The bitter taste of olives is mainly caused by the phenolic compound named oleuropein and the mechanism of its hydrolysis during the processing of natural green olives was studied. First, a rapid chemical hydrolysis of oleuropein takes place at a high temperature of 40°C and at a low pH value of 2.8, but the chemical hydrolysis of the bitter compound is slow at the common range of pH for these olives (3.8-4.2). However, decarboxymethyl elenolic acid linked to hydroxytyrosol and hydroxytyrosol have been found in a high concentration during the elaboration of natural green olives. When olives were heated at 90°C for 10min before brining, these compounds are not formed. Hence, the debittering process in natural green olives is due to the activity of β-glucosidase and esterase during the first months of storage and then a slow chemical hydrolysis of oleuropein happens throughout storage time.

Brownish colourations in Natural green table olives (non-treated with alkali) make this product less attractive to consumers than Spanish-style green table olives (treated with alkali), which develop a more appreciated bright golden-yellow colour. These colour differences were studied in relation to changes in the composition of chlorophyll and carotenoid pigments, as well as polyphenolic compounds and polyphenol oxidase enzyme (PPO) activity. Natural green olives showed a different chlorophyll profile than Spanish-style. However, all the chlorophyll pigments formed in both processing types were Mg-free derivatives (mostly pheophytins) with similar colourations, ranging from grey to green brownish. In the carotenoid fraction no appreciable differences were found between both processing types. The fruit's brownish colour was mainly due to polymeric substances with a size of >1000 daltons and polyphenolic nature, resulting from an enzymatic oxidation by PPO of the o-diphenolic compounds present in the fresh fruits.

29 Páginas; 4 Tablas; 3 FigurasThe presence of phenolic compounds in raw olives, particularly the bitter glucoside oleuropein, requires transforming this substance into other non-bitter to make the fruit palatable. An alkali treatment is currently carried out to hydrolyze the oleuropein but a high volume of wastewater is generated. The aim of this study was to develop a natural product without an alkali treatment and with appropriate organoleptic characteristics similar to those of Spanish-style green olives. Mild heat treatments (60 °C for 10 min) were sufficient to inactivate the β-glucosidase activity which prevented the formation of antimicrobial compounds and thereby the growth of lactic acid bacteria was promoted. By contrast, heating released a high concentration of oleuropein in fruits that remained at a very high level even after 6 months of fermentation. The inoculation of the brines of Manzanilla olives with selected lactic acid bacteria with oleuropeinolytic activity was insufficient for reducing the high concentration of the bitter glucoside. However, favorable results were obtained with varieties such as Gordal and Aloreña, which have lower oleuropein contents. The product obtained showed a very attractive color, similar to that of Spanish-style green olives and lighter than natural green olives, which is a positive aspect for consumer acceptability.This work was supported by Project AGL 2009-07512 from the Spanish government and European Union (European Regional Development Funds). E.R. thanks MINECO for her FPI fellowship.Peer reviewe