SOP: Total Solids in Dairy Products

Total solids is the backbone number in dairy plants: it checks standardization, predicts yield, and verifies that the evaporator did its job. The forced-draft oven method here is the reference against which rapid instruments are checked.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine total solids in milk and dairy products by oven drying for composition control and yield calculation.

Scope

Applies to raw and pasteurized milk, cream, condensed milk, and yogurt. For cheese and high-fat products, use the prescribed sand-dish variant to prevent crust formation.

Equipment and Materials

• Forced-draft oven at 102 plus or minus 2 C
• Flat-bottomed aluminum dishes with lids
• Analytical balance, 0.1 mg
• Desiccator
• Boiling water bath (for pre-drying liquid milk)
• Sea sand, acid-washed and ignited (for cheese)

Procedure

1. Dry empty dishes with lids at 102 C for 1 hour, cool in the desiccator, and weigh. Record the tare.
2. For milk: pipette 5 mL of well-mixed sample into the dish and weigh. Evaporate on the boiling water bath until apparently dry, about 20 minutes.
3. For cheese: weigh 3 g of grated sample mixed with 20 g of prepared sand into the dish to spread the solids.
4. Transfer dishes to the oven at 102 C and dry for 2 hours.
5. Cover, cool in the desiccator for 30 minutes, and weigh.
6. Return to the oven for 1-hour intervals until constant weight (change under 0.5 mg).
7. Run duplicates; agreement within 0.05 percent absolute is required for milk.

Recording Results

Percent total solids equals (dish plus dry residue minus empty dish) divided by sample weight, times 100. Report to two decimal places. Solids-not-fat equals total solids minus fat percent when both are determined.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• Crust formation trapping moisture: the water-bath pre-drying step was skipped or the layer is too thick. Use the sand variant for viscous products.
• Browning of residue: oven too hot. Verify 102 C with a calibrated thermometer; lactose caramelizes above 105 C.
• Weight gain on cooling: desiccant exhausted. Regenerate and cool for the full 30 minutes.

Safety Notes

• Steam from the water bath and hot dishes cause burns. Use tongs and gloves.
• Keep the oven area clear of flammables.