Treatment of straw with urea. 1. Ureolysis in a low water medium

Abstract

The aim of this work was to gain a better understanding of the reaction of ureolysis, with or without added ground dehulled soya bean, during the process of straw (wheat varieties 1,2,3) treatment by urea as an ammonia source in conditions limiting the addition of water. In Experiment 1 the reaction was made at 30 °C in an open liquid (without straw) medium at pH 9.0, avoiding product (NH 3) inhibition and in a closed medium (substrate and product inhibition) with and without straw 1 mixed with urea, soya bean and water to reach 30% moisture. Initial urea concentrations ( U i ) ranged from 2 to 400 g l −1. Soya bean concentrations ( S) were 10 g l −1 and 30g l −1 in open and closed media respectively. In open medium the kinetic parameters of ureolysis in dilute solutions (maximal specific initial rate of reaction V M = 16.6 μM urea g −1 soya protein s −1, Michaelis-Menten constant Km = 63.8 mM 1 −1) were close to those of pure urease. However, U i to reach V M (75 g l −1) was higher than for pure urease and beyond this value, the kinetics were different from that calculated on a basis of uncompetitive inhibition by the substrate. In a closed medium, the specific initial rates ( V i ) of the reaction were lower than in open medium beyond U i = 50 g l −1, and the reaction slowed down considerably beyond 200 g l −1. This may be an upper limit in conditions simulating a stack. In Experiment 2, it was shown that at U i = 75 g l −1 in open medium, the initial velocities (in μM s −1) were proportional to S, when S varied from 5 to 100 g l −1. Experiment 3 was designed to determine the effect of straw 3 on the development of the reaction. The treatments were made in tightly closed glass jars, with U i = 175 g l −1 (e.g. 8 g U per 100 g straw, 85% DM) and S = 30 g l −1, at 30 °C, with or withou'straw 3 (30% terminal moisture). The straw was either mixed with the urea-soya bean-water homogenate or placed on a sieve over it (indirect method). The accelerating effect of the straw was clear after the first hours of the reaction and was demonstrated better by the indirect method than by the mixing method. In Experiment 4, straws 1 and 2, with or without soya bean, were treated by the mixing method as above. With added urease, the reaction was nearly complete after 1 month for both straws, but without soya bean, it took 3 months to have total or only 77% ureolysis with straws 1 and 2, respectively. Because of the complex phenomena occurring during the ureolysis in a concentrated and heterogeneous medium, the kinetics can only be roughly predicted by V i when the same (i.e. mixing) method is compared. They are better characterized by other parameters, such as time to achieve 50 or 99% of ureolysis. It is concluded that in the process of straw treatment by urea, the addition of an external erude urease may be useful to reduce both the amount of water necessary and the treatment duration.