Olive Fermentation


Four fundamental types have been established;

1. green

2. turning color

3. natural black ( for these types they refer to color of fruit as a raw material, color does not change during processing)

4. black ( harvested as turning color and turns to black during oxidation in  an alkaline during processing)

• Bitterness of fruit can be eliminated completely and quickly by alkaline hydrolysis, that is, by treatment of olives with sodium hydroxide solutions prior to fermentation and by storage in brine or dry salt.

• These fruits are generally called pickled olives.

• Bitterness may also be eliminated slowly and partially without alkaline treatment during the acid fermentation by placing the fruit directly into brine or by preserving it with dry salt.

• These types are generally known as olives in brine or olives in dry saltOlive fruit

• Pulp    70-90 %

• Stone   9-27 %

• Seed    1-3 %

Pulp                                                        %

Moisture                                              50-75

Oil                                                          6-30

Soluble reducing sugar                      2-6

Soluble non-reducing sugar             0.1-0.3

Crude protein                                     1-3

Fiber                                                     1-4

Ash                                                        0.6-1

Others                                                  6-10

Major soluble sugars are glucose, fructose, sucrose, xylose and rhamnose.

• Also contains 0.5-1 % mannitol.

• Pulp contains 1-3 % tannic acid.

• Oleuropein is the compound responsible for the bitterness of the fruit.

• Catechol  oxidase is the enzyme responsible for the color change of fruit from green to brown.

• Treatment of fruit prior to fermentation or conditioning and the fermentation or storage in brine can affect the composition of final product.

• Fermentation tanks are used to be wood but not now.

• Concrete tanks are still in use (height should be less than 2.5 m to eliminate pressure problems on olives)

• Concrete tanks should be painted with synthetic paints or covered with polyesters.

• Polyester and fiber glass tanks are the best ones and nowadays commonly used.

•  One can have over-ground fermentation tanks ( easy to fill and empty the tank , but investment cost is high and temperature control is difficult), or underground-fermentation tank ( low investment cost, easy temperature control

Spanish style pickled green olives in brine

• ( 1/3 of all production, lactic fermentation is fundamental)

• The fruits are picked when they are still firm and light green in color

Starting phase:

• Alkaline ( 1.3-2.6 % weight/ volume NaOH, 6-10 hours, 28 C )

• Hydrolysis of oleuropein and formation of organic acids from sugars.

• The freshly picked firm green olives stay in the lye 9 to 12 hours.

• Test the penetration of the lye into the olive berries by slicing one berry open after 6 hours to see how far the neutralization has progressed. Use phenolphthalein indicator (1gm phenolphthalein dissolved in 50ml alcohol in 50ml water, is an excellent indicator). The penetrated part of the olive flesh will turn into a bright magenta, the acidic flesh near the stone will still be white.


Main phase:

• Closed containers help prevent the development of surface yeasts and molds.

• Fermentation in brine. Mainly lactic acid fermentation. Lactobacillus plantarum (homofermentative) is dominant culture.

• L. brevis ( hetero fermentative is also exist but when salt concentration is high it is inhibited.

• Yeasts like Candida, Saccharomyces, Deboramyces, Torulopsis are also observed.

• Glass fiber or polyester fermenters are used where anaerobic condition is maintained.

• Initial growth of coliforms can be retarded by acidification or CO2 injection into brine.

• Process starts with 5-6 % salt and ends with
7 % salt

Final phase:   

• Conservation in brine.

•  Bottling.

• Growth of propionibacterium is undesired at this stage because it uses lactic acid produced.

•  It can’t grow over 8% salt concentration

• So optimum final values are : pH:3.8-4.2, Lactic acid: 1%, Salt :7 %,

• To increase shelf life > 8 % salt necessary.

Natural black olives in brine (Greek Method):

• (1/3 of all production, Yeast dominates and minor lactic fermentation).

• The Greek black olive industry uses naturally ripe, fully matured and dark-purple fruits

Starting phase:  No starting phase.

                               Main phase:

•  Fermentation in brine. (6 % salt, concrete tanks lined with paraffin or epoxy resins).

• Yeast predominate (Candida, Pichia, Kleockera , Torulopsis, Deboramyces)

•  Lactic acid bacteria sometimes present ( if salt concentration is less than 7-8 %).

• Slow loss of sugars and bitterness (3-6 months) occur accompanied with the formation of organic acids, ethanol, acetaldehyde and ethyl acetate.

• Strict anaerobic condition should be maintained to prevent film forming yeasts. Fermentation is very slow, diffusion of components is very slow because it wasn’t treated with alkaline solution.

• The dark-purple anthocyanins of the olives turn light red during fermentation.

• If Lactobacillus dominates, pH drops to 4 ( > 0.6 % L. acid ) color will be lighter ( Similar to black olives treated with alkaline , it is not desired.)

• Final pH is 4.5-4.8, at the end salt concentration is > 10 %

• After fermentation is completed, it exposed to air to oxidize which improves the skin color.

Final phase:

• Conservation in brine.    Antifungal treatment trials of
‘Greek-style’ black olives indicate that sorbic acid 0.075% is the most efficient preservative, followed by 0.075% benzoic acid and 0.032% calcium proprionate.

• The spoilage organisms encountered are zapatera, galazoma, and film-forming pectolytic organisms which disintegrate the ‘meat’ of the fruit.

• This last problem can be prevented by means of a thin layer of paraffin oil on the surface of the brine.

Pickled black (ripe) olives in brine ( American style):

    Straw-yellow to cherry-red olives are graded according to color and size to insure uniform lye penetration

• Alkaline treatment ( 1-3 % NaOH )

• place into water,

• inject air under pressure to oxidize ( oxidation of polyphenolic compounds permits complete blackening of the skin , first two steps will be repeated 3-5 times)

• wash several times

• add 0.1 % iron gluconate to last wash water to stabilize color

• put in to brine ( 3 % salt )

• bottled in tin cans or glass bottles

• sterilize (116 C for 1 hr)

• Final pH: 5.8-7.9, salt 2-3 %, taste is very different than fermented ones.

Some Turkish style olives produced

Gemlik style black olive production :

• Harvest olives: when skin color is black and pulp color is purple,

• process within 14 hours.

• If it is going to be stored longer before processing keep it in a solution ( of 0.67 % L acid, 1 % acetic acid, 0.03 % sodium benzoate, and 0.3 % potassium sorbate .)  

•  grade

• washing ( 2-3 days)

• Put them into fiber glass tanks ( 10 tons size tanks)

• add brine solution 10 % salt,

• 0.15 g / kg iron gluconate or iron oxalate can be added to improve color,

• 1 % inoculum L. plantarum

•  Fermentation tank has a solution circulation system.

• After fermentation started, within 2-3 days salt concentration decreases to 5-6 %.

• During first two months, twice a week control salt concentration and adjust to 10 % until it stays constant

• Fermentation temperature is approximately 20 C.

• Try to prevent contact of olives with air at the surface of tank.

• If sugar concentration is not sufficient add 0.5-1 % molasses during third or forth week of fermentation.

• Lactic acid concentration should be not less than 0.9 %.

• Mold growth on the surface of the tank should not be permitted. It uses lactic acid produced and product softens.

• fermentation time is approximately 6 months

• Fill cans under N gas or CO2 and pasteurize.

• Keeping them in low salt concentration for first 20 days then increasing to 10 %, shorten fermentation time to 2-3 months.

            Cut (çizme) green olives  

• Olives are cut from 2-4 place

• Put into 2-3 % salt solution ( no alkaline treatment)

• Change its solution twice a day until bitterness is removed

• put into 8-10 % salt solution and 8-10 days of fermentation

• packaging ( in 5-8 % salt solution, 1 % citric acid, olive oil)

                Olive produced in baskets ( sele zeytini)

• Ripe olives are placed in a basket as one layer olive and one layer dry salt until to the top of basket and covered with cloth

• turn these olives within couple days  (olive release its water)

• In 3-4 weeks no bitterness left (weight loss is 20-30 %, no air movement is wanted to prevent drying of olives)

• Wash

• pack and cover with olive oil

                Hurma zeytin

• Single olive type that can be consumed immediately after harvest.

• ( Grown at Urla and Karaburun but very small amount is produced so economically it is not important )

• Mold ( Phomoolea) grows on the tree and it breaks down oleuropein on the tree so olive is no longer bitter when it is harvested.


1- Film formation on the surface of tanks ( soft pickles):

• Film formation ( called kefeke in Turkish)  is observed on the surface of the tanks, especially during summertime. (color of the film changes as White to green to red to finally gray).

• Microorganism in this film breaks down lactic acid produced and this supports the growth of microorganism which breaks down pectin so that olive softens.

• This softening also occurs if sufficient lactic acid is not produced or salt concentration is very low.

•  Also breakdown of fatty acids occur which gives bitter taste, it is called yağlanma in Turkish.

To prevent film formation we should tightly close surface of the tank.


2-Slime formation

• Due to low acid and salt, slime forming m.o. growth cause this problem.



• Occurs in green olive pickles due to contact with air. Also iron contamination cause darkening by reacting with polyphenols and forming iron tannat.

• If darkening is not intense, keeping olives in 1-2 % H2SO4 solution can be helpful to remove dark color


4-Air space formation: (blistering)

• Due to high temperature and high alkaline concentration air space forms just underneath the skin of olive

• Faulty fermentation and growth of Klebsiella, EscherichiaCitrobacter, Aerobacter, Aer-obacillus, ClostridiumEnterobacter, Aeromonas, SaccharomycesIt may be prevented by adding lactic acid ( 0.5 %) and salt ( 3-6 % ), adding starter, satisfying hygenic conditions


5-Off flavor:

• Occurs due to butyric acid formation.

• If occurred, remove saline solution, wash olives and put them into solution containing 7-8 % acid.


6- Zapatera sickness:

•  Due to formation of caprilic acid and butyric acid by propionibacterium and clostridium .

• Since it occurs when pH > 4.2 , add acid to reduce pH<4 to prevent the sickness.


7-White spots under skin of green olives

• These are colonies of L. plantarum ( Same problem occurs in tomato pickles)

• Olive is edible, it only affects appearance.

• To prevent, add acid at the beginning of process and don’t use very hot alkaline solution.

8- Softening

• Softening can occur either prior to or during processing.

• It may be caused by overheating of the fruit, by enzyme action of the fruit or of microorganisms such as molds, yeasts, and some bacteria, or by excessive lye treatment.

• Control is by strict monitoring of all stages of olive harvesting, handling, storage, and processing


9. Shriveling.

• This is caused by a high initial salt content in the brine,

• To release of carbon dioxide from the fruit when they are removed from the fermentor. Accumulation of this gas in the anaerobic process results from olive respiration at the beginning of the process as well as   microbial activity

• It can be avoided by strictly controlled brief aeration of the brine.

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