Laboratory Activity 3: Teacher Notes

Continued

Part 4

  1. The stated procedure in Part 4 is not a very effective way to remove the moisture from the milk sample, but it has been bench tested and has produced good results. However, you can consider using a small convection oven if one is available.

Pre-Laboratory Discussion

Discuss solvent extraction, polar and nonpolar solvents, two layers and how to know which is which. Discuss precipitate formation, evaporation of a liquid and the effects of heat on the rate of evaporation.

Teacher-Student Interaction

Circulate in the laboratory. Demonstrate use of a pipet bulb. Buchner funnels will be new to students; show them how to remove filter paper from the funnel.

Anticipated Student Results

  1. Laboratory values to be expected:

    1. The reported values are averages. Each sample of milk will vary from the average values. Experimental error can account for some of the differences: balance accuracy and precision, extraction errors, etc.
    2. The average values to not total 100% because of the uncertainties in the reported values and the presence of small quantities of other components, such as vitamins and minerals.
    3. Student values total 100% because the carbohydrate value was obtained by subtracting the total of the other measurements from 100. The assumption was made that the contribution of the other components in milk was negligible and that the carbohydrate percentage could be determined indirectly by difference.
  3. Fats are nonpolar and dissolve more readily in nonpolar solvents.
  4. The extraction of nonfat milk components could be incomplete and yield high results for the percentage of milk fat.
  5. Some of the fat-soluble layer would be weighed with the bottom layer and give low results for the percentage of milk fat.
  6. Acids cause denaturation of proteins (see Enzymes module). Proteins lose their three-dimensional configuration that allows them to be water soluble, and they precipitate out of solution.
  7. The milk solid residue will collect on the stirring rod while stirring the mixture. Since it would be impossible to remove all of the solids from the rod in order to weigh the residue, the weight of the rod is counted from the beginning to minimize error from this experimental step.
  8. The milk would char or burn if subjected to extreme temperatures such as the direct flame of a burner. Losing some of the milk solid would cause a higher percentage of water to be reported.

Continued

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