Chemical reactions - Chemistry - page 113

● In the production of yoghurt, milk is initially
heated to 90 °C for 15-30 minutes to kill any
bacteria in the milk. After cooling to 40 °C, a
starter culture of Lactobacillus bacteria is added
and the mixture incubated at 40 °C for eight hours
(Figure 7.20). The bacteria ferment the lactose
in the milk to lactic acid, which causes the milk
protein to become solid.
● In cheese making, milk is initially heated to
kill bacteria and then cooled. A starter culture
of Streptococcus bacteria is then added, which
coagulates the milk into curds and whey
(Figure 7.21). The curds are put into steel or
wooden drums and pressed and allowed to dry.
Figure 7.21 Cheese is separated into curds and whey by the addition
of bacteria. The liquid, whey, is separated from the curds which are then
pressed.
In industry, enzymes are used to bring about
reactions at normal temperatures and pressures that
would otherwise require expensive conditions and
equipment. Successful processes using enzymes need
to ensure that:
● the enzyme is able to function for long periods of
time by optimising the environment
● the enzyme is kept in place by trapping it on
the surface of an inert solid (some processes
immobilise the enzymes when the process is
complete)
● continuous processes occur rather than batch
processes.
Questions
1 When using biological washing powders what factors
have to be taken into consideration?
2 Enzymes in yeast are used in the fermentation of glucose.
Why, when the temperature is raised to 45 °C, is very
little ethanol actually produced compared with the
amount formed at room temperature?
Figure 7.20 Yoghurt is incubated in these tanks, and allowed to mature.