SOP: Determination of Total Solids in Dahi (Sodium Hydroxide Method)
GIFSQ — Global Institute for Food Safety & Quality
1. Principle
In cultured dairy products such as dahi or yoghurt, total solids are determined after neutralization of developed acidity with alkali. In this procedure, NaOH is used for neutralization of developed acidity, followed by determination of total solids by evaporation at 100 ± 2°C.
2. Apparatus
Shallow flat-bottom dishes of aluminium, nickel, stainless steel, porcelain or silica, 7–8 cm diameter, about 1.5 cm in height, and provided with easily removable but closely fitting lids.
3. Reagents
a. Sodium hydroxide (0.1 N solution, used in the procedure)
b. Phenolphthalein indicator solution: dissolve 1 g of phenolphthalein in 100 mL of ethanol (95%, v/v). Add 0.1 N NaOH solution until one drop gives a faint pink colouration. Dilute with distilled water to 200 mL.
4. Procedure
Heat the clean dry empty dish and lid in an oven maintained at 100 ± 2°C for one hour, cool in a desiccator and weigh
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Quickly weigh, to the nearest 0.1 mg, 4–5 g of prepared sample of curd or dahi; replace the lid and weigh again
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Add 1–2 drops of phenolphthalein solution to the sample in the dish and neutralize with 0.1 N sodium hydroxide solution to a faint pink colour. Note the volume of 0.1 N sodium hydroxide required to neutralise the sample
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Place the dish without lid on a boiling water bath until the water is removed from the sample
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Wipe the under-surface of the dish and place it in the oven maintained at 100 ± 2°C for 3 hours
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Remove the dish along with the lid, cool in a desiccator and weigh
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Continue heating and re-weighing at hourly intervals until successive weighings do not vary by more than 0.5 mg
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Deduct half the weight of the 0.1 N sodium hydroxide added to neutralize the sample from the residue after drying, and calculate total solids as below
5. Calculation
a = (N × T.V. × 40) ÷ (1000 × 2)
Total solids percent w/w = 100 × (W₂ − a) ÷ W₁
Where,
– N = normality of NaOH
– T.V = titre value
– W₂ = weight in g of residue left after drying
– W₁ = weight in g of the prepared sample taken
– a = half of the weight of the 0.1 N sodium hydroxide added (from the formula above)
6. Total milk solids in Dahi sweetened with cane sugar
First determine the total solids as described above, and then estimate the cane sugar separately by the method for determination of sugar content in condensed milk. Deduct the amount of sugar from the total solids.
References
Manual of Methods of Analysis of Foods, Milk and Milk Products, FSSAI 2016, 4.4.1.