Velp Scientifica
– UDK139 Operating Manual
10004762/B5
103
32 - Method to determine the residual urease activity in soya beans
To evaluate if a soya bean sample, submitted to thermal treatment (toasting), has
been treated in a correct way, it is possibile to measure the residual activity of the
enzyme urease. This enzyme is present in uncooked soya bean and is progressively
destroyed by heat.
The result is expressed as milliequivalents of ammonia produced from urea by 1 g of
grinded soyabeans during 1 hour at 30 °C (Rass
mussen’s method).
In uncooked soyabean the urease activity is 5 milliequivalents/g about, the correctly
roasted soyabean shows values between 0,5 and 1 milliequivalents.
A too strong heat treatment lowers values under 0,2 milliequivalents.
Procedure
1) sample:
introduce into a test tube a 1 g soya sample finely grinded, add 0,20 g of
urea and 150 ml of distilled water.
Close immediately the test tube.
2) Incubation:
shake the closet test tube and bring to 30 °C
(water bath). Allow the enzymatic reaction develop for 60 min, shaking at intervals.
3) Distillation:
analytical parameters:
H
2
O=0ml. H
3
BO
3
=0 ml. NaOH=0 ml.
Distillation time = 4 min
Steam power: 100%
Introduce 30,0 ml of titrated 0,1 N solution of hydrochloric acid in the Erlenmeyer
flask used to collect the distillate; place it in position with the tip of the silicone tube
dipped in the acid.
If it is not dipped add 30-50 ml of ammonia free distilled water. At the end of the 60
min of reaction add in the test tube about 2 g of magnesium oxide (MgO) (pH 10,3),
place in position in the steam distillator and start distillation to obtain 100 ml of
distillate.
4) Titration:
titrate the residual acid in the flask by an alkalyne solution of known titre
and a suitable indicator (eg. Sodium hydroxide 0,1 N and methyl red).
5) Computation:
the result is expressed as milliequivalents of ammonia produced
from urea per each gram of soyabean:
3
NH
meq
10
x
g
weight
soya
d
neutralize
N
0,1
HCl
ml
6) Note:
a strong foaming can develop during distillation, particularly with poorly
roasted soyabeans. Control distillation and if necessary stop it at intervals.
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