GB 5413.29-2010National food safety standard Determination of solubility in foods for infants and young children, milk and milk products (English PDF)

4.1 Silicone defoaming agent: Mass fraction of silicone emulsion is 30 %.

The applicability of the silicone defoaming agent shall be tested according to the

procedures mentioned in 6.4.5 (without adding sample). After the test, the visible silicone

liquid at the bottom of the centrifugation tube shall not exceed 0.01 mL.

6.4.5 Add 3 drops of silicone defoaming agent to the mixture in the cup. Mix the contents in the

cup with the flat spoon (5.10) for 10 s (no excess). Then pour the mixture in the

centrifugation tube (5.12) to the scale of 50 mL. That is, the level of the top is consistent

with the scale mark of 50 mL.

6.4.6 Place the centrifugation tube in the centrifuge (symmetrically). Make the centrifuge to

rotate quickly and produce an acceleration of 160 gn at the bottom of the tube. Then rotate

at a temperature of 20 °C - 25 °C for 5 min.

6.4.7 Take out the centrifugation tube. Remove and pour down the upper fatty substance in the

tube with the flat spoon (5.10). Hold the centrifugation tube vertically. Remove the

supernatant with the siphon or suction tube (5.13). For the drum dried product, suck until

the top of the liquid coincides with the 15 mL marking-line. If it is the spray dried milk

powder, suck until the top of the liquid coincides with the 10 mL marking-line. Pay

attention not to stir the insolubles. If the volume of the precipitate exceeds 15 mL or 10 mL

obviously, do not perform the lower-part operation. Record the insolubility index as "15

mL" or " > 10 mL". Mark the recovery temperature as described in Chapter 7; otherwise

operate according to 6.4.8.

6.4.8 Add 24 °C or 50 °C water to the centrifugation tube till the level reaches 30 mL. Fully stir

the precipitate with the stirring rod (5.14). Put the stirring rod against the tube wall. Add

some water of the same temperature. Flush the liquid on the stirring rod down, till the

liquid-level coincides with 50 mL marking-line.

6.4.9 Plug the centrifugation tube with the rubber stopper. Turn over the centrifugation tube for

6.4.10 Take out the centrifugation tube. Hold it vertically. Use appropriate background for

contrast (see note). Make the eyes in the same level with the top of the precipitate. Read

the volume of the precipitate with the help of the magnifier (5.15). If the volume of the

precipitate is less than 0.5 mL, it shall be accurate to 0.05 mL. If the volume of the

precipitate is more than 0.5 mL, it shall be accurate to 0.1 mL. If the top of the precipitate

inclines, its volume shall be estimated. If the top of the precipitate is uneven, the

centrifugation tube shall be vertically placed for a few minutes. Usually, the top of the

precipitate will become flatter, so the number will be easier to read. Record the

temperature of recovered water.

Note: Observe the centrifugation tube with light or dark background for contrast. The top of the

precipitate will be more striking and easier to read.

0.10 mL (24 °C)

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