This is my second installment of food analytical methods for quality control purposes. The first one is Sugar Determination by Refractometer. Please bear with me, there are more to come.
The two next methods are useful for pH and titrable acidity adjustment.
1. Calibrate the pH meter using two buffer solutions near the range of the test sample. The pH of most fruits plays near four, so you can select buffers ranging from three to seven.
2. Place sample in a clean beaker. Dip the electrode of pH meter until the reading becomes stable. Different equipment may have different way of indicating stability, so read the manual before proceeding.
3. It is always good to do three trials and compute the average. Rinse electrode tip after every trial.
Measurement of Titrable Acidity
1. Pipette 10mL of sample into a 50mL beaker.
2. (A)Insert the pH probe into sample or (B) add 3 drops of phenolphthalein indicator. Choose only one option.
3. Fill the burette with 0.1 N NaOH standard solution. Take note the initial reading.
4. Begin titrating by slowly rotating the burette stopcock. Swirl the beaker so that sample and NaOH mix well.
5. (A) Continue titrating until the pH is approaching 7.5. Stop titrating and let the pH stabilize (keep swirling). Then add NaOH drop-wise until the pH approaches 8.2. Or (B) until the solution become pink for 30 seconds.
6. Note the final burette reading to an accuracy of 0.1mL. Subtract the initial from the final to calculate the volume required for the titration.
7. Compute as follows:
Where V is volume of sodium hydroxide solution used for titration (ml); N is normality of sodium hydroxide solution; meq. wt. is milliequivalent weight of the standard; and v is sample volume (ml).
Table 1. Standard Acids of Some Foods and milliequivalent weight(meq. wt.)
Apple – Malic , 67
Apricot – Malic , 67
Banana – Malic , 67
Blueberry – Citric, 64
Cherry – Malic , 67
Cranberry – Citric, 64
Grapefruit – Citric, 64
Grape – Tartaric, 75
Lemon – Citric, 64
Lime – Citric, 64
Orange – Citric, 64
Peach – Malic , 67
Pear – Malic , 67
Pineapple – Citric, 64
Plum – Malic , 67
Raspberry – Citric, 64
Strawberry – Citric, 64
Tomato – Citric, 64
Wine – Tartaric, 75
1. Standard chemical reagents and equipment can be procured from your nearest chemical and laboratory suppliers.
2. Ask nearest university or research institution to verify the accuracy of your test.