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From: edbw@unixg.ubc.ca (Edgar Wickberg)    source
All homemade herb or vegetable flavoured oils have doubtful safety if they are not refrigerated immediately and kept in the refrigerator. The reason for concern is botulism. Clostridium botulinum, the organism that makes the deadly poison that results in botulism, grows in airfree environments. It is only when it grows that it produces the toxin. Putting anything up in oil produces a really great airfree (oxygen-free) environment and therefore the risk of toxin production. Clostridium botulinum is a very common organism in all of our environments, but the organism itself isn't dangerous to us. It requires, besides the airfree environment, a low acid one and temperatures above fridge temps. This is probably more than you ever wanted to hear about why not to make garlic flavoured oil. Why not dry most of your garlic so it will last and make just a little oil and keep it in the fridge. To make the kind that you store in the fridge, just heat a small quantity of oil (of your choice), put a few peeled cloves of garlic in a small bottle, pour the hot oil over, cool for half an hour, cover and refrigerate.

From: baker.325@magnus.acs.ohio-state.edu (gwen baker)
Take the cleaned herbs and place into a jar (a mason jar will do). Heat the oil to just warm (too hot and you will cook the herbs instead of extracting the taste), pour the warm oil over the herbs and let set. Check every 3 days to once a week, straining and adding additional herbs until the desired flavor is reached.

HeK comment to above: keep in refrigerator until desired flavor is reached and thereafter

Editorial Note: Foodborne botulism is a paralytic illness caused by the ingestion of botulism toxin, a neurotoxin produced by the ubiquitous spore-forming bacterium C. botulinum. Although botulism toxin in food can be destroyed by heating to boiling (212 F [100 C]) for 10 minutes, the spores are heat resistant and can survive prolonged boiling. To destroy C. botulinum spores, food must be heated under pressure to temperatures substantially greater than 212 F. Certain environmental conditions, such as absence of oxygen (anaerobic conditions), pH less than 4.6, warm temperatures (generally less than 39 F [ less than 4 C]), high moisture content (water activity), and lack of competing bacterial flora promote production of botulism toxin in foods contaminated with C. botulinum spores. The process used to produce the eggplant epidemiologically implicated in the outbreaks in Italy probably failed to remove C. botulinum spores and may have provided such conditions.

Covering foods in oil may provide the anaerobic conditions required for the production of botulism toxin. Outbreaks of botulism in the United States and Canada have been caused by covering vegetables with oil or grease. For example, in 1983, onions covered in grease and left overnight on a grill caused a large outbreak of botulism in Illinois (1), and commercially processed garlic in oil caused outbreaks in 1985 and 1989 (2,3 ). As a result of these outbreaks, the Food and Drug Administration recommended the addition of antimicrobial growth inhibitors or acidifying agents to such products (4 ) (such as citric or phospheric acid )

Bertolli recommends heating garlic and herbs in olive oil to release flavors as an alternative to infusing oils. Wash and crush herbs or garlic, saute in olive oil and strain. Crushing garlic around the rim of a salad bowl and adding olive oil can also achieve a similar taste to an infused oil. The flavored oil should be made fresh for each use.

It is now a relatively common practice to bottle vegetables and herbs and spices in either oil, vinegar or a mixture of both. This is done both commercially and domestically and the products should be refrigerated below 4°C.

Products which are treated in this way include chopped garlic, garlic cloves, sun-dried tomatoes, chilli, ginger, egg plant, capsicum, mushrooms and various mixtures of these and similar materials. While these products are safe if refrigerated, they represent a potential food poisoning hazard unless certain basic precautions are taken in their preservation.

This fact was unfortunately highlighted in Canada and the United States in the 1980s when two serious outbreaks of botulism occurred in which chopped garlic in oil was clearly identified as the source of botulism toxin. Botulism is a rare disease, particularly in this country, but because of its severe, debilitating symptoms and relatively high mortality rate remains a major hazard in home preserves.

Authorities in Canada and the United States reacted to the above incidents by preventing the sale of garlic-in-oil products in which refrigeration was the only barrier to the growth of the bacterium, Clostridium botulinum the organism which causes botulism. These products are now required to contain an additional barrier to C. botulinum such as acidification.

In 1991 Australian authorities took similar precautions by regulating that this class of product must not have a pH greater than 4.6. The pH of a product is a measure of its acidity. Foods with a pH below 4.6 do not in general support the growth of food poisoning bacteria including C. botulinum. The necessary pH adjustment for these products can be achieved only by adding acid to the vegetable material. Vinegar, which is a solution of acetic acid, is the usual choice. Citric acid and lemon juice are other possible acidifying agents.

Attempts to preserve these products without acidification seem to be based on two false assumptions. The first of these is that the addition of oil has a preservative effect. This is incorrect. The only function of the oil is to prevent oxidation from the air in the container which can lead to discolouration of some foods. By excluding air from the surface of the vegetable, one is establishing anaerobic conditions which actually favour the growth of some types of bacteria. Unfortunately, C. botulinum is one of these bacteria.

The other incorrect assumption which is often made is that some herbs and spices, and especially garlic, have significant anti- microbial properties. The preservative effect of these materials including garlic is slight and inconsistent as the botulism incidents in Canada and the United States prove.

It is therefore essential that sufficient acid is added to the vegetable before oil is poured on so that any C. botulinum or other potentially dangerous bacteria can not grow. Vinegar prepared for domestic use contains 4 percent acetic acid. Vinegar should be added to the vegetable component of these preserves before any oil is added so that the ratio of vegetable to vinegar by weight is not greater than three to one. For example, to make 400 grams of preserved garlic, one would mix 300 grams of garlic with 100 grams of vinegar. The resultant mixture will then contain approximately one percent acetic acid which would ensure a final pH below 4.6. This will not guarantee that the prod- ucts will not spoil if not kept properly refrigerated, but it will ensure they do not become toxic.

If vegetable products e.g. tomatoes, are dried prior to being stored in oil, a different set of circumstances applies. Correctly dried vegetables and herbs will not support the growth of food poisoning bacteria but they may still support the growth of spoilage organisms such as yeasts and moulds. Moulds will usually only be a problem on exposed surfaces but yeasts bring about fermentation in the absence of air.

Vegetables and herbs to be packed in oil without treatment with vinegar should be dried almost to crispness. Tomatoes, including sun-dried tomatoes, are a special case. The pH of fresh tomatoes is normally just below 4.6. When the tomatoes are dried, the natural acid components are concentrated and the pH is reduced. It will often be close to 4.0 in the dry product and therefore the risk of food poisoning is eliminated. No such safeguard exists with other vegetables, however, and these must be either acidified or properly dried before being covered with oil. This includes small quantities of garlic or herbs which may be added to other preserved vegetables as flavourings.

Discolouration of bottled garlic in vinegar

Users of bottled garlic in vinegar are occasionally alarmed to find that the product has turned green or blue-green. These colour changes do not make the product unsafe but are obviously undesirable.

The problem of garlic changing colour is associated with the addition of acid which changes the normal pH of the product. This is precisely what is required to ensure that the garlic remains safe but the change in acidity brings about chemical changes in pigments in the garlic.

All plant materials contain various pigments some of which change colour as the pH of the plant tissue is changed by the addition of vinegar or other acids. The most common of these pigments are the anthocyanins which may be blue, colourless or red depending on the pH.

These pigments may be involved in some colour changes observed in preserved garlic but American scientists have identified another more general explanation.

Certain amino acids, natural components of foods, are responsible for many of the pigment characteristics of the onion family which includes garlic. The American scientists have shown that the outstanding difference in composition between garlic which turns green and garlic which does not is the presence of much higher levels of one particular amino acid in the green garlic.

It is not possible to tell by looking at untreated garlic whether it is likely to become green on crushing and acidification. However the work reported indicates that if garlic bulbs are stored for four weeks at a temperature above 23°C prior to processing, the production of the green pigment is prevented.

This may not be practical at the domestic level, but could be a valuable precaution for commercial producers.

Information Services, Food Science Australia, PO Box 52, North Ryde NSW 1670

The practice of keeping cheese covered in olive oil at Room Temperature

By Linda J. Harris, Ph.D.
Food Safety/Microbiology Specialist
University of California, Davis

I have been asked to comment on the practice of keeping cheese covered in olive oil at ROOM TEMPERATURE. The following is my response.

Covering cheese with olive oil and storing at room temperature is a common practice in the Mediterranean. Recently, it has become a popular practice in some of the trendy delis and restaurants and possibly in some homes.

In discussions with Dr. John Bruhn, Dairy Specialist, the California Department of Food and Agriculture (CDFA) Milk and Dairy Foods Control Branch, is not recommending this practice (room temperature storage) when the pH of the cheese is greater than 4.6. The final pH of cheeses can vary and this practice cannot be recommended if you do not have a means of measuring the pH. Adding fresh herbs and garlic for "flavor" is as hazardous as it is when making flavored oils.

My own analysis of this practice is as follows:

Most cheeses will keep better if stored in the refrigerator. Vacuum packaging (and covering in oil) excludes air which prevents the growth of the yeasts and molds that typically spoil cheeses.

There are a number of pathogens that could be a problem in cheeses. Botulism is not common, but an outbreak was reported earlier this year involving marscarpone cheese (see below). This could definitely be a factor in cheeses that are pH of greater than 4.6. Listeria monocytogenes, if present, will grow much better at room temperature than at refrigerated temperatures, will grow in the absence (or presence of oxygen), and can grow at pH as low as 4.0.

If you want to serve cheeses in olive oil (even flavored oil), make small batches right before serving. Treat as for all potentially hazardous foods don't leave at room temperature for longer than 2 h, store leftovers promptly in the refrigerator, and use up within a reasonable period of time (1 to 2 weeks).