Chemistry Reference
In-Depth Information
oestrus, lactation stage, the presence of other diseases, milking interval, time
of day and milk composition (Norberg et al., 2004; Norberg, 2005). In 1998,
an extensive analysis of published data carried out by the International Dairy
Federation (Haman and Zecconi, 1998) concluded that EC measurement
could not identify mastitic quarters or cows, or detect subclinical mastitis,
with sufficient accuracy to be useful. Currently, most automatic milking
systems and some manual systems incorporate sensors for measuring the
EC of quarter milk and software for processing the data generated (Norberg,
2005). The development of sophisticated statistical modelling of the data,
which involves comparisons between the quarters of the individual cow over
successive milkings, has led to an improvement in the sensitivity of detection
of subclinical and clinical mastitis. In the case of automatic milking systems
the farmer is dependent on EC records for detecting mastitic cows (Norberg,
2005).
Several studies have indicated that there is a correlation between the EC
of milk and the genetic susceptibility of cows to mastitis. Much further
investigation will be required to determine whether or not this correlation
will be useful in breeding programmes that include selection for resistance to
mastitis (Norberg, 2005).
The EC of fresh milk and cream decreases with increasing fat content
because the fat globules (themselves non-conducting) occupy volume that
would otherwise be filled with the conducting aqueous phase of the pro-
duct, impeding the mobility of the conducting ions and increasing the
distance that migrating ions have to travel (Prentice, 1962; Fern ´ndez-
Martin and Sanz, 1985; Mabrook and Petty, 2003). Prentice (1962) deter-
mined the following empirical equation relating EC to the volume fraction
of fat in milk, for fat contents of 2-6.1% (w/w) at room temperature
(unspecified):
¼
s
ð
1
fat
Þ
1
:
5
(69)
where
s
¼
electrical conductivity of skim milk prepared from the same batch
of milk;
fat
¼
volume fraction of fat in the milk.
A more recent study of the EC of milks, creams and model dairy
emulsions (Robin et al., 1994) found that 'the presence of ... fat globules
reduced the conductivity of the milks and creams approximately by a power
of 3/2 [
¼
1.5] of the dispersed fat fraction present', thus confirming the result
of Prentice (1962).
