4-10
(
This is an indirect reading of starch by measuring dextrose liberated from the hydrolysis of
starch. To use the method described, your sample must be dextrose-free. If free dextrose is
present, see Special Considerations below.
First, dissolved starch is hydrolyzed externally by the enzyme Amyloglucosidase to produce
dextrose. Dextrose is read at the enzyme sensor. The enzyme Glucose Oxidase is immobilized
in the YSI Dextrose Membrane.
H
2
O
Amyloglucosidase
β
-D-Glucose (dextrose)
De O
2
Glu Oxidase
H
2
O
2
+ D-Glucono-
δ
-Lactone
System Buffer
YSI 2357
Calibrator Std
YSI 2776 (2.50 g/L dextrose)
Linearity Std
YSI 1531 (9.00 g/L dextrose)*
Black Probe
YSI 2365 Dextrose Membrane
White Probe
n/a **
Sample Size
25
µ
L
Cal Station
Station #1 Call Well
Black Probe Parameters
Chemistry
Dextrose
Unit of Conc
g/L
Cal value
2.50 g/L
End Point
30 sec
* For applications requiring linearity performance to 25.0 g/L, YSI 2777 (25.0 g/L dextrose)
may be used as a linearity standard provided the sample size is 10
µ
L.
** If you have a dual channel unit, either Black or White, or both Black and White probes can
be configured for this chemistry.
Special Considerations:
»
An amyloglucosidase enzyme such as Sigma # A7420 works with many samples.
»
Solubilizing starch is key to obtaining reproducible results. A variety of methods are in
the literature. YSI has documented a couple of methods in YSI Application notes.
»
Prepare a starch solution using the diluent described below, add 1 to 2 mg
amyloglucosidase per mL of starch solution. Allow at least 20 minutes for the enzyme
to react, and the liberated dextrose to reach mutarotational equilibrium, before
measuring the sample. If your reading is higher than expected, you may need to wait
slightly longer, or re-evaluate the possibility of free-dextrose in the sample.
Diluent: 40 g/L NaH
2
PO
4
, 10 g/L Na
2
HPO
4
, in reagent water.
»
If free-dextrose is a concern, first measure free dextrose. Then externally hydrolyze the
starch, and measure the total dextrose after hydrolysis. Subtract the free-dextrose from
the total dextrose and multiply by 0.9 to account for water of hydrolysis.
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