Microwave

Thursday 28th April 1994 00:00

AIR FILTERS

Regular cleaning is essential for filters as grease and dirt can block the air intake, possibly causing components to overheat and break down.

BAR CODES

One of the most recent innovations is a microwave oven from Bradshaw which can read up to 250 bar codes and automatically set the timing accordingly.

BRITISH

STANDARD

BS5175 is the safety standard applicable to commercial microwave ovens.

COMBIS

Combination microwave ovens may also offer convected heat or a grill, or both, in combination with microwave energy. Zanussi's Combi Wave provides a combination of convection, steam and microwave power. As well as offering speed, combis can make food crisp and brown - unlike a microwave.

Combis may be operated on microwave-only programs, but they are generally not as quick as standard microwave ovens. They are substantially more costly than standard microwaves. As a result, combis are probably best as alternatives to convection ovens in situations where speed is essential.

CONTROLS

Most microwaves now feature microprocessor controls with digital displays which are more accurate than reading timings from a dial. They may be operated with dials, push-buttons or touch pads. Dials are probably the simplest to use and are often liked by chefs who choose a wide variety of different timings. Push-buttons and touch pads are particularly suited for use in self-service and vending where timings are pre-set. An added bonus of touch pads is that they are smooth and easy to keep clean.

DEFROST

There is no standard wattage for the defrost setting, so never assume two different ovens will defrost at the same speed. During defrosting the power is "pulsed" on and off to aid even thawing. More sophisticated ovens may offer a "graduated defrost", giving an initially high level of power, followed by a lower level to provide quick defrosting without the risk of edge burning.

DISPLAYS

Diagnostic and error messages are shown on some ovens' displays as well as the time programed - features which can cut down service calls and speed up any necessary repairs. Visually, the clearest displays are normally considered to be provided by light-emitting diode (LED) and vacuum- fluorescent (VF) systems.

DOMESTIC OVENS

Ovens made for the home are just not up to life in a commercial environment. Their construction is too flimsy which can lead to faults such as broken doors and cracked interiors. Also, they lack sufficient power to consistently heat food to required temperatures in a busy outlet and may even cut out if continuously used. Typically, domestic microwaves are rated at around 600W, compared to most commercial models which are 1-2kW or more.

DOORS

When siting a microwave oven, make sure there is enough space to allow for the door to open fully and so it is easy to insert and remove dishes. Most have hinged doors, but some ovens from Merrychef feature "pop-up" doors which are especially useful in confined spaces. Commercial microwave ovens are designed to cope with a minimum of 200,000 door operations, which is twice the target for domestic ovens.

ELECTRIC SUPPLY

Small microwaves can usually be plugged into a 13-amp socket, but larger ones may need a 20- or 30-amp supply.

EMPTY

Increasingly there are trips on microwave ovens to prevent them from being operated when there is no food in the cavity - a practice which may result in damage to the magnetron.

FANS

Fans are needed to keep the components of an oven cool so they can work properly. A domestic microwave oven probably will not have a sufficiently good fan to do the job over the prolonged periods that most caterers require. This may cause the magnetron to overheat, which could in turn lead to under-cooked food.

FIRE SENSORS

Some models have fire sensors that automatically switch off the power. If your oven does not have this feature and food ignites, immediately disconnect the power at source. Leave the door shut so that the fire is starved of oxygen and so extinguished. Get the oven professionally checked after a fire.

FOIL

Foil containers used to be the subject of a blanket ban for microwave use, but now they can be used in some ovens under some circumstances. If you need to used foil containers, consult the individual oven supplier.

GUARANTEE

The guarantees of domestic microwave ovens are invalidated by commercial use. Guarantees for commercial models vary quite substantially in what they offer in terms of time and cover.

HOT AND COLD SPOTS

Poor appliance design may result in uneven heating. It is also common in low-powered ovens, and turntables are often used to overcome the problem in domestic models. More commonly, however, hot and cold spots are caused by food being thoughtlessly arranged. For example, a mound of mashed potato will not be fully heated in the same time as a thin slice of meat in gravy.

INTERIOR

Ideally the oven should have a stainless steel or other non-chip cavity. Rounded corners are also desirable because they make it easier to clean the appliance. Sealed base shelves ensure that there are no traps for spilled liquid. Check that the cavity is big enough to accommodate your dishes - some are extra-wide to take two plates side by side.

KILOWATTS

Both the input and output powers of microwave ovens are measured in Watts or kilowatts - 1kW is equal to 1,000 Watts.

LEAKAGE

Emission outside the cavity is most likely to occur if the oven has been badly knocked or dropped, damaging the door or its seals. If a door has been damaged, the oven should not be used until it has been professionally checked. Checks for leakage are part of any good maintenance programme offered by a reputable supplier. DIY test kits are rarely a good idea - some are so poorly calibrated that even a television can provoke a reading from them.

LEVELS OF POWER

Most ovens can be operated at several different power levels. For example, full power, 75%, half power and defrost. Lower power levels can be useful for prime cooking of delicate foods like fish or for getting an even temperature when reheating dense foods such as lasagne.

LOAD

Check that the oven is suitable for your particular menu because some are designed specifically for snacks, while others are better suited to large loads.

MAGNETRON

Microwave energy is produced by magnetrons and up to four may be used in a single oven. Extra magnetrons do not necessarily mean better speed or power, but are simply one way of designing the appliance so that the energy is balanced. Over a period of time or heavy use, the power of a magnetron will decline.

OUTPUT POWER

Very broadly, the output power of a microwave oven is 50-60% of the input power, so it is important not to confuse the two. When ovens are described as "900W" or "1.4kW", it is the output power that is being referred to.

PRE-SETS

It is helpful to have pre-set timings if you regularly cook or heat the same menu items. In vending applications these pre-sets are invaluable.

QUICKNESS

Speed of cooking depends primarily on the output power of the microwave oven. Caterers wanting a high throughput need ovens with higher power ratings. The following list, from manufacturer Valera, shows times and throughputs in reheating medium-sized, cooked jacket potatoes in ovens of different ratings. An extra five seconds has been added to place and remove each potato.

RATING

Various ratings tests have been used over the years producing different verdicts on the output power of appliances. In one test an oven could be rated at 1.2kW, while another might say it was 1.4kW. Most suppliers now use the IEC 705 standard test. This enables different models to be compared, but it has also resulted in some ovens being rated up to 23% higher than they would have been under other tests. So check that replacement ovens really do have the same power as old models.

SERVICE

It is worth checking on servicing arrangements in and out of the guarantee period, plus the availability and stock of spare parts. In a busy kitchen, the oven may be used 1,000 times a day so a rapid response to emergency call-outs is a must.

SHELVES

All microwave ovens have a shelf about 30mm above the base of the cavity. Made of pyroceramic material, they enable the microwave energy to pass through then reflect back off the metal of the base. This results in some microwave penetration from the bottom of the food, as well as from the top and sides. Ideally the shelf should be sealed so that spills cannot get underneath.

STACKING

Some microwaves, for example some models from Crypto Peerless, can be stacked to save space.

STAND-BY

A stand-by facility cuts fans and lights if the oven is not being used, saving energy and prolonging their life.

TURNTABLES

Only domestic microwave ovens have turntables. They are unnecessary in commercial ovens because of their greater efficiency and undesirable because they are easily broken in the hurly-burly of a commercial kitchen.

USES

Most catering kitchens can benefit from installing a microwave oven. It is estimated that about 80% of caterers use one. Surveys indicate that reheating accounts for three-quarters of their catering use.

VENDING

Features which are particularly useful for vending applications include pre-set time and power controls that cannot be tampered with.

WAVE STIRRER

A wave stirrer or waveguide is a component used to help to create an even heat pattern in the cavity. Typically it rotates at the top of the cavity and is often mistaken for the fan.

ZAP

US slang for microwaving!


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