Kettles are high-production, high-volume steam-heated pots most frequently used by large institutional food service operations like those found in hospitals and school cafeterias. They are well suited for the production of soups, sauces, chili, vegetables, braised and simmered meats, eggs, noodles, rice, and many deserts. The steam heat cooks efficiently and in about one-third the time of a stockpot on a hot top. Kettle capacities range from roughly 1 quart to 200 gallons. They are available in floor mounted, cabinet mounted, or wall mounted models.
Kettles provide an operator a method for cooking large quantities of liquid foods such as soups, stews, jellos, etc. quickly and efficiently. They also are designed so that these products can be readily decanted or removed from the cooker without the back breaking lugging of traditional stock pots.
One of the primary advantages of cooking with a kettle is that it is much less laborious than cooking over a hot stove. Also, since the kettle is essentially a double boiler, there is no danger of scorching food, as there is when cooking with direct heat. There is an inner shell and an outer shell in the construction of the kettle. The outer shell can extend up the sides half-way, two-thirds, or all the way to the top of the inner shell. This allows the inner jacket to be heated up the sides to one-half, two-thirds, or full (to the top). This provides many more square inches of heating surface than can be obtained with a stock pot on a range. The stock pot can only be heated on the bottom, and foods are subject to scorching and sticking. This is why kettles require little supervision during the cooking process, and thereby save labor.
Kettles are either one-half, two-thirds, or fully jacketed. One-half and two-thirds jacketed kettles make up the majority of the installations. The steam jacket surrounds two-thirds of the kettle's product-contact surface. These units are usually deep and use a minimum of floor space. Fully jacketed kettles are shallower and are usually selected for special-purpose cooking situations because they reduce food crushing, as is experienced when boiling potatoes, and they provide easier access to cooked products.
Many kettles are equipped with an accessory rack, which fits across the top of the kettle and holds a stainless steel basket. This basket is lined with a nylon mesh. It is designed to allow simultaneous preparation of several bland food items, such as potatoes, pasta, or cauliflower.
Some kettles have built-in automatic stirrers, blending adapters, or mixers. Some units are even equipped with separate piping systems for pumping food into quick-chill containers. Accessories can have a significant impact on labor costs. A mixer on a kettle, for instance, can permit one worker to operate multiple kettles, where kettle operation would normally require one person per kettle. As a result, such mixers are usually a sound investment.
Larger wall- and floor-mounted kettles are normally equipped with a tilting device. Tilting gas kettles have a safety switch that disconnects the gas when the holding vessel is tilted more than 5° or 10°F.
Electric Vs. Gas
There are many factors to consider when selecting a kettle: first cost, food preparation productivity, ease of operation, and heat generation in the kitchen, as well as the energy source used: electricity or gas. Remember that energy only accounts for 3 to 5 percent of a food service establishment's total costs. So, while one fuel may be less expensive in a BTU to BTU comparison, the best choice in cooking equipment is the one that minimizes total operating costs, not just energy costs. Features that reduce labor costs or result in higher food product yield will nearly always outweigh any energy considerations. Make sure that you include all of these factors in any equipment evaluation.
Let's take a closer look at the energy-use issues, remembering that comparisons should be made on the basis of similar equipment where only the energy input is being changed.
- Electric and gas powered kettles operate at similar efficiencies. The main advantage of the electric kettle is that it is easier to clean and maintain than a gas unit. Gas units generate sooty carbon deposits, which must be removed from the components and cooking surfaces regularly. Gas units also release combustion gas into the kitchen, which must be removed by the ventilation system.
- Kettles that can use existing boiler capacity will nearly always prove less costly to operate than either electric or gas powered units.
There are two types of kettles available to the foodservice operator.
Kettles come in a variety of sizes from 3 gallon to 120 gallon capacities. Most kettles have a draw off value (tangent) in the bottom to remove product. If it is a tilting kettle, there may be a draw off value in addition to a pouring up. The pouring up helps to direct the product flow into a pan or pot.
Tabletop kettles are mounted on a table, stand, or wall and are simply smaller versions of the larger kettle. Stationary and tilting types are available. Some can be ordered with mixer attachments and most larger kettles should be ordered with a lid.
A trunnion kettle is a 20 to 40-quart cooker used predominately by military and family-style food service establishments for re-heating pre-cooked foods. See the discussion of "cook-chill" systems to see why these are growing in popularity.
In military base kitchens, pre-cooked food arrives from the commissary in a bag. These bags functions as both a package and a cooking container. To prepare, they are placed directly into the kettle's boiling water for re-heating. This helps assure uniformity in the food preparation and minimize the potential for bacterial contamination.
Twenty- to forty-quart trunnion kettles are available as either table top or special cabinet-mounted units. Medium-sized models have tilting capabilities, so they are perfect for cooking and serving vegetables such as green beans and corn. They also work well for foods that are cooked and held for serving, such as soups.
Large kitchens, offering a wide selection of menu items every day, often use several large kettles. This may present some interesting operational challenges. Kettles exceeding 60 gallons are actually so large they require the user to stand on a foot-stool to access the kettle contents. Very large kettle units, up to 120 gallons in capacity, are wider and lower to the ground than smaller units. This helps reduce potential dangers such as the cook falling into the kettle.
Generally, the larger kettles, 80 gallon and above, require the addition of a kettle mounted mixing motor. Hand mixing that much product becomes impractical. For 60 gallons and under, a large paddle that looks like a ship oar is used for stirring.
Kettles are steam-heated vessels that cook by the transfer of heat from the steam through condensation against the kettle walls. They are intrinsically simple devices. Most kettles have a steam jacket that covers at least two-thirds of the vessel. Some models have a steam jacket that covers 100% of the kettle but these units are rare and are considered specialty applications by most manufacturers.
The steam pressure in the vessel jackets is precisely controlled. Zero pound (also called gauge pressure) steam at sea level produces 212°F heat, and 50-pound steam produces about 298°F. Higher pressures cook food faster, but could run the risk of scorching. Many kettles have mixing arms to scrape the inside heated surface to reduce scorching and to mix the food being cooked to create a more uniform temperature. Safety valves are located around the top of kettles to vent steam in the event that pressure rises above its setpoint, preventing rupture of the steam jacket.
Condensed steam is drained from the vessel jackets using special control valves and some kettles are piped, so that they are capable of alternating between steam and cold water energy sources. These designs are popular for the "cook-chill" concept discussed elsewhere in this program.
Common sense will eliminate most energy waste when using kettles. The most common wastes are associated with improper steam system water chemistry, which can result in premature heating-element failure, deviations in cooking time and product quality, and even food scorching.
Operational common sense includes turning the heat source on only when needed and repairing steam leaks as they are identified. Lids should also be closed when possible to reduce heat losses.
If the customer is cooking highly acidic based foods like tomato sauces, they should order a kettle with a 316 type stainless steel inner liner. This will prevent the metal from becoming pitted. Normally manufacturers use 308 type stainless steel which is more than sufficient for non-acidic based products.