Cacao starch has the usual properties of ordinary kinds of starch, namely:
1. It is gelatinised by hot water, that is to say, the water penetrates between the layers of starch granules, separating them and causing by its penetration a swelling up of the starch whereby a transparent mass know as “starch paste” is produced. It has been supposed that cacao starch is less easily gelatinised than the starch of other plants. According to investigations of Soltsien’s[72], which Zipperer unreservedly endorses, this is not the case, for under certain essential conditions, cacao starch gelatinises just as readily as other kinds of starch.
The blue coloration of starch with iodine.
This is said to take place more slowly with cacao than with other starches, though we have always found that once the cacao starch is gelatinised, a blue coloration appears immediately on adding a sufficiently strong solution of iodine.
There are certainly other materials in the cacao bean, such as fat, which by more or less enveloping the starch, prevent access of water to the starch granules and thus hinder gelatinisation; or again, the albumen and cacao-red may exert some retarding influence on the iodine reaction, especially if the iodine solution used is very dilute. Yet it is impossible to describe the reaction as slow.
According to Soltsien, if a mixture of two parts of cacao bean with one part of calcinated magnesia and water is heated, a clear-filtering decoction is obtained, which immediately assumes the blue colour on addition of iodine solution. On neutralising the filtrate with acetic acid, and adding 3-4 parts of strong alcohol, its starch is precipitated.
By boiling with dilute acids as well as by the action of ferments like the saliva, diastase etc., starch is converted into starch sugar (glucose, dextrose). The empirical formula for starch is C6H10O5, that for starch sugar is C6H12O6, so that in the conversion one molecule of water is introduced, wherefore its chemical nature is greatly changed, and especially in its becoming freely soluble in water. That alteration allows of starch being quantitatively determined, as the dextrose thus produced has the property of reducing an alkaline solution of copper sulphate (known as Fehling’s solution, after the discoverer); that is to say, the copper sulphate is converted into insoluble red cuprous oxide. As dextrose always precipitates a definite amount of cuprous oxide, the quantity of starch present can in that way be determined.
The chemical determination of starch is only in a limited degree effectual in the recognition of an admixture of foreign starch in cacao preparations. If more than 10-15 percent of starch (calculated on the crude bean) has been found, then it must be assumed that there has been an admixture of foreign starch, but chemistry affords no means by which foreign starch can be distinguished from the genuine starch of the cacao bean. For that purpose the foreign starch must be minutely observed under the microscope, which not only serves to detect its presence, but gives an approximate estimation of the amount present, and its origin. Great caution should be exercised, or the result may be easily exaggerated.
7. Cellulose or crude fibre.
We have already made the acquaintance of this material as the chief constituent of the cell walls and vascular tissues. Recent chemical investigations have shown that it consists of the anhydrides of hexose and pentose (sugar compounds) incrustated with many impurities, such as cacao-red, gum, mucilage etc. From a chemical point of view, cellulose has the same formula as starch, viz. C6H10O5, or one of its multiples represented in formula. One of its chemical properties is solubility in ammonio-cupric sulphate, and affinity for alkalis such as potash, soda, ammonia, causes it to swell when they act on the cell fibres.