7 min readConcept sheetOften a 5-mark answer; UHT and commercial sterility are 2-mark questions

Sterilisation and UHT processing

Sterilisation destroys all microbes and spores, usually at 121 °C under pressure. UHT does it in seconds at 135-150 °C and then packs the food aseptically.

Standard condition121 °C (250 °F) for about 15 min, steam at about 15 psi (about 1 kg/cm²)

UHTAt least 135 °C, usually 135-150 °C, for a few seconds

Commercial sterilityNo microbes able to grow in the sealed food in normal storage

Packing after UHTAseptic packing into sterile multilayer cartons

Sterilisation and commercial sterility

Sterilisation means destroying all living microorganisms, including spores. Steam under pressure is used because water boils at only 100 °C at normal pressure; at about 15 psi (about 1 kg/cm²) above atmospheric pressure, steam reaches 121 °C. Total sterility is hard to prove, so food processors aim for commercial sterility: the sealed food has no microbes or spores that can grow in it during normal storage without refrigeration, and no pathogens. Canned foods and UHT milk are commercially sterile.

In-container sterilisation vs UHT

In-container sterilisationFood is sealed first, then heated in a retort (canning, bottled sterilised milk). Heat must travel slowly to the centre of the pack, so there is more cooked flavour, browning and nutrient loss.

UHT (ultra-high temperature)Food is heated first in continuous flow (135-150 °C for a few seconds), cooled, then filled into sterile packs under sterile conditions (aseptic packaging). Much less cooked flavour and vitamin loss; suits liquids such as milk, juices and soups.

As temperature rises, spores are killed much faster than flavour and vitamins are damaged; so very high heat for a very short time sterilises with better quality.

Ways of UHT heating

Method	How	Notes
Direct: steam injection	Clean food-grade steam is injected into the milk	Very fast; the added water is removed in a vacuum chamber
Direct: steam infusion	Milk is sprayed into a chamber full of steam	Gentle; less burning-on
Indirect	Plate, tubular or scraped-surface heat exchangers with steam or hot water on the other side	Simpler; more deposit on surfaces, a little more cooked flavour

Aseptic packing, advantages and limits of UHT

Pack: multilayer carton of paperboard, polyethylene and aluminium foil (Tetra Pak type), sterilised with hydrogen peroxide and filled in a sterile zone.
Advantages: keeps for months unopened without refrigeration; reaches places with no cold chain; less cooked flavour than canning; no preservative needed.
Limits: costly plant and packs; once opened it spoils like pasteurised milk, so chill and use within days; slight cooked taste; some loss of heat-sensitive vitamins; may thicken or settle on long storage.

In food and nutrition

UHT milk and juices are useful for disaster relief, travel, hostels and areas without refrigeration. Their protein and calcium are unchanged; the loss of heat-sensitive vitamins is smaller than in bottled sterilised milk. Once opened, chill them like fresh milk.

Exam angle

2 marks: “What is commercial sterility?” The state of a sealed processed food in which no microbes or spores able to grow during normal non-refrigerated storage, and no pathogens, survive.

5 marks: “Write a note on UHT processing.” Definition (135-150 °C, seconds); direct and indirect heating; aseptic packaging; advantages and limits; examples.

10 marks: “Explain sterilisation of foods. How does UHT processing differ from in-container sterilisation?” Define sterilisation and commercial sterility; 121 °C at 15 psi; compare in-container and UHT; aseptic packing; merits and limits.

Remember it

121 for 15 at 15: 121 °C, 15 minutes, 15 psi. UHT: 135-plus for seconds, then aseptic.