200 mg of sample. Digestion takes place
within a 100 ml graduated glass syringe
installed on a rotor and maintained at
39°C. Gas volumes are read to ±0.5 ml. If a
large volume of gas is to be measured, the
syringe can be emptied after a measured
volume is collected, and the fermentation
can then be continued.
2.The system can be used both for
digestibility measurements (24 h incuba-
tion) and for rate measurements (readings
taken after 4, 8, 12, 24 and 32 h). Recent
work has sometimes used more extensive
sampling.
3.Blanks (four syringes with no substrate)
and standards (three syringes with hay, hay
- starch, concentrate) are run with each
experiment. The standards each have a
known gas production determined by
averaging many replicates and using
different ruminal fluid inocula. If the
standard included within a run produces
between 90 and 110% as much gas as the
‘average’ value for that standard, then the
ruminal fluid is scored as normal and all
the measured gas volumes are corrected by
the factor ‘average standard volume/run
standard volume’. If, however, the run
standard volume lies outside this 90–110%
range, the run data are discarded on the
grounds that the ruminal fluid was abnor-
mal. The average blank volume is sub-
tracted from all samples. This volume is
normally 6–12 ml or 13–27% of the final
reading, quite a high blank value.
4.The above treatment of blanks and
standards is claimed to correct for atmos-
pheric pressure differences between runs.
As applied, the correction converts gas
volumes to the average pressure at which
the standards were determined. Unless this
pressure is reported, it is difficult to
compare gas volume data from different
laboratories.
Manual methods of recording data,
whether by reading a manometer or a
syringe, necessarily restrict the amount of
data collected. If one is interested in
digestibility values, the problem is manage-
able because only a few readings need be
taken from each sample. However, for a
kinetic analysis of feedstuff digestion,
many timed readings are necessary, and
manual methods become burdensome.
Research workers in this field therefore
looked for an appropriate automatic data
recording system. The first hint of a
possible solution was published in 1974,
well before Menke’s syringe method
appeared. This report described the use of
an electronic pressure sensor and strip
chart recorder to follow the growth of gas
producing bacteria such as Enterobacter
aerogenesand Escherichia coli in closed
test tube cultures (Wilkins, 1974). Taya et
al.(1980) reported an application of this
technique. They studied the use of a
pressure sensor to control the fermentation
of cellulose by Ruminococcus albuson an
industrial scale. These workers showed
that the total gas yield was proportional to
the amount of cellulose digested (480 ml
g^1 at 39°C).
When microcomputers became readily
available in the early 1980s, the stage was
set for the electrical signal from a pressure
sensor to be combined with a computer-
based data collector. Beaubien et al.(1988)
described a method to measure gas flow
from a bioreactor. A schematic diagram of
the method is shown in Fig. 10.1.
A three-way solenoid valve is con-
trolled by a separate flow meter circuit
that responds to a voltage signal from the
pressure sensor. The valve is set so that
ports 1 and 2 are normally open and
communicate with the transducer and
ballast. Port 3 is normally closed. When
the pressure in the reactor and ballast
rises to a pre-determined set point, the
controller activates the valve for about 2 s,
closing port 2 and opening port 3 to vent
the ballast and restore atmospheric
pressure. The valve is then deactivated
and the cycle repeated. The computer
records the time of each open–close cycle
and counts the number of cycles. Each
cycle will correspond to a fixed gas
volume that depends on the ballast size
and the pressure sensor trip point. This
incremental volume can be varied over a
wide range, depending upon the experi-
mental need.Gas Production Methods 211