Processing of Horticultural Crops (1+2)

Exercise

Practical 6 - Determination of pH in food products

Aim: To determine the pH of food samples.

Theory: pH is defined as the negative logarithm of its hydrogen-ion concentration in gram per litre. It can be measured by using pH meter.

The hydrogen-ion concentration of a food is a controlling factor in many chemicals and microbiological reactions. Pure water has equal concentration of H+ and OH- ions, each having the value of 1x107g/litre at room temperature that is why water is regarded as neutral. The pH of pure water is 7.0, the solution having pH below 7.0 is regarded as acidic while solution with pH above 7.0 is alkaline. Most fruits and fruit products have pH below 4.0 while vegetable, milk and meat products have pH more than 4.0 or above. pH value of some fruits and fruit products is given in Table 6.1. Thus with the increase in acidity in the solution, the pH value decrease and vice versa. However, presence of buffer salts helps in maintenance of pH. Estimation of pH of the food is important as the processing conditions for different food products are categorized on the basis of their pH.

Table 6.1: pH value of some fruits and processed products

Commodity	pH value	Commodity	pH value
Apple	3.2-3.7 (3.4)	Apple cider	3.3-3.5 (3.3)
Apricot	3.6-3.9 (3.7)	Apple Sauce	3.2-4.2 (3.6)
Asparagus green	5.4-5.7 (5.5)	Asparagus Puree	5.0-5.3 (5.2)
Beans green	5.2-5.7 (5.4)	Beans Puree	5.0-5.2 (5.1)
Black berries	3.3-3.5 (3.4)	Carrot Puree	4.9-5.2 (5.1)
Carrot	5.0-5.4 (5.2)	Corn, brine packed	6.1-6.8  (6.3)
Cherry	3.3-3.5 (3.3)	Cherry Juice	3.4-3.4 (3.4)
Figs	5.0-5.0 (5.0)	Cranberry juice	2.4-2.8 (2.6)
Goose berries	2.8-2.9 (2.9)	Orange juice	3.5-4.0 (3.7)
Grapes	(3.1)	Grapes juice	3.0-3.4 (3.2)
Grapefruit	3.0-3.4 (3.2)	Grapefruit juice	3.0-3.4 (3.3)
Mushroom	5.8-5.9 (5.8)	Lemon Juice	2.7-3.3 (2.9)
Peach	3.6-4.1 (3.8)	Pickle, Dill	2.6-3.8 (3.1)
Pear, Bartlett	3.6-4.7 (4.1)	Cucumber Pickle	(4.4)
Prunes	2.5-4.2 (3.7)	Pickle, Sour	(3.1)
Potato	5.4-5.6 (5.5)	Pine apple juice	3.4-3.5 (3.5)
Pineapple crushed	3.2-3.5 (3.4)	Plum, green gaze	3.6-4.0 (3.8)
Pumpkin	4.8-5.2 (3.7)	Sauerkraut	3.4-3.7 (3.5)
Raspberry	3.2-4.1 (3.7)	Spaghetti in tomato sauce	4.7-5.5 (5.1)
Spinach	5.1-5.9 (5.4)	Tomato Juice	4.0-4.5 (4.3)
Strawberry	3.0-3.9 (3.4)	Tomato Puree	4.0-4.3 (4.2)
Tomato	4.0-4.6 (4.4)

Principle: Working of a pH meter is based on the principle of measuring the Electro Motive Force (EMF) or potential formed from a reference electrode, test solution and a glass electrode sensitive to H+ ions. The potential developed is directly proportional to the concentration of the H+ ions in the given solution.

Apparatus and reagent’s required

1. pH meter
2. Standard buffer solution (pH 4.0 and pH 7.0)

Procedure for measurement of pH

1. Weigh a known quantity of the sample
2. Macerate with known volume of distilled water
3. Allow the mixture to stand for 30 minutes
4. Decant the supernatant in a beaker. (In case of juice, squash, cordial, measure pH directly without dilution/maceration).
5. Caliberate the pH meter with the help of standard buffer solutions (pH 4.0 and 7.0)
6. Wash the electrode with distilled water and wipe with tissue paper
7. Dip the electrode in the test solution; keep stirring the solution with a glass rod till a constant pH is recorded.

Note: For rough measurement, the pH indicator paper can be used. Indicator papers of different pH ranges are available. The indicator paper slip is dipped in the test solution and change in colour of the paper is compared with colour of indicator paper to note the pH.

Observations:

Sample No.	pH
1.
2.
3.
Mean

Results: The pH meter directly gives the pH value of the product and can be calculated directly by taking average of three values.