Plants & People: The Intersection of Botany and Human Experience

Carob: From Pod to Pantry

Nomenclatural History

The scientific name Ceratonia siliqua L. is derived from the Greek work keras, meaning “horn”, and the Latin word siliqua, which refers to the shape and fitness of the pod. And, of course, the L. is in reference to Carl Linnaeus. The binomial name was first recorded by Linnaeus in Species Plantarum 2 in May 1753, with a locality of Apulia and Sicily (Italy), Crete (Greece), Cyprus, Syria, and Palestine. The designated lectotype is located at the Linnean Society of London Herbarium (Missouri Botanical Garden, n.d.). The common name for this plant is carob, and has its roots in ancient Mediterranean/Middle Eastern cultures. Carob originates from the Hebrew word kharuv, which means “locust”. This relates to two other common colloquial names for carob: St. John’s bread and locust bean. These two names stem from the biblical references to St. John the Baptist’s consumption of locusts during his travels. The “locusts” he was consuming were likely actually the pods from carob trees, hence the adoption of common names that have to do with the word locust. As the carob tree spread throughout the Mediterranean and other parts of the globe, other names were adopted in the new regions, such as the Arabic kharrūb, Spanish algarrobo or garrofero, Italian carrubo, and many more (Batlle & Tous, 1997, p. 9).

Evolution & Domestication

Evolutionary History
Ceratonia siliqua is a member of the Leguminosae family, the third largest angiosperm family and possibly originating in the late Cretaceous period (110.5-66 mya). (Wojciechowski et al, 2004 ). Within this classification, carob belongs to the subfamily Caesalpinioideae which, along with the other two Leguminosae clades (Mimosoideae and Papilionoideae), is evidenced to have been present since the Eocene (55.8-33.9 mya) (Wojciechowski et al, 2004). C. siliqua is then a member of the Caesalpinioideae subfamily that diverged even further into a smaller group known as the Umtiza Clade, to which C. siliqua belongs. The use of fossil records has allowed researchers to estimate that the Umtiza Clade was likely present as far back as the Late Paleocene, dating it to about 56.5 mya, making it one of the oldest Caesalpinioid clades. (Bruneau et al, 2008).
It was long thought that C. siliqua was the only member of the genus Ceratonia, but in the mid-1900s another species of the Ceratonia genus was discovered: C. oreothauma, which is native to Arabia and Somalia. C. oreothauma has been proposed as the potential wild ancestor of the cultivated C. siliqua due to its smaller, less evolved pollen form. It is distinct from the cultivated species since its flowers are strictly unisexual. (Batlle & Tous, 1997, p. 9, 14).

Domestication History 
 

Ecology & Life History

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Morphology

Vegetative Morphology
Carob is a sclerophyllous evergreen shrub or tree with a thick trunk with rough bark, and sturdy branches that may grow up to approximately 10 m high, though wild trees may grow a few meters taller (Batlle & Tous, 1997, p. 10). Some features of sclerophyllous trees include hard, leathery leaves and short internodal distances, which allow them to adapt to long periods of drought and heat (Alm, 2016). The leaves of the carob tree are 10-20 cm long, alternate, pinnately compound with 4-10 oppositely arranged pairs of leaflets, with or without a terminal leaflet. Each leaflet is 3-7 cm long, ovate or elliptic, dark green on the top with slightly undulate margins. The sclerophyllous leaves have a thick epidermis layer to protect from water loss (Batlle & Tous, 1997, p. 10). Carob trees also have a broad, semi-spherical crown that requires low maintenance and are valued as an ornamental shade tree throughout California, Australia, and elsewhere. However, their strong, extensive root system that allows them to adapt to dry conditions in the wild are considered invasive in landscaping (Batlle & Tous, 1997, p. 28).

Reproductive Morphology
Most carob trees are dioecious with female and male flowers on separate trees, though a small number are hermaphroditic. The prevalence of flower types varies by region. For example, hermaphrodite trees are found more frequently in the Mediterranean Spanish coast than in Southern Spain, though they are still rare with respect to male and female trees. Generally, the flowers are 6-12 mm long, arranged along an inflorescence axis in racemes, with coloration that ranges from green to red. They display radial symmetry with no petals and a disc-shaped calyx consisting of 5 hairy sepals. Flowers typically begin bisexual but one sex is repressed during later development, hence the presence of rudimentary stamens or pistils on females or males, respectively. All three types have a nectarial disc, though females produce the highest volume of nectar (Batlle & Tous, 1997, p. 12).

Female flowers feature a fully developed pistil that is 6-8.5 cm long with 2 stigmatic lobes, 2 carpals, and 5 rudimentary stamens. Male flowers have 5 stamens and a central rudimentary pistil. The filaments may either be of either short or long type, while the anthers may range from yellow to red. Hermaphrodite flowers are a combination of both male and female types with a fully developed pistil and 5 fully developed stamens (Batlle & Tous, 1997, p. 12, 14).

The carob fruit is an indehiscent pod or legume 10-30 cm long, 1.5-3.5 cm wide, and about 1 cm thick; they are compressed, with a straight or curved shape depending on the cultivar. Pods are soft and green but become hard, brown, and leathery upon ripening. Carob pods have two main components: the pulp and the seeds. The pulp consists of an outer, dark and leathery pericarp and an inner, soft mesocarp that separates the seeds. The seeds range from 8-10 mm long, 7-8 mm wide, and 3-5 mm thick. They are composed of a hard external coat, an embryo/germ, and the endosperm (Batlle & Tous, 1997, p. 12, 24).

Cultivation Practices

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Consumption & Nutritional Value

Carob is commonly known and utilized as a cacao substitute to its high sugar content (48-56%) and lack of theobromine and caffeine. Currently, the main use is for gum extraction. However, carob pods are an excellent source of bioactive compounds and contain low levels of fat (0.2-0.6%). To get a better idea of the nutritional value carob offers and its wide variety of uses, it’s convenient to split it into its two components. As mentioned previously, the carob fruit can be split upon crushing the pod into the pulp and the seeds, which approximately make up 90% and 10% of the pod’s weight, respectively (Batlle & Tous, 1997, p. 23).

Pulp
The pulp can be ground into large pieces called kibbles, or ground into powder. Kibbles are typically used for animal feed, carob syrup or liqueur, while the powder is used as a cacao substitute or flour for baked goods or sweets. The pulp can also be roasted whole and eaten as a snack (Batlle & Tous, 1997, p. 26-27). 
Carob pulp is high in sugar content (48-56%), but also contains cyclitols, fiber, polyphenols such as tannins, amino acids, minerals, and vitamins. The main cyclitol in the pulp is D-pinitol, which has anti-diabetic effects due to the compound's ability to increase insulin sensitivity and regulate blood sugar levels. The fiber and polyphenols in carob are also known to enhance lipid metabolism, lower total and LDL cholesterol to counter hyperlipidemia, and produce anticancer effects that involve anti-tumor, anti-proliferative, and proapoptotic activity. The main polyphenols are tannins, which have anti-diarrheal effects. Carob pulp is also considered a good source of amino acids by the World Health Organization as it contains all 7 essential amino acids at levels that meet their standards. Carob is also a strong source of calcium, and potassium, though it also contains other nutritionally important minerals such as iron, copper, zinc, or manganese (Goulas et al., 2016). Some prominent vitamins in carob pulp include vitamin E, D, C, niacin (B3), B6, and folic acid (B9), with lower levels of A, B2, and B12 (Papaefstathiou et al., 2018).

Seeds
Upon de-hulling the seeds, they are composed of the embryo, which is used in germ meal, and the endosperm. The endosperm is ground into carob bean gum (CBG) also known as locust bean gum (LBG), which is the most common use of commercially grown carob. LBG is used as a stabilizer, thickener, or a binder and gelling agent in a wide variety of food products such as ice creams, soups, sauces, cheese, pies, canned meats, or pet food (Batlle & Tous, 1997, p. 26-28). Germ meal is high in protein content (50%), while LBG is also used for pharmaceutical purposes due to its inhibition of gastrointestinal diseases like gastroesophageal reflux (Batlle & Tous, 1997, p. 24; Goulas et al., 2016).

Human Experience

Historical Applications
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Future Applications
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References

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