Farm Animal Metabolism and Nutrition

Temperature control

Microbial activity, and gas volumes and
pressures all change with temperature.
Close temperature control, using either a
water bath or an incubator maintained at
39°C, is therefore needed for in vitrodiges-
tion studies.

Atmospheric pressure changes

The amount of a gas in moles is defined by
the volume, the temperature and the pres-
sure (Equation 10.1), and all three of these
quantities must be known or measured in a
gas production experiment. We return later
to the problem posed by changes in atmos-
pheric pressure during the course of an
experiment.

Stirring

Carbon dioxide has a strong tendency to
form supersaturated solutions in aqueous
media. If this occurs, then either pressure- or
volume-based gas readings will be incorrect.
Fortunately, this tendency is countered by
the presence of particulate matter in an in
vitrosystem and can be reduced further by
occasional gentle stirring or shaking (Pell
and Schofield, 1993).

Closed versus open systems

The amount of gas produced in a chemical
reaction can be measured by holding the
volume constant and observing the pressure
change (closed system) or by maintaining a
constant (usually atmospheric) pressure
and measuring the volume change (open
system). The vented system of Beaubien et
al.(1988) can be considered a compromise
between these two extremes. Factors affect-
ing system choice are considered below.

Information desired

Possible applications of gas fermentation
data include testing of plant varieties for

plant breeding, feedstuff comparisons,

digestion rates for modelling studies and

feed component interactions including

inhibitory effects of plant secondary com-

pounds. The nature of the application will

dictate the experimental design and may

influence the choice of gas method.

Pros and Cons of

Alternative Methods

For convenience, we divide the methods

into three groups: the syringe method, and

the open and closed automated methods.

Syringe method

● Pro. Simple and relatively inexpensive.

No excess pressure accumulates and

thus equal molar amounts of CO 2 and

CH 4 give equal volume changes.

● Con. Insensitive (maximum precision

±0.5 ml) and subject to error from

sticking plungers. Not convenient for

detailed kinetic studies because of

manual recording of data.

Closed automated method

● Pro. Simplest automated method to con-

struct and maintain because it requires

no valves. Good for measuring small gas

volumes because the sensitivity of the

system depends on the bottle size and

the sensor range, both of which can be

made small.

● Con. Not readily adapted to handle

sample sizes >250 mg because of the

need to keep the maximum pressure

increase down to about 0.5 atmospheres.

This arbitrary limit is imposed for two

reasons. First, because gas leaks are less

likely when the pressure is low. Second,

because some data have suggested that

more scatter in pressure readings occurs

at higher pressures (Theodorou et al.,

1994). We have not observed a trend of

this kind (Schofield and Pell, 1995a).

A correction may be required to deal

with the different sensor response to a

214 P. Schofield