Product reheating

Packaging issues relevant to product reheating

By Michael Bonin and Greg Hooper - 9 November 2015

Product reheating is the warming of ready-cooked foods by consumers and food retailers. The requirements of reheating will vary in terms of the temperature required to give an assurance of food safety and palatability, and this is dependent on the nature of the food, the processing the food has undergone, previous storage conditions, and the packaging used to contain, preserve and protect the food.

Reheating instructions on product labels are developed from a ‘safety first’ perspective, but there is often scope for optimising quality without compromising on product safety. The overall objective of reheating is to provide enough heat to ensure food safety, without overheating the product or packaging.

As a rule of thumb, minimising the amount of heating (time and temperature) required will optimise quality and this can be facilitated by ensuring, as far as is possible, that all parts of the product attain the required temperature at the same time. The arrangement of the components in a food product can assist in management of this objective, particularly in microwave-reheated foods, while the design of the packaging is also crucial in ensuring reduced heating times.

Package design has significant implications for those producing instructions for product reheating in both microwaves and conventional ovens. In microwave ovens the microwave energy will behave in different ways according to the geometry of the pack as well as the nature of the packed product.

Actual microwave penetration depth is dependent on various factors, including food composition (especially ionic ‘salt’ content), the physical state of the food (frozen or thawed) and the density of the food. In both conventional and microwave ovens, the heating of food depends on other factors such as pack dimensions and geometry, thermal diffusivity/specific heat capacity of the product and packaging, as well as the convection, conduction and radiation of thermal energy through the packaging and product.

'Hot Spots' in heated food may lead to deformation of overheated packaging, reducing pack integrity and resulting in thermal injury risks for consumers handing the packaging, while thermal breakdown of the packaging materials may increase chemical migration from the pack into the food. Microwaves in particular can generate very high temperatures in food products and determination of the maximum product temperature is therefore vital in assessing the resistance of the packaging to deformation and/or melting. Thermal imaging is a very useful tool in the determination of the location and magnitude of any developed hotspots.

Uniformity of product temperature in reheating is another significant issue in terms of selection of packaging materials and for packaging design.

Packaging can be designed to minimise temperature uniformity issues. Reducing product thickness in any dimension to approximately 2cm or less can allow faster product heating, while reduction of the depth at the geometric centre of the product can increase the rate of heating at this point, thereby reducing overheating issues in more peripheral areas of the product. Depending on the viscosity and pliability of the food, packaged products tend to mould themselves into the shape of the pack. Avoiding packaging with sharp edges or corners may reduce overheating in those protruding areas and the use of a shallow pack format with a raised base can reduce the required heating time (and thus reduce overheating issues). Because of the differences in heating performance it is imperative to thoroughly test the product and packaging in a wide range of disparate microwave ovens. The development of packing using a limited number (and type) of microwave ovens can lead to issues when a wider range of microwave ovens are used by consumers.

About Greg Hooper

Greg has worked here at Campden BRI since 1990, after studying Applied Science (Physics and Chemistry). As part of his extensive knowledge and experience in thermal processing and microwave cooking, he was instrumental in setting up the microwave heating category rating system used in the UK, and has travelled internationally assisting and advising on the safe development of microwave products and rating systems.

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