Explain what pH is and how it relates to bacterial growth.
The tartness or sour taste of grapefruit, sauerkraut, yogurt, and pickles is the result of the intensity of acidity in these products. The acidity of foods has been used for centuries to preserve foods. Acidity plays a primary role in the preservation of fermented foods and combined with other factors such as heat, water activity, and chemical preservatives acts to prevent food deterioration and spoilage.
The acidity of a food may occur naturally as in citrus fruits, apples, tomatoes and strawberries or it may be produced in foods through microbial fermentation. Selected acid producing bacterial cultures added directly to foods can produce desirable products like yogurt, buttermilk and fermented meat products. Acid may also be added directly to a food; an example is the addition of acetic acid to fish and vegetables, lactic acid to Spanish-type olives and citric acid to beverages.
The intensity of acidity of a food is expressed by its pH value. The pH of a food is one of several important factors that determine the survival and growth of microorganisms during processing, storage and distribution. Consequently, food processors are interested in determining the pH of foods and in maintaining pH at certain levels to control microbial growth and prevent product deterioration and spoilage.
The pH scale was developed from mathematical calculations based on the dissociation temporary breakdown) of water. These complex calculations allow us to measure pH on a scale that runs from 0 to 14. The values that are less than 7 are acidic, while those greater than 7 are alkaline. A pH value of 7 is neither acid or alkaline and is considered neutral. Pure water has a pH of 7 and is neutral. The pH scale is based on the hydrogen ions concentration [H+] in the food. The more hydrogen ions present, the more acid the food and the lower the pH. The diagram below shows the pH scale and the taste of foods at acid and alkaline pH values.
Foods can be classified according to their pH as follows:
In food pH is used to control microorganisms by: (1) directly inhibiting microbial growth, and (2) reducing the heat resistance of the microbes.
Most fruits are naturally acid and may be given a mild heat process in which the temperature does not exceed 212°F and does not require pressure. Vegetables are predominately low-acid foods and require a severe heat process to destroy all spores of Clostridium botulinum. For foods that may be acidified like cucumbers, artichokes, cauliflower, peppers and fish, it is essential that the pH be allowed to equilibrate (stabilize) thoroughly before the heat treatment. This involves the addition of sufficient acid, proper mixing, and enough time for the pH to fall to 4.6 or below, at the center of solid foods.
Every microorganism has a minimum, an optimum and a maximum pH for growth. Most microorganisms grow best at pH values around 7.0 while only a few grow below pH 4.0. Yeasts and molds are generally more acid tolerant than bacteria and can grow at lower pH values. Foods with pH values below 4.5 are usually not easily spoiled by bacteria but are more susceptible to spoilage by yeasts and molds. Microorganisms can grow in wide pH ranges and these ranges are probably the difference between different bacterial strains, types of food or growth medium and the type of acid or base used to adjust pH.
For a listing of the approximate pH of foods and food products, go to: http://www.fda.gov/Food/FoodSafety/FoodborneIllness/FoodborneIllnessFoodbornePathogensNaturalToxins/BadBugBook/ucm122561.htm