National Food Safety Standard — Determination of Fat in Foods
1 Scope
This standard specifies the methods for determination of fat in foods.
Method I of this standard is applicable to the determination of free fat in fruits, vegetables and derived products, grains and grain products, meat and meat products, egg and egg products, marine lives and their products, bakery products, candy and other foods.
Method II of this standard is applicable to the determination of free fat and total bound fat in fruits, vegetables and derived products, grains and grain products, meat and meat products, egg and egg products, marine lives and their products, bakery products, candy and other foods.
Method III of this standard is applicable to the determination of fat in foods for infants and young children, milk and milk products.
Method IV of this standard is applicable to the determination of fat in foods for infants and young children, milk and milk products.
Method I Soxhlet Extraction
2 Principle
Fat is freely soluble in organic solvents. After the sample is extracted by anhydrous diethyl ether or petroleum ether, and the solvent is evaporated, dried, the substance left is the content of free fat.
3 Reagents and Materials
3.1 Reagents
Unless otherwise specified, reagents used in this method are all analytically pure, and water is the Grade 3 water as specified in GB/T 6682.
3.1.1 Anhydrous diethyl ether (C4H10O).
3.1.2 Petroleum ether (CnH2n+2): boiling range of petroleum ether is 30℃ to 60℃.
3.2 Materials
3.2.1 Quartz sand.
3.2.2 Absorbent cotton.
3.2 Sea sand: boil sea sand or river sand (clean the mud by water) with hydrochloric acid (1+1) for 0.5h, and then clean it to neutral by water; then boil it with caustic soda solution (240g /L) for 0.5h, and then clean it to neutral; dry it at 100℃±5℃ for standby.
4 Apparatus and Equipment
4.1 Soxhlet abstractor.
4.2 Thermostat water bath.
4.3 Balance: with sensibility of 0.001g and 0.0001g.
4.4 Electrothermal blowing dry box.
4.5 Dryer: filled with effective drying agents, for example silica gel.
4.6 Filtration paper cylinder.
4.7 Evaporation dish.
5 Analysis Procedure
5.1 Treatment for sample
5.1.1 Solid sample: weigh, to the nearest 0.001g, 2.00g to 5.00g fully mixed samples and move them into a filtration paper cylinder.
5.1.2 Liquid or semisolid samples: weight, to the nearest 0.001g, 5.00g to 10.00g and place it in evaporating dish; add about 20g quartz sand and evaporate to dryness on boiling water bath; dry it in electrothermal blowing dry box at 100℃±5℃ for 30min; take it out grind to fine, and move it to the filtration paper cylinder. Clean the evaporating dish and glass rod with sample by absorbent cotton with acetic ether and place cotton in filtration paper cylinder.
5.2 Extraction
Place the filtration paper cylinder into the extracting cylinder of Soxhlet abstractor, connect the receiving flask (dried to constant quantity); add diethyl ether dried or petroleum ether from the upper end of extractor condenser pipe to 2/3 of the internal volume; heat it in the water to reflux and extract the acetic ether or petroleum ether continuously (6 times/h to 8 times/h); generally extract for 6h to 10h. At the end of extraction, collect 1 drop of extracting solution with a ground glass rod, no oil spot on the rod shall be considered as extraction completed.
5.3 Weighing
Take down the receiving flask and recycle the acetic ether or petroleum ether; evaporate the receiving flask to dryness in water bath when the acetic ether in receiving flask is 1mL to 2mL; dry it at 100℃±5℃ for 2h; cool it in dry for 0.5h and then weigh it. Repeat above operation till constant quantity (until the differences between two weights not exceed 2mg).
6 Result Calculation
……………………………(1)
Where:
X — the crude fat content in sample, in gram per hectogram (g/100g);
m1 — the quality of receiving flask and crude fat, in grams (g);
m0 — the quality of receiving flask, in grams (g);
m2 — the sample quality (if it is the sample after determining the water content, the quality before determining water content shall be counted), in grams (g).
100 — the conversion coefficient.
The calculation result is accurate to one decimal place.
7 Precision
The absolute difference in the two results measured independently under repeated conditions shall not exceed 10% of the arithmetic mean value.
Method II Acid Hydrolysis Method
8 Principle
Bound fat in foods must be dissociated by strong acid, the dissociated fat is freely soluble in organic solvents. After hydrochloric acid hydrolysis, the sample is extracted by anhydrous diethyl ether or petroleum ether. The quantity after eliminating the solvent is the total content of the free fat and bound fat.
9 Reagents and Materials
9.1 Reagents
Unless otherwise specified, reagents used in this method are all analytically pure, and water is the Grade 3 water as specified in GB/T 6682.
9.1.1 Hydrochloric acid (HCl).
9.1.2 Ethyl alcohol (C2H5OH).
9.1.3 Anhydrous diethyl ether (C4H10O).
9.1.4 Petroleum ether (CnH2n+2): boiling range of petroleum ether is 30℃ to 60℃.
9.1.5 Iodine (I2).
9.1.6 Potassium iodide (KI).
9.2 Preparation of reagents
9.2.1 Hydrochloric acid solution (2mol/L): weigh 50mL hydrochloric acid and add into 250mL water, mix well.
9.2.2 Iodine solution (0.05mol/L): weigh 6.5g iodine and 25g potassium iodide, dissolve them in little water, and then dilute to 1L.
9.3 Materials
9.3.1 Litmus blue test paper.
9.3.2 Absorbent cotton.
9.3.3 Filter paper: medium speed.
