compounds act as both oxidizable sub-
strates and oxidizing agents. If we ignore
microbial cell production, then fermenta-
tion of glucose in the rumen produces a
stoichiometrically balanced mixture of
acetate, propionate, butyrate, CO 2 and CH 4
(Wolin, 1960). It is helpful, at this stage, to
look at an example of a theoretical
balanced equation for this kind of glucose
fermentation (Baldwin, 1970):
1 glucose →1.2 acetate
+ 0.4 propionate + 0.2 butyrate
+ 1 CO 2 + 0.6 CH 4 + 0.4 H 2 O (10.1)In this example, fermentation of 1 mol of
glucose produces 1.6 mol of gas (CO 2 +
CH 4 , direct gas) plus 1.8 mol of volatile
fatty acid (VFA). If the fermentation takes
place in a bicarbonate buffer at pH 6.5,
then the VFA will react with the buffer to
give an equimolar amount of CO 2 (indirect
gas). The total gas yield will then be
3.4 mol mol^1 of glucose.
Let us also review the physical law
that governs gas behaviour. The general gas
law,
PV = nRT (10.2)tells us that, for an ideal gas, pressure (P)
and volume (V) are inversely related. If one
of these is held constant, the other is
directly proportional to both the molar
amount (n) of a gas and the absolute tem-
perature (T). The universal gas constant (R)
has the units (pressure volume moles^1
temperature^1 ) and the numerical value
of R depends on the units selected for
pressure, volume and temperature. Most
students remember that 1 mol of a perfect
gas occupies 22.4 litres at one atmosphere
pressure and 0°C. We can use this informa-
tion to calculate Rfrom Equation 10.2:
R= (1 atm 22.4 l)/(1 mol 273°K) =
0.0821 atm l mol^1 .°K^1Equation 10.2, or simplified forms of it,
will be needed to convert measured gas
volumes to a standard temperature and
pressure, to derive volumes from pressure
measurements, and to relate gas volumes to
substrate disappearance. The gas law also
reminds us that the volume of a fixed
molar amount of a gas is defined only
when both temperature and pressure are
defined.
If we now apply Equations 10.1 and
10.2 to the fermentation of 1 g of glucose
equivalents (formula weight = 180 18 g
mol^1 ) at 39°C and one atmosphere pres-
sure, the predicted gas volume is:
(1/162) mol 22.4 l/mol
3.4 mol/mol (312/273) °K/°K
= 537 ml
In practice, the gas yield from fibre
substrates such as cellulose and neutral
detergent fibre is always less than this
figure and is approximately 350–400 ml
g^1 glucose equivalent (Pell and Schofield,
1993). The reasons for this discrepancy
will be discussed later.
The above calculation provides a
useful sense of the size of gas volumes
produced in an in vitro system that
measures the gas output from fibre
fermentation. The design of the equipment
is dictated to some degree by the volume
considerations noted above. Several
different approaches have been taken to
measure the gas produced during in vitro
fermentation. The paper by Blümmel et al.
(1997b) contains references to the early
history of the gas method. The following
survey is restricted to techniques currently
in use.Techniques – an Overview
If an in vitrofermentation is carried out in
a syringe, the volume of gas produced at
the prevailing atmospheric pressure is
automatically made evident by plunger
displacement. The syringe technique has
been used extensively by Menke and col-
laborators at the Institute for Animal
Nutrition at the University of Hohenheim
in Germany. The original method was
described in 1974 and revised and
reviewed in 1988 (Menke and Steingass,
1988). Some important features of the
method are:
1.The standard incubation contains 10 ml
of ruminal fluid, 20 ml of buffer and210 P. Schofield