The word acid originates from the Latin word acere and essentially means to taste sour, associated with natural acidic solutions such as vinegar and lemon juice. A base, or alkali, is derived from Arabic al-qali that means roasting or ash because alkalis where originally sourced from burned plant material (eg, wood) ashes. In 1887, the Swedish scientist Svante Arrhenius claimed that in an aqueous (aq) solution, an acid produces hydrogen ions (H+) (eg, HCl[aq] → H+[aq] + Cl–[aq]) and a base produces hydroxide ions (OH–) (eg, NaOH[aq] → Na+[aq] + OH–[aq]). In 1923, the proton theory of acids and bases (ie, an acid is a proton, H+ contributor, and a base is a proton, H+ acceptor, eg, HF + H2O ↔ H3O+ + F–) was introduced by the Johannes Brønsted and Thomas Lowry. Gilbert Lewis, an American physical chemist, proposed the electron pair theory of acids and bases. He suggested that an acid accepts a pair of electrons, whereas a base donates a pair of electrons (eg, NH3 + H+ → NH4+).
Figure 12.1 shows the effect of acid and base in water. Generally, acid reacts with metals, metal oxides, metal hydroxides, and metal carbonates, and a salt is produced. Acids also react with alkalis to form salts and water, otherwise called neutralization reactions. The pH scale measures the extent to which a substance is acidic or basic. The term pH is a measure of the hydrogen ion [H+] concentration of a solution and is defined as the negative logarithm of ion concentration. Solutions with a high concentration of hydrogen ions have a low pH and are called acidic solutions; solutions with low concentrations of H+ ions are considered as high pH and are called alkaline (also known as caustic or basic) solutions. The pH value varies from 0 (most acidic) to 14 (most basic), and 7 is considered as its neutral point (Figure 12.2). Although the pH range seems like a linear scale, it is important to remember that it is in the logarithm function. A 10-fold change in concentration of hydrogen ion happens when one pH unit is changed. Therefore, a pH of 11.0 is 10 times as basic as a pH of 10.0. This definition of pH was introduced in 1909 by the Danish biochemist Søren Peter Lauritz Sørensen. It is expressed mathematically as pH = -log [H+], where [H+] is hydrogen ion concentration in mol/L. In special cases, it is found that commercially available concentrated hydrochloric acid (HCl) solution (37% by mass) has pH ≈ -1.1, whereas saturated sodium hydroxide solution has pH ≈ 15.0, which indicates that more than 14 pH values and negative values exist.1,2
The value of pH is important for living system, for example, gastric (stomach) acid (pH 1.5-3.5), lysosomes (pH 4.5), urine (pH 6.0), cytosol (pH 7.2), blood (pH 7.34-7.45). The severity of periodontal (gum) disease is determined by the pH of saliva. All living cells are naturally maintaining pH balance homeostasis. Therefore, abnormality in pH balance (acidity and alkalinity) can be harmful to any organism. In a broad sense, pH can be used as a regulator for numerous applications, for example, agronomy, agriculture, medicine, biology, civil engineering, chemistry, forestry, food science, nutrition, environmental science, oceanography, chemical engineering, water treatment, and water purification. In many of those applications, pH can be used as a preservative as well as a disinfectant to control pathogens or other microorganisms.3 Several factors can affect the general efficacy of extremes of pH as disinfectants, such as concentration, contact time, organic load, temperature, relative humidity, and microbial characteristics.4,5 In this chapter, the modes of action of acids and alkalis (ie, pH in disinfection processes) are discussed.
CRITICAL LIMITS FOR MICROBIAL GROWTH
Most bacteria are neutrophiles in nature. They develop optimally at a pH within one or two pH units of the neutral pH of 7 (Figure 12.3). Most common bacteria, like Escherichia coli, staphylococci, and Salmonella are neutrophiles and do not proliferate under extremes of pH (eg, in the stomach’s acidic pH). On the other hand, there are many pathogenic strains such as E coli, Salmonella ser Typhi, and other types of intestinal pathogens that are much more resistant to stomach acid. In comparison, fungi thrive at slightly acidic pH values of 5.0 to 6.0.
FIGURE 12.1 Acid and base in water. Abbreviations: H+, hydrogen ion; H2O, water; OH–, hydroxide ion.
Helicobacter pylori is a primary cause of peptic ulcers and gastric cancer. Although its natural habitat is the acidic gastric mucosa, Hpylori is a neutrophile (ie, thriving in a relatively neutral pH range). The bacterium can survive brief exposure to pHs of <4, but growth occurs only at the pH range of 5.5 to 8.0, with optimal growth at neutral pH.6 Also, the composition of the gut bacteria community in the stomach and colon is distinctive, which is because of various physicochemical conditions such as pH value, nutrients, intestinal motility, and host secretions (eg, gastric acid, bile, digestive enzymes, and mucus). The organic acids affect bacterial growth in the colon by affecting colonic water absorption and decreasing fecal pH.7,8S aureus can colonize the human host over a range of pH. Examples include blood at pH 7.4, mouth at pH 5 to 7, nose at pH 6.5 to 7, lungs at 6.8 to 7.6, the vagina at pH 4.2 to 6.6, in abscesses at pH 6.2 to 7.3, urinary tract at pH 4.6 to 7, and the skin at pH 4.2 to 5.9.9
FIGURE 12.3 The curves show the approximate pH ranges for the growth of the different classes of pH-specific prokaryotes. Each curve has an optimal pH and extreme pH values at which growth is much reduced. Most bacteria are neutrophiles and grow best at near-neutral pH (center curve). Acidophiles have optimal growth at pH values near 3, and alkaliphiles have optimal growth at pH values above 9.
Microorganisms that grow optimally at pH <5.5 are called acidophiles. For example, the sulfur-oxidizing Sulfolobus species, isolated from hot springs in Yellowstone National Park and in sulfur mud fields, are extreme acidophiles. These archaea can survive at pH range of 2.5 to 3.5. Species of the Archaean genus Ferroplasma have been identified to grow in acid mine drainage at pH range of 0 to 2.9. An important bacterium of the normal microbiota of the vagina, Lactobacillus, can tolerate acidic environments at pH ranges 3.5 to 6.8 and participates in controlling the pH (acidity) of the vagina (pH of 4, except at the onset of menstruation) through their metabolic production of lactic acid. The acidity of the vagina plays a vital role in inhibiting other microbes that are not tolerable to acidity. Acidophilic microorganisms have been shown to have some critical adaptations to allow them to survive in strongly acidic environments. For example, proteins show increased negative surface charge that stabilizes them at low pH. Surface proton pumps in the cells actively expel H+ ions out of the cells. The changes in the composition of phospholipids in membrane probably reflect the need to maintain membrane fluidity at low pH level.10
At the opposite end of the range are the alkaliphiles, or microorganisms that grow best at pH range 8.0 to 10.5. Vibrio cholerae, the pathogenic agent of disease cholera, survives best at the slightly basic pH of 8.0. It can survive at pH of 11.0 but can be inactivated by the stomach acid. Extreme alkaliphiles have adapted to their harsh environment through modification of lipid and protein structure and compensatory mechanisms to maintain the proton motive force in an alkaline environment.11 For example, the alkaliphile Bacillus firmus derives energy for transport reactions and motility from a Na+ ion gradient rather than a proton motive force.12 Many enzymes from alkaliphiles showed a higher isoelectric point due to an increase in the number of basic amino acids as compared to homologous enzymes from neutrophiles. Table 12.1 shows the optimum pH tolerance of some foodborne bacteria.
Acids such as acetic acid, benzoic acid, and the parabens and bases such as sodium hydroxide, potassium hydroxide, and sodium bicarbonate are used directly as preservatives and disinfectants. Note that stronger acids and bases have limited use due to safety concerns (eg, user safety and material compatibility). Acids are often considered unsafe and some (eg, concentrated sulfuric acid, nitric acid, HCl) are extremely destructive. But, many acids are commonly used in every day life, such as acetic acid (ethanoic acid) in vinegar, citric acid in fruits (ie, oranges and lemons), malic acid in apple, and folic acid in strawberries. Similarly, a large portion of concentrated alkali substances are considered poisonous, and some might be destructive (to hard surfaces, as well as human tissues such as the skin and eyes). Used at higher concentrations, temperatures, and contact times, they can even degrade full carcass. Mild alkalis, such as aluminium hydroxide or magnesium hydroxide, are used as oral drugs (as antacids in liquid or tablet form) or in toothpastes. Foods (for the purpose of preservation) can also be classified as being naturally high acidic, such as pickles, berries, and sauerkraut (pH 3.7 and lower), whereas low acidic foods include tomatoes and pears (pH 3.7-4.5), pumpkin and spinach (pH 4.6-5.3), milk, corn, and seafood (pH 5.4 and higher). The human stomach contains gastric juice that is acidic and aids in food digestion, whereas blood is slightly alkaline (ie, 7.35-7.45). Acidic or alkaline eating behavior is considered important for a healthy life.17 A list of common acids and alkalis are listed in Table 12.2. Many of these are more commonly known as food/beverage preservatives by their corresponding International Numbering System for Food Additives (INS), such as in the European Union by their “E” number, such as E-200 (sorbic acid) and E-270 (lactic acid). Note that it is typical for the INS number to be preceded by an E in the European Union, an A in Australia, and a U in the United States.
METHODS OF CONTROLLING pH
Acids and alkalis are considered either “strong” or “weak.” For acids, this relates to the number of free hydrogen ions in solution for a given concentration. Thus, nitric acid is a strong acid because all of the acid molecules are dissociated into active H+ and nitrate ions. Acetic acid is a weak acid; a solution of this acid of the same molar concentration as nitric acid would have a very different pH value because most of the acetic acid molecules do not separate into H+ and acetate ions. Both nitric acid and acetic acid have the same total available acidity and, therefore, each requires the same amount of neutralizing base. Similarly, there are strong and weak bases. Sodium hydroxide is a strong base, whereas ammonia and sodium carbonate are weak bases. Equal molar concentrations of these bases possess the same capacity to neutralize a given quantity of acid. Thus, acids and bases are commonly used to control the pH of the cleaning and disinfectant solutions. Generally, the increment or decrement of pH can be achieved by two methods. The first one is a direct method, when the product is acidified or made more basic by adding one or more acids or bases respectfully. The second is an indirect method based on the use of microorganisms, such as that used in fermentation processes.18,19
TABLE 12.2 Examples of common types of acids and alkali
Name
Acid/Alkali
Remarks
Acetic acid
Acid
Found in vinegar, widely used in the pickling industry as a natural preservative.
A 1% solution is considered an effective antiseptic.
Citric acid
Acid
Citric acid was originally extracted from lemons and limes, but it is now produced commercially by a fermentation process. Widely used for flavoring and as a preservative in food and beverages; used in effervescent salts, as a chelating agent, for surface passivation, and in cleaning solutions
Fumaric acid
Acid
It is manufactured synthetically from malic acid; widely used as a food/beverage additive for taste (sourness or vinegar) and as a preservative (eg, in tortillas)
Lactic acid
Acid
Lactic acid is widely used in the production of boiled sweets, pickled foods, and as a raw material in the manufacture of important emulsifiers for the baking industry intravenous fluids; widely used as a food preservative, flavoring agent, in cleaning formulations (for descaling), and as a surface disinfectant in the meat industry
Malic acid
Acid
Malic acid is found naturally in apples, pears, tomatoes, bananas and cherries; used for food flavoring and as a preservative
Phosphoric acid
Acid
Phosphoric acid is manufactured commercially from mined phosphate rock; used as an inorganic soil cleaning agent (eg, rust removal or descaling), cleaning/disinfectant excipient, food flavoring/preservative, and as a mild disinfectant
Tartaric acid
Acid
Tartaric acid is used in baking, in pharmaceuticals as effervescent salts, as a chelating agent, and in cleaning solutions
Sorbic acid
Acid
Sorbic acid and sodium and potassium sorbate are used to inhibit the growth of molds and yeasts in foods/beverages.
Formic acid
Acid
Used as a preservative and antibacterial agent in livestock feed and other food applications as well as in cleaning products (eg, for descaling or stain removal); can be an irritant at concentrations >2%
Benzoic acid
Acid
Benzoic acid, such as in the form of sodium benzoate, is widely used as a food preservative. Inhibits the growth of fungi and some bacteria. Typically used in the 0.05%-0.1% range in foods; also used as a topical antiseptic for the treatment of fungal skin infections such as athlete’s foot
Propionic acid
Acid
It is found in Swiss cheese at concentrations of up to 1%. It is effective against molds and bacteria at concentrations of 0.1%-1%; widely used as a food preservative and for flavoring
Calcium hydroxide
Alkali
Commonly called lime, used to neutralize excess acid in soil. Widely used as a flocculant in water/sewage treatment and in clarifying foods/beverages; sometimes used as a viricide and surface cleaning agent (in formulation)
Sodium hydroxide
Alkali
Used in making soaps and detergents, as well as a viricide/bactericide and surface cleaning agent (in formulation), particularly for protein removal/degradation. Widely cited as being an effective cleaning and inactivation agent for prion decontamination; used for tissue and whole carcass digestion
Ammonia
Alkali
Used as a general purpose cleaner, as a preservative (eg, in foods), and as a disinfectant
Sodium bicarbonate
Alkali
Commonly known as baking soda, which is widely used in cooking. It is also considered a weak disinfectant (or preservative), being particularly used for odor control or activity against fungi; used as an antiseptic in toothpastes and mouthwashes as well as a mild cleaning agent
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CRITICAL LIMITS FOR MICROBIAL GROWTH
TYPES OF ACIDS AND BASES
METHODS OF CONTROLLING pH
