Brief Summary from
the Encyclopedia of Life
by Leo Shapiro
Traditional fermentation and curing of Olives
An olive vat room used for curing at Graber Olive House
Raw or fresh olives are naturally very bitter; to make them
palatable, olives must be cured and fermented, thereby removing
oleuropein, a bitter phenolic compound that can reach levels of 14% of
dry matter in young olives. In addition to oleuropein, other phenolic
compounds render freshly picked olives unpalatable and must also be
removed or lowered in quantity through curing and fermentation.
Generally speaking, phenolics reach their peak in young fruit and are
converted as the fruit matures. Once ripening occurs, the levels of
phenolics sharply decline through their conversion to other organic
products which render some cultivars edible immediately. One example of
an edible olive native to the island of Thasos is the throubes black olive, which when allowed to ripen in sun, shrivel, and fall from the tree, is edible.
The
curing process may take from a few days, with lye, to a few months with
brine or salt packing. With the exception of California style and
salt-cured olives, all methods of curing involve a major fermentation
involving bacteria and yeast that is of equal importance to the final
table olive product. Traditional cures, using the natural microflora on
the fruit to induce fermentation, lead to two important outcomes: the
leaching out and breakdown of oleuropein and other unpalatable phenolic
compounds, and the generation of favourable metabolites from bacteria
and yeast, such as organic acids, probiotics, glycerol, and esters,
which affect the sensory properties of the final table olives. Mixed
bacterial/yeast olive fermentations may have probiotic qualities.
Lactic acid is the most important metabolite, as it lowers the pH,
acting as a natural preservative against the growth of unwanted
pathogenic species. The result is table olives which can be stored
without refrigeration. Fermentations dominated by lactic acid bacteria
are, therefore, the most suitable method of curing olives.
Yeast-dominated fermentations produce a different suite of metabolites
which provide poorer preservation, so they are corrected with an acid
such as citric acid in the final processing stage to provide microbial
stability.
The many types of preparations for table olives
depend on local tastes and traditions. The most important commercial
examples are:
Spanish or Sevillian type
(olives with fermentation): Most commonly applied to green olive
preparation, around 60% of all the world's table olives are produced
with this method. Olives are soaked in lye (dilute NaOH, 2–4%) for 8–10
hours to hydrolyse the oleuropein. They are usually considered
"treated" when the lye has penetrated two-thirds of the way into the
fruit. They are then washed once or several times in water to remove
the caustic solution and transferred to fermenting vessels full of
brine at typical concentrations of 8–12% NaCl. The brine is changed on
a regular basis to help remove the phenolic compounds. Fermentation is
carried out by the natural microbiota present on the olives that
survive the lye treatment process. Many organisms are involved, usually
reflecting the local conditions or "Terroir" of the olives. During a
typical fermentation gram-negative enterobacteria flourish in small
numbers at first, but are rapidly outgrown by lactic acid bacteria
species such as Leuconostoc mesenteroides, Lactobacillus plantarum, Lactobacillus brevis and Pediococcus damnosus.
These bacteria produce lactic acid to help lower the pH of the brine
and therefore stabilize the product against unwanted pathogenic
species. A diversity of yeasts then accumulate in sufficient numbers to
help complete the fermentation alongside the lactic acid bacteria.
Yeasts commonly mentioned include the teleomorphs Pichia anomala, Pichia membranifaciens, Debaryomyces hansenii and Kluyveromyces marxianus. Once fermented, the olives are placed in fresh brine and acid corrected, to be ready for market.
Sicilian or Greek type
(olives with fermentation): Applied to green, semiripe and ripe olives,
they are almost identical to the Spanish type fermentation process, but
the lye treatment process is skipped and the olives are placed directly
in fermentation vessels full of brine (8–12% NaCl). The brine is
changed on a regular basis to help remove the phenolic compounds. As
the caustic treatment is avoided, lactic acid bacteria are only present
in similar numbers to yeast and appear to be outcompeted by the
abundant yeasts found on untreated olives. As very little acid is
produced by the yeast fermentation, lactic, acetic, or citric acid is
often added to the fermentation stage to stabilize the process.
Picholine or directly-brined type
(olives with fermentation): Applied to green, semiripe, or ripe olives,
they are soaked in lye typically for longer periods than Spanish style
(e.g. 10–72 hours) until the solution has penetrated three-quarters of
the way into the fruit. They are then washed and immediately brined and
acid corrected with citric acid to achieve microbial stability.
Fermentation still occurs carried out by acidogenic yeast and bacteria,
but is more subdued than other methods. The brine is changed on a
regular basis to help remove the phenolic compounds and a series of
progressively stronger concentrations of salt are added until the
product is fully stabilized and ready to be eaten.
Water-cured type
(olives with fermentation): Applied to green, semiripe, or ripe olives,
these are soaked in water or weak brine and this solution is changed on
a daily basis for 10–14 days. The oleuropein is naturally dissolved and
leached into the water and removed during a continual soak-wash cycle.
Fermentation takes place during the water treatment stage and involves
a mixed yeast/bacteria ecosystem. Sometimes, the olives are lightly
cracked with a hammer or a stone to trigger fermentation and speed up
the fermentation process. Once debittered, the olives are brined to
concentrations of 8–12% NaCl and acid corrected, and are then ready to
eat.
Salt-cured type
(olives with minor fermentation): Applied only to ripe olives, they are
usually produced in Morocco, Turkey, and other eastern Mediterranean
countries. Once picked, the olives are vigorously washed and packed in
alternating layers with salt. The high concentrations of salt draw the
moisture out of olives, dehydrating and shriveling them until they look
somewhat analogous to a raisin. Once packed in salt, fermentation is
minimal and only initiated by the most halophilic yeast species such as
Debaryomyces hansenii. Once cured, they are sold in their natural state
without any additives. So-called Oil-cured olives are cured in salt,
and then soaked in oil.
California or "artificial ripening" type
(olives without fermentation): Applied to green and semiripe olives,
they are placed in lye and soaked. Upon their removal, they are washed
in water injected with compressed air. This process is repeated several
times until both oxygen and lye have soaked through to the pit. The
repeated, saturated exposure to air oxidises the skin and flesh of the
fruit, turning it black in an artificial process that mimics natural
ripening. Once fully oxidised or "blackened", they are brined and acid
corrected and are then ready for eating.
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© Leo Shapiro
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