The chill factor

Blast chillers and blast freezers are relatively expensive compared with other forms of refrigeration equipment, but they are a necessity on grounds of food safety in any catering business using cook-chill or cook-freeze operations rather than cook-serve.

Large institutional kitchens have had blast chillers and freezers for many years, but they're now considered just as essential for food safety in hotels where the banqueting system uses cook-chill.

While traditional banqueting service has been of the cook-serve variety, with silver service, this is rapidly losing favour with hotels. The problems it poses are in the co-ordination of making all items ready at the same time, maintaining food temperature, good portion control, chefs losing control over plate presentation, and the increasing difficulty of finding casual banqueting staff able to do silver service properly. In comparison, cook-chill delivers on all these points.

While the combi-oven is the first and last part of the cook-chill banqueting process, the blast chiller is the vital part in between. While a high cooking temperature kills almost all bugs in food, airborne bacteria are impossible to exclude from the kitchen and, during the process of cooling hot food, there's a high-risk temperature zone - typically between 25¡C and 65¡C. In this temperature range, any airborne bacteria that settle on food will multiply at an alarming and dangerous rate. Of course, bacteria will also multiply at temperatures above and below this range, but more slowly.

It's wrong to assume cooking kills all traces of bacteria, says Roger Hart, a consultant environmental health officer for manufacturer Total Refrigeration. "Many bacteria can create spores that can survive the cooking process," he adds.

Chilling food to a safe temperature as quickly as possible is the role of the blast chiller. Food held in a fridge is still subject to harmful bacterial growth, but the faster the food has been chilled to fridge temperature, the safer it will be. Blast chiller guidelines say that any cooked food should be brought to 3°C in not more than 90 minutes.

Letting food cool unaided is highly dangerous. Putting cooked food in a standard fridge is equally unsafe, since not only is a fridge incapable of pulling down the temperature of hot food, the temperatures of other foods in the fridge will be raised, creating another food-poisoning risk.

In simple language, a blast chiller is little more than a fridge with a beefed-up motor to drive the refrigerant liquid around the condenser panels, and powerful fans. The principle can be demonstrated by wetting your hand and exposing it to a howling wind on a bitter winter's night - it would not take long for your hand to cool down through a combination of conduction (wetness) and convection (wind) chilling.

Just as some foods need a delicate touch in the combi-oven during the cooking process, so do some foods in the rapid chilling process. Blast chillers thus have two modes of operation: hard and soft chilling.

Hitting the high-chill button for everything is wrong, according to Lawrence Hughes, UK sales manager for Williams Refrigeration. "Using the hard blast chill button for all food types can lead to more delicate products being frozen as a result of the air temperature going below zero, resulting in damaged appearance and texture," he says. "The hard blast chill process is designed for more dense products, such as mashed potato, that can tolerate the lower blast temperatures with no impact on texture, flavour or appearance. In the soft chill function, the air temperature remains above zero and eliminates the risk of damage to the texture and appearance of delicate foods such as fish and pastries."

Blast freezers work on the same construction principles as blast chillers, but they are more powerful, able to bring food temperatures to -18°C, the usual temperature for holding frozen food. In a central production unit such as at a hospital, the blast freezer would be a walk-in unit with cooked food, in either single- or multi-portion foils, wheeled in on production trolleys.

Correct thawing is just as important as correct blast freezing. The safe way to do it is either in a standard fridge, for small amounts, or a dedicated tempering fridge, if large amounts of food are being thawed. A tempering fridge, instead of maintaining a constant temperature during a thawing cycle, will slightly raise cabinet temperature in the first part of the defrost cycle to kick-start thawing, then automatically lower to holding temperature.

While blast-chillers are usually associated with banqueting, they do have a use in smaller establishments. The Ellerby Inn near Whitby, North Yorkshire, has a busy food operation with up to 150 diners in the restaurant and bar areas on busy Sundays. The problem is meeting the food quality demanded by owner David Alderson, combined with the speed of food delivery expected by pub customers.

The kitchen has a Foster blast chiller, used in conjunction with a vacuum packer. For the huge demand for Sunday roast dinners, joints are cooked in a combi-oven on a Friday, chilled, portioned, vacuum-packed and held in refrigeration. Regeneration is done in the combi-oven to order throughout Sunday service - and, being vacuum-packed, the food suffers no loss of succulence or flavour. Alderson admits the blast chiller is an expensive item compared with standard refrigeration, "but if you're going to run the system we do, you have to have one," he says. "You can't compromise on food safety." n

Blast chiller safety rules

Food that has been correctly cooked and then chilled to 3°C within 90 minutes should be safe to reheat and eat, for as long as five days. However, because of the storage implications, most kitchens turn the food around in no more than two or three days.

Dense foods such as meats and pasta dishes will take longer to chill than lighter foods such as rice or vegetables. Food produced in gastronorm trays shouldn't be deeper than 50mm.

Avoid overloading on both blast chillers and blast freezers, as this will cause the food to remain longer in the bacterial danger temperature range of 40°C to 60°C.

While rigorous cleaning is important with every item of refrigeration equipment, it's particularly important with blast chillers and freezers because of the length of time the food is likely to be stored in them.

Buy or lease?

One of the arguments against buying a blast chiller, when cook-chill is not part of the normal food production cycle in a kitchen, is the cost compared with that of other refrigeration equipment. An increasingly popular route around this problem is leasing.

Christian Williams, commercial manager of Bristol-based Dawsonrentals, says that, apart from removing the need for capital expenditure, there are accounting benefits to leasing, with long-term rental equipment not counted as an asset on the balance sheet and not having to be written off through a depreciation policy. Payments are treated as an operating expense and are paid from cash flow.

Contacts

CESA (Catering Equipment Suppliers Association)
020 7233 7724
www.cesa.org.uk

CEDA (Catering Equipment Distributors Association)
01274 826056
www.ceda.co.uk

Foster Refrigerator
0500 691122
www.fosterrefrigerator.co.uk

Dawsonrentals Display Refrigeration
01179 373310

Total Refrigeration
08456 448311
www.totalrefrigeration.co.uk

Williams Refrigeration
0800 526517
www.williams-refrigeration.com