Zulkowsky soluble starch
Around 1875, Karl Zulkowsky made the discovery that starch dissolves easily in hot glycerine . It changes into a water-soluble form, which can be separated from the glycerol solution by precipitation with concentrated ethanol . Potato starch , which easily converts into the soluble form, proved to be particularly suitable . Wheat starch and rice starch are much more difficult or even impossible to convert in this way. Starch produced by this process is commercially available (e.g. from Fluka or Merck).
Soluble starch according to Zulkowsky is preferred to produce aqueous starch solutions, as it is much more soluble in water than ordinary soluble starch. Such starch solutions are used in some analytical determinations in the laboratory, for example in iodometry or the enzymatic analysis of amylase .
Procedure for the laboratory scale
Approx. 60 g of ground starch are stirred into 1 kg of concentrated glycerine. The whole thing is heated in a porcelain bowl with constant stirring.
| temperature | observation |
|---|---|
| <130 ° C | strong swelling of the starch granules |
| <130 ° C | steam escapes from the mixture for some time |
| 130 ° C | the consistency increases significantly, stirring can only be continued with difficulty, shapeless mass |
| 170 ° C | Consistency decreases, mass appears thin, starch is completely dissolved, clear liquid |
| 190 ° C | Glycerine begins to evaporate, starch has changed into soluble form |
Whether the conversion has taken place to a sufficient extent can be tested as follows: A few drops of the hot solution are dropped into water, depending on whether a cloudiness occurs and how strong it is, the conversion can be concluded.
The liquid is allowed to cool to 120 ° C. and slowly poured into two to three times the amount of ethanol. The liquid above the precipitate is poured off and new ethanol added, making the precipitate thicker. The precipitate is washed free of glycerol with ethanol.
For cleaning, the preparation obtained is immediately dissolved in warm water, filtered and the clear solution is poured into an appropriate amount of ethanol while filtering.
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- ↑ K.Zulkowsky: Ber.Deutsch.Chem.Ges, 13, 1395, 1880 (PDF; 5.4 MB).