On its own, PVC is hard and rigid — it’s often used for drains and for guttering and downpipes. Plasticisers are added to soften it and make it flexible enough to form an airtight seal against the glass rim of the jar.
Plasticisers can make up as much as 40% of the plastic material. They’re relatively small molecules that can easily migrate from the plastic into the food, and because plasticisers dissolve readily in fats and oils, fatty foods are especially vulnerable to contamination.
Most common types
The plasticisers most frequently used to make the seals for screw-capped jars are epoxidised soybean oil (ESBO) and a number of different phthalates.
This is one of the most frequently used additives to PVC, especially for containers or packaging for food. It functions as a stabiliser as well as a plasticiser. Lid seals are formed at high temperatures, which causes the PVC in the seal to partially break down and release hydrogen chloride.
ESBO reacts with the hydrogen chloride and thereby prevents further breakdown of the plastic, but in doing so it forms compounds called chlorohydrins. Chlorohydrins make up, at most, 5% of the ESBO but they can be toxic. There’s no evidence that ESBO itself is harmful, but an expert committee appointed by the European Union (EU) to review the evidence about ESBO concluded that:
“In the absence of adequate analytical and toxicological data on ESBO derivatives, no advice can yet be given on the significance for health of such derivatives in foods.”
The EU has set guidelines specifying a limit of 60 ppm (parts per million) of ESBO in food generally and 30 ppm for baby foods. Infants are at greater at risk because of their smaller body size — and they’re often given food from screw-capped jars. In other words, we can’t be sure whether our health is at risk from the use of ESBO, but let’s opt to be on the safe side.
Phthalates have become a health hot spot. The most commonly used of this class of compounds, DEHP (see How we tested for the full chemical names), can interact with people’s hormone systems and affect reproductive development.
Phthalates aren’t permitted at all in food packaging materials in the US, and the EU recently banned the use of DEHP in all toys and articles used for childcare. They've also banned two other phthalates — DNOP and DINP, often used as plasticisers in toys and childcare articles that children might put in their mouths, such as teething rings and dummies. The EU has also specifically set very low limits for phthalates in food — 1.5 ppm (parts per million) for DEHP and 9 ppm for DIOP and DINP.
How to avoid plasticisers
Research has shown that plasticisers are more likely to migrate into food that contains more than about 4% free oil or fat. And migration could be high, even with less fat, if the product is strongly heated after the container’s been filled — as is typical for infant foods. Foods without much fat (such as honey) or foods that set hard and don’t touch the lid (such as mustard or firm mayonnaise) are unlikely to pick up significant contamination.
So if you want to minimise your risk of exposure to plasticisers, check the nutrition-information panel on the label and avoid any product with more than about 4g of fat per 100g of food — especially if it’s runny enough to slop against the inside of the lid.