FOOD INDUSTRY INFORMATION SERIES
Understanding Microwave Browning Dishes, In-Pack Food Crisping and other Microwave Active Food Packaging Heating Techniques.
The criticism levelled at microwave energy that it does not brown or crisp foods was initially seen as a barrier to oven growth. In reality, this barrier has been somewhat overcome by using various techniques that can be used to create a browned food appearance, in many instances similar to those of conventional cooking, but at microwave enhanced speed. In the early days of microwave sales many leading manufacturers included a comprehensive microwave cookery book with each oven, explaining how to achieve the best results from their appliance.
With most meat, fish and poultry products, browning techniques included using powered coatings or made-up pastes comprising butter, salt or sugar, colouring agents, and various mixed spices. Some condiment companies established proprietary microwave sprinkle-on additive brands for applying to protein food items. This had the effect of concentrating energy at the foods surface, increasing its temperature, and thereby creating a near conventional cooked appearance. In many instances and depending on the specific food item, also achieving an acceptable degree of crisping.
In an endeavour to create microwave browning, leading oven manufacturers experimented with much higher (9GHz) microwave frequencies, having a wavelength and similar heating characteristics to those of thermal infra-red frequencies. However, it was found more practical to achieve the higher temperatures by using cost-effective conventional grills and hot-air ovens. Nowadays, these functions are included within a microwave combination oven.
During the 1970s both Litton and Corning Glass introduced a new concept of ceramic browning grills, which were able to absorb the ovens microwave energy converting it into heat. These dishes, depending on the output power and oven set heating time duration, could reach operating temperatures of around 350°C. They were used to grill meat cutlets, fish and poultry items, and freshly toasted cheese and ham sandwiches!
They were soon followed by other speciality cookware manufacturers who introduced metallic microwave absorbent browning dish variations.
Microwave In-Pack Food Crisping Systems
The fundamental difference between the ceramic and metallic browning dishes, and consequently the food packaging materials that followed, is in their ability to absorb different aspects of the microwave energy.
The microwave transparent ceramic dishes have a metallic oxide coating beneath the food heating area that absorbs the electric (E) field much in the same way as food. Due to the lack of microwave electric field strength at a metals surface, metallic browner dishes use a ferro-magnetic material bonded to the metal dish base, which absorbs the magnetic (H) field energy and converts it into heat. This is then transmitted to the dish cooking surface by thermal conduction.
The same basic principles employed with browning dishes are currently utilised within food crisping in-pack systems, using more advanced film materials and techniques.
The susceptor material consists of a polyester film onto which an optically thin layer of metallization has been deposited. The deposition thickness is critical and applied so that it effectively reacts with the microwave energy to generate heat. Unlike browner dishes, the low mass susceptor material is a one-shot device, where food storage life-span can be affected by oxidation and its heating is less effective when wet.
Low-cost aluminium is preferred for single-mode E field applications. The film is adhered to paper or a fibreboard substrate for pizza bases, pouches and cartons, and used as a thin film for breaded and pastry products where surface texture is important. It can also be used as a heating patch within popcorn applications, to melt the flavour ingredients and coat the individual kernels prior to popping the corn, and thereby ensuring uniformity of flavour throughout the eating experience.
These in-pack crisping examples work particularly well with many frozen food items. This is due to the microwave energy being preferentially absorbed by the susceptor film, generating higher temperatures prior to the product defrosting, and thereby creating a more effective crisped appearance.
Stainless steel inconel susceptors can be similarly applied to achieve much higher temperatures and improve heating performance uniformity, by using microwave dual-mode E and H field applications.
It is well known that small pieces of aluminium foil can be used to shield delicate food areas such as the thin part of a chicken drum-stick or wing portion, and prevent them from becoming overheated. This technique has now evolved into a more complex packaging system, where pack designs can be configured to internally modify the ovens energy field to meet a specific microwave heating need.
The basic objective of field modifcation is to alter the microwave energy distribution within the pack to meet the heating requirement. This can mean controlling everything from shielding the product to promote even defrosting throughout, or protect an individual item that needs to remain cold during use, together with in-pack even-heating, browning and crisping. This is achieved by using selective film demetallizing techniques that allow accurate control over the heating environment.
Other systems are able to modify the energy field within the pack and thereby intensify the microwave impact at the foods surface. The central rectangular foil tray and accompanying field modifying lid showing receptor patches (Insert No.15) was used for a frozen potato topped product to enable it to be cooked and surface browned by microwaves alone. Similarly the oval fibre-board pack using metalized transmission and shielding techniques, together with its microwave field intensifying film cover, has been used to evenly defrost and cook a frozen chicken breast portion within the pack.
The top circular pack together with the bottom red coloured equivalent, have been used for intensified in-pack browning of frozen meat and fruit pies. If properly used under good quality microwave energy distributions conditions, it is possible to bake frozen pastry products from raw when using this type of pack!
© February 2011 GAMA Microwave Technology Ltd.