Zinc acetate solution, 1 M: dissolve 21,9 g crystallized zinc acetate dihydrate Zn(C2H.O2)2.2H2O and 3 ml glacial acetic acid in water and make up to 100 ml with water.

Potassium hexacyanoferrate (II) solution, 0,25 M: dissolve 10,6 g crystallized potassium hexacyanoferrate (II) trihydrate K4[Fe(CN)6]. 3H2O in water and make up to 100 ml with water.

Hydrochloric acid solution, 6,35 ± 0,2 M (20 to 22 %) or 5,0 ± 0,2 M (16 to 18 %).

Ammonia solution, 2,0 ± 0,2 M (3,5 %).

Acetic acid solution, 2,0 ± 0,2 M (12 %).

Bromothymol blue indicator, 1 % (m/v) solution in ethanol.

Balance, sensitivity 10 mg.

Polarimeter tube, 2dm, of exactly calibrated length.

1. (a)

   Polarimeter with sodium light or mercury green light (mercury vapour lamp with prism or the special Wratten Screen No 77 A), to be read with an accuracy of at least 0.05 angular degrees,

2. (b)

   Saccarimeter with international sugar scale, using white light passing through a filter of 15 mm of a 6 % solution of potassium bichromate, or sodium light, to be read with an accuracy of at least 0,1^o on the international sugar scale.

Water bath, regulated at 60 ^oC ± 1 ^oC.

In order to standardize the procedure, reagents and apparatus, carry out a control determination in duplicate as described below using a mixture of 100 g of milk and 18 g pure sucrose or a mixture of 110 g of skimmed milk and 18 g pure sucrose, each corresponding to 40 g of condensed milk containing 45 % sucrose. Calculate the sugar content using the formulae under 7, substituting for M, F and P respectively in formula 1 the quantity of milk taken and the fat and protein content of this milk, and in formula 2 for M, the value of 40,0. The mean of the values found shall not differ by more than 0,2 % from 45,0 %.

Calculate the sucrose content by means of the following formulae:

1. (1)
   v = $\frac{M}{100}\left(\mathrm{1,08F\; +\; 1,55P}\right)$
2. (2)
   S = $\frac{\mathrm{D\; -\; 1,25l}}{Q}\times \frac{\mathrm{V-v}}{V}\times \frac{V}{\mathrm{L\times M}}\%$

where:

S

sucrose content;

M

mass of the weighed sample in grams;

F

percentage of fat in the sample;

P

percentage Of protein (N x 6.38) in the sample;

V

volume in ml to which the sample is diluted before filtration;

v

correction in ml for the volume of the precipitate formed during clarification;

D

direct polarimeter reading (polarization before inversion);

I

polarimeter reading after inversion;

L

length in dm of the polarimeter tube;

Q

inversion factor, the values of which are given below.

Remarks:

1. (a)

   When exactly 40,0 g of condensed milk are weighed and a polarimeter with sodium light, angular degrees and a 2dm polarimeter tube at 20,0 ^oC ± 0,1 ^oC is used the sucrose content of normal condensed milk (C = 9) can be calculated from the following formula:

   S = (D — 1,25 I) x (2,833 — 0,00612 F — 0,00878 P)

2. (b)

   If the invert polarization is measured at a temperature other than 20 ^oC, the figures should be multiplied by:

   (1 + 0,0037 (T — 20).

The following formulae give accurate values for Q, for various sources of light with corrections for concentration and temperature:

Sodium light and polarimeter with angular degrees:

Q

0,8825 + 0,0006 (C — 9) — 0,0033 (T — 20).

Mercury green light and polarimeter with angular degrees:

Q

1,0392 + 0,0007 (C — 9) — 0,0039 (T — 20).

White light with dichromate filter and saccharimeter with international sugar scale degrees:

Q

2,549 + 0,0017 (C — 9) — 0,0095 (T — 20).

In the above formulae:

C

Percentage of total sugars in the inverted solution as polarized,

T

Temperature of the inverted solution in the polarimetric reading.

Note 1:

The percentage of total sugars C in the inverted solution may be calculated from the direct reading and the change on inversion in the usual manner, using the usual values for the specific rotations of sucrose, lactose and invert sugar.

The correction 0,0006 (C — 9) etc., is only accurate when C is approximately 9; for normal condensed milk, this correction can be neglected, C being close to 9.

Note 2:

Variation in temperature from 20 ^oC of 1 ^oC makes little difference in the direct reading, but variation of over 0,2 ^oC in the invert reading necessitates a correction. The correction - 0,0033 (T — 20) etc., is only accurate between 18 ^oC and 22 ^oC.

The difference between results of two determinations carried out simultaneously or in rapid succession on the same sample, by the same analyst, under the same conditions, shall not exceed 0,3 g of sucrose per 100 g of condensed milk.