FOOD PROCESSING AND PRESERVATION: THERMAL PROCESSING
Pasteurization and sterilization: two levels of heat
Pasteurization
Mild heat, below 100 °C, kills growing cells but not spores
versus
Sterilization
Severe heat, 115 °C to 121 °C under pressure, kills all microbes including spores
Figure 1: Pasteurization and sterilization are two levels of the same weapon, heat. The two boxes are alternatives, not steps in order.
Introduction
Pasteurization and sterilization both use heat to make food safe, but they aim at different targets. Pasteurization is mild heat that kills disease microbes and most spoilage microbes, but not bacterial spores, so the food still needs cold storage. Sterilization is severe heat that kills all microbes, including spores, so the food keeps at room temperature in a sealed container. The difference is the strength of the heat, and the shelf life it buys.
Quick comparison
Table 1: Pasteurization compared with sterilization
| Aspect | Pasteurization | Sterilization |
|---|---|---|
| Purpose | Kill disease microbes and most spoilage microbes | Kill all microbes, including bacterial spores |
| Temperature | Milk: 63 °C for 30 minutes (LTLT) or 72 °C for 15 seconds (HTST); always below 100 °C | 115 °C to 121 °C, reached only under pressure in a retort |
| Spores | Spores survive | Spores are destroyed |
| Storage after | Cold storage needed, at about 4 °C | Room temperature in a sealed container |
| Shelf life | Days to weeks in a fridge | Months to years on a shelf |
| Food quality | Flavour, colour and vitamins mostly kept | More flavour, colour and vitamin loss from long heating |
| Typical foods | Milk, fruit juice, beer, wine, liquid egg, some ready meals | Canned beans, meat, fish, most vegetables, UHT milk |
| Equipment | Plate heat exchanger with holding tube, or batch tank | Retort (pressure vessel), or aseptic line with sterile filling |
Scientific explanation
Definition
Pasteurization is a mild heat treatment that kills the harmful microbes that can grow in a food and cuts the spoilage microbes enough to give a useful chilled life, without cooking the food into a different product. Sterilization aims higher: it kills all microbes, including spores, so the food keeps at room temperature for months. Canned food is heated in its sealed container in a retort, a pressure vessel that reaches about 115 °C to 121 °C with steam.
Working principle
Heat kills by breaking the things a cell is made of. Proteins unfold and lose their shape, which switches off enzymes and breaks the cell’s working parts. The cell membrane leaks, and the cell’s DNA is damaged. A growing, or vegetative, cell dies at moderate heat, which is why simmering milk for a short time makes it safer. A bacterial spore is a tough, dry, resting package with thick coats, and it survives boiling water, drying, and many chemicals. Killing spores needs much higher temperature under pressure. The two standard milk treatments show the trade-off: low temperature long time, or LTLT, holds milk at 63 °C for 30 minutes, while high temperature short time, or HTST, holds it at 72 °C for 15 seconds. Both give the same killing of the target microbes, because hotter for shorter can equal cooler for longer, and HTST is preferred in practice because it damages flavour and vitamins less.
Apparatus
Pasteurization of milk uses a plate heat exchanger: thin stainless steel plates stacked with narrow gaps, hot milk on one side of each plate, cold milk on the other, flowing in opposite directions, followed by a holding tube that keeps every drop at temperature for the set time, then cooling plates that chill the milk to about 4 °C. A flow diversion valve sends milk back if the temperature falls below the set point, even for a second. Sterilization of cans uses a retort, a pressure vessel that heats sealed cans or pouches with steam under pressure above 100 °C. Aseptic processing takes a different route: the food is sterilized in a fast continuous flow, cooled, and filled into sterile containers in a sterile room. Ultra high temperature, or UHT, milk is heated to about 135 °C to 150 °C for a few seconds, then packed aseptically.
Measurement procedure
A heat process is proved with temperature, not with hope. Thermocouples and recorders track the temperature at the cold spot, usually the centre of the can, during a test run, and the holding time is checked by timing the flow through the holding tube. The lethal effect is summed as an F value: the time at a reference temperature that gives the same kill as the real, varying temperature history. For low-acid canned foods, the safety target is the 12D concept: twelve log cycles of killing of Clostridium botulinum spores, which works out to an F value of about 3 minutes at 121.1 °C. Pasteurization is checked with the alkaline phosphatase test in milk: the enzyme has a similar heat resistance to the target microbes, so its absence shows the treatment worked.
Factors affecting readings
The pH of the food decides the process. Below pH 4.6, botulinum spores cannot grow or make toxin, so fruit, pickles, and tomato products can be processed in boiling water. Above pH 4.6, low-acid foods such as beans, meat, and most vegetables need the full pressure process at 115 °C to 121 °C. Can size, food thickness, and acidity change the time needed, because heat must reach the cold spot of every can. The D value of the target microbe also changes with acidity, sugar, fat, and the strain, so real D values come from lab tests on the real food and the real organism.
Formulas and calculations
The D value is the time at one temperature needed to kill 90 percent of a population of one microbe, which is one log cycle. The z value is the rise in temperature needed to cut the D value to one tenth. The F value is the total lethal effect of a whole process, stated as equivalent minutes at a reference temperature, usually 121.1 °C.
Worked example 1: the 12D target. The D value of Clostridium botulinum spores at 121.1 °C is 0.1 to 0.3 minutes, with 0.21 minutes as the usual working value. Twelve log cycles need 12 x 0.21 = 2.52 minutes, and the standard F₀ of 3 minutes adds a safety margin on top.
Worked example 2: cooler means much longer. Take a process at 115 °C with z = 10 °C. Its lethal rate is 10^((115 – 121.1) / 10) = 10^(-0.61) = 0.25 per minute, so each minute at 115 °C is worth only a quarter of a minute at 121.1 °C. To reach F₀ 3, the hold must be 3 / 0.25 = 12 minutes. Drop 10 °C more, to 105 °C, and the rate falls to 0.025, so the same F₀ needs 120 minutes.
Worked example 3: a 5 log reduction in juice. Suppose lab tests give a target organism with a D value at 72 °C of 3 seconds. A 5 log reduction, which cuts the count by a factor of 100,000, needs 5 times the D value, which is 15 seconds at 72 °C. That is the same 15 seconds as HTST milk pasteurization.
Industrial applications
- Pasteurization: milk plants (HTST lines and batch tanks), fruit juice plants, breweries, wineries, liquid egg plants, and ready-meal lines that need a chilled shelf life.
- Sterilization: canneries for beans, meat, fish, and vegetables; UHT milk and long-life juice in cartons; aseptic soups and sauces; low-acid foods that must keep without a fridge.
Advantages and limitations
Pasteurization, advantages: keeps fresh flavour, colour, and vitamins; cheap and fast; suits liquids that flow through a heat exchanger. Limitations: spores survive, so cold storage is needed; shelf life is short; a broken cold chain ruins the food.
Sterilization, advantages: long shelf life at room temperature; no cold chain needed; safe for low-acid foods. Limitations: long heating damages flavour, colour, and vitamins; needs costly pressure equipment; a failed seal or an under-processed can is dangerous.
FAQ
Why does pasteurized milk still need a fridge? Because pasteurization kills growing cells but not spores, so the surviving spores and microbes would grow again if the milk were kept warm.
Can boiling water sterilize a can of beans? No. Boiling water at 100 °C cannot kill Clostridium botulinum spores, so low-acid foods need 115 °C to 121 °C under pressure.
What is the pH 4.6 line? Below pH 4.6, botulinum spores cannot grow or make toxin, so acid foods can be processed in boiling water, while foods above pH 4.6 need a pressure process.
Why is HTST preferred over LTLT for milk? Both give the same killing, but HTST holds the milk hot for 15 seconds instead of 30 minutes, so it damages flavour and vitamins less.
What does F₀ 3 mean? It means the process has delivered the same killing as 3 minutes held at 121.1 °C, which is the 12D safety target for botulinum spores in low-acid canned foods.
References and further reading
FTN subjects: Principles of Food Preservation, Unit 2 (Preservation by Heat); Principles of Food Processing, Unit 2 (Heat Transfer and Thermal Processing). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice; Stumbo, C. R., Thermobacteriology in Food Processing; Frazier, W. C., and Westhoff, D. C., Food Microbiology; Rahman, M. S., Handbook of Food Preservation. Google Books page read: Spray-freeze-drying of Foods and Bioproducts by S. Padma Ishwarya (books.google.com) for drying context used across this series.
