Method according to Luff-Schoorl

The Luff-Schoorl method is a method for determining the content of reducing sugars in a sample. It is based on the reduction of the copper (II) ions in an alkaline environment by the sugar and the subsequent back titration of the reagent added in excess. In food analysis, the most important monosaccharides D - glucose and D - fructose as well as the most important dissacharides D - lactose and D - maltose can be determined with this method . Non-reducing sugars, for example D - sucrose , can only be quantified if they can be broken down into reducing monosaccharides by acid hydrolysis.

chemistry

Luff's solution

After separation and clarification, reducing sugars in a sample are reacted with Carrez-I solution (blood liquor salt) and Carrez-II solution (zinc acetate) and with what is known as Luff's solution while boiling. Luff's solution is a defined mixture of three different partial solutions. Partial solution 1 contains citric acid . It serves as a complexing agent to avoid the formation of copper (II) hydroxide Cu (OH) ₂ in the alkaline. Partial solution 1 is mixed with a partial solution 2 containing sodium carbonate . Finally, the third partial solution is added to this. It contains copper (II) sulfate, which is important for the reaction . The pH of the mixture can be adjusted to pH 9.3-9.4 through the amount of sodium carbonate.

Reactions

The reducing sugar reacts with the copper (II) ions in Luff's solution and is oxidized in the process, while Cu ^{2+ is} reduced to Cu ⁺ :

${\displaystyle \mathrm {2\ Cu^{2+}{\xrightarrow[{Zucker}]{red.}}Cu_{2}O\downarrow } }$

The excess of Cu (II) is determined iodometrically. For this purpose, the remaining copper (II) ions in the sample are mixed with potassium iodide after acidification (usually 25% sulfuric acid ) :

${\displaystyle \mathrm {\ Cu^{2+}+2\ I^{-}\rightarrow \ CuI_{2}} }$

The resulting copper (II) ion is reduced to poorly soluble copper (I) iodide and an iodide is oxidized to iodine :

${\displaystyle \mathrm {2\ CuI_{2}\rightarrow 2\ CuI\downarrow +I_{2}} }$

The resulting iodine is then titrated with a standard sodium thiosulphate solution until a pale yellow color appears . The starch solution serves as an indicator because it forms a blue iodine-starch complex with iodine . The titration is therefore continued until the blue color disappears:

${\displaystyle \mathrm {I_{2}+2\ S_{2}O_{3}^{2-}\rightarrow 2\ I^{-}+S_{4}O_{6}^{2-}} }$

In the same way as the main experiment, a blank value without sugar is set. The difference in the volumes of standard solution in the blind and main experiment provides the amount of copper consumed in the main experiment.

evaluation

Since the reaction between reducing sugars and copper (II) ions is neither stoichiometric nor linear (double the amount of sugar in the sample does not use double the amount of copper), empirically recorded, tabulated values ​​are required for the quantitative evaluation of the experiments. From these it can then be read off which sugar content corresponds to the measured solution consumption. The sugar content in liquid samples is given in g per liter, while weight percentages (g per 100 g) are used for solid samples.

Determination of sucrose

Sucrose is a non-reducing disaccharide and cannot be determined directly using the Luff-Schoorl method. It is therefore split into glucose and fructose by acids ( invert sugar ). If the sample to be examined also contains other reducing monosaccharides, the sample must be examined before and after inversion. The difference, which is multiplied by a correction factor of 0.95 due to water accumulation, then corresponds to the sucrose content.

literature

• Reinhard Matissek, Gabriele Steiner: Food analysis: basics, methods, applications. 3rd, completely revised Edition, Springer-Verlag, Berlin / Heidelberg 2006, ISBN 3-540-62513-5 , p. 115ff.