When the first combination ovens appeared from Germany in the late 1980s they revolutionised oven cooking by allying a pressureless steam-generating boiler with a hot-air convection oven. This gave chefs composite cooking processes that were able to achieve lower meat shrinkage, better nutrition retention, low-temperature steaming of delicate foods and other menu-enhancing benefits.
Modern versions of these early ovens still dominate today's combi market but any caterer looking to update their kitchens needs to be aware of the dichotomy in oven design of the past decade. While it's not always clear from product literature, a growing number of combi-oven makers have taken the radical step of eliminating steam-generating boilers.
Instead, steam is produced directly by drawing water through a pipe into the oven cavity and squirting it at the centre of the ventilator fan, which circulates hot air around the cavity. As the water hits the hot surfaces, it turns into saturated steam. The result of this "boilerless" method is a simpler design of oven costing 8% to 12% less because it requires fewer components.
Typically, ovens without boilers cost less to run too, since steam is produced only on cooking cycles that need steam. For example, Leventi Combimat ovens with direct injection systems (sold in the UK through Valera) use no water at all during standby, while features such as double insulation and door coating further contribute to lower water usage.
Combis with boilers, on the other hand, need to keep the boiler operating so that fresh steam can be produced immediately it is needed.
So is there an open-and-shut case for boilerless combis? Combi market leader Rational believes that direct water injection entails various problems. First, water entering the cabinet is prone to a temperature drop, which in turn influences cooking results adversely. There is also thought to be greater stress imposed on the metal inside the cabinet when cold water hits hot surfaces, as well as increased scaling of surfaces, with consequent higher cleaning expenses. This in turn makes oven operation more cost-intensive because of the need for additional water treatment.
Graham Russell, Lainox brand manager at Dawson Foodservice Equipment goes further. "When water is sprayed directly on to a hot surface to produce steam, a chemical reaction can happen with the chlorides in the water producing acids that can attack the cavity." His company used to sell boilerless models but now prefers a boiler design.
Nick Bates, development chef at Angelo Po, defends the boilerless combi against the charge that spraying water inside an oven increases the problems of scaling. "The cavity is the only place you can get scale and, because it is completely visible, it is easily treated with our cleaning product." By comparison, the pipe that feeds boilers can harbour unseen scale.
Hobart UK concludes that both types of oven are susceptible to scale and should - without fail - be installed on a food-safe, filtered water supply. "Yet time and time again operators either fail to install one or fail to maintain it," observes Richard Hurst, Hobart product director.
Combi manufacturers are bringing refinements into this area, notably monitoring systems that warn of internal scale build-up on boiler models. For example, Fagor's Evolution combi series, introduced in October 2006, features automatic decalcification and also automatic draining and refilling of the steam generator every 24 hours.
Is water consumption higher on a boiler model? Graham Russell claims that a Lainox electric six-grid combi-oven, which has a separate boiler holding 2.5 litres of water, uses up 10 litres of water per hour, and then only if it is producing 100% steam for that hour - "which would be very doubtful" he points out. If a user needs to do a lot of convection-only cooking, it is possible to switch off the boiler to reduce running costs.
Nick McDonald, marketing and export director of Lincat, which sells the German-made 24-model Opus series, maintains that steam from a built-in boiler can be more accurately measured and therefore controlled for more consistent cooking, with desired humidity maintained to an accuracy of +/-1%. "This would simply not be possible with the somewhat hit-and-miss nature of spraying cold water into the cooking cabinet," he points out.
One outcome with direct systems is, he says, an immediate drop in cabinet temperature when the cold water is introduced. "So 100% steam is not always available," he says. "This can have a detrimental effect on the cooking process, for example causing brown discoloration to sensitive vegetables such as Brussels sprouts or cauliflower."
"Horses for courses" is the conclusion of Roger Flanagan, managing director of Universal Foodservice Equipment, which imports Italian-made Baron combi-ovens available in both boiler and boilerless versions. "In banqueting, for example, it makes economic sense to choose a model with a boiler which generates steam that is instantly available." But he concedes that only 30% of users actually need this and others can get by with the less costly boilerless option.
Best of both
Another way of getting the best of both worlds is seen on the Retigo range of combi-ovens now imported from the Czech Republic by Bradshaw. These have boilers but an exclusive additional "double protection" facility where the oven switches automatically to direct steam production should any problems occur in the boiler. This enables the chef to carry on cooking until the problem is rectified. According to Tony Challice, Bradshaw's group technical services manager, a pipe drawing water into the cavity serves to preheat the water before it is atomised, enabling very rapid steam production preheating is aided by a recirculation system which utilises waste steam.
Preheating of the incoming water prior to atomising is also seen as a key point of difference on the boilerless combis made by Germany's Eloma, which have been introduced to the UK in the past year by Aga Foodservice. The company's Multi-Eco ovens are said to reduce water consumption by up to 40% and energy consumption by 15% compared with comparably sized ovens with conventional boilers.
These gains arise partly because the system draws water only when steam is needed and partly because the water is passed through a heat exchanger. This uses heat exhausted from the cavity to push up water temperature as high as 70°C before it is sprayed on the heat elements in the cavity.
Latest Henny Penny SmartCombi ovens just launched by ServEquip are claimed to bridge the gap between boiler and boilerless combi ovens. A patented system called Advanced Steam Technology is designed to provide greater control of relative moisture and the cooking climate, and contribute to reduced maintenance costs.
Users need to consider their specific needs, particularly the relative importance of dry heat, steaming and low-temperature cooking and also door opening if it's very frequent, there is general agreement that a boiler-type combi will cope better.
- Price: 8-12% higher
- Boiler feed prone to scaling
- More precise low-temperature steaming
- Less affected by door opening
- Slower steam production
- Visible scaling in cavity
- Less technically complex
- Fewer servicing issues
Key Maintenance points
Combination ovens have become critical equipment in almost every kitchen, so don't ignore the simple signs that something is wrong before doing something about it:
- If the fan or elements are cream coloured when hot (or grey when cold) there's scale on them.
- If you don't have cleaning agents approved by the manufacturer, get some - don't use a scourer to clean the oven, inside or outside.
- If the fan or elements show a build up of limescale, this may affect the cooking temperature control, and overall efficiency.
- If the electronic message says "Limescale", you need to have the steam generator descaled.
- If you have a cartridge filter fitted, check the water meter reading with your supplier to make sure it's not exhausted.
- If the programme says to start the cleaning cycle, don't ignore it - do it soon.
- If the door seal is split it won't fix itself, but it will leak steam and get worse.
- If it's a gas combi, get it checked for gas safety at least once a year.
Source: AFE Serviceline 01438 363 000