Food Analysis (Quantitative)

To introduce a stronger emphasis on quantitative and, especially, simple instrumental methods, I created this exercise to use in place of the more qualitative exercise described elsewhere. Provided students have enough spectrophotometers, the exercise can easily be completed in a three-hour lab period. In brief, students determine protein and lactose colorimetrically using the biuret method for protein and the dinitrosalicylic acid (DNSA) assay for reducing sugar.

For the sugar analysis, students are given lactose solutions of 1, 2, and 3 mg/mL and an unknown dilution of milk. Using calibrated 1-mL plastic droppers, they put 0.25 mL of water, each lactose solution, or the diluted milk into separate test tubes. Duplicates of all samples requires ten tubes. They then add 8 drops of DNSA reagent to each tube and heat in a boiling water bath for 10 minutes. When tubes are cool, students dilute each tube with 4.0 mL of water for determination of absorbance.

For the protein analysis, students are given gelatin solutions of 2, 4, and 6 mg/mL and an unknown dilution of milk. Using calibrated plastic droppers, they put 0.5 mL of water, each gelatin solution, or the diluted milk into separate test tubes. Duplicates of all samples requires ten tubes. They then add 2.0 mL of biuret reagent to each tube. Tubes are ready immediately for determination of absorbance.

Students determine the absorbance of each sample at 540 nm using a Spectronic 20 (or whatever may be available). I explain absorbance simply as a quantitative measure of color intensity that is proportional to the amount of sugar present. They tabulate and graph their data and determine the concentrations of lactose and protein in their respective diluted samples. Finally, they are told the dilution factor for their unknown samples (25-fold for lactose and 10-fold for protein) and asked to determine how many grams of lactose and of protein are in a cup (240 mL) of milk, which they can compare with the amounts given on the nutrition label.

Milk dilutions are prepared in distilled water using nonfat dry milk. Full-strength milk is 97 g/L, so 25-fold uses 0.39 g/100mL and 10-fold uses 0.97 g/100mL.

Revised 8/31/06