e r cent FeCla ppt.

A c c o r d i n g to U l r i c h ' s m e t h o d ~

1

On original sample

P e r cent a s h in ppt.

P e r cent F e in ash

15.91

1]..95

11.96

40.6

P e r cent Fin ash

cent

Exgess

Fe and per cent P in ash calculated to per cent FePo~

Fein ash caleul a t e d to per c e n t Fe203

17.77

13.08

11.48

39.2

t~

tt

~t

t¢

17.17

13.84

11.95

42.3

75.9

20.3

Ct

~t

t¢

¢t

16.29

11.57

]2.76

41.4

83.1

15.2

~

¢;

tt

tt

7.42

5.43

18.24

44.8

71.0

16.50

13.35

12.78

45.2

72.9

10.28

4.55

17.48

42.2

65.2

16.02

6.95

14.51

40.8

71.0

20.7

]1.55

45.6

73.4

26.3

9.80

39.3

90.4

8.3

¢¢

tt

~l

it

16.53 U s e d W h a t m a n No. 42 filter p a p e r washed ppt. with cold 50-50 acetic acid. Finished w a s h i n g with hot w a t e r U s e d filter crucible. Washed ppt. s a m e m a n n e r as for 9a A c c o r d i n g to U l r i c h ' s m e t h o d *

15.71

U s e d W h a t m a n No. 42 filter p a p e r w a s h e d ppt. with cold acetic acid + HC1 acid. Conc e n t r a t i o n of acid m i x t u r e same as p r e c i p i t a t i n g solution. F i n i s h e d w a s h i n g with h o t water U s e d filter crucible. W a s h ppt. in m a n n e r as f o r 10a

!

13.54 13.06

I

r

Excess

Pin ash calcu]ated to per cent P:O5

26.4 26.0

i

25.9

I ~fl

16.55

13.80

9.50

38.3

88.0

8.1

13.78

10.91

11.80

45.4

74.4

25.6

13.53

10.50

7.51

33.3

90.0

9.8

10.67

7.13

31.6

85.4

12.6

13.57

i

U s e d W h a t m a n No. 42 filter p a p e r a n d w a s h e d ppt. w i t h h o t water u n t i l filtrate g a v e n e g a t i v e test for F e with K~Fe(CN)~.

TANNIC SUBSTANCES IN COMMERCIAL COCOA POWDERS

901

SUiV£MARYAND CONCLUSIONS 1. F i f t e e n samples of c o m m e r c i a l cocoa p o w d e r s were a n a l y z e d , pres u m a b l y for chcao-purple, b y U l r i c h ' s f e r r i c chloride p r e c i p i t a t e method. The f e r r i c chloride p r e c i p i t a t e s r a n g e d f r o m 3.48 to 15.59 p e r cent, w i t h a n a v e r a g e of 11 p e r cent. The a v e r a g e p e r c e n t a g e of f e r r i c chloride prec i p i t a t e o b t a i n e d f r o m D u t c h cocoas was a p p r o x i m a t e l y o n e - t h i r d of the a m o u n t o b t a i n e d f r o m the u n p r o c e s s e d cocoas. 2. The a m o u n t of f e r r i c chloride p r e c i p i t a t e does n o t m e a s u r e a c c u r a t e l y the a m o u n t of c a c a o - p u r p l e or other t a n n i c s u b s t a n c e s i n cocoa, p a r t l y because the ash c o n t e n t of the p r e c i p i t a t e was f o u n d to be r e l a t i v e l y high, a v e r a g i n g 13.45 p e r cent. 3. The ash consisted m a i n l y of i r o n a n d p h o s p o r u s , p r e s e n t e i t h e r as s e p a r a t e oxides or i n c o m b i n a t i o n as f e r r i c phosphate. 4. A m o d i f i c a t i o n of the m e t h o d of w a s h i n g the f e r r i c chloride p r e c i p i tate has been suggested, which r e d u c e d its ash coute~lt. 5. R e s u l t s o b t a i n e d f r o m this s t u d y i n d i c a t e t h a t the f e r r i c c h l o r i d e p r e c i p i t a t e w o u l d m e a s u r e the t a n n i c s u b s t a n c e s c o n t e n t of cocoa m o r e a c c u r a t e ] y if the m o d i f i c a t i o n of the w a s h i n g p r o c e d u r e were fol]owed a n d c o r r e c t i o n m a d e for the ash c o n t a i n e d . RE FERENCES (1) A1)A~r, W. B. ])et(,rmination of the Colour-Producing Constituents of the C:~c.w Bean. Analyst, 53: 369-372. 1928. (2) FINC,KE, II. Pigments of Cac'lo Beans and Cacao Products. Ztschr. f. 1Tntcrsuch, der I,cbensmitl., 55: 559-566. 1928. (3) FINCKE, t[. ttandbu('h der Kakaocrzcognissc. Julius Springer, Berlin, 1)p. 465-466. 1936. (4) HEWUSCHKA,A., .~ND BIENEaT, B. Cacao Red. J. Prakt. Chem., 117: 262-272. 1927; and 119: ]99-208. 1928. (5) KNAI~P,A.W. C,qcao Fermentation. John Balc, Sons and Curnow, Ltd., London, pp. 83-91. 1937. (6) LY'~'IIGOE~I{. C. l{eport on Cocoa and Cocoa Products. Jour. A. O. A. C., 1. No. 4: 550-552. 1916. (7) MUELLER,W.S. The Significtmce of Tannic Substances and Thcobrominc in Chocolate Milk. JOI~a. DAIRY SCI., 24, No. 3: 221-230. ~942. (8) ULRICH, C. Detection of Shell in Cocoa Preparations. Arch. 1)lmrm., 249: 524-597. 1911. Cited from: Jour. Soc. Chem. Indus., 30, No. 24: 146(i-1467. 1911.