So you think you know about… combi-ovens

A combi-oven combines several cooking functions in one oven. It uses dry heat - either still or fan-driven - and steam, which is injected into the oven when the food being cooked needs it. It can use both dry heat and steam in scores of combinations for different cooking functions.

An alternative name for the combi-oven is the combi-steamer. Manufacturers can't agree on which name best fits, but we'll stick with combi-oven.

Up to a third of the weight of a piece of meat can be lost during dry roasting through loss of water content. Having gentle steam in the oven during roasting minimises weight loss and produces a more tender joint.

The amount of steam injected into the oven cavity can be very closely controlled and the chef needs to guard against injecting too much steam and cooking for too long. A prime rib of beef roasted on a bed of root vegetables will end up being boiled beef and carrots if the chef just turns up the steam and heat and works on the "20 minutes a pound plus 20 minutes and a bit more for luck" system. The meat will have lost so much structure it'll just fall into shards at any attempt to carve it. Spoiled food is the reason why some chefs reject the combi-oven as too complicated.

A combi-oven is an enclosed cooking environment - a chef can't keep opening the door and peeking at what's happening as with an oven without a fan under a cooking range or on some island suites. Timing and humidity controls in a combi-oven are rocket science, not guesswork.

Fortunately, combi-oven manufacturers make rocket science easy, with on-board computer-driven cooking cycles that can be pre-programmed by pushing buttons which have symbols and with information displays that warn of disaster or success.

With a traditional oven under a cooking range, different joints of meat need different core temperatures before being both safe to eat and at the stage of cooking the chef wants.

Combi-ovens can deliver meat at exactly the stage of cooking the chef wants, and can do it every time with different sizes of joints. The oven does this through the use of a meat core temperature probe, separate from the oven with some smaller and cheaper combi-ovens, or connected to the oven control system with the bigger and more expensive ones, inserted into the deepest part of the joint throughout the cooking period.

In combi-ovens with integral core temperature probes, if the chef wants lamb or beef to be cooked pink, the probe will switch the oven either off or on to a gentle heat-and-hold cycle when the required core temperature is reached. If it's an oven full of beef fillets for a banquet and the restaurant manager insists on not a trace of redness, no problem. The oven can cook so that the meat is well done, yet still retains moisture because of the steam injection. The cooking cycle is preset and left until the bell rings, buzzer sounds or LED lights start flashing.

These are other functions a combi-oven can deliver:

Fish - roasting, poaching, grilling and steaming all become easy in a combi-oven when a large quantity of fish is being cooked. The oven can be programmed for a high steam content and lower heat than would be needed for cooking meat so the fish has the texture of poaching without the flavour loss or breakdown of texture which can happen with poaching in fish stock. If fish is to be grilled, the combi-oven can reach a temperature to flash-grill plaice fillets, or a lower temperature for roasting items like salmon portions. Using the temperature sensor probe allows perfect cooking of whole salmon for buffets, while finishing au gratin for sauced fish portions is at the push of a button.

Vegetables - by cooking in steam instead of boiling water, vegetables keep more flavour, nutritional value and colour. If a precook, chill and regeneration for service system is required, steamed vegetables regenerate as if freshly cooked. Bulk quantities of roast potatoes can be produced through dry roast, or jacket potatoes by first softening with a hot steaming then crisped on a hot convection cycle.

Baking - if the combi is operated as a fan-driven convection oven, baked goods are evenly and crisply cooked. A slight injection of steam can also enhance some baked foods such as bread. Banqueting items such as individual Yorkshire puddings can be crisped in bulk. Most combi-ovens have variable fan speeds, so delicate items such as souffl‚s or crŠme caramel get a gentle waft of air rather than gale-force winds which would cause uneven cooking and setting.

Regeneration - food that's been precooked and correctly chilled before service can be rapidly brought up to serving temperature, avoiding the need to hold food hot for long periods, which leads to flavour loss and drying-out. Combi-ovens are ideal for busy banqueting operations and can handle both ready-plated meals and multiportion containers.

So what does the future hold for combi-ovens?

They will get increasingly clever. There are already models which can link into office management computers to keep a record of cooking temperatures and cleaning cycles to assist with HACCP protocol. In addition to convection heat and steam, microwave energy is coming into combi-oven technology.

Let's get technical

What do I need in the kitchen to install a combi-oven?
A water supply, an energy supply and at least £4,000. Combi-ovens will run off electric, mains gas, Calor and other forms of liquid gas.

How do I calculate the size of combi-oven?
An oven manufacturer or distributor will arrange this. The size needed is calculated by the amount and types of food to be cooked on a regular basis and the nature of the kitchen operation. A fine-dining restaurant may still need a combi-oven, but not the same size as a hotel with a busy banqueting operation.

The size of oven will be quoted in grids. This refers to the number of shelving slots in the oven cavity.

Spacing allowed between grids varies between manufacturers and size of oven, but the average is 65mm and a length and breadth dimension of one gastronorm.

Isn't a combi-oven for big food operations?
Combi-ovens come in a range of sizes and all manufacturers build ovens for the small independent caterer as well as the very high-volume outlets.

What technical questions should I ask?

• What are the performance and cost implications of steam coming from a water boiler or spraying water on to heated elements in the oven?
• Is it necessary to fit a water filtration system to the oven to remove dissolved salts in the water and prevent scaling?
• Does the oven have a high preheat function to enable fast heat recovery when cold food is put in?
• How easy are the oven cavity and door seals to clean, and what self-cleaning features are there?
• What are the programming features, are they easy to understand, do they meet my needs? Is there a self-diagnostic facility?
• Is there a food core temperature probe, rapid cool-down feature or a reversible fan?

The steam debate

Steam generation is at the heart of the performance of a combi-oven and there are two basic systems for producing the steam. There can be an internal steam generation boiler where the steam is held until needed, then injected into the oven cavity. The competing system is called direct injection or spray-on, where water is sprayed on to heated elements within the oven cavity to generate instant steam.

Arguments for a separate steam boiler

• Water filtration systems should be fitted to all combi-ovens, irrespective of prevailing water hardness, since this can change. Also, the absence of a water filtration system can contribute to corrosion because of chlorines in the water.
• The elements on to which the cold water is sprayed can suffer heavy limescale build-up if no water filtration system is fitted.
• Cold water sprayed on to hot elements cools the elements, forcing them to regain the correct temperature, leading to longer cooking times and higher energy costs.
• The amount of steam introduced into the cooking cabinet cannot be guaranteed with a spray-on system as water is fed into the cabinet, not steam. Steam boilers offer very tight control, essential for delicate work such as pastry.
• If no filtration system is fitted to the spray-on steam combi, any particles in the water are sprayed on to the food and limescale build-up can occur in the cooking cavity.
• The metal stresses caused by continually spraying cold water on hot steel elements can lead to high maintenance costs if replacement elements are needed. Three to four years is a typical lifetime for an element.

Arguments for spray-on steam

• There's no need to have an expensive steam boiler, so the overall cost with two comparable ovens is less with a spray-on steam system.
• Spray-on is quicker, whereas steam boilers need time to heat up.
• It uses only the water it needs and heats only the water it needs, saving on costs.
• Any scale problems can be seen early, so the cost of dealing with them is less.
• Kitchens in soft water areas may not need a water treatment system as most scaling problems occur in the steam generator and its pipework.
• Steam boilers can regularly go wrong on a combi-oven and maintenance is essential.

Cooking temperatures

While the combi-oven is an exact science, what constitutes rare, medium and well-done protein is not, depending on the chef's preference and warnings from the food safety police.

But here are some widely used temperature guidelines to achieve the degree of cooking preferred.

Lamb and beef joints Very rare 50°C
Medium-rare 56°C
Medium 62°C
Well done 75°C

Pork
British, with full traceability 72°C
Unknown source 75°C
Roast hams 70°C

Poultry 75°C

How do I find out more?

CESA (Catering Equipment Suppliers Association)
Tel: 020 7233 7724
E-mail: enquiries@cesa.org.uk
Web: www.cesa.org.uk

CEDA (Catering Equipment Distributors Association)
Tel: 01274 826056
E-mail: secretary@ceda.co.uk
Web: www.ceda.co.uk