Food Chemistry. Dennis D. Miller

into the hemiacetal and alcohol forms. However, they may be split into the alcohol and hemiacetal by acid‐catalyzed hydrolysis.

3.3 Apparatus and Instruments

      1 Test tubes

      2 Volumetric flasks, 50 ml

      3 Hot plate

      4 Large beaker, 600 ml

      5 Water bath, 37 °C

      6 Water bath, boiling

Figure 3.3 The formation of sucrose, a 1,2‐glycoside, from glucose and fructose.

3.4 Reagents and Materials

      1 Crystalline glucose, fructose, sucrose, lactose, and sorbitol

      2 HCl, 0.5 N

      3 NaOH, 1.0 N

      4 Benedict's solution (CuSO4 in citrate/carbonate buffer) [2].To prepare Benedict's solution: Dissolve, with stirring, 17.3 g sodium citrate and 10 g Na2CO3 in 80 ml distilled water. Dissolve 1.73 g CuSO4 .5H2O in 10 ml water. Add the CuSO4 solution to the citrate/carbonate solution. Transfer to a 100 ml volumetric flask. Dilute to 100 ml and mix well.

      5 Starch solution (3% soluble starch in water)

      6 Amylase solution (~1% amylase in water)

3.5 Procedures

      1 Transfer 3 ml starch solution to each of two test tubes.

      2 Add 5 drops of amylase solution to one of the tubes from Step 1 and incubate both tubes for 15 minutes in a 37 °C water bath.

      3 Prepare solutions of glucose, fructose, sucrose, lactose, and sorbitol (50 ml of each, 0.1 mol l−1, in water).

      4 Prepare 50 ml of 0.1 mol l−1 sucrose in 0.5 N HCl.

      5 Transfer 5‐ml aliquots of the sucrose‐HCl solutions (from Step 4) to two test tubes.

      6 Heat one of the test tubes from Step 5 in a boiling water bath for 10 minutes, cool. Keep the other at room temperature. Neutralize the HCl in the tubes by adding 1 ml of 1.0 N NaOH to each tube.

      7 Transfer 3 ml of each of the solutions, including the starch solutions and the heated and unheated sucrose solutions, into separate test tubes.

      8 Add 8 drops of Benedict's solution to each tube (including the starch solutions and a water blank). Mix well.

      9 Place tubes in boiling water bath for three minutes.

      10 Remove tubes from bath and observe color and appearance of solutions.

3.6 Study Questions

      1 Draw the structure of each of the sugars you tested. Label them reducing or nonreducing.

      2 What structure is characteristic of reducing sugars? Explain.

      3 Compare your results from the 3 sucrose solutions (in water, in HCl, in HCl with heating). Explain any differences.

      4 Compare the structures of glucose and sorbitol (a sugar alcohol).

      5 Fructose is not an aldose, yet it is a reducing sugar. Explain. (Hint: Remember that tests for reducing sugars are conducted in alkaline solution and recall the concept of keto‐enol tautomerism.)

      6 Name and draw the structures of 5 glycosides that may be present in foods.

      7 Are all disaccharides nonreducing sugars? Explain.

      8 Is starch reducing? Explain. Is starch treated with amylase reducing? Explain.

3.7 References

      1 1 Huber, K.C. and BeMiller, J.N. (2017). Carbohydrates. In: Fennema’s Food Chemistry , 5e (eds. S. Damodaran and K.L. Parkin ), 91–169. Boca Raton: CRC Press,Taylor & Francis Group.

      2 2 Benedict, S.R. (1909). A reagent for the detection of reducing sugars. Journal of Biological Chemistry 5: 485–487.

3.8 Suggested Reading

      1 Brady, J.W. (2013). Introductory Food Chemistry . Ithaca: Comstock Publishing Associates. 638 p.