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Total Solids

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

Preface
    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

Principle
    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 milk weight.

Apparatus

  1. 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.

  2. 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.

  3. 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.

  4. Water bath.  For tempering milk samples prior to weighing, sample temperature should be 38+1 degree Celsius.

  5. 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.

  6. Tongs.

  7. 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.

Determination

  1. 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, dry tongs.

  2. Sample preparation.  Prepare by tempering milk to 38+1oC and mixing.  Sample and weigh milk at 38+1oC.

  3. 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.

  4. Blanks.  Weigh 2 empty, predried dishes. 

  5. 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.

  6. 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.

Calculation
    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 solids

                Sr = 0.019                SR = 0.042

                RSDr = 0.149%        RSDR = 0.327%

                r value = 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 Milk

Principle
    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.

Determination

  1. Conduct total solids analysis of milk by the direct forced air oven drying method.

  2. Conduct fat analysis of milk by the modified Mojonnier ether extraction method.

Calculation
Percent solids-not-fat = Percent total solids - Percent fat

Maximum recommended difference between duplicates 0.06% total solids

                Sr = 0.019                SR = 0.041

                RSDr = 0.218%        RSDR = 0.466%

                r value = 0.055          R value = 0.117

 

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