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33.2.43 AOAC Official Method 990.19 Solids (Total) in Milk By Forced Air Oven Drying after Steam Table Predry First Action 1990 Final Action 1993 IDF–ISO–AOAC Method

Results of the interlaboratory study supporting acceptance of the method: sr = 0.017; sR = 0.047; RSDr = 0.136%; RSDR = 0.370% A. Principle

Total solids are determined by weighing milk, drying milk, and weighing dried milk residue. Test portion is predried 25 min on steam bath and then dried 3 h at 100° ± 1°C in forced air oven. Total solids content of milk is weight of dried milk residue as % of original milk weight. B. Apparatus

(a) Weighing dishes.—Flat bottom, ³5 cm diameter, without cover. If disposable aluminum dishes are used, they should be ones manu fac tured by “oil-free” pro cess to al low more uni form spreading of test portion in dish. (b) Calibration weights.—Class S standard calibration weights to verify balance accuracy within weight ranges for weighing empty dish and dish plus test portion. (c) Analytical balance.—Readability to near est 0.0001 g. Accuracy on verification within 0.0002 g. Check periodically and whenever balance is moved or cleaned. Keep record of balance calibration checks. Digital electronic balance is recommended for fast response time for weighing warm milk. (d) Water bath.—For tempering milk test samples before weighing. Test sample temperature should be 38° ± 1°C. (e) Steam table.—With individual openings that expose full bottom surface of each test portion dish to steam. (f) Desiccator.—Room temperature. For cooling weighing dishes after preliminary drying of empty dishes and after final drying of dish plus residue. Use coarse desiccant (6–16 mesh) that contains minimum of fine particles and that changes color when moisture is absorbed. Desiccator should seal tightly and contain a thermometer. (g) Tongs. (h) Forced air oven.—Capable of rapid temperature recovery after loading and of maintaining 100° ± 1°C. To evaluate temperature recovery, heat empty oven to 100° ± 1°C and stabilize for 15 min. Turn oven off and fully open door for 2 min. Close oven door, turn oven back on, start timing, and record time for oven to reach 99°C. Time should be £7 min. C. Determination

(a) Weighing dish preparation.—Number clean weighing dishes and dry ³2 h in 100° ± 1°C forced air oven. Store dishes in desiccator

until needed (must be used within 1 week or redried). Only touch predried dishes with clean, dry tongs. (b) Test portion preparation.—Temper milk to 38° ± 1°C and mix until homogeneous as in 925.21 (see 33.2.02). (c) Initial weighing.—(Note: Remove and weigh milk at 38° ± 1°C. Record all weights to 4 decimal places.) Remove metal shelf from oven and place next to balance. Weigh predried weighing dish. Pipet ca 3 g of 38° ± 1°C prepared milk test portion directly into preweighed dish. Do not chase drift. Record test portion weight to nearest 0.1 mg immediately after weight stabilizes and just before start of steady drift to lower numbers. Remove dish from balance and place on oven shelf for transport to steam bath. Check balance zero between weighings. (d) Blanks.—Weigh 2 empty, predried dishes. (e) Predrying.—Place dishes on steam bath so that full surface of dish bottom is exposed to steam. Be sure that all holes in steam bath contain a dish or are covered during predry period. Expose blank dishes to steam as for dishes containing test portions. Do not have steam so high that dishes float or tip over. Predry milk until there is little or no free liquid movement in dish (£25 min). (f) Drying.—Preheat oven to 100° ± 1°C. After steam table predry, place all dishes (including blanks) on oven shelf, turn oven off, and place shelf plus all dishes into forced air oven. (Note: Do not place dishes on solid trays; this does not allow heat to circulate evenly around dishes. Do not place any thing else in oven.) Immediately after loading all dishes in oven, close door and turn oven on again, and dry 3 h at 100° ± 1°C. Do not open oven door until 3 h drying is completed. (g) Cooling.—Remove dishes from oven and cool to room temperature in desiccator (³30 min). Weigh dish plus dry milk on same balance that was used for initial weighings. D. Calculations

Always use blank values in calculation. If mean blank is negative, be sure to add blank weight instead of subtracting it. Cause of negative blank weight should be de termined and cor rected. Negative blank weight is usually caused by pans not being completely dry before use in analysis. Total solids, % =

(W 2 - W ) - B ´ 100 W1 - W

where W weight of dish; W1 = weight of dish + milk test portion; W2 = weight of dish + dry milk; and B = mean blank weight. Maximum recommended difference between duplicates = 0.05% total solids. E. Repeatability and Reproducibility

Repeatability (r) and reproducibility (R) values established in the collaborative study were 0.049 and 0.133, respectively. Reference: JAOAC 72, 712(1989).

2006 AOAC INTERNATIONAL

ã 2005 AOAC INTERNATIONAL