The determination of the refractive index in Zeiss butyrorofractometer is of value for ascertaining the purity of cacao butter, and it serves as a control on the iodine value, for according to Roques[43] the refractive index and the iodine value stand in equal relation, so that fat having a high refractive index gives a high iodine value and vice versa. The refractive index of cacao butter ranges between 1·4565-1·4578 at 40°C. corresponding to 46-47·8 on the scala of the Zeiss butyro-refractometer. The use of the latter is recommended by Filsinger as a preliminary test for cacao butter, since with a normal refraction it is not necessary to proceed further and determine the iodine, saponification and acid values, nor the melting point. In conclusion we annex table 12, where the respective constants for different varieties of cacao butter will be found tabulated.[44]
For further information on all these methods, the reader is referred to the excellent work of R. Benedict, entitled “Analysis of Fats and Waxes”: VII. Edition, Berlin.
Table 12.
Physical and Chemical Analyses of the Various Kinds of Pressed Stollwerck Cacao Butter.
| Accra | Ariba | Bahia | Guayaquil | Cameroon | |
|---|---|---|---|---|---|
| a) Fat | |||||
| Point of refraction at 40° C | 64·3 | 46·1 | 46·9 | 46·5 | 46·0 |
| Melting Point (Polenske)(1) | 33·1 | 33·2 | 31·95 | 32·5 | 33·65 |
| Freezing Point (Polenske) | 20·0 | 21·55 | 19·35 | 19·8 | 20·95 |
| Variations(2) between Melting Point and Freezing Point (Polenske) | 13·1 | 11·65 | 12·60 | 12·5 | 12·70 |
| Reichert-Meissl number | 0·49 | 0·33 | 0·38 | 0·55 | 0·33 |
| Polenske(2) number | 0·50 | 0·50 | 0·60 | 0·42 | 0·40 |
| Köttstorfer number | 192·4 | 191·7 | 191·4 | 190·8 | 193·2 |
| Hübl’s iodine value | 35·24 | 34·89 | 37·87 | 36·54 | 34·0 |
| Bellier’s reaction(4) | violet | as 1 | as 1 | as 1 | as 1 |
| R. Cohn’s reaction(5) | |||||
| a) Fresh fat(6) | negative | " | " | " | " |
| b) Rancid fat | strong positive | weak positive | positive | weak positive | positive |
| b) Fatty Acids(7) | |||||
| Refractive index at 40° C | 34·60 | 34·55 | 34·50 | 34·40 | 33·70 |
| Melting Point(8) | 52·90 | 52·95 | 51·80 | 52·90 | 52·00 |
| v. Hübl’s iodine value | 35·88 | 36·27 | 38·78 | 37·78 | 36·02 |
| Puerto Cabello | Thomé | Trinidad | Fluctuations of Analyses Values | ||
| from | mean | ||||
| a) Fat | |||||
| Point of refraction at 40° C | 46·0 | 46·8 | 46·3 | 46·0-46·9 | 46·4 |
| Melting Point (Polenske)(1) | 32·7 | 32·95 | 32·9 | 31·95-33·65 | 32·9 |
| Freezing Point (Polenske) | 20·8 | 18·60 | 20·66 | 18·6-21·55 | 20·2 |
| Variations(2) between Melting Point and Freezing Point (Polenske) | 11·9 | 14·35 | 12·30 | 11·65-14·35 | 12·7 |
| Reichert-Meissl number | 0·41 | 0·55 | 0·55 | 0·33-0·55 | 0·45 |
| Polenske(2) number | 0·40 | 0·55 | 0·55 | 0·4-0·6 | 0·49 |
| Köttstorfer number | 191·6 | 191·7 | 191·5 | 190·8-193·2 | 191·8 |
| Hübl’s iodine value | 32·72 | 37·24 | 33·72 | 32·72-37·87 | 35·28 |
| Bellier’s reaction(4) | as 1 | as 1 | as 1 | — | — |
| R. Cohn’s reaction(5) | |||||
| a) Fresh fat(6) | " | " | " | — | — |
| b) Rancid fat | opal escence+ | opal escence+ | opal escence+ | — | — |
| b) Fatty Acids(7) | |||||
| Refractive index at 40° C | 33·50 | 34·70 | 33·50 | 33·5-34·7 | 34·18 |
| Melting Point(8) | 51·45 | 52·05 | 52·50 | 51·45-52·95 | 52·32 |
| v. Hübl’s iodine value | 33·85 | 39·60 | 36·02 | 33·85-39·78 | 36·90 |
Remarks 1) Exact point of liquefaction difficult to observe; therefore the average of several readings must be taken.
2) Work from the Imperial Office of Health 1907, 26, 444-463.
3) Work out of the Imperial Office of Health 1904, 20, 545-558.
4) Central Journal for Germany 1908, 36, 100.
5) Journal for Popular Chemistry 1907, 16, 308.
6) Obtained at the expiration of a four weeks’ treatment as recommended by Erlenmeyer.
7) Non-volatile fatty acids, insoluble in water, from the determination of the Reichert-Meissl number.
8) Obtained as under a). Freezing Point in various cases, 1 to 8 equals 47·8—Melting Point minus Freezing Point: 52·3-47·8 4·5.
We have already stated that there is also cacao fat in the shells, and though it only amounts to some four or five percent, it has long been the care of experimenters to recover and realise that little as fully as possible. It is commercially known as Dutch IIa or artificial cacao butter, and cannot be obtained like the fat of the kernel by mechanical means, but is obtained by some cheap solvent like benzene. The traces of benzene are very difficult to hide, and consequently this shell butter has little commercial value and its manufacture is unremunerative.
Filsinger[45] gives the iodine value of shell butter as higher than that of kernel butter, and fixes it between 39 and 40: its acid value, especially if the fat is rancid, can reach 50-60° Burstyn, i. e. 50 to 60 ccm. normal alkali for 100 grammes of fat.[46] If the free acid of shell butter be counteracted with sodium or magnesium carbonate, the neutral fat then has the normal iodine value of pure cacao butter, namely 36·5. In a sample giving an abnormally high iodine value it is always necessary to determine the acid value, and if the latter be too high, the fatty acids must be removed, when if the sample be unadulterated, the normal iodine value will be obtained. It may be noted in passing that the high acid values occurring in shell butter may be due in part to the acidity of the benzene employed as a solvent.
Cacao butter has a considerable commercial value, and is consequently liable to adulteration with many inferior fats of vegetable origin. Among these are especially beef and mutton tallow, the purified fatty acids of palm-nut oil, wax, paraffin, stearic acid, dicka fat (nucoa butter, possibly) and cocoa-nut fat, as well as the numerous preparations of the last named, variously known in commerce as Mannheim cocoa-nut butter, vegetaline, lactine, finest plant butter, chocolate butter, laureol vegetable butter, palmin, kunerol etc. Other but less commoner are the sesame cotton-seed, arachidic, margarine and hazelnut oils.
For the detection of these and similar adulterates, the reactions and analytical methods described are all-sufficient. Benedict[47] discovers that the presence of wax and paraffin considerably diminishes the saponification value, cocoa, nut fat increases it and lowers the iodine value, whereas stearic acid raises the acid value.
| Melting point °C. | Melting Point of fatty acids °C. | Iodine value | |
|---|---|---|---|
| Cacao butter | 30-34·5 | 48-52 | 34-37·5 |
| Oil of Almonds | — | 14 | 93-101·9 |
| Sesame oil | — | 26-30 | 106·4-109 |
| Earth-nut (Arachis) oil | — | 27-31 | 92-101 |
| Hazelnut oil | — | 17-25 | 83·2-88 |
| Cotton-seed oil | — | 38-40 | 106-111 |
| Oleo-margarine | 32·4-32·5 | 42 | 43·8-48·5 |
| Beef tallow | 43-49 | 43-46 | 35·4-36·5 |
| Wax | 62-64 | — | 8·0-11 |
| Paraffin | 38-82 | — | 3·9-4 |
| Stearic acid | 71-71·5 | — | — |
| Sebin | 37·6-37·8 | — | 43·7-43·8 |
| Cocoa-nut fat | 20-28 chiefly 26·2-26·4 | 24-25 | 8-9 |
| Saponification value | Acid value | Refractive index in Zeiss’s butyrometer | |
| Cacao butter | 192-202 | 9·24-17·9 | 46-47·8 at 40° C. |
| Oil of Almonds | 189·5-195·4 | — | 64-64·8 at 25° C. |
| Sesame oil | 187-192 | — | 67-69 at 25° C. |
| Earth-nut (Arachis) oil | 190-197 | — | 65·8-67·5 at 25° C. |
| Hazelnut oil | 191·4-197·1 | — | — |
| Cotton-seed oil | 191-197 | — | 67·6-69·4 at 25° C. |
| Oleo-margarine | 195-197·4 | — | 48·6 at 40° C. |
| Beef tallow | 193·2-198 | — | 49 at 40° C. |
| Wax | 97-107 | 19-21 | — |
| Paraffin | — | — | — |
| Stearic acid | 195-200 | 195-200 | — |
| Sebin | 192·4-192·6 | — | — |
| Cocoa-nut fat | 254·8-268·4 | — | 35·5 at 40° C. |