                                                                                        TBS/ AFDC 27 (6606 ) P3         
                                         
 
 
   
 DRAFT TANZANIA STANDARD  
 
Code of practice for the prevention and reduction of inorganic tin 
contamination in canned foods  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS                                                                                                                                 TBS/AFDC 27 (6606) P2          
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 0 FOREWORD  
Tin is relatively less toxic than mercury, cadmium and lead. Th e principal concern in relation to tin in food is  the 
possibility of high levels potentially present in canned food in incorrectly manufactured tins, where tin  present in the 
can has leached into the food. This has been shown to occur in the case of acidic  foodstuffs  such as canned 
tomatoes, and consumption of the affected foodstuff h as resulted in gastrointestinal irritation and upsets due to the 
acute toxic effects of tin. These short -term effects may occur in some  individuals at concentrations above 200  mg/kg. 
Only limited data are available on the toxicological effects of  inorganic tin present in canned food, resulting from the 
dissolution of the tin coating.  
 
This code is therefore aiming  to guide the manufacturers, producers , distributors  as well as  consumers to  prevent 
and reduce the inorganic tin contamination in canned foods.  
 
In the preparation of this code of practice the assistance was drawn from  CAC/RCP 60 -2005 code of practice for the 
prevention and reduction of inorganic tin contamination  in ca nned foods . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 1 SCOPE  
This code of practice recommends the  good practice for the prevention and reduction of solely migration of inorganic 
tin from the internally plain (i.e. not lacquered) tin coating of tinp late cans  in canned foods . Compliance with this code  
does not confer immunity from relevant statutory and legal requirements  
 
This code of practice is applicable to thermally processed canned human foods (including fruit and vegetable juices) 
which are pac ked into plain tinplate cans. It is considered that this description covers both:  
i) Hot fill and hold products; and,  
ii) Hot or cold fill and retort products.  
 
This code of practice is not intended to apply to tin exposure from any other source and is spe cific to inorganic tin . 
Also d ry goods and 100% oil products are not included, because they do not experience tin migration.  
 
2 TERMS AND DEFINITIONS  
For the purposes of this document, the following terms and definitions apply.  
 
2.1 Annealing  
heating proce ss used in tinplate manufacture to soften the steel strip after cold rolling and to impart the required 
hardness; the process can either be continuous (continuous annealing or CA) or in batches (batch annealing or BA).  
 
2.2 Beads, Beading  
corrugations roll ed into can walls to give added strength to the can body  
 
2.3 Closer  
machine  used to seal an end onto a can  
 
2.4 Closing under vacuum  
applying a vacuum to the closing chamber of the can  closer, whilst sealing the end  
 
2.5 Corrosion  
chemical action of disso lving the surface of a  metal (eg. tin in food medium)  
 
2.6 Corrosion accelerator  
chemical species with the ability to accept electrons, which will increase the rate of a corrosion reaction  
 
2.7 Corrosion mechanism  
specific chemistry of any corrosion reacti on; especially for tinplate when 2 metals (tin and iron) are coupled and 
where one or bot h has the potential to dissolve  
 
2.8 DR Tinplate  
‘double reduced’ tinplate where a second rolling is used to reduce steel thickness in order to produce a thinner but 
stronger product  
 
2.9 Electrolyte  
substance which dissociates into ions when dissolved in a suitable medium; hence a tin rich electrolyte is used in 
tinplate manufacture (see Electro -Tinning); the food in contact with an internally plain can may also be  described as 
an electrolyte  
 
2.10 Electrolyte Tinplate  
low carbon mild steel strip coated on both top and bottom surfaces with an electrolytic deposition of tin; the 
deposited tin exists as an alloyed and free tin and has a passivated surface as well as a coa ting of oil.  
 
2.11 Electro -Tinning  
act of plating tin from a tin rich electrolyte onto a continuous steel strip t o produce electrolytic tinplate  
 
2.12 Embossing  
use of a die to stamp a product code or manufacturing date into a can end  
 
2.13 Filler  
machine used to automatically fill a can with the desired weight or volume of food  
 
2.14 Fill Temperature  
temperature at which the food is filled into the can                                                                                                                                 TBS/AFDC 27 (6606) P2          
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2.15 Food Acids  
organic acids, naturally occurring in foods, especially in fruits and vegetables; also u sed to impart flavou r and to 
modify the pH of foods  
 
2.16 Headspace  
space left in the top of the can after filling and end sealing, in order to allow for product expa nsion during thermal 
processing  
 
2.17 Hot Fill and Hold  
process where a high acid food pro duct (usually juice or liquid) is filled at high temperature, the end sealed and cans 
held for a period of time before cooling; commercial sterility is ach ieved without retort processing  
 
2.18 Inject Coding  
use of an ink jet to print a product code or manu facturing date on the can end  
 
2.19 Internal Corrosion  
corrosion occurring wi thin a food can (see Corrosion)  
 
2.20 Lacquers  
inert organic coatings used to give additional protection to tinplate; usually applied in liquid form and ‘cured’ at high 
temperatur es 
 
2.21 Pack Testing  
storage and regular sampling of canned foods under controlled temperature conditions to determine internal 
corrosion character istics and potential shelf life  
 
2.22 pH 
measure of acidity  
 
2.23 Plain Cans  
cans made from plain tinplate  
 
2.24 Plain Tinplate  
bright tinplate without  any additional lacquer coating  
 
2.25 Process Time  
calculated time at a particular temperature (process temperature) for which a specific can size and food product 
need to be heated in order to achieve commercial sterility  
 
2.26 Product Line  
maximum level or height of the product in the can; the headspace is above the product line  
 
2.27 Reducing Environment  
conditions expected inside a plain processed food can, whereby the contents are protected from oxidative reac tions 
such as colour change  
 
2.28 Shelf Life  
the term or period during which canned foods stored at recommended conditions remai ns suitable for consumptions.  
 
2.29 Sidestripe  
thin band of lacquer designed to protect the we ld of a can body from corrosion  
 
2.30 Stock Rotation  
method of ensuring the oldest canned products are identified, removed first from warehouse storage and are  first 
onto the retailers shelf  
 
2.31 Thermal Processing  
use of any heat process to ensure the commercial sterility of filled cans  
 
2.32 Tin Coating Mass  
mass of tin, expressed in g/m2, which is applied to each side of the steel base; standard coating masses generally 
range from 2.8 to 11.2g/m2 in increments of 2.8g/m2; the internal tin coating mass of plain cans is usually either 8.4  
or 11.2g/m2                                                                                                                                TBS/AFDC 27 (6606) P2          
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3. RECOMMENDED PRACTICES  TO MINIMISE TIN UPTAKE BY FOODS PACKED INTO 
PLAIN TINPLATE CANS  
3.1 General  
3.1.1  There are many factors which may influence the level of product tin uptake in plain tinplate cans. Some are 
very minor and others, usua lly specific to the chemistry of the processed food, may have a significant effect on 
internal can corrosion and product tin dissolution. The recommendations contained below are based on an attempt 
to identify all of these factors, no matter how minor, and  to suggest specific areas where monitoring or other controls 
would be beneficial.  
 
3.1.2 In summary the factors which have been identified can be grouped as follows:  
i) choice of tin coating mass and passivation  level;  
ii) damage to tin coating or passiva tion; 
iii) type of food product, pH and acid content;  
iv) presence of corrosion accelerators, such as nitrates, in the raw food ingredients;  
v) presence of sulphur compounds in the food;  
vi) presence of oxygen within the sealed can;  
vii) p rocess times and temperatures;  
viii) s torage times and temperatures; and,  
ix) storage humidity.  
 
3.2 Packaging manufacturer  
3.2.1 Tinplate supplier  
3.2.1.1 Tinplate customers should state the end use when ordering tinplate. The tinplate supplier should have 
sufficient expe rtise to ensure that specifications for the tinplate are appropriate to the stated end use and notify the 
customer  should there be any concerns (e.g. with regard to the passivation level or the requested tin coating mass).  
 
3.2.1.2 The tinplate manufacture r should have quality procedures in place to ensure that every tinplate order 
conforms to the required standard. Incorrect tin coating masses or passivation levels could result in abnormal 
corrosion and increased product tin levels. Low oil levels may lead  to abrasive damage to the tin coating during 
transport and can manufacture.  
 
3.2.2 Can maker  
3.2.2.1 Can makers should approve tinplate suppliers on the basis that each supplier has demonstrated compliance 
to agreed standards and ordering requirements.  
 
3.2.2.2 The can maker should have sufficient expertise to ensure that the customer’s ordering requirements (i.e. 
passivation and tin coating mass) are appropriate for the end use and should notify the customer of any concerns.  
 
3.2.2.3 The can maker should assist the customer in determining the correct can specification for any new food 
product or recipe change. Such changes should be tested to ensure that product tin uptakes are not excessive.  
 
3.2.2.4 Machine settings for processes where metal working occu rs (e.g. beading) should be such as to minimise 
damage to the tin coating.  
 
3.2.2.5 If a sidestripe is applied to 3 piece cans then excessive heat should be avoided when curing the stripe.  
 
3.3 The canner  
3.3.1 Raw Materials  
3.3.1.1 The canner should work closely with the can supplier to ensure an appropriately specified can is supplied for 
any given application. Procedures should be in place to ensure that cans are supplied to specification.  
 
3.3.1.2 The canner should consult with the can maker to determin e the correct specification can for any new product 
or any recipe change of an existing product. It is extremely important that sufficient pack testing is conducted to gain 
a thorough knowledge of the corrosion mechanism, likely product tin uptakes and ove rall suitability of the can 
specification for the product.  
 
3.3.1.3 Canners should be knowledgeable about the shelf life of all their products with respect to likely tin uptakes. It 
should be noted that fruits and vegetables in particular may have a signif icant variation in their chemistry, dependent 
on variety, maturity, time , or place  or conditions of harvest, soil chemistry and agricultural practices. These are 
difficult for the canner to control and may ultimately impact on the level of tin uptake by th e product.  
 
3.3.1.4 Quality procedures should be in place to ensure that product batches conform to recipe specification.                                                                                                                                 TBS/AFDC 27 (6606) P2          
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3.3.1.5 Particular attention should be paid to the pH of the food and the addition of food acids. It should be 
recognised that corros ion is pH dependent and that too large drop in pH may give a significant change in corrosive 
behaviour and tin uptake. Different food acids (e.g. citric, malic , fumaric and acetic) behave in different ways with 
respect to internal corrosion and any ingredi ent change from one type of acid to another should be thoroughly tested. 
Acetic acid is particularly aggressive towards tin.  
 
3.3.1.6 The presence of a chemical species with the ability to accept electrons will increase the rate of the corrosion 
reaction. Nitrate is a corrosion accelerator and its presence, even at low levels (1  mg of NO 3- will yield nearly 8  mg of 
Sn2+) causes rapid de -tinning. In a 400  g can, 10  mg of NO 3- will rapidly react to give approximately 80  mg of Sn2+ 
or, in other words, a produc t tin concentration of 200 ppm. In about one year 100 ppm of nitrate will completely de -tin 
a No. 303 can with an inside coating weight of 11.2 g/m2. Nitrates originate from overzealous  use of fertilizers and 
some fruits and vegetables can accumulate high levels (e.g. tomatoes and pineapples). It is essential, when nitrates 
are likely to be a problem that the canned food manufacturer and his suppliers have a system in place to ensure 
fruits, vegetables and other ingredients are acceptable for use in canning . 
 
3.3.1.7 Sulphur residues have also been known to cause corrosion problems in plain tinplate cans. These residues 
can be of agricultural origin or may have resulted from bleaching or preserving agents used in some ingredients. The 
canned food manufacture r and his suppliers should again carry out any necessary testing and make sure that the 
raw materials are fit for purpose.  
 
3.3.1.8 Some foods, especially protein rich meat and fish and, to a lesser extent, vegetables (e.g. peas, beans, corn 
etc.) contain naturally occurring sulphur compounds. These can react with a plain tinplate surface to give a purple -
black stain of tin sulphide. Although the stain is harmless, it may serve to change the passivation of the tinplate 
surface, which, in turn, could alter t he rate of tin uptake. Degree of sulphide staining is also influenced by pH, 
process time and temperature and the presence of certain cations. Al3+ and Fe3+, Fe2+ ions, found in some treated 
potable water;  act as catalysts for the breakdown of naturally oc curring sulphur compounds. Subsequently the 
presence of these ions increases the rate and severity of sulphide staining. Clearly the canner must have an intimate 
knowledge of his  product; the likely variations that could occur in raw materials and process;  and the range of effects 
that these variations could produce within the can. That knowledge should be used to set controls where necessary 
and to determine consistent supply.  
 
3.3.1.9 All raw materials from all suppliers should be well documented especial ly when a supplier is changed or a 
raw material is obtained from another source or location. In the unlikely event that unexpectedly high product tin 
levels occur, documentation makes it easier to track back to any specific changes and to take appropriate action.  
 
3.3.1.10 Water quality should be monitored as some water supplies may contain corrosion accelerators such as 
nitrates.  
 
3.3.2 Processing  
3.3.2.1 The canned food manufacturer should take all necessary steps to eliminate oxygen from within the can p rior 
to closing and to ensure an adequate can vacuum. Oxygen is a corrosion accelerator and its presence in a can after 
closing can lead to early tin dissolution, especially from the headspace area. Oxygen can be present in the 
interstices of the product a nd steam exhausting plus a high fill temperature will help its removal. Minimising 
headspace, whilst still allowing for product expansion, also helps eliminate oxygen. Another control method is closing 
under vacuum. Steam injection to the headspace must be  consistent and controlled. Line stops and delays between 
filler and closer should be avoided.  
 
3.3.2.2 The primary method used for removing oxygen is closing under vacuum  and s eam exhausting . 
3.3.2.3 Chemical reactions, such as corrosion, are accelerated by increasing temperature. Canners should avoid  
excessive processing times at high temperatures that may have an effect on advancing tin uptake.  
 
3.3.2.4 Inadequate cooling and drying should be avoided because this means, for a large mass of cans, that the y will 
remain at an elevated temperature for  a considerable period of time. Cans  should be cooled to 35 –40 °C. Cans 
cooled to a lower temperature may not dry adequately leading to external rusting. Cans that are not adequately 
cooled can be subject to  contamination  by thermophilic bacteria or products may suffer a loss in quality.  
 
3.3.3 Finished goods storage  
3.3.3.1 Internal can corrosion, like any chemical reaction, is temperature dependent. In general,  for every 10 °C rise 
in temperature the reaction ra te will double. The expected level of tin uptake from a can stored at high temperature 
(i.e. 40 °C) would be significantly higher than from a can stored at lower temperature (i.e. 10 °C) for the same period 
of time. Canned food manufacturers should conside r the location of their finished goods storage areas when 
determining maximum storage times. For example: - what is the likely maximum temperature; are some areas 
heated more by the sun; how many days per annum at relatively high temperatures etc.                                                                                                                                 TBS/AFDC 27 (6606) P2          
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3.3.3.2 Stock control is required to ensure finished canned goods from earlier production dates are used first.  
 
3.3.3.3 Warehousing be done under conditions where the temperature can be controlled. Large swings in 
temperature can lead to condensation of moisture  on the exterior of cans which can lead to rusting.  
 
3.3.4 Other considerations  
Can damage should be minimised as this can lead to local areas of de -tinning. For this reason,  it is preferable to use 
ink jet coding rather than embossing.  
 
3.4 Transport and warehousing  
3.4.1 Please refer to sub clause 3.2.3.2 and 3.2.3.3  in sub clause 3.2.3 Finished Goods Storage.  
 
3.4.2 Temperatures encountered during transport need to be considered if the canned goods are likely to remain at 
these temperatures for any lengt h of time (i.e. during shipping). If possible, it is preferable to export stock from a 
more recent production date if high temperatures are likely to be encountered during shipping or at the final 
destination.  
 
3.5 Retailer  
The retailer should maintain cor rect stock rotation and good handling practices to ensure that shelves are stocked 
with cans in production date sequence  and to avoid can damage . 
 
3.6 Consumer  
3.6.1 The consumer should choose a storage location for canned foods that is not subject to exce ssive heat. 
Cupboards should not be close to ovens or heaters and should preferably not be in direct sunlight.  
 
3.6.2 Unused food or juice left in plain tinplate cans may rapidly accumulate tin in the presence of air. It should be 
transferred immediately t o a clean plastic or glass container and stored in the refrigerator . 