New Issues with Acidified Foods

Pickling—the preservation of foods by acidification, either added or created by fermentation—is an old process, but relatively recent research has raised some new issues and challenges. Probably the most common pickles are made from cucumbers, typically acidified or fermented in bulk to create the familiar cured whole pickles that are then further processed into slices, relishes, and various packaged forms. Different flavors added to the packaging brines give sweet or garlic versions.

Acid in these products is mostly lactic, generated by bacterial fermentation of sugars in the cucumbers. Salt is typically added to the storage brine to help leach out the sweet juice and to inhibit the growth of competing microbes.

One objective of the acidification is to prevent growth of Clostridium botulinum, which does not produce toxin at pH below 4.6. The fermentation also changes the color, texture, and flavor of the fresh cucumber (or other substrate) in characteristic ways. Fruits and vegetables besides cucumbers can be pickled in much the same way.

A firm texture—“crunchiness”—is desirable in many pickled products and can be achieved with fresh-pack pickles, which are packaged in salt brine with added flavor and, sometimes, added vinegar. Some of these products are preserved by refrigeration while others are pasteurized in the jars and may be shelf stable or refrigerated.

Pasteurization—relatively mild heating for relatively short times, as compared with low-acid canning—helps preserve acidified foods by killing spoilage microorganisms. At the same time, the heating can soften the texture, leading to a less-crisp pickle.

For a long time, the primary food safety concern in pickles was botulism, which is prevented by pH control. It takes time for added or generated acid to thoroughly penetrate a relatively large, whole cucumber, but in the traditional process, where cucumbers are stored for months in very large vats, the time is available. Often the vats are outdoors and natural; winter cold helps preserve the cucumbers as they ferment. The low temperature also slows the rate of reaction, but, again, there is plenty of time.

Fresh-pack pickles need to be held to allow the packing brine to penetrate and equilibrate with the substrate—the fruit or vegetable. Slices, with their smaller dimensions and exposed flesh equilibrate relatively quickly. Whole cucumbers in unacidified salt brine must be refrigerated and have relatively short shelf lives. These are most often used in delis rather than consumer packages.

Acid-tolerant Pathogens
Fred Breidt is a microbiologist with the USDA Agricultural Research Service (ARS) in Raleigh, N.C. (phone 919-513-0186), and also on the faculty at North Carolina State University. His ARS group has been studying pickling for many years and has published more than 300 papers on the topic. (Search with Breidt’s name to find his publications and links to the others from the group.)

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The somewhat alarming news from the ARS research is that some strains of Escherichia coli O157:H7, the virulent variety of an otherwise common and harmless microbe, are quite tolerant of acid in foods. The group has found that pH below 3.3 is required to achieve a 5 log reduction in the most resistant strains in cucumbers at room temperature. This reduction occurs over a period of about 48 hours. E. coli is apparently the most acid-tolerant of the other common foodborne pathogens—Salmonella and Listeria, for instance. However, it is also one of the more dangerous, believed to cause potentially fatal disease at very low doses.

This avenue of research arose in part because of the outbreak of disease caused by E. coli in unpasteurized apple juice and by Salmonella in orange juice, thought until then to be safe because, as with most fruits, their pH is below 4.6. It is likely that the apples and oranges had been contaminated by animal droppings after they fell from trees. Better practice for juice from tree fruits is only to use clean fruit picked from the tree, not fruit fallen to the ground. There was also an incident of disease caused by E. coli surviving in fresh squeezed carrot juice, which does not have a low pH.

These incidents led to the Food and Drug Administration (FDA) issuing guidelines for juices, which essentially require that all juices be pasteurized or otherwise undergo a 5 log reduction in pathogens likely to occur for that product. Heat pasteurization is most common, but that treatment causes changes in flavor and color for most juices. Alternative processes, such as high-pressure processing (HPP) and pulsed electric fields (PEF), have been applied with some success. Citrus fruits can be safely processed into juice if sound,ripe fruit is picked and then washed before the juice is extracted.

The connection between fruit juices and pickles may not appear obvious, but once the assumption of safety due to pH below 4.6 was challenged, other high-acid foods became suspect, and, sure enough, acid-resistant pathogens were found.

One way to assure safety is heat pasteurization, but this is not desirable in some products and may not even be possible in other cases.

Acidified Bulk Ingredients
Fruits and vegetables are pureed and made into juice for use as natural flavors and ingredients in other foods. Often these products are packaged in fiber drums or aseptic bags. Frequently, they have salt and acid added as preservatives and as part of their formula. Because of the packaging, they cannot be heat processed after filling. They can be aseptically processed and filled into sterile bags, using relatively expensive equipment. What of the more common “cold fill” into fiber drums?

All acidified foods are regulated by FDA in the low-acid and acidified-foods regulations, which require that preservation processes be filed with the FDA. Filing does not imply approval by the FDA. Rather, acceptance of the filing simply means that the process is on the record and that the manufacturer can be held responsible for carrying it out accurately. Filings are done electronically, using a form that is primarily designed for thermal processes.

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In hot-fill or post-packaging thermal processing, both the container and the product are heat treated, so any possible bacterial contamination on the container or lid is reduced. Some cold-fill bulk products are pasteurized to extend shelf life, but others are not. Fortunately, the work of Breidt and co-workers shows that the product itself kills pathogens, given enough time and a low pH. It also appears that acetic acid, found in vinegar, is more potent than other acids. Thus, in the case of these pickled vegetable ingredients, the preservation process is acidification to a pH below 4 (usually even less) and then holding for sufficient time—usually 48 hours minimum—to achieve equilibration and adequate reduction in pathogens. Most manufacturers hold for five days or more to allow time for their quality control tests anyway.

The FDA is said to be revising and improving its electronic filing procedures, but, meanwhile, processors need to explain the science supporting their processes.

Other Acidified Products
Most of the preserved fruits and vegetables in Moldova and Albania are glass-packed pickles, including small hot peppers, whole cucumbers, shredded cabbage and carrot salad, and whole fruits with skin and seeds. These are normally heated in boiling water after filling by hand.

The fruit products are called compotes and are rather more loosely packed than we normally see in the United States. The packing brine or syrup is also thinner than the heavy syrups found in California. It was surprising to learn that the syrup is viewed as a refreshing drink.

There are no manufacturers of the glass jars in these countries; all the jars are imported. Since the bulk density of the various products varies, the apparent volume required to fill a given weight, say 500 grams, also varies, leading to the inclusion of more brine or syrup than usual in many cases.

The plants tend to be old, built by the Soviets in Moldova and by a repressive Communist (but not Russian) regime in Albania. Typically the ceilings are low, bay spacing is close with large, poured-concrete columns, and the floors are bare concrete. Most preparation is by hand labor, including women chopping cabbage with knives and others filling jars with cucumbers by hand. Some preparations are carefully arranged in the jar to be colorful, while others are more random. It was a surprise to see stems and pits included with cherries, skins and pits included with plums, and skins and seeds with slices of apple. The products have a nostalgic appeal to the dispersed citizens working across Europe, but are not exported in the former quantities to Russia. Albania hopes to join the European Union and improve its trade balance as a result. Moldova expects to remain neutral and unallied.

Acidified and fermented foods are safe and nutritious and can be novel and delicious. New understanding of their microbiology actually is reassuring of their safety, when the knowledge is properly applied.

J. Peter Clark, Contributing Editor,
Consultant to the Process Industries,
Oak Park, Ill.
[email protected]