Processing of Horticultural Crops (1+2)

Exercise

Practical 7 - Estimation of Titratable acidity

Aim: To estimate the titratable acidity of fresh and processed fruit and vegetable products.

Theory: Most of the fruit, vegetables and their products contain acid or mixture of acids. The acids may occur naturally in the fruit and vegetables or may be added during manufacture of different products or by lactic acid or acetic acid fermentation. Generally citric acid is added in most fruit products while in pickles, sauces and ketchup acetic acid is used. The acids are mainly responsible for the tartness or sour taste, thus estimation of acidity is used as the measure of tartness.They also helps in preservation by lowering the pH of the finished products.

Principle: Acidity in the sample is measured by titrating a given sample against a standard alkali solution of known concentration using phenolphthalein as an indicator to a light pink colour . However, for highly coloured products like tomato, mixed fruit jam, accurate determination of end point may be difficult by using indicator, thus for such samples, acidity is measured by titrating the sample against a standard alkali to a pH 8.1 using pH meter or using electrometric titrimeter or the sample is further diluted to almost colourless. The acidity is expressed in terms of predominant acid present in the product using standard expression. The list of common predominant acids and their equivalent weight present in different fruit and fruit products is as under:

Predominant acids in some fruits and processed products

Acid	Fruit/vegetable/products	Equivalent weight
Acetic acid	Sauce, ketchup, pickle in vinegar	60.05
Citric acid	Citrus fruit, most fruit products, mango,guava	64.04
Lactic acid	Curd and sauerkraut	90.08
Oleic acid	Olive	282.46
Malic acid	Apple, pear, apricot, peach, plum	67.05
Tartaric acid	Grapes, tamarind	75.04

Apparatus, reagents and glassware required

1. Sodium hydroxide - 0.1 N
2. Phenolphthalein solution -1.0 %
3. Volumetric flask - 100 ml capacity
4. Conical flask - 250 ml capacity
5. Burette - 10-100ml capacity
6. Magnetic stirrer
7. pH Meter

Preparation of reagents

1. N/10 Sodium hydroxide: Dissolve 40.005g NaOH pellets in water and dilute to one litre in a volumetric flask. Standardize by titrating against standard H2SO4 solution (N/10). For standardization of H2SO4 against Sodium carbonate (Na2CO3), heat analytical grade Na2CO3 at 1050 C for 8 hours.
2. N/10 Na2CO3 Solution: Pipette 25ml of 0.1 N H2SO4 solution ¬into 250ml conical flask. Add about 100ml water and few drops of bromophenol blue. Add standard 0.1 N Na2CO3 solution from burette to a blue end point or pH 4.1 using pH meter.
3. 1% Phenolphthalein solution: Dissolve 1g phenolphthalein in 100ml ethanol.

Sample preparation: Crush the sample (fruit, vegetable, jam, pickle etc) in a blender or pestle & mortar and mix thoroughly to obtain pulp. Weigh the material, add some water and boil for 1hr replacing the water lost in evaporation. Cool and transfer to a volumetric flask, make up to a known volume. Filter if necessary. For juice, squash, cordial etc dilute (if necessary) without boiling or crushing.

Procedure for determination of acidity

• Dilute an aliquot of sample to known volume and place in titration flask.
• Add few drops of 1% phenolphthalein as an indicator
• Titrate with N/10 NaOH to faint pink colour using burette or pipette.
• Note the titre value and calculate % titratable acidity as predominant acid.

Note: For highly coloured products, dilute small volume of the sample (5 ml) with large volume of distilled water (300-400ml). The colour becomes so pale that the indicator colour change during titration can be observed easily.

Procedure for determining acidity of coloured products using pH meter:

1. Pipette 10ml of sample in 250ml beaker and add 90ml distilled water.
2. Agitate diluted sample using magnetic stirrer.
3. Immerse electrode of pH meter into the solution and titrate with N/10 NaOH from a burette to a pH value 8.1. At this pH, phenolphthalein turns from colourless to pink.
4. Note the titre value and calculate titratable acidity as percent of predominant acid.

Observations

Sample No.	Weight of sample taken	Volume made up	Volume used for estimation	Volume of NaOH used	Titratable acidity (%)
1.
2.
3.
Mean

Calculation

Titre × normality of alkali × volume made × equivalent weight of acid × 100

% Acidity = -----------------------------------------------------------------------------------

Wt of sample × volume of aliquot ×1000