Acidified Vegetables

For nonfermented, acidified vegetable products, acetic acid is commonly used as an acidulant. At a concentration of 3.6% or greater, acetic acid-acidified foods can be preserved without the addition of other antimicrobial agents or use of heat treatments [62,63]. For pickled pepper products, acidification with 2% acetic acid to pH values around 3.2 was found to prevent microbial growth for 6 months or more [64]. In general, preservation by organic acids alone results in products that can only be consumed in small amounts, as condiments, or as ingredients in other foods. Many acidified vegetable products contain between 0.5 and 2% acetic acid and are pasteurized to prevent spoilage, as well as to ensure safety. For nonfermented pickled vegetables, the combination of heat treatments, acid, and sugar concentration (for sweet pickles) serves to prevent microbial growth. Fresh-pack cucumber pickle products typically contain between 0.5 and 1% acetic acid. A recommended pasteurization procedure consists of heating to an internal temperature to 74° C for 15 minutes [65].

Both acidified and fermented vegetable products have enjoyed an excellent safety record with few or no reported cases of foodborne disease resulting from consumption of these products. Recently, however, there have been reports of disease outbreaks in juice products with pH values below 4.0, in the same range as many fermented and acidified vegetable products. Escherichia coli O157:H7 and salmonella serotypes have caused serious illness and death from the consumption of apple cider and orange juice [66,67]. These disease outbreaks have raised questions about the safety of acidified and fermented vegetable products. While pathogenic microorganisms have not been found to grow in these products due to the low pH (typically below 4.0), these microorganisms may adapt to acid conditions and survive for extended periods [68]. Acid types and concentrations vary considerably for acidified foods. Factors affecting acid inhibition of microbial pathogens include the pH of the product, as well as specific effects of the acid or acid anion on cellular enzymes or membranes, and the ability of bacteria to transport protons and organic acids out of the cell interior [69-72].

14.4.1 Definitions and Regulations for Acid and Acidified Foods

Acid foods are defined in the U.S. Code of Federal Regulations (21 CFR part 114) as foods that have a natural pH value at or below 4.6. These foods include fermented vegetables; vegetable fermentation is considered a ''field process'' and typically results in a product with a final pH below 4.6. A pH value of 4.6 is used in the definition of acid foods because this is a limiting pH at or below which C. botulinum spore outgrowth and neurotoxin toxin production is prevented [73]. Foods with pH values above 4.6 are defined as low-acid foods, and, when packaged in hermetically sealed containers, must be made commercially sterile as defined in 21 CFR part 113. Acidified foods are defined in 21 CFR part 114 as foods to which acid or acid food ingredients have been added that have a water activity (aw) greater than 0.85 and have a finished equilibrium pH value at or below 4.6. The regulation requires producers of acidified foods to verify that the final equilibrium pH is maintained at or below 4.6 to ensure safety. This regulation governing acidified foods in the U.S. was promulgated by the U.S. Food and Drug Administration (FDA) in 1979. At that time, vegetative pathogenic microorganisms were not considered to be a significant risk for acidified or fermented food products. Included in the regulation, however, is the requirement for a heat process ''to the extent that is sufficient'' to destroy vegetative cells of microorganisms of public health significance or those of nonhealth significance capable of reproducing in the product. The regulations governing acidified foods are, therefore, based primarily on the pH needed to prevent botulism, and do not include any specification about the type or concentration of acid needed to meet the pH requirement.

In a study of beef carcass wash water, a treatment with 0.2% (33.3 mM) acetic acid and a pH of approximately 3.7 showed that an E. coli O157:H7

strain survived for up to 14 days at 15° C, while cell numbers dropped about 4 log cycles [74]. In that study, competitive microflora were also present and could have influenced the survival of the E. coli strains. A statistical analysis of several published studies showed that, under typical storage conditions for apple cider (which typically has a pH value less than 4.0 and contains malic acid), the acid conditions alone were not sufficient to ensure a 5 log reduction in the cell numbers of E. coli [75]. From these and other studies [68,75-79], it is clear that the potential for E. coli to survive for extended periods in acidified vegetable products with a pH below 4 clearly exists, and pasteurization for some acidified food products may be needed to ensure safety.

14.4.2 Pathogenic Bacteria

After recent outbreaks of E. coli O157:H7 in apple cider and salmonella in orange juice [66,67], the FDA in 2001 proposed that all new process filings (which are required for the production of acidified foods) should include a heating or pasteurization step. Of primary concern was E. coli O157:H7 because of its low infectious dose and lethal sequelae which can result from infection [80,81]. Escherichia coli and other food pathogens have been shown to have inducible acid resistance mechanisms [76,82-85]. If only pH is considered, acid-resistant pathogens might, therefore, pose a potential threat to acidified foods. It is likely that the organic acids present in these products have contributed to their excellent safety record because some acidified products have been produced safely for many years without heat treatments [84], although quantitative measurements of the independent effects of organic acids and pH on the killing of pathogens in these products are lacking. In response to the pathogen outbreaks in juice products, 21 CFR part 120 was promulgated in 2001. This regulation mandated a HACCP (hazard analysis critical control point) system with a processing step designed to deliver the equivalent of a 5 log reduction in target pathogen populations in juices. Typically, a heat pasteurization process is used, based on thermal destruction time data for inactivation of E. coli O157, which was found to be the most heat- and acid-resistant pathogen in fruit juices [86]. In recent experiments (Breidt, unpublished data), the thermal resistance of E. coli O157:H7 and L. monocytogenes was found to be identical under the conditions typical of acidified pickle products, and salmonella strains were significantly less heat-resistant. Similarly, salmonella was found to be less heat resistant than L. monocytogenes or E. coli O157:H7 in fruit juices [86]. For the variety of acidified vegetable products currently available, the time and temperature needed to ensure a 5 log or greater reduction (although a 5 log reduction is not currently mandated by existing federal regulations) in numbers of microbial pathogens will depend on the type and concentration of organic acid present, the composition of the brine or suspending medium during heating, heat resistance of the microorganisms, and other factors.

Some pickled pepper products with high concentrations of acetic acid (greater than 2% acetic acid) and pH values around 3.1 to 3.3 may not need a heat treatment to ensure the destruction of acid-resistant pathogens because sufficient acid is present. In a study of firmness retention with unpasteurized pickled peppers, which typically have pH values around 3.1 to 3.3, and cucumbers, using 2 to 5% acetic acid, microbiological stability was achieved for a 6-month period [64] for all products tested. A heat process is typically not used for these pickled peppers because sliced peppers are susceptible to softening during pasteurization. Historically, pasteurization treatments were designed to prevent spoilage by LAB in brined vegetables and inactivate softening enzymes. Currently, most commercial acidified vegetable products with pH values between 3.3 and 4.1 are produced using a pasteurization process to prevent spoilage. In addition, low water activity and preservatives can reduce the amount of acetic acid needed for preservation. A preservation prediction chart showing the effects of acid and sugar in preventing the growth of spoilage yeasts in sweet pickles was developed in the 1950s [62]. The acid concentrations that will ensure the death of microbial pathogens for many acidified foods remain to be determined.

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