Olives (Olea europaea) are primarily fermented in the Mediterranean countries of Greece, Italy, Morocco, Spain and Italy. Part of the reason for the process is to eliminate the acute bitterness of the olive that is due to the glycoside oleuropin. Soaking the olive in brine or dilute caustic leads to the hydrolysis and removal of this material.

Nowadays olives are mostly fermented in plastic-clad tanks of fibreglass or stainless steel, perhaps buried underground in the interests of temperature regulation. There are basically two fermentation approaches.

Untreated naturally ripe black olives in brine

The olives are picked when completely ripened (turned from green to black or purple) and are not treated with lye (alkali solution) so that they retain bitterness and fruitiness. They are put into the tanks with 6-10% sodium chloride solution and allowed to undergo spontaneous fermentation by an endogenous microflora comprising lactic acid bacteria and yeasts. The olives are subsequently sorted and graded before packaging.

Lye-treated green olives in brine

The olives are harvested when green or yellow and treated with a 1.3-3.5% lye solution for up to 12 h at 12-20°C to remove most of the bitterness. After washing with cold water, they are taken in stages up to a concentration of 10-13% sodium chloride, a gradual process so as to avoid shrivelling. Endogenous fermentation is allowed to progress for up to a month at 24-27°C, prior to sorting and grading and packaging into glass jars.

In olive fermentations there is no use of starter cultures, although a proportion of brine from a previous fermentation may be used to supplement the new brine.

In the early stages of fermentation, there is activity of the aerobic organisms Citrobacter, Enterobacter, Escherichia, Flavobacterium, Klebsiella and Pseudomonas. These organisms will not grow when the salt is increased beyond 6-10%. Stage two comprises the activity of the lactic acid bacteria (Lactobacillus, Lactococcus, Leuconostoc, Pediococcus), with the progressively dropping pH destroying the initial microflora. The onset of the third stage is once the pH reaches 4.5, with the predominant organism being Lb. plantarum, together with fermentative and oxidative yeasts (Candida, Hansenula, Saccharomyces).

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