DETERMINATION OF TITRATABLE ACIDITY IN MILK
1. Introduction
The pH of milk normally varies between 6. 5 and 6. 7 so it has a slightly acidic reaction.
PH values below 6. 5 indicate acidic milk.
Values above 6. 8 indicate mastitic milk.
Milk contains organic acids, in particular citric acid, and inorganic, carbonic and phosphoric acids and others linked to the micelles of casein (the main protein of milk).
The titrated acidity of milk is an important parameter to identify different criticalities, e. g. the state of preservation, the possible presence of breast diseases (mastitis), suitability for cheese making.
It is expressed in degrees °SH (Soxlet-Henkel) determined on 100 ml of milk, or 50 ml if SH 50 is taken into account. The Soxlet-Henkel grades represent the ml of sodium hydroxide needed to neutralize 100 ml of milk, or 50 ml of milk in the case of °SH 50.
Normally, for convenience, 50 ml are used and the value is expressed in °SH50.
Mastitis: inflammation of the udder due to biotic agents, mainly bacteria, or to factors such as incorrect nutrition, as a result of mastitis the characteristics of the milk are altered.
2. Objectives
The main objective is to determine the titratable acidity of the milk and to express an opinion on the quality characteristics of the milk accordingly.
3. Materials
-Gloves and personal protective equipment
-Burette graduated in ml 50 ml pipette with pro pipette (if not available use other capacity)
-Becker 100 ml magnetic stirrer with relative anchor (otherwise glass stirring rod)
-Sodium hydroxide (NaOH) solution normalquarto0,25N (N/4) (or 0,25 Molar (0,25M)
note: normality and molarity are methods to define the concentration of solutions
(0. 25N and 0. 25M are two identical expressions because in sodium hydroxide the normal solution and the molar solution are equivalent)
-1% phenolphthalein alcohol solution (this is the indicator that turning pink gives the end point of the titration and of the neutralization reaction)
4. Products
Acid-base neutralization reaction between the acids, mainly organic, present in the milk and the sodium hydroxide used in the titration.
5. Experimental procedure
1. Comply with safety measures.
2. Shake the milk sample to homogenize it.
3. Take 50 ml of milk, with pipette and pro pipette.
4. Place the milk sample in a beaker with a capacity of 100 ml.
5. Add 1ml of phenolphthalein.
6. Titrate drop by drop, keeping the milk stirring, with NaOH until the milk turns a persistent pale pink for 15 to 20 seconds.
6. Expected results
The millilitres of NaOH used represent the titratable acidity value of the milk expressed in °SH50.
The normal value of titratable acidity in milk in °SH 50 is around 3. 5; with such, the milk is suitable for consumption and cheese making.
Values below 3. 2 are an indication of disease in the cow (mastitis), milk unsuitable for consumption and cheesemaking.
Values greater than 3.7 may indicate colostrum or soured milk (e.g. sour milk for poor preservation). Milk unfit for consumption and cheesemaking.
Note: Colostrum is the milk secreted by the cow in the first 10 days after calving, it provides the calf with the antibodies that it lacks at birth.
Multiplying by 2 the value obtained we obtain the °SH degrees referred to 100 ml.
Acidity and pH are linked by an inverse relationship, the higher the acidity the lower the pH and vice versa.
Remark: by adding sodium hydroxide (NaOH) to milk, some H+ ions are released from the third function of phosphoric acid due to the over-run phenomenon which occurs according to the following reaction:
3Ca2+ +2H2PO4-→← Ca3(PO4)2+4H+
Calcium (Ca2+) present in milk precipitates phosphoric acid releasing H+ ions.
Because of the over-run, the neutralized milk, which had turned pink, slowly returns to white and a very slight acidity due to the hydrogenions (H+ ions) released by the previous reaction.
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