FOOD PROCESSING AND PRESERVATION: PACKAGING AND PRESERVATION
Canning in seven steps
Select and wash
Food prepared, blanched if needed
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Fill the cans
Hot food packed into cans
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Exhaust air
Air driven out before sealing
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Seal
Hermetic seal keeps microbes out
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Process
Retort at 115 °C to 121 °C, or boiling water for acid foods
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Cool fast
Slow cooling lets heat-loving spores grow
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Store and check
A swollen can is never tasted, never opened, never salvaged
Figure 1: Canning follows a fixed order, and the cold spot at the centre of the can decides the process time. Bottling follows the same idea in glass jars. The seven boxes are steps in order, so the arrows show a real sequence.
Introduction
Canning and bottling are close cousins: both fill food into a container, heat it to destroy enzymes and microbes, and seal it so nothing can get back in. The difference is the container and the heat it can take. Canning uses metal cans, which stand pressure heating in a retort at 115 °C to 121 °C, so it can sterilize low-acid foods such as beans and meat. Bottling uses glass jars or bottles, which suit acid foods such as fruit in syrup, and are usually pasteurized in boiling water, because glass cannot take the same pressure heating as a metal can. Bottling and canning are essentially similar processes in that food is filled into a container and heated to destroy enzymes and micro-organisms.
Quick comparison
Table 1: Canning compared with bottling
| Aspect | Canning | Bottling |
|---|---|---|
| Container | Metal can, sealed with a double seam | Glass jar or bottle with a lid |
| Heat | Retort at 115 °C to 121 °C under pressure for low-acid foods | Boiling water, 90 °C to 100 °C for 10 to 20 minutes, for acid foods |
| Foods suited | Low-acid foods: beans, meat, fish, most vegetables | Acid foods: fruit in syrup, pickles, tomato products, jam |
| pH rule | Above pH 4.6 needs the pressure process | Below pH 4.6, boiling water is enough |
| Seal | Hermetic metal seal; steam replaces air and cooling forms a partial vacuum | Lid sealed while hot; an internal vacuum forms when the jar cools |
| See-through | No: the food cannot be seen | Yes: glass shows the food and the fill level |
| Breakage | Dents, but does not shatter | Breakable glass; needs care in handling and transport |
| Shelf life | Months to years at room temperature | Months at room temperature for acid foods |
| Cost | Cans are cheap per unit, but retorts and boilers are costly | Glass costs more and weighs more, but the heating is simpler |
Scientific explanation
Definition
Canning is heating food in its sealed metal container in a retort, a pressure vessel that reaches about 115 °C to 121 °C with steam, so the food is sterilized and keeps at room temperature. Bottling is filling food into glass jars or bottles, sealing them, and heating them, usually by pasteurization in boiling water, so the food keeps in its sealed container.
Working principle
Both methods work the same way: heat destroys the microbes and enzymes, and the sealed container then preserves the food by preventing re-contamination and excluding air and sometimes light. Preservation depends on an adequate heat treatment and an air-tight, or hermetic, seal. Sterilization kills all microbes, including spores, so the food keeps at room temperature in a sealed container. For low-acid foods, meaning foods with pH above 4.6, such as beans, meat, and most vegetables, sterilization needs temperatures of 115 °C to 121 °C, which can only be reached under pressure in a vessel called a retort. Boiling water at 100 °C cannot do the job, because Clostridium botulinum spores survive it. Acid foods are different: below pH 4.6, botulinum spores cannot grow or make toxin, so fruit, pickles, and tomato products can be processed in boiling water. Fruits can be packed into jars with a hot, sugar syrup and vegetables can be packed into a hot brine. The filled jars are sealed and pasteurized so that an internal vacuum forms when they are cool.
Apparatus
Canning uses a retort, a strong pressure vessel heated by steam, with a boiler to make the steam and compressed air to hold pressure while cans cool. When foods are heated in sealed cans, the temperature of sterilization is above 100 °C and the pressure outside the can must equal that inside, to prevent the cans from exploding. Bottling uses glass jars or bottles, lids, and a boiling-water bath or a steam pasteurizer. The food is filled hot, air is driven out in a step called exhausting, the jar is sealed, and it is heated and cooled.
Measurement procedure
Heat reaches the centre of a container last, so the cold spot, usually the centre of the can, decides the process time: the process is timed on the slowest-heating point, not on the average, and a container with a cold centre is under-processed even if its outside spent long enough hot. The lethal effect is summed as an F value, and for low-acid canned foods the safety target is F₀ about 3 minutes at 121.1 °C, the 12D target for botulinum spores. Acidic fruits need relatively mild heating for pasteurization, for example 90 °C to 100 °C for 10 to 20 minutes, to destroy yeasts and moulds, whereas less acidic vegetables need more severe heat sterilization, for example 121 °C for 15 to 40 minutes, depending on the size of the container. Cooling fast matters: slow cooling lets heat-loving spores grow and cause flat sour spoilage, where the can looks normal but the food tastes sour.
Factors affecting readings
Container size changes the time: a large can heats slower at its centre than a small jar, so the process time grows with size. Food thickness, acidity, and the target spore all change the time needed. The pH line at 4.6 is the single most important number in canning: it decides whether a boiling-water process is safe or a pressure process is needed. Getting the pH wrong is how home canning goes dangerous. A poor seal lets microbes in after processing, so the can swells from gas made by the invaders. A swollen can is never tasted, never opened, and never salvaged.
Formulas and calculations
The F value is the total lethal effect of a heat process, stated as equivalent minutes at a reference temperature, usually 121.1 °C. 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.
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.
Industrial applications
- Canning: canneries for beans, meat, fish, and vegetables; canned soups and ready meals; low-acid foods that must keep for months without a fridge.
- Bottling: fruit in syrup in glass jars, pickles and tomato products, jams and sauces, and juices bottled and pasteurized for a chilled or shelf life.
Advantages and limitations
Canning, advantages: sterilizes low-acid foods, so they keep for months to years at room temperature; metal cans are light, cheap, and do not break; the seal is strong and hermetic. Limitations: needs a costly retort and boiler; long heating damages flavour, colour, and vitamins; the food cannot be seen, so a failed can is judged only by swelling or smell; canning is not suited to small-scale processing, because the time and temperature must be controlled with care.
Bottling, advantages: glass shows the food, so fill level and quality can be seen; suits acid foods with simple boiling-water heating; glass does not react with acid foods. Limitations: glass is breakable and heavy; it cannot take pressure heating like a metal can, so low-acid foods cannot be sterilized in glass by the same route; lids and seals must hold a vacuum, or the food spoils.
FAQ
What is the main difference between canning and bottling? Canning uses metal cans that stand pressure heating in a retort at 115 °C to 121 °C, so it can sterilize low-acid foods, while bottling uses glass jars that are usually pasteurized in boiling water and suit acid foods.
Why can fruit be bottled in boiling water but beans cannot be canned that way? Because fruit is acid, below pH 4.6, so botulinum spores cannot grow in it, while beans are low-acid, above pH 4.6, so only pressure heating at 115 °C to 121 °C destroys the spores.
What is exhausting? Driving air out of a container before sealing, by heat or vacuum, so the container does not swell and the metal does not corrode, and a vacuum forms on cooling.
What does a swollen can mean? That the process or the seal failed and microbes got in and made gas. A swollen can is never tasted, never opened, and never salvaged.
What is flat sour spoilage? Spoilage in which the can looks normal but the food tastes sour, caused by heat-loving acid-forming bacteria after an under-process or slow cooling.
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) and Unit 7 (Packaging Principles, Process Design). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice, including its chapter on filling and sealing of containers; Stumbo, C. R., Thermobacteriology in Food Processing; Frazier, W. C., and Westhoff, D. C., Food Microbiology. The bottling and canning facts on acid fruits at 90 °C to 100 °C for 10 to 20 minutes and vegetables at 121 °C for 15 to 40 minutes follow Fellows, as adapted in the food processing guidance for small-scale processors.
