7 min readPathway sheetOften a 5- or 10-mark answer; LTLT and HTST are 2-mark questions

Pasteurisation

Pasteurisation is mild heat below 100 °C that kills disease germs and most spoilage organisms. Milk: 63 °C for 30 minutes or 72 °C for 15 seconds, then chilling.

Named afterLouis Pasteur, who used it for wine in the 1860s

LTLT (holding)63 °C (145 °F) for 30 min; Srilakshmi gives 62.8 °C

HTST (flash)72 °C (161 °F) for 15 s; Srilakshmi gives 71.7 °C

Check testAlkaline phosphatase test: negative = properly pasteurised

What pasteurisation does

Pasteurisation is heating a food to a set temperature below 100 °C for a set time to destroy all disease-causing (pathogenic) organisms and most spoilage organisms, and to inactivate some enzymes, with little change in taste and nutrients. It does not sterilise: heat-resistant bacteria and spores survive, so pasteurised milk must be chilled (below about 5 °C) and used within days. For milk, the time and temperature are set to kill the most heat-resistant non-spore pathogen, Coxiella burnetii (Q fever); older books name Mycobacterium tuberculosis. The alkaline phosphatase enzyme in milk is destroyed by just this heat, so a negative phosphatase test proves correct pasteurisation.

Methods of pasteurising milk

Method	Temperature and time	How it is done	Notes
LTLT: low-temperature long-time (holding, batch)	63 °C (145 °F) for 30 min	Milk heated in a jacketed vat, held with stirring, then cooled	Small dairies; more cooked flavour
HTST: high-temperature short-time (flash)	72 °C (161 °F) for 15 s	Continuous flow through a plate heat exchanger with a holding tube and flow-diversion valve	Most city dairies; heat recovery saves energy
UHT: ultra-high temperature	135-150 °C for a few seconds	Continuous flow, then aseptic packing	Actually sterilises; keeps unopened at room temperature (next topic)

HTST pasteurisation of milk

1
Receive, filter or clarify and chill raw milk
removes dirt; about 4 °C
2
Regeneration: cold incoming milk is warmed by hot outgoing milk
saves energy
3★
Heat to 72 °C in the heating section of the plate heat exchanger
hot water on the other side of the plates
4★
Hold for 15 seconds in the holding tube
flow-diversion valve returns under-heated milk
5
Cool through regeneration and chilling sections
to about 4 °C
6
Pack in sachets or bottles and keep cold
phosphatase test checks the process

★ one-way, controls the pace

Pasteurisation vs sterilisation

PasteurisationBelow 100 °C; kills pathogens and most spoilage organisms; spores survive; needs refrigeration; keeps for days; little change in flavour and nutrients. Milk, juices, beer.

SterilisationUsually 121 °C under pressure (or UHT at 135-150 °C); kills all microbes and spores; sealed food keeps at room temperature for months; more cooked flavour and nutrient loss (less with UHT). Canned foods, sterilised milk.

Other pasteurised foods: fruit juices, ready-to-serve drinks and squashes (their acid makes mild heat enough), liquid egg, ice-cream mix, honey and beer.

In food and nutrition

Pasteurised milk keeps nearly all its protein, calcium and riboflavin; losses of vitamin C and thiamine are small. In hospital and school feeding, use only pasteurised milk, kept below about 5 °C and used within its date.

Exam angle

2 marks: “What is HTST pasteurisation?” High-temperature short-time method: milk is heated to 72 °C for 15 seconds in a plate heat exchanger, then cooled quickly to about 4 °C.

5 marks: “Differentiate pasteurisation and sterilisation.” Compare temperature, organisms killed, spores, storage, shelf life, effect on quality and examples, in a table.

10 marks: “What is pasteurisation? Explain the methods of pasteurising milk.” Definition and aim (pathogens, index organism); LTLT; HTST with a flow diagram; UHT; phosphatase test; other foods; merits and limits.

Remember it

63 for 30, 72 for 15: holding is slow and low, flash is fast and high.