olive debittering and fermentation

Raw olives remain strongly bitter even though bitterness weakens as they ripen. Table olives, eaten as a preserved food, are produced through partial or complete debittering followed by fermentation. A principal source of bitterness is oleuropein, a bitter secoiridoid, so debittering is a process that reduces this compound. Methods vary: the Spanish style soaks fruit in a 1.5–4.5% sodium hydroxide solution to break down oleuropein chemically, then uses lactic fermentation in brine; the Greek style ferments whole fruit directly in brine; and the alcaparras style repeatedly soaks halved fruit in water over one week to remove oleuropein by diffusion.

During debittering, oleuropein breaks down into the less bitter compounds hydroxytyrosol and tyrosol. In a storage experiment with natural green olives, one study found enzymatic hydrolysis early in storage followed by slower chemical hydrolysis. This is a different pathway from an external chemical treatment such as the rapid breakdown with caustic soda in the Spanish style. In a Spanish-style PDO product, it was also reported that alkali treatment completely removed oleuropein, leaving only hydroxytyrosol, tyrosol, and oleoside 11-methyl ester.

A study following Nyons-style black olives fermented naturally in brine alone without additives for about 15 months considered diffusion, chemical breakdown, and enzymatic hydrolysis as possible pathways of debittering. It concluded that diffusion from flesh into brine and the activity of enzymes naturally present in the olive were most involved in this fermentation. However, no strong association with a particular microorganism was found. This study did not establish the share of bitterness reduction attributable to each mechanism or a causal role for a particular microbe.