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SOP: Performance Test for Tinplate Containers by Test Packs

GIFSQ — Global Institute for Food Safety & Quality

Principle

Lab tests on plate and lacquer are only screening. The real verdict on acid and sulphur resistance comes from packed food: the candidate plate is canned up alongside a plate of known good performance and the two are compared over storage. A plate passes if it performs no worse than a standard plate in commercial use.

Materials for test packs

Sulphur staining tests:

  • Green peas (pH 5.6), cauliflower and meat loaf in plain cans
  • Meat-in-gravy in sulphur-resistant (SR) lacquered cans

Mildly corrosive foods:

  • Grapefruit juice (pH 3.2)
  • Plums in syrup and plum jam (pH 3.2)
  • Clingstone peaches (pH 4.0)
  • Tomato pulp (pH 4.6)
  • Green beans (pH 5.1)

Acid-resistant (AR) lacquered cans:

  • Acidified beetroot (pH 4.3)
  • Boysenberries (pH 3.3)

Procedure

Follow the recommended procedure for canning, with the following controls and precautions:

  • Use uniform raw materials from a single source
  • Mix bulk juices, brines and syrups thoroughly
  • Assign cans to treatments at random and fill, close and process in that order — this spreads uncontrollable variables across all treatments
  • Carefully control the fill-in weights, strength of covering liquid, exhaust temperature and cooling; avoid excessive variations of the parameters in cans packed with a particular product
  • At each testing, examine 4 to 8 cans; examine at frequent intervals

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Store the test packs at room temperature and at 37°C — a temperature-controlled room at 30°C is advisable, since room temperature varies considerably with season

↓

It is customary to store at elevated temperature and correlate the storage period at elevated temperature to room temperature:

  • Davis: shelf-life at 21°C and 30°C is roughly 4 times and 2 times that at 37°C
  • Sampson: the ratio between shelf-life at 37.7°C (100°F) and shelf-life at 21.1°C (70°F) is 1:3.5 for grapefruit juice, green beans and pork and beans; 1:3.5 for tomatoes (the print reads “1735” and “1:35” — carried as 1:3.5, the unambiguous intended reading; flagged. Two further product rows are garbled beyond use and are omitted)
  • Liebmann: since 5-hydroxymethylfurfural formed in fruit products at elevated temperature as a result of non-enzymatic browning accelerates corrosion, observations made on fruit packs stored at high temperatures may not permit a reliable prediction of storage life at lower temperatures

Assessment of performance

  1. Count swells as storage progresses.
  2. Loss of can-end concavity, measured with a spherometer.
  3. Vacuum loss, measured regularly without opening the cans using a flip vacuum tester — vacuum and concavity loss, usually from hydrogen generation, indexes container performance.
  4. Head-space gas analysis tracks the corrosion reaction, but can-to-can variation is large, especially in lacquered cans.
  5. Take the time until a set share of cans in a batch turn into hydrogen swells under controlled storage as the performance measure — the Continental Can Company uses 36 cans per treatment and records the time to 25% swells. The time to first failure or to 25% failures is the performance index. The drawback: cans must be stored until they fail, which can take long, especially in plain-can packs.
  6. Tin and iron pick-up takes a lot of analytical work and is not usually done at every examination. In plain cans, corrosion of iron liberates hydrogen, while dissolving tin evolves no significant gas — so in plain cans not yet stored long enough for iron attack to start, vacuum loss alone can mislead. Following tin pick-up gives more reliable results: estimate tin content in 4 to 6 cans at intervals through storage. Iron pick-up matters more in lacquered cans; can-to-can variation in iron is usually large, so analyse more cans at each examination to get a reliable figure.

References

  1. Davis, E. G., Food Pres. Quart., 17, 64 (1957); 18, 66 (1958).
  2. Davis, E. G. & A. G. L. Elliott, Food Technol. (Chicago), 12, 473 (1958).
  3. Sampson, D. F., Proc. Tech. Sess., Conv. Nat. Cann. Ass., 46, 16 (1953).
  4. Liebmann, H., Tinplate Corrosion of Tinplate Containers, Third Sci. Congress on Canned Foods, Rome–Parma, 1956.
  5. Kefford, J. F. & E. G. Davis, Food Pres. Quart., 14, 8 & 46 (1954).