10 min readPathway sheetVery often a 10-mark answer; spoilage of canned foods is a 5-mark note

Canning

Canning seals food in airtight containers and heats it to destroy spoilage and disease organisms. Low-acid foods need 121 °C in a retort; acid fruits only 100 °C.

Father of canningNicolas Appert (France); hence 'appertisation'

Acid linepH 4.5 (US rules use 4.6): below it, C. botulinum cannot grow

12D processCuts C. botulinum spores by 12 log cycles; about 2.5-3 min at 121 °C

Key stepsPrepare, blanch, fill, syrup or brine, exhaust, seal, process, cool

Steps in canning fruits and vegetables

1
Select and grade
fresh, firm, ripe-but-not-soft produce of even size and colour
2
Wash
removes dirt, spray residues and many surface microbes
3
Peel, core and cut
by hand, steam or hot dilute lye (caustic soda), then rinse well
4
Blanch (vegetables and some fruits)
inactivates enzymes, removes air, softens for filling
5
Fill the cans
lacquered cans for coloured or sulphur-rich foods; leave a small headspace
6
Add hot syrup (fruits) or dilute brine (vegetables)
fills spaces, adds taste, helps heat transfer
7★
Exhaust
heat or vacuum removes air: prevents bulging, corrosion, oxidation and vitamin C loss
8
Seal
airtight double seam made by a can-seaming machine
9★
Process (heat)
acid fruits: boiling water, 100 °C; low-acid vegetables: retort, about 115-121 °C
10
Cool quickly
in cold water to about 35-40 °C, so cooking stops and heat-loving bacteria cannot grow
11
Dry, label and store
cool, dry store; reject swollen or leaking cans

★ one-way, controls the pace

Acidity and the 12D concept

Canned foods are divided at pH 4.5 (US rules use 4.6). Clostridium botulinum, whose toxin causes the deadly botulism, cannot grow below this pH. Acid foods (most fruits, tomatoes, pickles) need only boiling water (100 °C). Low-acid foods (peas, beans, corn, meat, fish) must be processed in a retort at about 121 °C. Heat kills microbes in a logarithmic way: the D value is the minutes at a set temperature that kill 90% (one log cycle) of them, and the z value is the temperature change that alters D ten-fold (about 10 °C for C. botulinum). A 12D process gives enough heat to cut C. botulinum spores by 12 log cycles, so that only about one spore in 10¹² could survive.

Worked example: the 12D botulinum cook

Problem

The D value of Clostridium botulinum spores at 121 °C is 0.21 min (the standard value quoted by Fellows). Find the minimum process time at 121 °C for a 12D cook. If a can starts with 100 spores, how many survive?

Working

12D means 12 × D.
Process time = 12 × 0.21 min = 2.52 min (about 2.5 min).
This is the classic minimum botulinum cook (F0 about 2.5-3 min). Real processes are longer because heat takes time to reach the centre of the can.
Each D cuts the number ten-fold, so 12D divides it by 10¹².
Spores left = 100 ÷ 10¹² = 10² ÷ 10¹² = 10⁻¹⁰ per can.

Answer

Minimum 12D time = 2.52 min at 121 °C. With 100 spores per can, the chance of one survivor is about 1 in 10¹⁰ (ten thousand million) cans.

Worked example: syrup strength for canned fruit

Problem

A can holds 300 g of pineapple pieces at 12 °Brix and 200 g of syrup. After the sugar evens out, the contents should be 25 °Brix. What syrup strength should be used? Use the mass-balance formula given in Girdharilal, Siddappa & Tandon: Y = (W2 × Z − W1 × X) ÷ W3.

Working

W1 = fruit = 300 g; X = fruit °Brix = 12; W3 = syrup = 200 g; W2 = total = 300 + 200 = 500 g; Z = desired final °Brix = 25.
Total sugar needed in the can = 500 × 25 ÷ 100 = 125 g.
Sugar already in the fruit = 300 × 12 ÷ 100 = 36 g.
Sugar the syrup must supply = 125 − 36 = 89 g, in 200 g of syrup.
Y = 89 ÷ 200 × 100 = 44.5 °Brix (same as (500 × 25 − 300 × 12) ÷ 200 = 8900 ÷ 200).
To make 1 kg of this syrup: 445 g sugar + 555 g water.

Answer

Use syrup of 44.5 °Brix: 445 g sugar dissolved in 555 g water for every kilogram of syrup.

Spoilage of canned foods

Defect	What you see	Cause
Flipper	Can looks normal, but one end flips out when the can is struck sharply; it can be pressed back	Overfilling, poor exhausting, slight gas
Springer	One end stays bulged; pressing it in makes the other end spring out	More gas or overfilling
Soft swell	Both ends bulge but can be pressed in a little; they do not stay in	Gas from microbial growth
Hard swell	Both ends bulge hard and cannot be pressed in; the can may burst	Heavy gas from spoilage
Hydrogen swell	Bulging from hydrogen gas	Acid food reacting with the can metal
Flat sour	No bulge, but the contents are sour	Heat-loving spore-formers such as Geobacillus (Bacillus) stearothermophilus; B. coagulans in tomato
Sulphide spoilage ('sulphur stinker')	No bulge; blackened contents and rotten-egg smell	Desulfotomaculum nigrificans making hydrogen sulphide
Leaker	Contents ooze out; spoilage by many microbes	Faulty seam or rust holes; dirty cooling water drawn in

In food and nutrition

Canned peas, corn, fruit and fish keep their protein, minerals and most vitamin A, but lose some vitamin C and thiamine to heat and the liquid, so use the liquid in gravies. Never serve from a swollen, leaking or seam-dented can: low-acid foods carry a botulism risk.

Exam angle

2 marks: “What is exhausting in canning? Why is it done?” Removing air from the filled can by heat or vacuum before sealing, to prevent bulging, corrosion, oxidation and vitamin C loss and to form a vacuum after cooling.

5 marks: “Write a note on the spoilage of canned foods.” Swells (flipper, springer, soft, hard, hydrogen), flat sour, sulphide spoilage, leakers, with causes; botulism warning.

10 marks: “Describe the steps in canning of fruits and vegetables.” Flow chart; purpose of each step; acid vs low-acid processing (100 °C vs 121 °C) and the 12D concept; cooling; storage and spoilage checks.

Remember it

Canning order: Please Bring Five Sweets, Each Sealed, Properly Cooled = Prepare, Blanch, Fill, Syrup, Exhaust, Seal, Process, Cool.