Tannins
Carob pods and their by-products contain important amounts of tannins and the pods are considered an exceptionally rich source of non-extractable polyphenols (Silanikove et al., 2006). 15-20% of these tannins are polyphenols and 25-50% are more or less bound with cell walls (Kotrotsios et al., 2011). Scientific literature reports widely contradictory values for condensed tannins. Some authors report extremely large amounts of condensed tannins (16-20%, Silanikove et al., 2006; Battle et al., 1997), but others give values in the 3-5% range (Priolo et al., 2000), and very low values have also been proposed (< 0.4 % DM; Avallone et al., 1997). Methodological differences may explain theses discrepancies (Silanikove et al., 2006).
These tannins interfere with protein digestibility and may reduce the digestibility of other feeds used together with carob, thus reducing body weight gains (Calixto et al., 1982). The presence of tannins in carob pods has been shown to have deleterious effects on digestion, growth and milk production in ruminants, and growth in poultry (Silanikove et al., 2006). One proposed mechanism of action specific to carob tannins is that they trap fat and cholesterol, causing unwanted cholesterol excretion and resulting in hypercholesterolemia. Supplementation with PEG (polyethylene glycol) helps to release the cholesterol, thus countering this effect (Silanikove et al., 2006).
However, it should be noted that carob tannins also have anti-diarrheic properties and that toasted carob powder is used to that effect in children (Lizardo et al., 2002).
Palatability
In green non-ripe pods, condensed tannins cause a strong astringency taste in the mouth, making the pods unpalatable. This astringency decreases during ripening, due the polymerization and condensation of tannins, making the ripe pod more palatable (Silanikove et al., 2006). Also, carob pods contain free iso-butyric acid (about 0.6-0.9% DM) that give them a specific cheese-like odour, which may have positive or negative consequences on palatability (Berna et al., 1997). However, much lower values (<0.05% DM) have been reported for deseeded pods (Iipumbu, 2008).