Other methods for determining fat in food (3)

2.
Geber method (according to GB 5413.
3-2010)

The Gaber method (acid method) is the most commonly used milk fat determination method in China
.

(1) Principle: Adding sulfuric acid to cow milk destroys the gel quality of cow milk and the protein outer membrane covering the fat globules, and the volume is measured after the fat is separated by centrifugation
.

(2) Reagent

①Sulfuric acid: The relative density is 1.
820～1.
825
.

②Isoamyl alcohol: analytically pure

(3) Instruments

① Cream centrifuge
.

②Gaibo milk fat meter: the smallest scale is 0.
1%, see Figure 3-5
.

Figure 3-5 Geber Milk Fat Meter

(4) Operation steps

①Place the milk fat meter on the milk fat meter shelf, and pour 10 mL of sulfuric acid into the Gabor milk fat meter (do not wet the bottle mouth with sulfuric acid)
.

②Carefully add 5.
0 mL of the well-mixed sample along the tube wall
.

Aspirate 6mL of water, carefully wash the pipette that aspirates the sample, pour the lotion into the milk fat meter, and then add 1mL of isoamyl alcohol

③Tighten the stopper, wrap the bottle mouth with a cloth (to prevent the acid from rushing out during shaking), press the rubber stopper with your thumb, make the bottle mouth down, make the fine sulfuric acid flow to the enlarged part of the milk fat meter, vigorously shake Shake to make a uniform brown liquid, let stand for a few times

④ Put it in a 65～70℃ water bath for 5min, take it out and wipe dry, put it in a centrifuge (800～1000r/min) and centrifuge for 5min, take it out and place it in a 65～70℃ water bath (note the water bath) The water surface should be higher than the fat layer of the milk fat meter), take it out after 5 minutes (the meter mouth is downward and vertical), and read immediately, which is the fat content
.

(5) Result calculation: According to the percentage multiplied by 2.
2%, it is the fat content of the sample
.

(6) Tips

①The density of sulfuric acid must be 1.
820～1.
825 relative density at 15℃
.

If the concentration is too high, the protein will be carbonized and the reading will be difficult; if the concentration is too thin, the protein will not be dissolved, the water phase and the fat phase will not separate well, and the fat will not be detected

Generally, the relative density of commercially available sulfuric acid is 1.
84, which needs to be adjusted
.

The relationship between relative density and concentration of sulfuric acid is as follows:

Relative density of sulfuric acid: 1.
84 1.
83 1.
825 1.
82 1.
815

Sulfuric acid concentration: 96% 92% 91% 90% 89%

②The milk fat meter should be placed symmetrically in the centrifuge during centrifugation, and the speed should be slow to fast
.

When the specified speed is reached, the speed should be kept uniform, timing for 5 minutes, and stop centrifuging

③Be careful when adding acid to prevent it from spilling on the table, clothes or hands
.

④Isoamyl alcohol is added to promote the precipitation of fat and reduce the surface tension of fat globules, so as to facilitate the formation of a continuous fat layer
.

3.
Acid hydrolysis method (according to GB/T 5009.
6-2003)

(1) Principle Heat and hydrolyze the sample with hydrochloric acid solution to free the bound or encapsulated fat in the tissue, then extract it with an organic solution, and weigh it after drying
.

Calculate as follows:

Where x-fat content,%

m——The mass of the receiving bottle, g

m ₁ ——The mass of receiving bottle and fat, g

(2) Tips

① This method is suitable for the determination of fat in various foods.

For solid, semi-solid, viscous liquid or liquid food, especially foods that are easy to absorb moisture, agglomerate, and are not easy to dry after processing, when Soxhlet extraction cannot be used, The effect of this method is better

② When the extraction was added ethanol can play precipitated protein, lower the surface tension, promote polymerization of fat globules, dissolution alcohol substance
.

Petroleum ether is added to reduce the solubility of ethanol in ether , so that the ethanol solubles remain in the water layer, and the separation is clear