(7) pH and Titratable Acidity

Question 1

define pH

Answer 1

The unit of measure which describes the degree of acidity or alkalinity in an aqueous system

Question 2

Function of pH

Answer 2

Used to make quantitative statements about the nature of solutions

pH = -log[H+]

Question 3

Accurate measurement of pH with pH meter

Answer 3

• voltage generated by a pH probe is predicted by the Nernst equation; voltage is a linear function of pH, the electrode potential being essentially +59 mv per pH unit
• from equation, a change in temperature would induce a change in pH (pH is sensitive to temperature)
• for accurate readings, the actual temperature has to be recorded (advisable to use temperature controlled room)

Question 4

Used and care of pH meter (4)

Answer 4

1. calibration with 2 pH standard buffers pH 4 and pH 7 or pH 7 and pH 10
   (4 and 10 covers a wide spectrum of food samples; buffer solutions should bracket expected pH sample)
2. set and record temperature
3. rinse the electrodes with deionised water before and after use
4. store in diluted 7.0 buffer between readings; for longer storage period use KCl solution

Question 5

Problem with pH meter: decreased response time/poor slope (error in calibration)

Answer 5

> knowing how an instrument works helps troubleshooting to maintain effective performance

Problem: Decrease response time or poor slope (error in calibration)

Cause: bound contaminants reduce proton binding to glass electrode

Solution: Clean/replace electrode

Question 6

Roles of acidulants (food acids) (5)

Answer 6

1. Flavouring
2. Food safety
3. maintaining pH
4. minimising lipid oxidation
5. altering structure of food

Question 7

Properties of acidulants (7)

Answer 7

1. acidity
2. taste
3. metal chelating activity (oxidation)
4. anti-microbial activity
5. solubility
6. moisture absorption
7. cost

Question 8

Define pKa value

Answer 8

The pKa value of an acid is the pH value at which 50% of the acid is dissociated

• depends on how many CO2H groups in the acid (if have 3 will have 3 pKa values)
• used to define acidic nature of organic acids (inorganic dissociates completely)

Question 9

how does acid titration work

Answer 9

• Titration of acids with alkali detects acids by removing protons from solution
• this raises the pH to a set point usually flagged by a colour change of an indicator compound
• weak acid indicators: phenolphthalein pKa 9.4 so change in colour is seen at pH 8.4-10.4

Question 10

Problems with titrating food with organic acids

Answer 10

Weak acids do not dissociate completely

|&raquo_space; If acid is partially dissociated, then titratable acid =/= pH

Question 11

Factors affecting the amount of titratable acid in food (3)

Answer 11

1. pH value
2. types of acid present
3. temperature

Question 12

Uses of acid titration

Answer 12

1. useful in quality control: change in titratable acidity may indicate manufacture failure
2. in quality control, the titration assay is useful to look at relative differences between samples

Question 13

activity vs concentration

Answer 13

activity: measure of expressed chemical reactivity
concentration: measure of all forms (free and bound) of ions in solution

Question 14

Instruments for pH determination: pH electrode

Answer 14

• generates an electrical potential as a function of pH in the surrounding
• in acidic solutions: more protons bind to the glass electrode, giving it a more positive voltage
• in basic solutions, fewer protons bind to the glass electrode, giving it a more negative potential

Question 15

Instruments for pH determination: combination electrode

Answer 15

-has both test and reference electrodes combined in the same housing

Question 16

Instruments for pH determination: solid state electrodes

Answer 16

• ion sensitive field effect transistor (ISFET)

- made from stainless steel which makes it robust for food industry (harsh conditions; cant use glassware)

Question 17

Roles of acidulants: Flavoring

Answer 17

to provide a desired taste and serve to intensify, enhance, blend or modify the overall flavor of the product (pickles)

Question 18

Roles of acidulants: food safety

Answer 18

• reduction of the pH to prevent/retard growth of MO and germination of spores

Question 19

Roles of acidulants: maintaining pH

Answer 19

• serve as buffering agents

- usually a combo of free acids and salts are used

Question 20

Roles of acidulants: minimise lipid oxidation

Answer 20

• by chelation of metal ions (Cu, Fe) which contribute to oxidation of food
• reduce color changes and controlling texture in some fruits and veg

Question 21

Roles of acidulants: altering the structure of foods

Answer 21

• gels made from gums (pectin)
• proteins and emulsifiers to modify the structure of foods (doughs)
• altering heat stability of proteins
• emulsifier in processed cheese
• sugar crystallisation in hard candy manufacturing

Question 22

pH vs titratable acidity

Answer 22

• The pH of a wine or juice is a measure of the concentration of free hydrogen ions in solution, while the TA is a measure of the total amount of hydrogen ions.
• Titratable acidity is a better predictor of acid’s impact on flavor than pH. However, total acidity does not give all relevant information on a food. For instance, the ability of a microorganism to grow in a specific food is more dependent on the concentration of free hydronium ions, H3O+, than titratable acidity.