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MAFF UK - TIN IN CANNED TOMATO PRODUCTS

Index to MAFF UK Food Surveillance Information Sheets, 1999

This survey was carried out to establish the extent of a problem with elevated concentrations of tin in canned tomato products discovered in earlier JFSSG tests. High tin concentrations in food may cause short-term stomach upsets in some people but without any lasting harm.
Tin concentrations were within the legal limit of 200 mg/kg in 98 per cent of the 185 products tested, but three canned spaghetti products and one canned tomato product contained mean tin concentrations above this limit. At JFSSG's request these four products were immediately withdrawn from sale and consumers warned to return any unused cans to the store. These actions took place between December 1998 and February 1999 as the test results became available.
No samples of baked beans or tomato soup were affected.
JFSSG is working with industry and food safety enforcement authorities to investigate the cause of this problem and to prevent its recurrence.

Summary

A survey of tin in a wide range of canned tomatoes, pasta in tomato sauce, baked beans in tomato sauce and tomato soup on sale in the UK was carried out between December 1998 and February 1999. The purpose of the survey was to determine the extent to which these products were affected by a problem of high tin concentrations, identified in certain batches of canned tomatoes and canned spaghetti tested in a JFSSG research project and in subsequent tests by industry and by enforcement authorities in Scotland.¹ High tin concentrations in food may cause short-term health effects in some people, including stomach upsets, abdominal cramps, nausea and/or diarrhoea.

Tin concentrations in canned foods can vary considerably between individual cans in the same production batch. For this reason, tin concentrations were measured in three separate cans from the same batch for each product tested and the mean (average) tin concentration across the three cans was used to determine whether the batch was affected by elevated tin levels.

Overall, tin concentrations were similar to those normally expected in these products and, in all but four products (i.e. in 98 per cent of those tested), mean tin concentrations were below the legal limit of 200 mg/kg for tin in food set by the Tin in Food Regulations 1992 (S.I. [1992] No. 496).² However, three out of 39 canned spaghetti products and one out of the 61 canned tomato products tested contained mean tin concentrations above the legal limit. Because there is risk of short-term health effects in some individuals from elevated levels of tin in food, JFSSG immediately informed the retailers and suppliers of these products who agreed to withdraw them from sale and to issue notices in stores and the national press to advise consumers to return any unused cans to the store. The Department of Health and Territorial Departments issued Category D Food Hazard Warnings ('for information only') to inform enforcement authorities of these results and, because the affected products originated from another Member State of the European Union (Italy), the European Commission was also informed. Retailers of these products were also invited to submit brief statements on their results for inclusion in this Food Surveillance Information Sheet, and all the statements that were received are reproduced in Annex 1.

Two further samples of canned tomatoes and one of canned spaghetti were identified for which the results for the three individual cans tested for each product were unusually variable. These products all had mean tin contents below the 200 mg/kg legal limit but in each case one individual can had a tin content above the limit while both the other two cans had tin contents well below this limit. JFSSG contacted the retailers of these products and asked them to investigate these results. These retailers of these products were also invited to submit brief statements on their results and all the statements that were received are reproduced in Annex 1.

The tin concentrations in all the samples of baked beans and of tomato soup tested were below the legal limit.

JFSSG has informed the enforcement authorities of these results. JFSSG is working with industry and with enforcement authorities to ensure that the cause of this problem is identified and that all necessary steps are taken to prevent its recurrence. JFSSG will carry out a further survey of the canned tomatoes and canned pasta produced from the next season's crop to ensure that these steps have been effective. Enforcement authorities are continuing to test canned tomato products and JFSSG will work with them to ensure that any further products that are found to contain elevated tin concentrations are withdrawn from sale and that consumers are informed.

Background

Health effects of tin in food
High concentrations of tin in food irritate the gastrointestinal tract and may cause stomach upsets in some individuals, with symptoms which include nausea, vomiting, diarrhoea, abdominal cramps, abdominal bloating, fever and headache. These are short-term effects with recovery expected soon after exposure.³ These effects may occur in some individuals at tin concentrations above 200 mg/kg with an increased risk of effects at concentrations above 250 mg/kg. Long-term effects are not expected from tin in the diet.³

As there are health concerns regarding high concentrations of tin, the UK Tin in Food Regulations 1992 limit the maximum amount of tin in foods sold in the UK to 200 mg/kg.² The Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT) (which gives the Government independent expert advice on the safety of chemicals in food), has supported this limit, which is intended to protect consumers against the potential adverse effects of tin.³

The Joint Expert Committee on Food Additives of the Food and Agriculture Organization and the World Health Organization (JECFA) has recommended a Provisional Tolerable Weekly Intake (PTWI) for tin of 14 mg/kg body weight/week to protect against the risk of any chronic (long-term) effects.⁴ This PTWI is equivalent to 120 mg/day for a 60 kg person. The PTWI is an estimate of the amount of a substance that can be ingested over a lifetime without appreciable risk to health.

Most foods contain very low concentrations of tin, usually below 10 mg/kg, although canned foods may contain higher concentrations as a result of the slow dissolution of the tin coating used on the inside of some food cans to protect the steel body of the can from corrosion. In the 1994 Total Diet Study, which is the most recently reported of JFSSG's regular surveys of the levels of metals in the UK diet, average tin concentrations in all the food groups were below 1 mg/kg, except for Canned vegetables (44 mg/kg tin) and Fruit products (which include canned fruits; 17 mg/kg tin).⁵ Together, Canned vegetables and Fruit products accounted for 97 per cent of the average dietary exposure of UK consumers to tin of 2.4 mg/day, estimated from this survey. This total exposure represents only 2 per cent of the JECFA PTWI for tin, which supports the view that long-term health effects are not expected from tin in the diet.³

Tin in canned foods
There are many factors that influence the amount of tin that is taken up by canned foods from the internal coating of the can. These include: the type and composition of the product itself (including the acidity, the presence of organic acids and pigments and certain ions such as nitrate); the type of can (including the quality and thickness of the tin coating, the amount of the tin coating that is exposed to the can contents and the presence or absence of a layer of lacquer over the tin plate); the canning procedure (including the amount of air in the container at sealing and the internal vacuum and 'headspace' between the fill level of the product and the lid) and storage (time from canning and temperature during storage).⁶

Under normal circumstances the shelf life of canned foods is set so that the tin content of the food remains well below the legal limit throughout the product's shelf life. However, under certain circumstances tin dissolution can become accelerated and this can lead to unacceptably high tin concentrations in the food within its shelf life.

A MAFF survey of canned fruits and vegetables carried out in 1997 showed that, whilst the majority of products contained concentrations well below the legal limit, some products did contain significant amounts of tin.⁷ Asparagus, tomatoes, apricots, grapefruit and gooseberries all contained mean tin concentrations above 100 mg/kg. For gooseberries, two of the three composite samples (two cans combined) exceeded the 200 mg/kg legal limit. Relatively high tin concentrations of 140 mg/kg to 190 mg/kg were also found in three samples of canned tomato soup tested in a MAFF survey of metals and other elements in snack and convenience foods carried out in 1996 and 1997.⁸

As a result of these findings, JFSSG commissioned a research project to investigate the sources of tin in canned tomatoes, tomato soup, asparagus, apricots, grapefruit and gooseberries and the reasons for the relatively high tin concentrations found in these foods in previous surveys. This project was carried out by Campden & Chorleywood Food Research Association (CCFRA) in 1998. The results of this project showed that, while tin concentrations in these foods were generally well controlled, elevated tin concentrations above 200 mg/kg were present in certain batches of canned tomato products.^{9, 10} As a result of these findings, further tests were carried out by JFSSG as well as by industry and by enforcement authorities in Scotland. These tests revealed that further canned tomato products and canned spaghetti products were also affected.¹

All the products found to be affected in these tests were withdrawn from sale. JFSSG called a meeting on 20 November 1998 with consumer groups and food industry representatives to discuss the problem. MAFF issued a Press Release on 20 November 1998 to ensure consumers were informed of the details of the affected batches and to advise them to return unused cans to the place of purchase.¹¹ Retailers and suppliers also placed notices in stores and in the national press to advise customers.

The high concentrations of tin discovered in the JFSSG study could not be readily attributed to factors related to the cans themselves or to the canning procedure, but were more likely to be related to some property of the product. One possible explanation is higher than usual nitrate concentrations in the product at the time of canning.^{9, 10} Since these results suggested that the problem might possibly affect other canned tomato-based products, JFSSG commissioned the much wider survey of canned tomato products reported here. The survey was also carried out by CCFRA and covered a wide range of whole and chopped tomatoes, baked beans, canned spaghetti and other pasta in tomato sauce, and tomato soup. These products were selected because they contained a significant proportion of tomatoes and are widely consumed.

Brand names
In accordance with the MAFF policy for the release of brand names when reporting the results of food chemical surveillance,¹² details of the individual samples of canned tomato products obtained in this survey are given in full in Table 1. It should be noted that the absence of any particular brand from this survey means only that the brand has not been included in the survey. No further meaning should be read into the absence of any product or brand from this Food Surveillance Information Sheet.

Methodology

Sample plan
The sample plan was designed to cover the major product types, brands and retailers within each product category, as well as smaller brands and retailers, according to available market share data.^{13, 14} The sample set consisted of 61 samples of whole or chopped tomatoes, 39 samples of canned spaghetti or other pasta in tomato sauce, 39 samples of baked beans in tomato sauce and 46 samples of canned tomato soup. This gave a total of 185 samples. Tin concentrations in canned foods can vary considerably between individual cans in the same production batch. Therefore, tin concentrations were measured in three separate cans from the same batch for each product sampled, and the mean (average) tin concentration for the three cans was used to determine whether the batch was affected by elevated tin levels. Each sample consisted of four cans with identical batch codes (three for analysis and one held in reserve), giving a total of 740 cans.

Samples were purchased by CCFRA from retail outlets in three geographical areas of England: the north (Grimsby area), the Midlands (Evesham/Kidderminster area), and the south (Bristol area). This spread was intended to ensure that brands more common in particular areas were represented as well as nationally-distributed brands. The survey was not designed to examine whether there were any differences between tin concentrations in these products in different regions and this aspect is not discussed further.

Further details of the sampling plan and of sample instructions are given in the contractor's final report for this survey,¹⁵ which is available to the public in MAFF's Library (see 'Further Information').

Sample preparation and analysis
For each product sampled, three individual cans were analysed separately for tin, with the fourth can being retained for possible future examination. Cans for analysis were opened and the contents transferred immediately to a sealed glass or plastic vessel (samples were not left in the cans after opening). The entire can contents were homogenised prior to analysis and the tin content was determined by a method based on flame atomic absorption spectrophotometry. The emptied cans were immediately cleaned and dried and stored under stable conditions. All cans analysed were within their best before date when they were opened.

The internal vacuum and headspace depth (which indicates the filling level) of each can were also measured on opening. These parameters can be used to check whether any anomalies have occurred during the canning procedure that might cause elevated tin concentrations. To check any influence of the can type and the quality of the internal coating on tin concentrations in the food, the internal surfaces of the empty cans were inspected visually and the type and condition of internal coating (e.g. tin plate only, partial or complete lacquered coating) were noted for each can.

Quality Control
CCFRA is accredited by the United Kingdom Accreditation Service (UKAS) for the determination of tin in food by the method used in this survey. CCFRA has taken part in all rounds of the Food Analysis Performance Assessment Scheme (FAPAS) for tin and has consistently performed well. The method used has a limit of detection (LOD) of 5 mg/kg and a repeatability of plus or minus 5.0 mg/kg at tin concentrations below 100 mg/kg and plus or minus 5 per cent (relative) at tin concentrations above 100 mg/kg. All samples were analysed in duplicate and duplicate results falling outside these repeatability criteria were rejected and the analyses repeated. However, repeatability for duplicate samples in this work was typically less than 1 per cent (relative). Each batch of analyses included at least one spiked sample, the recovery of which had to fall between 85-115 per cent, otherwise all analyses in the batch had to be repeated. In this study, the requirement for recovery concentrations was met in all cases.

Results

The results of this survey are summarised in Table 2 below, which shows the average, minimum and maximum tin concentrations found in each product type tested in this survey. Full details of the individual products tested, including brand names and batch numbers are given in Table 1, together with the tin concentrations found in each of the individual cans and the mean tin concentration for each product tested. These results are discussed in more detail below.

Interpretation

Overall, tin concentrations were similar to those found in previous studies^{5,7, 8} and in all but four products (98 per cent of those tested) average tin concentrations were within the legal limit of 200 mg/kg for tin in food. The three canned spaghetti products and one canned tomato product containing mean tin concentrations above the legal limit are discussed separately below.

All the samples of baked beans and of canned tomato soup contained tin concentrations within the normal range and below the legal limit.

The overall mean concentration in all 185 samples tested in this survey is 63 mg/kg, which is slightly higher then the mean tin concentration of 44 mg/kg found in the Canned vegetables food group in the 1994 TDS.⁵ This is as expected and reflects the fact that the TDS Canned vegetables group comprises composite samples containing a mixture of a wide variety of canned vegetable products, many of which may have been packed in fully lacquered cans, whereas the present survey focused on products which are known to have a potential to accumulate higher tin concentrations and are normally packed in plain tinplate cans (i.e. without a lacquer coating).

The mean tin concentration in the 61 canned tomato products tested in this survey is 77 mg/kg. This is similar to the mean tin concentration of 89 mg/kg found for the 77 samples tested in an earlier JFSSG research project^{9, 10} and generally consistent with that of 41 mg/kg for three samples tested in an earlier survey of snack and convenience foods, bearing in mind the small number of samples in the latter survey.⁸

The mean concentrations found in this survey for canned pasta in tomato sauce (73 mg/kg, 39 products) and baked beans (32 mg/kg, 39 products) are similar to those of 70 mg/kg found for two samples of spaghetti in tomato sauce and 41 mg/kg found for three samples of baked beans included in the earlier survey of snack and convenience foods.⁸

The mean tin concentration in canned tomato soup (64 mg/kg, 46 samples) was lower than that of 158 mg/kg found for three samples tested in the earlier survey of snack and convenience foods.⁸ As before, the small number of samples in the earlier survey limits any conclusions that can be drawn from this comparison. The present results are reassuring however because this much more extensive survey of more recently purchased samples shows no evidence of elevated tin concentrations in this product.

There was no obvious relationship between tin content and the headspace depth or vacuum in the can.¹⁵ As expected, products in fully lacquered cans (which have no exposed tin on the interior of the can) had a lower mean tin content (7 mg/kg, 20 products) than those in unlacquered or partially lacquered cans (70 mg/kg, 165 products).

Products with elevated tin concentrations
Three canned spaghetti products and one canned tomato product tested contained mean tin concentrations above the legal limit of 200 mg/kg for tin in food. Because of the risk of short-term health effects from elevated concentrations of tin in food, as soon as these results were confirmed JFSSG immediately informed the retailers of these products who agreed to withdraw them from sale and to issue notices in stores and the national press advising consumers to return any unused cans to the store. The Department of Health and Territorial Departments issued Category D Food Hazard Warnings ('for information only') to inform enforcement authorities of these results and, because the affected products originated from another Member State of the European Union (Italy), the European Commission was also informed. These withdrawals and corresponding notifications took place between December 1998 and February 1999 as the analytical results for each product became available. The retailers of these products were also invited to submit brief statements on these results for inclusion in this Food Surveillance Information Sheet, and all the statements that were received are reproduced in Annex 1.

Two further samples of canned tomatoes and one of canned spaghetti were identified for which the results for the three individual cans tested for each product were unusually variable. These products all had mean tin contents below the 200 mg/kg legal limit, but in each case one individual can had a tin content above the limit while the both other two cans had tin contents well below this limit. JFSSG contacted the retailers of these products and asked them to investigate these results. These retailers were also invited to submit brief statements on these results and all the statements that were received are reproduced in Annex 1. The tin concentrations in all samples of baked beans and of tomato soup tested were below the legal limit.

Causes of elevated tin concentrations
The individual cans whose contents contained elevated tin concentrations were not unusual either in terms of their headspace depth or vacuum in the can, or can type, or in the condition of the cans as apparent on a visual inspection. These findings are consistent with those of the earlier JFSSG research project and suggest that the cause of these elevated concentrations was most likely to be a product related factor rather than any problems with the can itself or the canning procedure.^{9, 10,15} Elevated tin concentrations were seen only in canned tomatoes and canned spaghetti in tomato sauce and not in baked beans and tomato soup. This again suggests a product-related problem rather than one affecting canned foods in general.

Further investigation of the affected products in liaison with the food industry and retailers has provided further evidence that the problem is product related. The tomato sauce for canned pasta is produced from tomatoes from the same sources as those used for canned tomatoes. All the affected batches identified so far have been produced in Italy. The affected batches identified to date have come mostly from those produced from the 1997 tomato crop although this may simply reflect the fact that these batches predominated among those on sale at the time of sampling. One possible explanation is that higher than usual nitrate concentrations in some batches of tomatoes have led to an increased rate of release of tin into these products after canning.

Following the initial detection of this problem by JFSSG, the suppliers, canners and retailers of these products in the UK and their counterparts in Italy have informed JFSSG that they have initiated co-ordinated efforts to clarify the cause of this problem and to try to eliminate it. These efforts include intensified testing of products before import and sale and research programmes both in the UK and in Italy, which include investigating the relationship between nitrate concentrations in tomatoes and tin pick-up and measures to control nitrate concentrations in the product prepared for canning. JFSSG will monitor these initiatives to ensure that adequate controls are put in place to ensure that the tin content of these products remains within legal and safe limits throughout their useable life.

JFSSG has informed food safety enforcement authorities throughout the UK of its findings. Enforcement authorities will continue to test canned tomato products to ensure that their tin contents remain within the legal limit. If any further products are found to exceed the legal limit for tin in food, JFSSG will liaise with enforcement authorities to ensure that these products are immediately withdrawn from sale and consumers are warned.

Long-term exposure to tin in the diet
As noted above, long-term effects on health would not be expected even at the elevated tin concentrations found in four products identified in this survey. Nevertheless, JECFA have recommended a PTWI to protect against any risk of long-term effects of dietary exposure to tin. Long-term dietary exposures to tin from the products tested in this survey were therefore estimated. This was done by assuming that all canned vegetables contain tin at the mean concentration reported for this survey (63 mg/kg) and that all other foods contain tin at the mean concentrations found in the 1994 TDS,⁵ and using data on the patterns and amounts of different foods eaten by UK consumers from the National Dietary and Nutritional Survey of British Adults (NDNS).¹⁶ The data from the NDNS allow exposure to be estimated both for average consumers and for 'upper-range' consumers who have higher dietary exposures because they eat above-average quantities of particular foods.

This approach will probably result in over-estimates of long-term tin exposure because the current survey focused on canned foods that are known to be susceptible to higher tin concentrations. Even so, the total dietary exposures to tin estimated on this basis were 3.1 mg/day for an average adult consumer and 10 mg/day for a 'upper-range' (97.5th percentile) adult consumer. These dietary exposures remain well below the PTWI for tin which is equivalent to 120 mg/day for a 60 kg person, and are similar to the exposures of 2.4 mg/day for the average and 7.9 mg/day for 'upper-range' (97.5th percentile) adult consumer estimated from the 1994 TDS.⁵ Therefore, the concentrations of tin found in this study are not a long-term health concern to consumers.

Actions

JFSSG ensured that retailers and suppliers withdrew the four products identified by this survey as containing unacceptable tin concentrations from sale and that they issued notices in stores and in the press advising consumers to return any unused cans to the store.
JFSSG informed food safety enforcement authorities of these results. They will continue tests on canned tomato products to ensure that their tin contents remain within the legal limit. JFSSG will liaise with enforcement authorities to ensure that any further batches found to be affected are immediately withdrawn from sale and consumers are informed.
JFSSG will continue to liaise with industry and enforcement authorities to ensure that the cause of the problem is identified and that all necessary steps are taken to ensure that it is prevented.
Industry has informed JFSSG that it is carrying out research into the cause of the problem and how to prevent it, and that they have intensified their own pre-sale testing programmes to identify any batches affected by elevated tin concentrations and ensure they are not sold.
JFSSG will carry out a further survey of tin in canned tomatoes and canned pasta produced from the new season's crop, when available, to check that these measures have been effective.
JFSSG informed the European Commission to ensure that other Member States were aware of the problem.

Units

kilogram (kg): one thousand grams (g)
milligram (mg): one thousandth of a gram (g)
mg/kg: milligrams per kilogram (equivalent to parts per million)
mg/day: milligrams per day

References

Ministry of Agriculture, Fisheries and Food/Department of Health (1999). Food Safety Information Bulletin No. 104.
The Tin in Food Regulations 1992 (S.I. [1992] No. 496). The Stationery Office, London.
Ministry of Agriculture, Fisheries and Food (1998). Cadmium, Mercury and other Metals in Food. Food Surveillance Paper No. 53. The Stationery Office, London.
World Health Organization (1989). Toxicological evaluation of certain food additives and contaminants. Thirty-third meeting of the Joint FAO/WHO Expert Committee on Food Additives. WHO Food Additives Series:24. Cambridge University Press, Cambridge.
Ministry of Agriculture, Fisheries and Food (1997). 1994 Total Diet Study: Metals and Other Elements. Food Surveillance Information Sheet No. 131.
Food and Agriculture Organization of the United Nations (1986). Guidelines for can manufacturers and food canners. Prevention of metal contamination of canned foods. FAO Food and Nutrition Paper No. 36. Food and Agriculture Organization of the United Nations, Rome.
Ministry of Agriculture, Fisheries and Food (1997). Survey of Lead and Tin in Canned Fruits and Vegetables. Food Surveillance Information Sheet No. 122.
Ministry of Agriculture, Fisheries and Food (1998). Concentrations of Metals and Other Elements in Selected Snack and Convenience Foods. Food Surveillance Information Sheet No. 146.
Campden & Chorleywood Food Research Association (1998). Investigation of Sources of Tin in Canned Foods. Final Report to MAFF, Campden & Chorleywood Food Research Association, Chipping Campden, Gloucestershire.
Campden & Chorleywood Food Research Association (1999). Investigation of Sources of Tin in Canned Foods: Part Two. Final Report to MAFF, Campden & Chorleywood Food Research Association, Chipping Campden, Gloucestershire.
Ministry of Agriculture, Fisheries and Food (1999). Tin in Canned Tomatoes. MAFF Press Release No. 456/98.
Ministry of Agriculture, Fisheries and Food/ Department of Health (1997). Food Safety Information Bulletin No. 88.
Mintel (1997). Frozen and canned fruit and vegetables, Market Intelligence: Food and Drink, June 1997. Mintel Ltd.
Langley, R. (ed.) (1997). Canned Foods 1997. Keynote Plus Market Report, 11th Edition. Key Note Ltd.
Campden & Chorleywood Food Research Association (1999). A Survey of Tin Content of Canned Tomatoes and Tomato-based Products. Final Report to MAFF, Campden & Chorleywood Food Research Association, Chipping Campden, Gloucestershire.
Gregory, J., Foster, K., Tyler, H. and Wiseman, M. (1990). The Dietary and Nutritional Survey of British Adults. The Stationery Office.

Further Information

Further information on this survey can be obtained from:

Dr Patrick Miller
Joint Food Safety and Standards Group, MAFF
Food Contaminants Division
Room 238 , Ergon House, c/o Nobel House
17 Smith Square
London SW1P 3JR

Tel: +44 (0)171 238 5751
Fax: +44 (0)171 238 5331
E-mail: pf.miller@fsci.maff.gov.uk

A copy of the final report of this survey has been placed in the MAFF Library, Nobel House, London, SW1P 3JR Tel. No. + 44 (0) 171 238 6575. If you wish to consult a copy please contact the library for an appointment giving at least 24 hours notice or alternatively copies can be obtained from the library: a charge will be made to cover photocopying and postage.

Spreadsheet tables

Table 1. Sample details and results for all samples of canned tomato products

Click here to view the Excel 5.0 version of Table 1
Click here to view the .csv version of Table 1 (if you have any other spreadsheet package)

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These pages were last updated on 28 May 1999