Plant 320

Ceratonia siliqua L. (Fabaceae)


Chocolate treats for dogs, stabilisers for industrial food, units of gemstone weights and measures of gold purity all have carob in common. Ripe fruits, dried and powdered, produce sweet, chocolate-flavoured carob flour that lacks the theobromine of proper chocolate. The ground seeds are an industrial source of locust gum, one of the most important thickening agents used by the food industry. Carob seeds were used as small weights by ancient cultures of the Mediterranean region, giving rise to the word carat.

Ceratonia siliqua is a small, evergreen tree that produces clusters of flowers directly from its branches and trunk (cauliflory). As a legume, it might be expected that carob takes nitrogen gas from the atmosphere and turns it into a form that can be used by plants, but the evidence carob can do this is equivocal.

Gender is also complex in carob. Individual flowers can be male, female or sometimes hermaphrodite; flower clusters comprise one or more of these flower types. Successful fruit production needs successful pollination. Male flowers have a faintly fishy odour, suggesting flies may be pollinators but bees and wind are also involved.

Over one-year, each pollinated female or hermaphrodite flower will produce a single dark brown, leathery, flattened bean-like, pulpy fruit. People prefer fruits from female flowers; they are larger and sweeter.

The genus Ceratonia comprises two species. Ceratonia siliqua is endemic to the eastern part of the Mediterranean Sea, although thousands of years of trade by peoples in the region has meant that it has become widely naturalised. The other species, discovered in the 1940s but not formally named until 1980, is found from Somalia to Arabia. Unconscious human selection has produced many cultivated carob types that differ from wild types in features such as seed size and the amounts of pulp and sugar in the fruits.

In 2017, global carob production was approximately 140,000 tonnes; Portugal, Italy, Morocco and Turkey were responsible for about 80% of this. Sugar-rich carob fruits contain a complex chemical cocktail that processing transforms into flavours associated with chocolate. Chocolate substitutes have become attractive to food industries where cocoa demand has increased, supply is erratic, and prices are volatile. Carob offers the possibility of caffeine-free, high-protein, low-sugar chocolate with high profit margins. The seeds are rich in galactomannan carbohydrates, which swell when wet. Locust gum is used in everything from ice cream though low-calorie and gluten-free products to pet food.

Further reading

Martins-Loução MA 1990. Carob (Ceratonia siliqua L.). In: Bajaj YPS (ed) Legumes and oilseed crops I. Biotechnology in agriculture and forestry. Springer, pp. 658-675.

Tucker, SC 1992. The developmental basis for sexual expression in Ceratonia siliqua (Leguminosae: Caesalpinioideae: Cassieae). American Journal of Botany 79: 318-327.

Turnbull LA et al. 2006. Seed size variability: from carob to carats. Biology Letters 2: 397-400.

Stephen Harris