Low-Acid Acidified Foods
A considerable safety margin can be introduced into processing time by acidifying the product. If the low acid food, for example cucumber, is acidified to a pH of 4.6 or less, it crosses the threshold that separates low acid foods from the high acid foods. In other words it becomes a high acid food and as such, it requires less severe thermal treatment to achieve sterility. It can, theoretically, be sterilized in a water bath canner (212° F, 100° C). As the thermal resistance of bacterial spores decreases in an acidic environment, they should not grow in foods with a pH below 4.6. Thermal treatment is needed to kill only spoilage bacteria, molds, yeasts and enzymes, all of which can be killed at 212° F, 100° C.
Adding citric acid, lactic acid, lemon juice or vinegar will lower the pH of the product and cooking media. Chicken that was marinated overnight with salt, vinegar, white wine or lemon juice will acquire some acidity and will be more hostile to any bacterial spores than a fresh chicken. A pH meter is needed to measure acidity levels accurately, the pH color strips are suitable for checking water pH in a fish tank or for general less critical applications. Meats, poultry, fish, vegetables and dairy products fall into a pH range of 5.0-6.8. These are low-acid foods so they must be processed at 240-250° F, 116-121° C, unless they become acidified to such an extent that the pH equilibrium of the finished product is pH 4.6 or lower. Acidified foods do not automatically fall into the high-acid food category, they become low-acid acidified products. As mentioned earlier, acidifying foods and establishing new processing times must be left to properly trained persons and a homeowner should follow the rules established by USDA guidelines without regard to the extra acidity that he may have introduced.