1
Acids, Bases, and Buffers

1.1 Learning Outcomes

After completing this exercise, students will be able to:

1. Explain the roles of acids and bases in food products.
2. Measure the pH of selected food products.
3. Prepare and evaluate a buffer system.
4. Measure the buffering capacity of a common beverage.

1.2 Introduction

Many food components may be classified as acids or bases due to their capacity to donate or accept protons (hydrogen ions). These components perform numerous important functions including flavor enhancement, control of microbial growth, inhibition of browning, alteration of texture, prevention of lipid oxidation, and pH control.

Acids and bases are key metabolites in living plant and animal organisms, for example as intermediates in the TCA cycle, and are mostly retained when the plant is harvested or the animal is slaughtered so they are naturally present in foods. They may also be added during processing or synthesized during fermentation to produce desired characteristics in the final food product.

The concentration and relative proportion of acids and bases determine the pH of a food, an extremely important characteristic. pH can affect the flavor, color, texture, stability, and behavior in food processing situations. For example, commercial sterilization of acid foods (pH less than 4.6) [1] can be achieved under milder processing conditions than in foods with a higher pH.

1.2.1 Acids

Acids serve a variety of functions in foods including flavor enhancement, control of microbial growth, protein coagulation, emulsification, control of browning, buffering action, and metal chelation (to control lipid oxidation). All acids have a sour taste but different acids produce distinctively different sour flavors. Thus, it is not enough to simply add any acid when attempting to produce a characteristic sour flavor in a food. Table 1.1 gives structures and pK values of some common food acids.

Table 1.1 Acids common in foods: structures and pKa values.

1.2.1.1 Food Acidulants

In the food industry, food additives that have acidic properties are commonly known as food acidulants. There are many approved food acidulants, but only a few are in wide use. They include organic acids like acetic acid, citric acid, fumaric acid, lactic acid, malic acid, and tartaric acid as well as the mineral acids phosphoric acid and sodium hydrogen sulfate. (See [2] for guidance in selecting food acidulants.)

1.2.1.2 Reactions of Food Acids

Most naturally occurring food acids are carboxylic acids. Carboxylic acids are weak acids compared with mineral acids such as HCl and H2SO4. Important reactions of carboxylic acids include the following:

Ionization:

Reaction with alcohols to form esters:

1.2.2 Bases

Bases are also common food additives and are added for a variety of purposes. They may be added to modify the flavor, color, and texture, enhance browning, induce chemical peeling, and produce CO2. Examples of bases used as food additives include dilute NaOH (to induce chemical peeling in fruits and vegetables, enhance browning, de-bitter olives, solubilize proteins), phosphate salts (to prevent protein coagulation in evaporated and condensed milks, produce a smooth texture in processed cheese), and NaHCO3 (to give chocolate a darker color, produce CO2 in leavening systems).

1.2.3 Buffers

Buffers stabilize the pH in foods. They are also used to neutralize foods which are too acidic. By using the salt of the acid already present, acidity is reduced without adding neutralization flavors. Many buffers are present naturally in foods. Animal products are usually buffered by amino acids, proteins, and phosphate salts. In plants, organic acids (such as citric, malic, oxalic, and tartaric) in conjunction with phosphate salts are the primary buffers. Table 1.2 shows the pHs of some common foods. Notice that most foods are buffered in the acidic range (pH?<?7).

Table 1.2 Approximate pH values for some common foodsa.

a Modified from [3] and [4].

See Appendix III or your chemistry and biochemistry textbooks for a review of acid and base chemistry.

1.3 Apparatus and Instruments

1. pH meter equipped with a pH electrode
2. Analytical balance
3. Household blender
4. Centrifuge
5. Centrifuge tubes
6. Pipette and pipette bulb, 10?ml
7. Volumetric flask, 200?ml
8. Beakers, 150?ml
9. Burette, 25 or 50?ml
10. Burette holder and stand
11. Thermometer
12. Funnel
13. Graduated cylinder, 100?ml
14. Squeeze bottle for deionized water
15. Tissue
16. Weighing paper
17. Spatula
18. Stirring hot plate with stirring bars

1.4 Reagents and Materials

1. Citric acid, monohydrate. MW = 210?g mole-1
2. KOH, 0.5?N
3. HCl, 0.5?N
4. HCl, 0.001?N
5. Sprite® (Coca Cola Company) or comparable lemon-flavored soda
6. Selected vegetables, e.g. fresh and canned tomatoes
7. Calibration buffers, pH 2 and 4

1.5 Procedures

1.5.1 Determining the pH of a Solid Food [5]

1. Cut a fresh tomato into small cubes and blend in a blender until a uniform slurry is formed, measure the temperature of the slurry.
2. Calibrate your pH meter.
3. Measure the pH of the slurry.
4. Centrifuge an aliquot of the slurry for 10?minutes at maximum speed.
5. Measure the pH of the supernatant.
6. Repeat Steps 1 through 5 using canned tomatoes.

1.5.2 Preparation of a Buffer and Determination of Buffer Capacity

1. Calculate the amounts of citric acid monohydrate and 0.5?N KOH required to prepare 200?ml of 0.05?M citrate buffer, pH 3....