Food Chemistry. Dennis D. Miller

pK₁ = 3.40 pK₂ = 5.05 Apples, apricots, grapes, oranges, tomatoes Oxalic acid

pK₁ = 1.27 pK₂ = 4.27 Spinach, potatoes, tomatoes Phosphoric acid

pK₁ = 2.12 pK₂ = 7.21 pK₃ = 12.32 Tomatoes, acidulant used in soft drinks Tartaric acid

pK₁ = 2.98 pK₂ = 4.34 Grapes Sodium hydrogen sulfate or sodium acid sulfate

pK = 1.99 Acidulant. Lowers pH without imparting acid taste. May be added to process water to enhance chlorine activity

      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 foods^a.

Food	pH	Food	pH
Lime juice	2.0	Yogurt	4.0–4.5
Lemon juice	2.2	Cheddar cheese	5.1–5.5
Vinegar	2.6	Beef, fresh	5.5–5.0
Rhubarb	3.0	Pork, fresh	5.6–6.9
Grape juice	3.1–3.2	Turkey, fresh	5.7–6.1
Wines	2.9–3.9	Tuna	6.0
Apple juice	3.5–3.9	Carrots, fresh	5.7–6.1
Strawberries	3.2–3.4	Potatoes, fresh	6.1
Peaches	3.8	Green beans, fresh	6.5–6.7
Pears	3.9	Milk, fresh	6.6
Grapefruit juice	4.0	Sweet corn, fresh	6.7
Orange juice	4.2	Egg yolk	6.0–6.9
Tomato juice	3.8–4.7	Egg white (pH increases as egg ages)	7.6–9.2

      ^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,