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From Rusty Returns to Flawless Seals: The Science Behind Food-Grade Capping

HOME  ꄲ  Knowledges  ꄲ  From Rusty Returns to Flawless Seals: The Science Behind Food-Grade Capping

Are you constantly facing customer complaints about rusted lids and leakage after filling? Or dealing with packaging deformation and product returns post high-temperature sterilization? As experts in food-grade glass packaging container solutions, we fully understand the dual challenges of quality preservation and packaging compatibility during high-temperature sterilization. Below we analyze common pain points and explain key considerations for mainstream sterilization methods and packaging selection to help you achieve safe, efficient production.

 

1. What Are Tinplate Lids Made Of 

Twist-Off closures are primarily made from doublesided tinplate. This metal closure with a compound inner lining, is designed to fit various glass finishes under various processing conditions. The closure, if properly used, is designed to ensure packaging integrity throughout the packaging’s lifecycle and to keep the product safe. In respect of the intended food product contact, sheets are lacquered on the reverse side with anti corrosion primer, as well as an adhesion-promoting Topcoat layer (organosol BPA NI). Depending on the decorative aspects, the sheets are printed on the face side with one or more layers of printing ink and finished with a clear varnish.

 

These semi finished sheets, called “finish plate”, is cut in circular blanks. The cut edges are open and unpainted. In the forming process that follows, these edges are carefully curled, lugs are formed, and the closure receives its final shape. In the last manufacturing step, the sealant, which produces the gas-and liquid-proof packaging system, is injected into the closure and finally dried in a curing oven. The tinplate used is of varying thickness and hardness according to the closure type and its intended application. Closure types with the requirement to withstand high mechanical stress during heat treatment are generally produced with thicker material. Usually, the thickness of sheets used for Twist-Off closures is in the range between 0.16mm – 0.18mm.

2. Causes of Tinplate Lid Rust

Rusting of tinplate lids is the result of electrochemical corrosion of the metal substrate (iron) when exposed to an oxidising environment, the specific causes of which can be classified as follows: 

Cause Type Specific Manifestations Associated Scenarios
Physical Damage Mechanical friction or collision during transport, screwing of lids leads to breakage of the tin layer and exposure of the iron substrate.     All sterilization methods, especially pressurized steam sterilization (high risk of cap deformation)
Chemical Corrosion Direct corrosion of the tin layer or coating by acidic/alkaline components, damaging the protective layer (e.g. highly acidic sauces pH ≤ 4.5).     High temperature water bath sterilisation (acidic + high temperature), pasteurisation (prolonged exposure to active ingredients).
Electrochemical Corrosion Lid in contact with glass bottle mouth forms a potential difference and accelerates localised corrosion (e.g. when salt is present).     Sauces with high salt content (e.g. bean paste), storage in humid environments.
High temperature and high humidity oxidation  Not thoroughly dried after filling, water beads remain in the gap between lid and bottle mouth, long-term contact with the substrate iron triggering oxidation  Inadequate cooling after high temperature sterilisation, low efficiency of drying equipment 
Coating Damage The internal anti-corrosion coating of the lid (e.g., organosolvents, primer) is scratched in the process of transporting, filling, or unscrewing the cap, resulting in the exposure of the substrate iron  The machine unscrewing the cap is too strong resulting in the rolled-up edge being flipped out, and the coating is scratched by the burr on the mouth of the glass bottle

In order to avoid customers encounter rust problems in the process of use, Shining Glass will ask the food factory (or sauce factory, filling factory) the following information before the production of lids, and then we will suggest customers to match the different filler according to the different contents of the customer's filling:

· Product?

· Product contains free oil?

· Will product have a thermal treatment? Which one?

· Closure is manual or by capping machine?

· Has customer the chance to pre-heat lids or use steam flow?

· Is Flip safety button requested?

· Does product contain aggressive elements?

 

We can find the correct type by matching the above packaging conditions in the following chart.

Product Catalogue

Contains fat and /or oil

Thermal Treatment

Compliance with oil

Compound

Notes

Honey, jams, cfruit

conserves, dry food, products w/o oil

No

None

Not Necessary

C&M

Cold Closing

Jams to be

pasteurized

No

PASTEURIZATION

EL-EF-ENI

but not necessary

PAST

 

Marinated vegetables

or preserved in oil,

other products w oil

Yes

PASTEURIZATION

EL-EF-ENI

PAST

 

Meat, fish, legumes,

truffle, foodstuff with

high bacterial load -

preserved in oil

Yes

STERILIZATION

EL-EF-ENI

STER

 

Products w relevant

contents of oil and long

expiry date (shelf life)

Yes

PASTEURIZATION

OR STERILIZATION

PVC FREE

PAST OR STER

 

AGGRESSIVE

PRODUCTS

(W AND W/O OIL)

RISKY

COMPONENTS

(AGGRESSIVE)

THERMAL

TREATMENT

COMPLIANCE

WITH OIL

COMPOUND

NOTES

Onions, gherkins,

pickles of any kind

and vegetables in

vinegar, asparagus

SO2 > 50ppm

pH > 4,0

Salt > 10%

Chloride and/or NaCl

> 50ppm

PASTEURIZATION

OR STERILIZATION

(depending on each

product)

EL-EF-ENI

or PVCFREE

PAST O STER

Need specific

protection inside,

production on

demand. To be

verified before

ordering.
 
REGARDING THERMAL TREATMENT THAT COMPOUND CAN STAND:
C & M: Jams, Marmelade and Honey. This compound is for cold closing, after filling with cold products, or
hot and left to cool. Compound is not for products containing fats-oil.
P: Pasteurization (~100°C Max)
S: Sterilization (~120-126°C Max)
 
REGARDING PLASTICISERS COMPLIANCE WITH OIL:
CEL: Esbo Light
EF: Esbo Free
ENI: Esbo Not Intended (not intentional presence of Esbo) for Pasteurization, it’s a compound made by Eviosys equivalent to Esbo Free
PVC-FREE: Without PVC or ESBO, but made of thermoplastic elastomers (TPE)
 
Identification of compound is an extremely delicate operation. It is done once on a product / lid / process lifetime, so it has to be done correctly. If a autoclave is used for the thermal treatment, both with pasteurization and sterilization temperatures, in presence of counter-pressure, use of sterilization compound has to be carefully analyzed.

 

3. Targeted Solutions

Based on the causes of rusting, combined with different sterilisation methods and food characteristics, the following solutions have been developed: 

Filling

Filling is a very important and delicate action. The good result of the subsequent operations strictly depends on filling. Therefore, for a correct and efficient filling, we suggest avoiding:
 
· Causing glass a thermal shock. Recommended difference, between product temperature and jar, is not over 35°C;
· Processing cold lids. On the contrary, it is suggested to harmonize lids to average temperature of +18°C before use (preferably at least 24 hours at room temperature), notwithstanding pre-heating during process;
· Product contamination on finish surface, dirty and/or product traces (both oily and fragments) can compromise lid hold;
· That product contains, in its mixture, air in excess. Air bubbles can impede vacuum formation and increase inner pressure during thermal treatment;
· Having excess of product, overfilling may cause loss of seal and contamination risk;
· Contact and sediment of products with high rate of starch or sugar with glass mouth, they can adversely affect jar opening;
· Contact with strongly acid food products, please refer to chart paragraph, to identify proper compound for aggressive products.
 
MANUAL FILLING AND COLD CLOSING
This section is dedicated to who does not have a capping machine, neither a lids heating system; it is possible
to maximize result, thanks to right expedients, despite manual processing. First of all, avoid putting closed jars
upside down. Correct jars position (as well as bottles) is always only upright, for all their life, from producer to
consumer. In an upside jar, product goes into direct contact with inner lid surface, and this may have negative
effects. The aim of this turning operation, by now outdated, was to warm compound up.
 
 
COLD FILLING, PASTEURIZATION, STERILIZATION
Generally speaking, products can be packed cold, or hot and left to cool down, or thermally treated with Pasteurization or Sterilization to preserve them. For cold-hot filling, please go back to specific section above.
 
PASTEURIZATION
It allows to obtain stability in products with pH less than 4.5 when temperature is same or higher than 85°C (185°F) in any area inside jar. For pasteurization operations using a tunnel (without over-pressure), filling product temperature should not be more than 10°C less than pasteurization temperature. Such temperature gap is essential for PT capsules. For pasteurization operations using a closed tunnel (with over-pressure), it is possible to have a lower filling temperature and a higher pasteurization temperature, but it should be done after a proper technical trial test. As already mentioned in section regarding head space, it needs to avoid product excess that, thanks to pasteurization that makes it increase volume, could pour out of the jar. Typical pasteurization temperature range: over 85°C until 100°C, respecting parameters indicated in technical sheet of the lid.
 
STERILIZATION
Products with a pH equivalent or larger than 4.5 can be sterilized with temperatures >100 °C until 121°C to extend their shelf life. This treatment offers a food safety guaranteed by the interaction of time and temperature, to have a sterilization optimised process; such combination is fixed by qualified organisations, for most of foodstuffs. It is fundamental to keep pressure under control, both inside and outside jar, to maintain package hermetically closed under any condition. Sterilization temperature range: until 121°C, in a process with counter pressure, respecting parameters indicated in technical sheet of the lid. In case of temperature out of range, a double check with producer technical lab is needed.
 
COUNTER PRESSURE
When a product requires a thermal treatment over 100°C, and it is used an autoclave equipment, it is necessary to have a counter pressure to balance inner pressure, caused by volume increase of product. Generally speaking, recommended counter pressure inside autoclave is 2,0-2,2 bar, but the setting target is to have it the closest possible to inner pressure. In order to avoid damages that may occur (compound cut, vacuum loss), because of excessive counter pressure, it should not be over 0,5-0,7 bar than the inner one.
 
VACUUM
Because of thermal treatments, a vacuum will be created, easily visible by pull down of safety button (see paragraph of profiles) of the lid top. Vacuum value, to be obtained, changes by the style of lid, by the filling temperature, and type of treatment. A vacuum between 0,3 and 0,5 bar, can be considered acceptable on average but, also for this parameter, it needs to take into account variables connected to diameter and temperature. To have precise referring values, verify producer technical sheet of item in use. Visual check of vacuum, that is button down, for consumer states guaranteed product integrity, and for producer is the certainty of a well-made package that preserves until expiry date. Therefore, it is always recommended a vacuum check system (dud detector) and automatic lids with button up reject, in the filling and production line. For a broad range check, it is ideal to have a dud detector right after capping machine, to avoid jars without vacuum entering Pasteurization/Sterilization section; and second detector at the exit of Pasteurization tunnel or autoclave, before jars arrive to labelling machine.
 

 

4. The Final Steps: JARS COOLING AND DRYING
 
COOLING
Cooling is an important aspect that, when overlooked, can cause problems, more in Sterilization than
Pasteurization. It must be absolutely prearranged a harmonic and gradual cooling so that, in temperatures
diagram, a curve line without net angles is generated. A drop of many degrees, when comes in in a very short
time, is surely cause of thermal shock, with the consequences initially mentioned also in filling stage: glass
molecular tension and/or possible breakage. Avoid letting in, at the end of thermal cycle, direct tap water
because its temperature is always far from jars one.
 
DRYING
Water drops settle, and stay until end of thermal treatment, inside lid, especially in lugs areas, is a potential risk
of rust formation after process. The only factor that originates lids oxidation is the presence of corrosive water
deposits. Therefore, it is necessary to meticulously dry jars and closed lids, after process, in order to remove
all water, including micro drops trapped in between outside glass finish and inner surface of lid. The drying
process can be executed with proper blowing air heads, better if placed both looking down on, and from
below to the top, and air flow must necessarily reach the entire mouth circumference.

Choosing a tinplate cap from Shining Packaging is not just about choosing a product, it's about choosing a complete rust prevention solution!
 

We provide ‘one-stop service’ from cap customisation, process optimisation to after-sales support, helping you to easily cope with production problems, so that every bottle of product can stand the test of time and consumers.

 

�� Inquiry Now：Get your FREE anti-rust solution + samples!
�� Hot Lines：+86 180 2416 0508  /  WhatsApp: +86 137 2499 2065

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