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Microbiological safety of green olives

The olive is the major fermented vegetable in western countries with the Mediterranean area being the major producer. The crop is becoming increasingly important in Australia both for fermented table products and oil production.

Traditionally, the production of green table olives (Spanish style) is a natural lactic acid fermentation that relies on lactic acid producing microorganisms on the raw material. Fermented green olives have an extended shelf life dependent on a number of factors including pH, titratable acidity, water activity, salt concentration and in some cases a terminal pasteurisation. The Codex Alimentarius contains a detailed standard for table olives which includes interdependent levels for minimum sodium chloride content and maximum pH (Codex Standard 66 Table Olives ).

Two recent reports act as a reminder that care must be taken in the production of this fermented product to ensure its safety:

• One report is of botulism attributed to consumption of poorly processed green olives in Italy.
• The other report discusses the potential for survival of Listeria monocytogenes in commercially prepared green olives.

Botulism from green olives

European workers report on an incident of sixteen suspected cases of botulism reported to health authorities by hospitals in three adjoining regions of central and northern Italy (Emerging Infectious Diseases 11 5 2005 1088). All patients had eaten at the same restaurant in the town of Molise in February, 2004. Fifteen were admitted to hospital and some to intensive care. None required ventilatory support and no deaths occurred.

Epidemiological and laboratory investigation confirmed Clostridium botulinum type B as the causative organism. None of the food items served on the days when diners became ill was available for sampling. However epidemiological analysis of the foods diners had consumed indicated that only risk associated with eating green olives was significant. A jar of olives prepared at the same time as those eaten on the implicated days had a pH of 6.2, well above the level of pH 4.6 normally required to prevent growth of C. botulinum. No salinity testing was performed.

Interviews with the restaurant proprietors indicated that the suspect olives were prepared on site. The olives were soaked in salt water for approximately 35 days. The olives were then decanted into jars and the salt water replaced with fresh water. Neither the amount of salt used in the brine nor the pH were standardised during preparation.

Production processes for fermented green olives range from traditional methods to large-scale controlled processes. There is nothing wrong with the use of traditional methods as long as manufacturers are aware of any hazards associated with the product and can manage them effectively. It is essential that sufficient acid is produced to ensure that pathogenic bacteria do not grow. The product has a good public health record but commercial manufacturers must be able to monitor and adjust the pH, total acidity and salt concentration during the lactic acid fermentation period and should ensure that the procedure they are using meets the Codex requirements. For those who wish to produce their own fermented olives at home, consultation with a reputable authority may be necessary as many domestic recipes are scant on detail.

Listeria presence and survival on green olives

Bacteria growing, or at least surviving, under environmental conditions once considered adequate to kill them quickly have sometimes been identified as the cause of foodborne illness. For example, Salmonella species and toxigenic Escherichia coli have been associated with illness following consumption of low pH fruit products. Listeria monocytogenes, an important foodborne pathogen, can grow under environmental conditions often considered sufficient to control foodborne pathogens: pH <4.6, in the presence of 10 percent salt, and temperatures <4°C.

No documented outbreaks of listeriosis have been attributed to consumption of olives but Italian researchers have recently shown that L. monocytogenes can survive and grow in green table olives produced under experimental conditions and may be present in commercially prepared green olives at least in that country (Journal of Food Protection 67 10 2004 2189–2194).

These researchers prepared green olives by the standard fermentation method using a 2 percent sodium hydroxide solution to reduce bitterness followed by washing in water and brining in an 8 percent sodium chloride solution. The brined olives were inoculated with two strains of Lactobacillus - one of which had been isolated from naturally fermented green olives.

A second experiment involved what the researchers term the biological method where washed olives were brined initially in 5 percent sodium chloride solution which was gradually increased to 7 percent over a two-month period. After the initial three days in brine, experimental samples were inoculated with the selected Lactobacillus strain(s).

In addition, ten samples of thermally treated green table olives in cans made by different producers were purchased and analysed. L. monocytogenes was isolated from all trial samples older than two months and from one of the ten thermally treated samples.

With one exception, the olive samples were pH <4.5 with one being as low as pH 3.76. Again with the one exception, titratable acidity (lactic) ranged from 0.2 percent to 0.45 percent. Sodium chloride concentrations in the brine varied from 6.2 percent to 7.5 percent.

The authors conclude that L. monocytogenes naturally present on green olives can survive in the fermented product despite its low pH, water activity and high salt concentration. They recommend that an appropriate heat treatment be applied to ensure a reduction in L. monocytogenes.