CHEMICAL AND BACTERIOLOGICAL CHANGES AT HIGH HEAT TREATMENT
When
milk is kept at a high temperature for a long time, certain chemical reaction products
are formed, which results in discoloration (browning). It also acquires a
cooked and caramel flavour, and there is occasionally a great deal of sediment.
These defects are largely avoided by heat treatment at a higher tempera for a
shorter time. It is important that the time - temperature combination is chosen
to that the spore destruction is satisfactory and at the same time the heat
damage to the milk is kept at the lowest possible level.
Figure
9.5
shows the relationship between sterilisation effect and browning reaction. The A
line represents the lower limit of time – temperature combinations which cause
the milk to turn brown. Line B is the lower limit of combinations for complete
sterilisation (destruction of thermophilic spores). The regions for
in-container sterilisation and UHT treatment are also marked in the figure.
FROM THE GRAPH
UHT: 137C/5S
- Line B (Lower temp Limit for Sterilization):
Above it i.e. within the UHT killing region killing for both Thermophilic and
Mesophilic spores.
- Line A (Lower temp Limit for Discolouration):
Below it i.e. outside of the region of chemical discolouration.
Retort @ 115C/12min(720S)
- Line B (Lower temp Limit for Sterilization): On
it i.e. just within the range of killing spores (again!).
- Line A (Lower temp Limit for Discolouration):
On it i.e. just within the range of chemical discolouration.
Considering that temperatures have a cumulative
chemical effect on discolouration it is clear that Retort @ 115C/720S + UHT @ 137C/5S will render discolouration
Retort @ 95C/15min(900sec)
- Line B (Lower temp Limit for Sterilization):
Below the spore killing zone so this temperature can be dropped since the
minimum micro kill temp required is 72C.
- Line A (Lower temp Limit for Discolouration):
Below the chemical discolouration range.