Standardizing Chemicals for Dairy Quality Assurance Tests

Quality management begins at the farm. The condition of the animal affects the quality of milk produced for processing. The animal must be very healthy, the animal feeds must be of high quality, the shed must be clean and the health of the person tending to the animals must be sound.

All these will ensure that they do not transmit zoonotic diseases to other milk consumers through the milk. One will need to carry out various tests to ascertain the quality of milk.

Some of the tests include:

Organoleptic tests: these are tests you carry out using your senses such as sight, smell, and temperature. Tasting of raw milk is generally discouraged. 

Chemical, physical, and microbiological tests are necessary to ascertain the quality of milk.

Various chemicals and reagents are involved in the quality assurance tests carried out on milk. We will look at the reagents needed for the dairy quality assurance tests, what they are used for, and how they are prepared.

Precaution!

Take a lot of care when handling any of these reagents as they are extremely corrosive. Ensure you have the appropriate personal protective equipment.

Reagents for Chemical Tests

1. Chromic Acid

Chromic acid is used for descalingglassware. It is prepared from potassium dichromate powder and sulphuric acid.

Reagents and apparatus

• Top loading balance
• 1000 ml volumetric flask
• 1000 ml measuring cylinder
• Potassium dichromate powder
• Concentrated sulphuric acid

Corrosive reagents!

Preparation Instructions

• Weigh 92gms of potassium dichromate and transfer into a 1000 ml volumetric flask.
• Add 250mls of distilled water while shaking to dissolve the dichromate.
• Add concentrated sulphuric acid slowly and carefully to the solution in the 1000mls mark.
• Immerse the flask under cold water during addition, as the process is exothermic.
• Label the reagent with the name and date of preparation.

2. Sodium Hydroxide N/9 (NaOH)

N/9 (normal 9) Sodium Hydroxide is used for the determination of acid strength. It is prepared from sodium hydroxide pallets.

Apparatus and reagents

• 50 ml burette
• 10 ml volumetric pipette
• 1 Lt volumetric flask
• Conical flask
• Top loading balance
• Glass funnel
• Sodium hydroxide- analytical reagent
• N/9 oxalic acid

Preparation Instructions

• Weigh 4.5gms of sodium hydroxide pellets using a top loading balance
• Transfer the pellets into a 1 Lt volumetric flask
• Add about 250mls of distilled water to dissolve the pellets
• Make up solution to volume using distilled water
• Take 10 ml of the above solution and transfer into a conical flask using a volumetric pipette add 2-3 drops of phenolphthalein indicator.
• Fill 50mls burette with N/9 oxalic acid. Run the acid into the sodium hydroxide until endpoint. This changes solution from purple to clear. Initially the oxalic acid is run rapidly but slowly towards the end to detect the end point.
• Record the volume of oxalic acid used. If more than 10mls of oxalic acid is used the NaOH is more concentrated.

Calculations and Standardization

• If 12 ml of oxalic acid is used, every 10 ml of NaOH requires 2 ml of distilled water.
• The volume remaining in the flask of NaOH is 990mls, divide 990 with 10 and multiply by 2. Thus, the amount of water to add is 198 ml.
• Add water and titrate 10mls of NaOH with 10 ml of oxalic acid, they should neutralise at a ratio of 1:1
• Transfer the re-agent into amber bottles or a suitable closable container, labelled with name and date of preparation.

3. Potassium Iodide (KI) 5%

5% potassium iodide solution is used for the detection of hydrogen peroxide in milk. It is prepared from potassium iodide granules.

Apparatus /reagents

• potassium iodide granules
• distilled water
• Top loading balance
• 100 ml volumetric flask
• Spatula, opaque material

Preparation Instructions

• Weigh 5 gm of potassium iodide and transfer into a 100 ml volumetric flask
• Add distilled water while shaking to dissolve the granules and the solutions made up to the mark.
• Label the reagents with date, name and who prepared it.
• The bottle is covered with opaque material as light affects this reagent.

4. Starch Solution 2%

2% starch solution is also used for detecting hydrogen peroxide in milk. It is prepared from starch powder.

Apparatus and reagents

• Starch powder
• Distilled water
• Top loading balance
• 100 ml volumetric flask

Preparation Instructions

• Weigh 2 gm of starch powder and transfer into a 100 ml volumetric flask
• Add distilled water while shaking to disperse the powder up to the 100 ml mark
• Label the reagent with name and date of preparation.

5. Hydrochloric Acid N/10

Hydrochloric acid solution (of the right concentration) is used for caustic strength determination in CIP detergents (lye). It is prepared from analar grade hydrochloric acid

Apparatus and reagents

• Hydrochloric acid
• Distilled water
• Chilled water in a water bath
• 1 litre volumetric flask
• 10 ml pipette
• 1 ml pipette
• Pipette filler

Preparation Instructions

• Pipette 9 ml of analar HCl and transfer into 1 lt volumetric flask
• add 250 ml of distilled water while shaking under chilled water
• Make up to the 1 lt mark using distilled water. The flask is stoppered and the reagent transferred into a suitable container.
• Label the reagent with name and date of preparation.

6. Phenolphthalein Indicator 2.5%

Phenolphthalein indicator is used to mark the end of an acid-base titration. The indicator solution is prepared from phenolphthalein powder

Apparatus and reagents

• Top loading balance
• 100 ml volumetric flask.
• Phenolphthalein powder
• Concentrated ethanol (95-96%)

Preparation Instructions

• Weigh 2.5gms of phenolphthalein powder and transfer into100 ml volumetric flask.
• Add concentrated ethanol (95-96%) while shaking to dissolve and after dissolution it is made upto 100 ml mark.
• Add N/9 sodium hydroxide solution until further drop of sodium hydroxide causes a faint pink colour to appear.
• Phenolphthalein indicator is colourless in acidic solutions and purple in alkaline solution.
• Label the reagent with name and date of preparation.

7. Ethanol 80% v/v

Used in the alcohol test of milk. Prepared from industrial grade ethanol.

Reagents and Apparatus

• Ethanol Absolute (96% – 99%)
• Distilled Water
• Measuring Cylinder( 1000ml)
• Alcoholmeter
• Thermometer

Preparation Procedure

• Ascertain the Ethanol concentration first.
• Prepare solution from the established concentration.
• If the initial concentration of Ethanol is 96 %, then to make 80 %;
• Put 833.33 of Ethanol in a measuring cylinder then top up to 1000 ml. the formula below shall apply;
• ^x/_y x   100, (where x is the required alcohol percentage divide by y which is the initial ethanol percentage i.e. ⁸⁰/₉₆ x 100 = 83.33).
• Confirm the Alcohol concentration by using an Alcoholmeter at 20°C

8. Sulphuric Acid 

Used in Gerber/Butterfat Test. Prepared from the industrial grade sulphuric acid.

Apparatus and Reagents

• Volumetric Flask
• volumetric flask 1000 ml
• Measuring cylinder 1000 ml
• Thermometer
• Acid Hydrometer
• Sulphuric Acid Technical Grade
• Distilled Water in a trough
• Chilled Water

Preparation Procedure

• Measure 100 ml of Distilled water using a measuring cylinder
• Put the distilled water into a volumetric flask
• Add Sulphuric slowly while immersing the flask in chilled water in a trough
• Make up solution to 1000 ml mark with Sulphuric Acid
• Let the Acid cool down to 20°C
• Check the density of the Sulphuric Acid using the Acid Hydrometer
• The Acid should be 1.815 ± 2 g/ml at 20°C

Important Note:

If the density is below 1.815 ± 2 g/ml at 20°C, add more Sulphuric Acid.

Add distilled water if the Acid density is above 1.815 ± 2 g/ml at 20°C.

9. Phosphatase Test Reagent

Used in carrying out the phosphatase test in pasteurized milk.

Reagents and Apparatus

• Sodium Carbonate
• Sodium Hydrogen carbonate
• p-Nitrophenyl Disodium Phosphate
• Distilled Water
• Weighing Balance
• 1,000 ml Volumetric Flask
• 100 ml Volumetric Flask
• Parchment paper
• Spatula

Procedure for Buffer Preparation

• Weigh 3.5 g of Sodium Carbonate using a spatula onto parchment paper.
• Transfer the sodium carbonate in to a volumetric flask.
• Weigh 1.5 g of Sodium Hydrogen Carbonate using a spatula onto parchment paper.
• Add the sodium hydrogen carbonate into the flask containing the sodium carbonate.
• Add Distilled water into the flask while shaking it for the content to dissolve.
• Top up to the 1,000 ml mark.

Procedure for Substrate Preparation

• To make the substrate, weigh 0.15g of p-Nitro-phenyl Disodium- Phosphate on parchment paper and put in to a 100 ml volumetric flask.
• Add the buffer (4.6 above) while shaking up to the 100 ml mark.
• You must prepare fresh substrate every week and discard the old solution.
• Label the reagent indicating the name and date of preparation.

10. Oxalic Acid N/9

Used for standardization of N/9 Sodium Hydroxide.

Apparatus and reagents

• Oxalic Acid
• N/9 Sodium Hydroxide
• 5% Phenolphthalein Indicator
• Distilled Water
• 10 ml pipette
• 1000 ml volumetric flask
• Beaker
• Funnel
• Weighing balance
• Spatula

Preparation Procedure

• Weigh 7 grams of Oxalic Acid on parchment paper
• Put the Oxalic Acid into a 1000 ml volumetric flask
• Add about 250 ml of distilled water to dissolve the Oxalic Acid
• Make up solution to 1000 ml mark using distilled water
• Take 5 ml of the above solution and transfer into a beaker. Using a dropper add 2-3 drops of phenolphthalein indicator.
• Titrate against N/9 Sodium Hydroxide to the end point.
• Record the volume of Sodium Hydroxide used. If it is more than 5 ml of NaOH, then the Oxalic Acid has a very high level of concentration.

Standardizing the Reagent

• If you use 6 ml of N/9 NaOH, then every 5 ml of N/9 Oxalic Acid requires 1 ml of distilled water.
• The volume remaining in the flask of Oxalic Acid is 995 ml, divide 995 by and multiply by 1: thus amount of distilled water to add is 199 ml.
• Add distilled water and titrate 5 ml of N/9 Oxalic Acid with 5 ml of N/9 NaOH, it should neutralize at a ratio of 1:1
• Transfer the re-agent into amber bottle or a suitable container, labeled and indicate the name, concentration and date of preparation.