Fundamental Food Microbiology, Third Edition - Fuad Fathir

484 FUNDAMENTAL FOOD MICROBIOLOGY
develop large varieties of fermented foods, especially in tropical areas where, unless
preserved, many foods spoil rapidly. 1
When the microbial involvement in food spoilage and foodborne diseases was
recognized, methods to control their growth as well as to kill them in food were
studied. It was observed that over a restricted pH range, many microorganisms
present in food can grow, but at lower pH ranges many of them die. Once this was
recognized, many organic acids were used as food additives. In addition to their
effectiveness as food preservatives, they are also used to improve acceptance qualities
of foods. The amounts and types of organic acids that can be added to foods are
governed by regulatory agencies.
Organic acids can be present in foods in three ways. They can be present
naturally, such as citric acid in citrus fruits, benzoic acid in cranberries, and sorbic
acid in rowan berries. Some, such as acetic, lactic, and propionic acids, are produced
in different fermented foods by desirable food-grade starter-culture bacteria. Many
acids are also added to foods and beverages to reduce the pH. Among the organic
acids used in food as preservatives are acetic, propionic, lactic, citric, sorbic, and
benzoic, their salts, and some derivatives of benzoic acid (e.g., paraben). The influence
of these acids in reducing food pH and producing antimicrobial effects are
briefly discussed in this chapter.
II. OBJECTIVES
The major antimicrobial objective of using weak organic acids is to reduce the pH
of food to control microbial growth. 1 As the pH drops below 5.0, some bacteria
become injured and die. However, the death rate in low pH is not predictable as in
the case of heat. Thus, it could not be used with the objective of destroying a
predictable percentage of a microbial population in the normal pH range of foods. 1–6
III. MECHANISMS OF ANTIMICROBIAL ACTION
The antimicrobial action of a weak organic acid is produced by the combined actions
of the undissociated molecules and the dissociated ions. 1–6 Microorganisms that are
important in food tend to maintain an internal cytoplasmic pH ca. 6.5 to 7.0 in
acidophiles and 7.5 to 8.0 in neutrophiles. The internal pH (pHi) is tightly regulated
and drops by ca. 0.1 unit for each 1.0 unit change in the environmental pH (pHo).
For nutrient transport and energy synthesis, the microorganisms also maintain a
transmembrane pH gradient (ca. 0.5 to 1.0 unit with alkaline pHi) and a proton
gradient (ca. 200 mV); together, they form the proton motive force (PMF).
When a weak organic acid is added to the environment (in a food), depending
on the pH of the food, the pK of the acid, and the temperature, some of the molecules
dissociate whereas others remain undissociated (Table 35.1). At the pH of most
foods (pH 5 to 8), except for paraben, the organic acid molecules remain generally
dissociated; as a result, [H + ] in the environment increases, which interferes with the
transmembrane proton gradient of microbial cells. To overcome this, the cells trans-