Pasteurization Htst 144o6z

Pasteurization • HTST • Components • Plate heat exchanger • Milk flow diagram • Regeneration of heat • Advantages and

disadvantages • UHT principles

HTST PASTEURIZATION  Modern method – Used when large volume of milk is handled  Milk heated to 72 o C for 15 sec and cooled to 4 o C  Developed by APV .CO in UK  Continuous method  Milk es through balance tank, pump, heating, holding and cooling.  Regeneration of heat is possible

Milk flow diagram- HTST

Milk flow diagram

Flow pattern in Plates

Components – balance tank • Float controlled balance tank (FCBT) • Maintains a constant head of milk for the milk pump • Receives the diverted milk from FDV (Flow diversion valve)

Components - Plate Heat exchangers (PHE) • • • •

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PHE is simple ,compact easily cleaned unit Plates used in heating, cooling, regeneration and holding sections Plates are ed in a press between terminal blocks in each section Every second channel is open to the same fluid These stainless steel plates have been machined and engineered so that milk will on one side of the plate while the heating or cooling medium flows in countercurrent direction on the other side Plates numbered and tightened into place Heat moves from the heating or cooling medium through SS plates. Space of @ 3 mm is maintained between the plates by non absorbent rubber gaskets

Plate Heat exchanger Very thin corrugated stainless steel heat transfer plates clamped together in a frame.  Every second channel is open to the same fluid.  Between each pair of plates there is a rubber gasket, which prevents the fluids from mixing and from leaking to the surroundings.  Heat is thus transferred from the warm fluid to the colder fluid via the thin stainless steel plate.  The corrugations the plates against differential pressure and create a turbulent flow in the channels.  In turn, the turbulent flow provides high heat transfer efficiency, making the plate heat exchanger very compact compared with the traditional shell-and-tube heat exchanger. In most cases the plate type heat exchanger is the most efficient heat exchanger.

Components - Plate Heat exchangers (PHE) Plates • Made of stainless steel • Raised sections corrugations in the form of knobs, diamonds, channels help provide the turbulent action and heat transfer • Pores are provided at the bottom and top to permit milk and medium to flow without mixing • By adding more number of plates the capacity can be increased

Heat transfer Plates

Regeneration of Heat • The cold incoming milk is partially and indirectly heated by the hot outgoing milk • Milk to milk regeneration • Improves the economy of the HTST process • Incoming milk requires less heating

Holding • Correct heat treatment requires that the milk is held at a specified time at the pasteurization temperature. • Holding tube ensures the milk is held for specified time - 15 sec • Pasteurization efficiency= time for the first milk to through theoretical time • Measures the most rapidly moving particle in the holder • A tube or Pipe or inbuilt section of plates used as holder

Flow Diversion Valve (FDV) • Three way valve • If milk is properly pasteurized it will forward the milk • Divert if not properly pasteurized, back to the balance tank • Operated by Air pressure working against a spring • When temperature falls air pressure is released and the valve snap shuts

Velocity Profile

Holding tube  Calculate dimensions of a holding tube for a holding time of 15 sec. with a capacity of 10,000 L/hour. The inner diameter of the tube is 48.5 mm. Efficiency factor is estimated to be 0.90.

UHT PROCESSING

UHT PROCESSING Principle • When milk is heated to high temperature, two groups of reactions take place. • Desirable effect of the destruction of pathogenic micro organisms in milk. • Undesirable effect of chemical changes which alter the colour and flavour of milk. • A most important observation is that the lethal action of heat on bacteria and bacterial spores increases much more rapidly with temperature than do the various chemical effects by destruction of vitamins, serum protein denaturation inactivation of enzymes etc.

UHT PROCESSING • In the range of 110 to 120o C, • the rate of spore destruction increases about 11 times for each 10 o C rise in the temperature. The rate of Maillard browning increases 3 times for every 10 o C rise in temperature. • Above 135 o C the ratio increases rapidly in relation to browning. • At 150 o C the bactericidal effect is about 3000 times greater than the browning effect. This improvement is the reason for the use of UHT process which involves heating to at least 130 o C