The goal of this exercise is to separate the two fractions of starch, amylose and amylopectin. I intended to have students use them to characterize the activities of alpha- and beta-amylase, which should give different hydrolysis products on complete digestion of amylose and amylopectin. It's based on one version of an old procedure (Methods in Carbohydrate Chemistry, vol. IV, pp. 25-27, 1964) that depends on the fact that unprocessed potato starch granules are formed of a matrix of amylopectin throughout which amylose is dispersed. Under mildly basic conditions, the amylose leaches out leaving the amylopectin network unaltered. The latter is recovered by sedimentation and the dissolved amylose is precipitated with butanol.

In a flask, students put 5 mL of a 10% (w/v) aqueous slurry of potato starch and 55 mL of 0.16 M NaOH and swirl gently until the suspension clears. After 5 minutes, they add 15 mL of 5% NaCl in 0.6 M HCl and mix gently but thoroughly. The precipitate is harvested by centrifugation at 10,000 rpm (SS-34 Sorvall rotor) for 15 minutes. The supernatant is saved in a separate flask and the pellet is washed by resuspending it in 20 mL of 1% NaCl and recentrifuging after standing at least overnight (or in the refrigerator for a week). The original supernatant contains amylose, which is precipitated by saturating it with 1-butanol and letting it stand for a few hours (or in the refrigerator for a week). It can be collected by centrifuging at 5,000 rpm for 15 minutes.

The only write-up I asked of the students was to determine and report the wet weights of amylose and amylopectin they recovered.

The details of the above procedure aren't hard and fast. The procedure as given in the reference is much more compulsive about times, temperatures, washing, filtering, etc. I only tried it once and results were somewhat variable. For example, the volume of the first amylopectin pellet varied, perhaps depending on how long the gel was allowed to swell in NaOH before neutralizing. All times and volumes might be varied somewhat. A more recent description of a similar separation can be found in Robyt and White, Biochemical Techniques: Theory and Practice (Waveland Press, 1990).

Revised 6/14/07