Direct Oven Drying Method for Determination of Total Solids
Content of Milk AND (Direct Oven-Ether Extraction) Method for
Determination of Solids-Not-Fat Content of Milk
The following procedures for determining total solids and
solids-not-fat content of milk have been approved by the Market Administrators'
Milk Test Procedures Committee. Both of these component test methods are
the result of collaborative studies which were presented to the Association of
Official Analytical Chemists (AOAC) in May 1988. These methods were
granted interim official first action approval in January 1989.
Ten laboratories analyzed nine pairs of blind duplicate raw
milk samples for total solids utilizing the direct forced air oven method.
Solids-not-fat testing was performed by nine laboratories which analyzed nine
pairs of blind duplicate raw milk samples for fat using the Mojonnier ether
extraction method and for total solids using the direct forced air oven
method. Solids-not-fat was subsequently determined by subtracting the
percent fat from the percent total solids.
The goal of the collaborative studies was to develop testing
procedures for solids and solids-not-fat determination that would yield highly
accurate test results and could be uniformly adopted and applied throughout the
dairy industry. The procedures presented here, when properly followed,
will yield comparable results between and within laboratories.
The full report of the total solids and solids-not-fat
testing methods is available from the office of any Federal Milk Market
Administrator or from the United States Department of Agriculture, Agricultural
Marketing Service, Dairy Division, P.O. Box 96456, Washington, DC 20090-6456.
Direct Forced Air Oven Drying Method for Total Solids in Milk
Total solids are determined by weighing milk, drying
milk, and weighing dried milk residue. The sample is dried in a forced air
oven for 4 hours at 100+1 degree Celsius. Total solids content of
milk is the weight of dried milk residue expressed as percentage of original
Weighing dishes. Flat bottom dish >5 cm
diameter without cover. If disposable aluminum dishes
are used they should be ones manufactured by the "oil
free" process to allow more uniform sample spreading in the dish.
Calibration weights. Class S standard calibration
weights to verify balance accuracy within weight ranges to be used for
weighing empty dish and dish plus sample.
Analytical balance. To read to the nearest
0.0001 g. Accuracy on verification within 0.0002 g. Check
periodically and whenever the balance is moved or cleaned. Keep a
record of balance calibration checks. A digital electronic balance is
recommended for fast response time for weighing warm milk.
Water bath. For tempering milk samples prior to
weighing, sample temperature should be 38+1 degree Celsius.
Desiccator. Room temperature. For cooling
weighing dishes after preliminary drying of empty dishes and after final
drying of dish plus sample. Use coarse desiccant (mesh 6-16) that
contains a minimum of fine particles and that changes color when moisture is
absorbed. Desiccator should seal tightly and contain a thermometer.
Forced air oven. Capable of rapid temperature
recovery after oven door is opened and closed after sample loading and
maintaining a temperature of 100 +1 degree Celsius (Blue-M model
490A-2 or equivalent). To evaluate oven temperature recovery: heat
empty oven to 100+1oC and stabilize for 15 minutes, turn off oven and
fully open door for 2 minutes, close oven door, turn oven back on, start
timing, and record the time that it takes the oven to reach 99oC. Time
should be < 7 minutes.
Weighing dish preparation. Number the clean
weighing dishes and dry for at least 2 hours in a forced air oven at 100+1
degrees Celsius. Store dishes in dessicator until needed (must be used
within one week or redried). Touch predried dishes only with clean,
Sample preparation. Prepare by tempering milk
to 38+1oC and mixing. Sample and weigh milk at 38+1oC.
Total solids determination. Remove metal shelf
from oven and place next to balance. Record all weights to four
decimal places. Weigh a predried sample dish. Pipet
approximately 3 g of 38+1oC milk (nearest 0.1 mg)
directly into a preweighed sample dish. Do not chase the drift.
Record the sample weight immediately after the weight stabilizes and just
before it starts its steady drift to lower numbers. Remove dish from
balance and place dish on oven shelf for transport to oven. Check
balance zeros between samples.
Blanks. Weigh 2 empty, predried dishes.
Drying. Preheat the oven to 100+1oC. The hot air exhaust vent should be open to allow
moisture to be removed from the oven. After weighing is completed,
turn off the oven and place shelf plus all dishes (including blanks) into
the oven, close the door, turn on the oven, and dry for 4 hours at 100+1oC. Do not open the oven door or place anything else into the
oven until the 4-hour drying is completed.
Cooling. Remove dishes from the oven and cool
to room temperature in desiccator (>30 minutes). Weigh the dish
plus dry milk on the same balance as was used for the initial weighings.
Always use blank values in the calculation. If blank
values are negative, be sure to add the blank value instead of subtracting
it. The cause of negative blanks should be determined and corrected.
Negative blanks are usually caused by the pans not being completely dry before
use in the analysis.
[(weight of dish + dry milk) - (wt of
dish)] - (mean blank)
Percent TS = ------------------------------------------------------------ x 100
[(wt of dish + milk) - (wt of dish)]
Maximum recommended difference between duplicates 0.05% total
SR = 0.042
= 0.149% RSDR =
= 0.054 R value = 0.118
Reference: J.L Clark, et al., 1989. Journal of
Association of Official Analytical Chemists, 72: 712-718.
(Direct Oven - Ether Extraction) Method for Solids-Not-Fat in
Solids-not-fat is determined by conducting total solids and
fat analyses. Percent fat is subtracted from percent total solids to
obtain percent solids-not-fat.
Conduct total solids analysis of milk by the direct forced
air oven drying method.
Conduct fat analysis of milk by the modified Mojonnier ether
Percent solids-not-fat = Percent total solids - Percent fat
Maximum recommended difference between duplicates 0.06% total
SR = 0.041
= 0.218% RSDR =
= 0.055 R value = 0.117
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