Boiling, Stewing and Steaming in Kitchen

The processes of boiling and steaming are different from those considered in the previous chapter in that the heat IL required for cooking is obtained from boiling water; the food is cooked under moist conditions rather than in the dry atmosphere previously described. Moreover, in baking and roasting temperatures of up to 500°F. are common, whereas in boiling and steaming at atmospheric pressure the temperature does not rise above boiling point, 212°F.

In boiling, food is immersed in water maintained at boiling point and the heat is transferred from the water to the food by conduction and convection. Naturally the outside parts of the food in contact with the boiling water quickly reach a temperature of 212°F., but the cooking process is not complete until the required temperature has been reached throughout the whole of the mass. The required internal temperature is not necessarily 212°F. as in some cases the food is satisfactorily cooked at a temperature below this figure. Boiled potatoes are completely cooked when they are soft throughout, à condition which is reached when the center temperature (at 150°F.-160°F.) is sufficient to burst the starch grains and soften the cellulose. When meat is being boiled, on the other hand not only must the temperature throughout be raised to 212°F. but it must be held at this point for some time in order to soften the fibrous substance, collagen, and convert it into gelatin. The length of time needed for this purpose varies according to the size and quality of the piece of meat being cooked.

The process of stewing is very similar to that of boiling in that the food is immersed in water, the only difference lying in the temperature at which the water is maintained during the cooking process. Here it is kept a few degrees below boiling point, with the result that the transfer of heat to the food is more gradual, even the outside parts do not reach 212°F. and the movement of the water is not so violent. These factors are important in cooking certain foods, such as fruit, which it is essential to serve whole and not broken.

Steaming is different from the other two processes in that although boiling water is employed, the food does not actually touch this water but is cooked in the steam generated by it. The temperature of steam at atmospheric pressure is 212°F. and thus the temperature conditions are similar to those of boiling. The absence of the weight of the water on the food prevents disintegration, and the leaching action in which some of the soluble constituents may go into solution is avoided two factors which are sometimes detrimental to quality in food which is boiled.

When referring to steaming as a method one usually means the cooking of food in steam at atmospheric pressure, but in large scale catering the term steaming is used rather loosely; frequently it means the cooking of food in steam maintained at pressure above atmospheric pressure. As is well known, the boiling point of water can be raised considerably above 212°F. merely by increasing the pressure on its surface, and the steam generated under such increased pressure is also of course much hotter. Thus at a pressure of 5lb. per sq. inch above atmospheric pressure the temperature of the steam is 227°F., at 10lb. 239°F., and at 15lb. 250°F. Catering equipment is available in which steam at these high pressures is employed and it is obvious that the cooking process in such steam will differ from that in steam at atmospheric pressure, if only in the reduced time taken to complete the operation. The whole question of the use of steam for cooking is a somewhat complicated one.

EQUIPMENT FOR BOILING AND STEWING

The actual requirements in equipment for boiling and stewing small quantities are comparatively simple. All that is needed is a metal container which can be heated and will hold water, and a cover to prevent undue loss of steam. When, however, large quantities of food have to be boiled the large-capacity containers required cannot be portable, mainly because of the great weight of water employed. When it is realized that 1 gallon of water weighs 10lb., it may be quickly appreciated that portable pans of more than about 3 gallons capacity are not suitable in a kitchen catering for large numbers. What is required in such cases is a pan which can be filled and heated without being moved. It is rather interesting to trace the history of this piece of equipment. During the Crimean War complaints were received about the quality of the food served to the troops both in the field and in the hospitals, one of the main causes being bad cooking. A chef of that period, Alexis Soyer, whose name became world famous, had already achieved renown for his exhibits at the Great Exhibition of 1851. Soyer agreed to go to the Crimea to investigate the problem, and quickly realized that one of the main difficulties in providing well-cooked food for the troops was the lack of suitable equipment in which large quantities of food could be cooked at one time. Very quickly he designed, made and put into use for the first time the boiler which was the forerunner of those used in catering today. An improved design of Soyer’s boiler, bearing his name is still used in the Army, and proved its worth in our own time for “blitz feeding” in many of our great cities after heavy aerial bombardment.

The essential feature of a Soyer boiler is the round iron open topped vessel of about 2’6″ diameter with a rounded bottom and a capacity of 10-20 gallons. It is supported on a cylindrical stand which forms the surround to the fire box beneath. A metal chimney and a metal cover for the pan complete the equipment.

The Modern Boiler
The boiler used today in catering is similar in principle to the Soyer boiler. Certain refinements have been added : special rust-resisting metals are used for the containers, and the surfaces are kept as smooth as possible, without pits and crevices to facilitate cleaning. The metals used are either stainless steel, an aluminium alloy or cast iron, the latter being finished either with enamel or by a process known as “bower barfing” which consists in subjecting the metal while red hot to superheated steam to produce a deposit of black oxide of iron on the surface. Both stainless steel and iron with a protected surface are satisfactory, the former being more hard wearing, attractive, and easier to clean. Although stainless steel is now gradually replacing iron with a bower barf finish, this latter is still widely available and with careful use is quite satisfactory. The purchaser should examine the interior of a new boiler carefully to ensure that the lining has been applied satisfactorily giving a smooth finish; an uneven surface is difficult to clean and therefore unhygienic. The surface should also be examined frequently during use to ascertain whether it is wearing satisfactorily. It is sometimes found that the superficial finish flakes off leaving pitted surfaces which do not resist rusting. Such flaking may be due to the use of sharp pointed instruments in cleaning, or to technical faults in finishing at the factory. It is not always possible to repair such pans by having them re-surfaced, and when the lining becomes too badly pitted a new pan must be bought and fitted in the existing stand.

Boiling pans are made in sizes ranging from 10 gallons to 100 gallons, the latter being very large and found only rarely in catering kitchens. The most common sizes are 20 gals., 30 gals., and 40 gals., with the smaller sizes gaining in popularity today, particularly for vegetable work where relay cooking is practiced. By this method frequent supplies of freshly cooked vegetables are always available and the nutritional disadvantages of keeping them hot over long periods are avoided. Overall dimensions of boiling pans naturally vary according to capacity. Diameters of 2,1″ are common for a 10 gallon pan and 3.4″ for a 40 gallon pan. Heights vary from 32″ for a 10 gallon pan to 18″ for a 40 gallon one. Since it is frequently necessary for women to stir the contents of pans during cooking, as for example in the making of custard and milk puddings, it is important that boilers should be a convenient height from the floor. The average woman cannot stir comfortably if the height is more than 36″ to 38″, ind if boilers of heights greater than this are installed a small standing platform becomes necessary. Cleaning also is made more difficult if pans are too high. Unless it is possible for a woman when standing on the floor to reach down to the bottom of the inside of the pan she will again require a standing platform, which is inconvenient and may even be dangerous.

The facilities for emptying the boiler are an important consideration, in view of the comparatively large quantities of both solid and liquid material which will be handled Solids must dearly be lifted out and this is frequently done by hand; sometimes, however, mechanical means are employed. A wire basket with a handle is fitted inside the boiler; when the food is cooked the basket with the food inside it is lifted out by means of pulleys running on an overhead girder. An additional convenience is obtained by arranging for the supporting member to swivel, so that the basket container with its contents, which might weigh as much as 1 cwt. or more, can be transferred to a side table for unloading or to a trolley for transport to another part of the kitchen. Vegetable baskets are sometimes supplied and used with boilers where mechanical means of withdrawal are not available, but the strain of lifting such heavy loads completely outweighs any other advantages they may have. It is better to take out the material in small amounts which can be lifted by hand easily. Liquids can be removed by a hand baler, but this is an inefficient method and it is better either to use a draw-off cock with strainer fitted at the base of the pan, or to have the pan so constructed that it can be tipped mechanically for pouring. Pans with a draw-off cock are more commonly in use because the design is simple and the pan less expensive to produce. The bore of the outlet in this type is, however, important. The usual diameters range from ” to 1{“; the larger bore has advantages in cleaning since access with brushes is easier, and the pan can be emptied more quickly, and the outlet cleaned more easily. The draw-off cock is not only useful for the withdrawal of cooked liquid foods but also for the removal of dirty water after cleaning, a necessary and frequent operation. It is a great advantage to provide in the floor immediately below the cock a waste channel connected to the main drainage system so that dirty water can be run off directly into the drain.

Pans designed for tilting have a broad lip to facilitate the flow of liquid and avoid drip. The pan is carried on trunnions, and is provided with a large cog wheel engaging with a pinion. By turning a handle on the pinion shaft the large wheel rigidity connected to the pan is made to rotate, carrying the pan with it. The leverage so obtained makes the process an easy one to carry out and the rate of pouring is always under control.

The design of the cover of the pan is another important feature. Metal lids of 30″-36” diameter are bulky, not only to lift but also to accommodate when they are not covering the pan. It is usual therefore for the lid to be fixed to the pan by hinges so that it can be raised easily and left in the vertical position when not required, and to make lifting easier it can in addition be counter balanced. Extended lifting handles make for easy manipulation of the cover and avoid the necessity of putting the arm and hand across the open pan, which may be steaming vigorously.

Boiling pans are designed to be heated by solid fuel, gas or electricity, and steam generated under pressure. The pans must be specially designed for this latter method of heating and tested to withstand the pressure of the steam which in normal practice is 151b. per sq. in. but may be 10lb. per sq. in. When the fuel is gas, coal or electricity the heat is concentrated mainly at the bottom of the pan although with coal and gas-fired boilers a certain amount of heating also takes place higher up from fiue gases produced. With steam heating, however, the whole of the outside surface of the pan is steam jacketed and not only is there a more even distribution of heat but there is also an increase in the speed at which heat is transferred to the contents.

In comparing the performance of steam-jacketed boilers it is sometimes found that there is an appreciable difference in the time taken to boil equal amounts of water in two identical boilers. This is usually the result of a block in the steam trap which operates on the condensate leaving the boiler. The result is to build up condensed steam in the jacket space around the pan thus restricting the amount of steam circulating in this space and lowering the effective temperature. This problem is dealt with in more detail in Chapter Eleven.

The performance of boiling pans as recommended by the Ministry of Power are as follows:
Steam: The boiling pan should be capable of raising its specified capacity of water from 70°F. to 212°F. in 30 minutes who supplied with saturated steam at 15 sq. ins.
Gas: Direct-heated boiling pans shall reach boiling point within the following times :
Appliance capacity / Time Up
Up to 20 gallons — 60 mins
21 to 30 gallons — 70 mins
31 to 40 gallons — 80 mins
Electric: The boiling pan shall be capable of raising its specified capacity of water from 70°F. to 212°F.

For certain special purposes in cooking, such as porridge and custard making, it is found advisable to surround the cooking vessel with water maintained at boiling point and so prevent burning which may be caused by the intense heat produced by flames impinging on a metal pan. A piece of equipment designed for this purpose is known as a double boiler or porringer. Double boilers for catering consist of a boiling pan similar to the one described above and a removable inner pan which fits inside. The inner pan is naturally of a smaller capacity than the outer to leave adequate space for the surrounding water, thus a 20 gallon pan will hold a 15 gallon, a 30 gallon pan will hold a 20 gallon, and a 40 gallon a 30 gallon inner pan. The outer pan is usually fitted with a steam outlet pipe and also with a filling cup, which not only provides a means for adding water without removing the inner pan but also indicates when the water level is reaching the danger level and replacement is necessary.

The material used for the inner pan is either stainless steel or tinned copper, both of which are very suitable. Whatever material is used, it must possess adequate strength and rigidity, must be light to facilitate handling and be resistant to attack by such chemical substances as are found in foodstuffs. It must also be capable of taking a surface finish which is smooth, without crevices, easily cleaned and resistant to frequent applications of mild abrasives. Any metal which is liable to develop microscopic cracks or surface porosity is clearly unsuitable.

Equipment for this purpose may be designed for use as a double boiler only, in which case the inner pan is usually fixed to the outer and cannot be removed. On the other hand it may be designed as a dual purpose boiler, the inner pan being removable to allow the outer one to be used for straight boiling. Where steam jacketed boilers are used there is no need for double boilers since, in addition to their normal use for straight fast boiling, they can also be used for slow cooking simply by admitting a smaller quantity of steam.

In kitchens where a supply of steam from a boiler house is not available, a self-generating steam boiling pan is a convenient piece of equipment. Such boiling pans are constructed with a sealed water jacket, in which the water when heated boils and produces steam which exerts its own pressure of up to a few pounds per $9. in. The sizes in which these pans are available range from 2-10 gallons; the smaller sizes are suitable for mounting on a work surface, and for use in making small quantities of sauce or other similar foods.

These disadvantages have to a great extent been met in the steaming oven which consists of a square or rectangular box, fitted with a door and into which shelves are fitted in the ordinary way. The supply of steam to the oven is obtained either by heating a well of water in the base or by admitting through a valve steam at pressure supplied by a distant generator.

Steaming ovens can broadly be classed in two groups, those which are designed to use steam at atmospheric pressure and those which use live steam, i.e. steam which is supplied by a high pressure boiler and reduced by valves to a pressure suitable for steaming. The most convenient pressure has been found to be 5lb. The former types are called wet steaming ovens : self-generating steam; the latter are usually known as wet steaming ovens: direct steam. There is one type of steaming oven however which belongs to neither of these groups and is in a way a compromise between the two. This oven is self-generating, but owing to its special design it operates at a small pressure.

The advantages of the pressure steamer lie in the speed of cooking owing to the higher temperature resulting from the increased steam pressure. As already stated, at 5lb. pressure the temperature is 227°F. But clearly unless live steam is readily available it would be uneconomic to install this type of oven. The self-generating type is therefore the only choice for many caterers and possesses one advantage over the other type in that no compulsory inspection and insurance is necessary, as for the pressure steamer. The self-generating pressure type of steamer would on the surface appear to possess the advantages of both types but as 5lb. pressure gives a temperature of only 2-3°F. above B.P., this effect is not great.

It is an interesting fact, not always appreciated, that live steam can be used in an oven operating at atmospheric pressure, known technically as a wet steaming oven; self-generating steam. In this case it is used indirectly to boil the water which generates the steam for cooking. Live steam at 151b. pressure circulates in a coil of pipes in the base of the steamer and during the process gives up heat to the surrounding water and causes it to boil. The question naturally arises as to why, if live steam is available, it should not be used direct as in the pressure steamer. The answer lies in the actual steam wastage which inevitably occurs when live steam is used direct in a pressure oven but does not occur in the other system. In both ovens the steam naturally condenses when it gives up its heat but whereas with the steam heated coils the condensate can be returned to the boiler through a system of pipes and used again, this is not possible with the pressure oven, because the steam picks up such impurities as grease from the food which make it unsuitable for return to the boiler. It must therefore be allowed to run away to the drain and is consequently wasted.

In the atmospheric type steamer the control of the water supply to the well in the base is of great importance. One method of course is to supply and replenish the water by hand, as is done in the portable type which operates over a gas burner or electric hot plate. This method obviously has one great disadvantage in addition to the labor of filling, since it is inevitable that the filling operation will at times be neglected and the water container allowed to boil dry, with possibly serious results. The usual method adopted with the fixed type of steamer is to have an automatic filling cistern fed from the main water supply. A astern complete with ball valve controls the supply of water to the base of the steamer and ensures replenishment as required.

The water is kept at the correct level even though a rapid change into steam is continually taking place and because the amounts which are added at any one time are small the water does not go off the boil. It is important also that the water should not boil more rapidly than is required to maintain the temperature throughout the oven at 212°F. since this wastes fuel and causes unnecessary loss of steam into the atmosphere. The amount of steam required to maintain this temperature is not fixed and will of course vary with the amount of cold material present in the oven. In the modern atmospheric type steamer a thermostat is installed which controls the supply of gas or electricity automatically so that just sufficient steam is generated to maintain a constant temperature of 212°F. throughout the whole of the oven, and there is no wastage of fuel.

The cleaning of both types of steamer is important particularly after the cooking of such foods as meat and puddings which contain fat. In the atmospheric type the water in the well becomes contaminated during cooking and must therefore be removed and the container cleaned before fresh water is put in. A tap is usually provided in the base to facilitate emptying. In the pressure type steamer there is no well in the base but there is instead an outlet pipe connected to a combined steam and grease trap as illustrated in Plate IX. Steam passes through the oven into the trap where it condenses, and also deposits the fat and other impurities which it has carried through from the oven. It is absolutely essential therefore that if the trap is to continue to function correctly it must be kept quite clean and free from grease. It it becomes blocked, condensation will occur in the oven instead of in the trap, the condensate will collect at the bottom of the oven and not only will the food be spoiled but the unfortunate person who opens the door may be scalded by the sudden escape of boiling water.

One other point must be mentioned in connect with pressure steam ovens, namely the use of a safety device which ensures that the oven door cannot be opened while the steam cork is turned on. Such a device is essential to protect the user from burns which might otherwise result from exposure to the live steam.

The range of sizes in atmospheric steam ovens is from 3 02. ft. to 7 cu. ft, and in the pressure type from 2 cu. ft. to 10 cu. ft. The smaller sizes of pressure ovens usually form one chamber of a multi-compartment steamer.

The recommended standard of performance for the cooking of a load of potatoes is 30 minutes for direct steam at 5lbs. pressure and 50-60 minutes for atmospheric steam.

Although the box type described above is the most common two other types exist which are worthy of mention. The illustration in Plate XI shows an atmospheric multi-deck type made with a circular water well and fitted decks, which are provided with hinges so that the compartments can swing out sideways. This piece of equipment has the merit of being very compact. Plate XXIX shows a Jet Cooker which has been designed specifically for high speed defrosting and heating of frozen foods, more particularly small packs of vegetables suitable for about 8-10 portions. It consists of a cooking chamber of approximately 30″ depth, 20″ height and 20″ width heated by steam at a pressure of 15 lbs. per sq. in., using either a direct steam supply or steam generated by a boiler heated by gas or electricity on which the cooking chamber stands. When once this boiler has been filled and brought up to pressure the time required to bring the temperature of food from 0°F. to 170°F. is less than five minutes. It is very suitable for relay cooking of vegetables where small quantities are required at frequent intervals.

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