Chapter 6 Ions: Charged Particles in Solution 103 Because most foods are complex biological systems, they contain substances that act as buffers. In plants, acids like citric and malic acid work with phosphate (PO3–) salts as buffering systems. Buffering salts are added to processed foods to stabilize pH. They include calcium phosphate, potassium phosphate, and sodium phosphate. Another group of compounds that can function as buffers is proteins. Food Preservation Food spoilage is caused by microorganisms growing in and feeding on food. Many foodborne illnesses are caused by bacteria growing in foods. Bacteria and other microorganisms grow best at a certain pH. For instance, most bacteria thrive in an environment that has a pH of 5.0 to 7.0. Preserving food and keeping it safe depend on an understanding of the pH environments of food products. The deadliest type of foodborne illness is botulism. It is caused by a toxin produced by the bacteria Clostridium botulinum. This bacterium will not produce toxin at a pH of 4.6 or lower. Any food that naturally has a pH of less than 4.6 will not support growth of this bacterium. Such high-acid foods can be safely canned in a boiling water canner. These high-acid foods include most fruits and some varieties of tomatoes. Clostridium botulinum is a spore-forming micro- organism that lives naturally in the soil. This bacterium is commonly found in many foods with a pH above 4.6. Such foods, which are often called low-acid foods, include corn, green beans, and other vegetables. These foods must be canned at a very high temperature. Otherwise, the Clostridium botulinum multiplies rapidly in the oxygen-free environment of the sealed food container. The normal temperature of boiling water (100°C) is not hot enough to kill Clostridium botulinum if the pH is above 4.6. Therefore, low-acid foods have to be pressure canned. A pressure canner increases the temperature of boiling water to kill more microorganisms. See 6-11. Many low-acid foods can be preserved by the pickling process. Through the pickling process, cucumbers become crisp with a sour bite. This process not only changes the texture and flavor of foods but the pH as well. Pickling involves soaking or heating foods in a vinegar (acetic acid) solution. The vinegar helps lower the pH to below 4.6 so harmful bacteria cannot grow. Beets, cabbage (sauerkraut), and water- melon rinds are some of the foods commonly pickled in addition to cucumbers. with ulcers may have bile salts in the stomach. Bile is normally found only in the duodenum, which is where the small intestine joins the stomach. Bile is fine in the duodenum, but it upsets the delicate chemical balance of the stomach. Bases can damage the mucous lining that protects the stomach from acid. When bile salts splash back into the stomach, the lining is weakened. This allows the hydrochloric acid to eat into the stomach lining and wall. Stomach acids do cause damage for people with bulimia. Bulimia is an eating disorder that often involves induced vomiting. Unfortunately for people with bulimia, there is no way to prevent stomach acid from coming up with the food. The strong acid eats away at tooth enamel and the delicate skin lining the mouth. This can cause mouth sores and permanent tooth damage. The Role of Buffers A buffer is a compound that helps stabilize pH by absorbing excess acids or bases in a solution. A buffer solution is made by combining a weak acid or base and one of its salts. An example is acetic acid (CH3COOH) and sodium acetate (NaCH3COO). The salt (sodium acetate) ionizes in solution to form Na+ and CH3COO– ions. If hydronium ions are added to the solution, the CH3COO– ions react with them to form acetic acid and water. Na+ + CH3COO– + H3O Na+ + CH3COOH + H2O (ionized sodium + hydronium sodium + acetic acid + water acetate) ion ion If hydroxide ions were added to the acid-salt solution, the acetic acid would react. It would release a hydrogen atom and form CH3COO– ions and water. CH3COOH + OH– CH3COO– + H2O acetic acid + hydroxide ion acetate ion + water Buffers play a key role in blood chemistry. The stability of the blood pH is so important that the body has three buffering systems to maintain the balance. In the body, lactic acid is a by-product of energy production. Ammonia is a result of protein digestion. These substances are balanced by phosphate ions (HPO4–2) and bicarbonate ions (HCO3–). The third buffering agent is proteins found in blood plasma. The structure of protein enables it to absorb either excess acids or bases. A fluctuation of as little as 0.2 in the blood pH can lead to death if not corrected. Buffers are also used to coat some aspirin tablets to make them easier to digest without damaging the digestive system. Copyright Goodheart-Willcox Co., Inc.