How to Determination of Total Solids in Curd
Shallow flat-bottom dishes of aluminum, 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.
Heat the clean dry empty dish and lid in oven maintained at 100º ± 2ºC for one hour, cool in a desiccator and weigh. Quickly weigh to the nearest 0.1 mg 4-5 g of prepared sample of curd or dahi, replace the lid and weigh again. Add 1-2 drops of phenolphthalein solution to the sample in the dish and neutralise with 0.1 N sodium hydroxide solution to a faint pink colour. Note the volume of 0.1N sodium hydroxide required to neutralize the sample. Place the dish without lid on a boiling water bath until the water is removed from the sample. Wipe the under-surface of the dish and place in the oven maintained at 100 ± 2ºC, for 3 hrs. Remove the dish along with the lid and cool in a desiccator and weigh. Continue heating and re-weighing at hourly intervals until successive weighings do not vary by more than 0.5 mg. Deduct half weight of the 0.1N sodium hydroxide added to neutralize the sample from the residue after drying and calculate total solids as for whole milk.
N × T.V. × 40
—————————— = a
1000 × 2
100 (W2 – a)
Total solids percent w/w = ——————————
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 volume of 0.1 N Sodium hydroxide added
(Ref :- Modified I.S 12333 – 1997/I.S.O 6731 – 1989 Milk, Cream and Evaporated Milk – Determination of Total Solid Content – Reference method/Pearsons Composition and Analysis of Foods, 9th edn. Page 570)