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"Phixing the pH
Most pathogens thrive within a fairly narrow pH range, but they can grow across a fairly wide range that coincides with that of many foods. For example, Staphylococcus aureus has optimal growth between pH 7.0 to 7.5, but will survive in a range of 4.2 to 9.3. A pH from 6.0 to 7.0 is optimum for Escherichia coli, but it will grow from 4.4 to 9.0.

Depending on the microorganism, a low pH can act as a bacteriostat (inhibit microbial growth) or a bacteriocide (kill microorganisms). In most cases the reduced pH acts in concert with other factors - such as increasing the effectiveness of heat treatments or optimizing preservatives - and provides a hurdle to limit growth rather than providing complete safety. That reduced pH doesn't provide complete safety has become painfully evident in the past several years, as acidic beverages such as unpasteurized apple and orange juices have been found to harbor E. coli 0157:H7 and Salmonella.

These acid environments might fall outside the microbial-growth norm, but low-acid foods are ideal for supporting microbial growth. The FDA defines low-acid foods as any that have a finished equilibrium pH greater than 4.6 and a water activity (Aw) greater than 0.85. Without complete sterilization to prevent microbial growth, particularly that of Clostridium botulinum in hermetically sealed packages, foods must have an acid pH. Some, such as tomatoes and oranges, come that way naturally. For those that don't, numerous food-grade acidulants can drop the pH.

The most commonly used acids in the food industry are citric, lactic, malic, acetic (vinegar), fumaric, tartaric, and phosphoric acids. With the exception of phosphoric, these are all organic acids and occur naturally in many foods. Other acidic ingredients, such ascorbic acid, citric acid-containing lemon or benzoic acid-containing cranberry juices, might be added. Additionally, acid-producing fermentations by friendly bacteria might also help achieve a target pH.

Fatty acids have also demonstrated the ability to prevent microbial growth. Lactic and acetic are considered short-chain fatty acids, and they inhibit gram-negative and gram-positive bacteria. Medium-chain saturated fatty acids (C8 to C14), especially lauric acid, restrict the growth of gram-positive microbes. Increased chain lengths, branching and other structural variations typically decrease their potency. Esterification often positively influences the antimicrobial effect; in many cases the monoglyceride is more effective than the fatty acid alone.

The antimicrobial activity of some acids does not come strictly from pH reduction. They also work through the antimicrobial effects of the undissociated ions in solution. Short-chain undissociated acids can pass through cell walls and disrupt cellular metabolism."

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In one study, conducted by researcher Susan Summer, department of food science and technology at Virginia Polytechnic Institute and State University, Blacksburg, fruits and vegetables were contaminated with Salmonella, Shigella and E. coli and then washed with a spray of hydrogen peroxide followed by acetic acid. While the H2O2 alone was fairly effective, the second spray of acetic acid improved the results tenfold.

Lactic and other acids can make effective washes for meat and poultry to keep the microbial count down. "Lactic acid is able to kill bacteria, giving a reduction between log one and log three," says DeVegt. "Manufacturers use about 2% lactic acid in a water solution as a final wash, before the carcass is chilled. It will kill up to about 99.9% of the bacteria. And if you leave it on the carcass surface, it will also have a bacteriostatic effect and inhibit the growth of bacteria." This same sort of treatment could also be used for cooked sausages, such as frankfurters, applied as a spray before peeling, he says.

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