A03018: Investigation into the effects of the freeze-thaw cycle on chemical migration from packaging into foods
Wednesday 18 June 2003
This research project investigates whether freezing and thawing could affect the migration of chemicals from packaging into food.
Background
Assessment of packaging materials for chemical migration, conventionally employs standard test conditions of time and temperature linked to the expected conditions of use. However, it is possible that dry or solid frozen foods do not behave in the same way as other foodstuffs when they are exposed to the food packaging. With such foods, intimate contact between the food and the package is small and migration may be dominated by the diffusion of volatile substances across the air gap as opposed to by direct contact.
The aim of this project was to test the possibility that the temperature gradients that may be experienced during the lifetime of a frozen food pack, and especially during the freeze-thaw cycle, affect the migration of substances from the pack to the food. Two possible scenarios exist by which migration could occur from the packaging material to the food:
- Mechanism One: release of volatiles by the packaging and cold trapping by the foods.
- Mechanism Two: ice crystal formation on the packaging surface with subsequent drip onto the food during thawing.
Research Approach
Model compounds (chosen to provide a range of volatilities and polarities) were incorporated into cartonboard and polyethylene film and the migration of these compounds was investigated into foods and food simulants. The migration into foods (beefburgers, green beans and chips) from impregnated packaging materials (cartonboard and polyethylene) was studied following freezing after packing, and 20, 60 and 240 minute thaw periods (i.e. Mechanism One).
In separate studies, a polar compound was incorporated into the packaging to assess migration and transfer via any condensate formed on the internal packaging surface (i.e. Mechanism Two).
Results and findings
Volatile compounds were lost from both cartonboard and polyethylene packaging, mainly to the surrounding air rather than being trapped by the food. Some migration of the model compounds was observed into foods. The presence of fat on the surface of the foodstuff enhanced the trapping of these volatile compounds.
Ice crystals formed on retail polyethylene and cartonboard packaging during the freezing of food. No condensate was observed on cartonboard. This is believed to be due to the cartonboard soaking up any moisture that condensed onto its surface. Low levels of a polar compound were detected in water condensate on an impregnated polyethylene surface. Condensation dripping onto the foodstuff was not observed but it is expected that it could be transferred during removal of food from the packaging.
In tests using frozen food simulants, distilled water and olive oil were poor simulants for frozen foods placed in indirect contact with cartonboard and plastic. Losses of volatile and semi-volatile substances from the packaging were similar, but the pick-up of substances by simulants was much less, generally, than that observed for foods. This is thought to be due to a combination of two factors. First, the simulant surface (a frozen liquid) presents a smaller effective surface area for trapping substances than does the food surface. Second, the absence of effective surfactants or emulsifiers in olive oil, in contrast to many foods, could allow the lipophilic surface of the frozen oil to become 'masked' by a superficial layer of water (ice) condensate.
No evidence was found for any special effect of the freeze-thaw cycle. Existing understanding of such parameters as time, temperature, volatility, and the relative affinity of the substance for the packaging and the food can be used to explain migration into frozen foods.
Findings that food simulants were poor models for frozen foods placed in indirect contact, are consistent with an earlier evaluation of chemical migration during low temperature storage of packaged foodstuffs with direct contact. That study concluded that standard testing protocols using short-duration contact with simulants at 5°C are appropriate both for chilled and frozen storage of foods.
Dissemination information
Final report is available from the FSA Library and Information centre.
To obtain a copy, please contact the Enquiry Desk, Dr. Elsie Widdowson Library and Information Services, Food Standards Agency (020 7276 8181/8182 or at infocentre@foodstandards.gsi.gov.uk).
See Also