History

Cochineal was used by the Greeks and Romans and was an important dye in 15th century England. It was derived from the swollen females of two scale insects Kermes vermilio and K. ilicis close relatives of D. coccus. These species use as host the kermes oak, Quercus coccifera, a native of the Mediterranean coast (for further details see J. Compton, The Garden, 1990, 115, 385).

Culture and life history

Each year eggs from the previous crop, which are protected during the rainy season by shelters placed over the plants, are ‘sown’ on the cacti (usually species of Opuntia) on which it is intended to breed. Both male and female insects emerge. The males are about 1 mm long and possess wings, while the females are about 2 mm long and without wings. After fertilization the females attach themselves to the cacti by means of their probosces, which become embedded in the tissues of the plant; the males then die. The females swell to about twice their former size, owing to the presence of developing larvae, and develop red colouring matter. The larvae mature in about 14 days and escape from the now dead body of the parent. Only a small proportion of the larvae develop into males. For the next fortnight the young females crawl about the plant and the males fly. The sequence of events described above is then repeated. The life cycle thus takes about 6 weeks and three to five generations of the insects may be produced in a season.

Collection and preparation

The insects are brushed from the plants with small brooms and killed, a certain number being left to provide for subsequent crops. The first crop of the season usually contains the most colouring matter. The insects are killed by plunging in boiling water, by stove heat or by exposure to the fumes of burning sulphur or charcoal. If heat is used, the insects change to a purplish-black colour and are known as ‘black grain’, while the fume-killed purplish-grey ones are known as ‘silver grain’. Small immature insects and larvae which can be separated by sieves are sold as ‘granilla’ or siftings.

Characters

Cochineal insects are 3.5–5.5 mm long and somewhat oval in outline. The convex dorsal surface shows from nine to 11 segments, but there are no constrictions between head, thorax and abdomen. The insect has a pair of seven-jointed antennae and three pairs of very inconspicuous legs. The surface bears tubular glands which secrete wax, the melting of which by heat accounts for the difference in colour between the silver grain and black grain varieties.

Cochineal should be examined microscopically after removing the colouring matter by means of solution of ammonia. Within each insect will be found from 60 to 450 eggs and larvae. For illustrations, see previous editions of this book.

Constituents

Cochineal contains about 10% of carminic acid, (Fig. 33.1), a brilliant purple, water-soluble colouring matter; it is a C-glycoside, anthraquinone derivative. The insects also contain about 10% of fat and 2% of wax. Recent research has shown that irradiation, even at the lowest level tested (1 KGy), is effective in eliminating the microbial count and has no significant effect on the stability of the pigment. The BP describes a test of absence of salmonellae andEscherichia coli and a colour value test in which the extinction of a diluted extract of pH 8.0 is measured at 530 nm.

Carmine, an aluminium lake, is prepared by precipitation by adding aluminium and calcium ions to an extract of cochineal; it contains about 50% of carminic acid. ‘Carmines’ are produced which vary enormously in shades and tinting strengths.