A bulletin for the Australian Food Industry November 2001
Contents: Food allergies and food sensitivities | Pathogens in fresh fruit and vegetables | Decontamination of fresh fruit and vegetables | Preservation of vegetables in oil and vinegar
Preservation of vegetables in oil and vinegar
We have written before on the potential hazards associated with this growing industry. The products are traditional in Italy and are now popular in many other countries including Australia. They are perfectly safe when manufactured to recognised standards which will prevent the growth of food poisoning bacteria.
However as Italian microbiologists have reported (Eurosurveillance 4 1999 7-9), from 1994 to 1998 there were 84 cases of foodborne botulism confirmed in that country. Six were fatal. Data from laboratory testing of implicated foods attributed 65 percent of the cases to homemade vegetable preserves in oil.
A number of small manufacturers involved in the production of vegetables in oil are essentially doing so on a homemade basis. It is essential if the risk of botulism and other foodborne disease is to be avoided that manufacturers of these foods understand the basis for the preservation process and implement the necessary controls.
It is a requirement of the Australian Food Standards Code, introduced following botulism incidents in the US over a decade ago, that this class of product has a pH below 4.6. In our experience a number of manufacturers are unaware of this requirement before contacting Food Science Australia. Usually these manufacturers already have their product on the market.
It is necessary with most vegetable products to include sufficient acid, usually vinegar, in the formulation to achieve this pH level. This will not guarantee that the products will not spoil if not handled correctly but it does ensure they do not become toxic.
In some situations the use of vinegar (or acetic acid) is avoided because it is incompatible with the vegetable product. When this is the case, a different set of circumstances applies and the water activity of the vegetable becomes the primary factor controlling the safety of the product. Correctly dried or semi-dried vegetables will not support the growth of food poisoning bacteria. The limiting water activity for strains of Clostridium botulinum, the organism which produces botulinum toxin, is 0.93.
Vegetables and herbs to be packed in oil without treatment with vinegar should be dried to a water activity below this figure. Ideally the water activity should be reduced below 0.85 to ensure that another pathogen Staphylococcus aureus does not grow. At this water activity, the vegetable product would still be classed as 'semi-dry' rather than 'dry' and the product would not be protected from the growth of all potential spoilage microorganisms.
Tomatoes, including sun-dried tomatoes, are a special case. The pH of fresh tomatoes is normally just below 4.6. When the tomatoes are partially dried, the natural acid components are concentrated and the pH is reduced. It will often be closer to 4.0 in a fully dried product and the risk of food poisoning is largely eliminated. However, as recent incidents have shown, acid tolerant strains of Salmonella spp. and pathogenic Escherichia coli are able to survive long periods at pH levels below 4.6 and these vegetable products must be produced in accordance with good hygienic principles.
Vegetables other than tomatoes are all low acid (high pH) products and must be either acidified or properly dried before being covered with oil. This includes garlic and herbs which may be added to other preserved vegetables as flavouring. It is important that manufacturers have the capability to monitor correctly the pH and water activity of these products.
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au