Ice-cream which is of good quality up to this stage in the manufacturing process may be spoiled or the quality impaired by improper hardening. The object, as its name indicates, is to harden the semi-frozen product after it is frozen in the freezer. During the time that the ice-cream is hardening, the flavors of the different ingredients blend to give the desired characteristic flavor.

114. Methods of hardening.

—The ice-cream may be hardened in the freezer, but this allows only one mix to be frozen and the machine cannot be used again until the product is consumed. This method is ordinarily followed with the small hand freezers. With the larger freezers, however, the ice-cream is hardened in pack-cans (Fig. 25) or bricks. (Fig. 58.) This allows the freezer to be used over and over. The ice-cream can be drawn directly from the brine freezer into the pack-cans or bricks; however, with the salt and ice freezer, the ice-cream must be dipped by hand. Various ladles or scoops have been devised for this purpose. (Fig. 53.) These are more or less rounded on the edge to scrape the sides of the freezing-can.

The ice-cream in the pack-cans or bricks may be hardened in any one of several ways: packing in an ice and salt mixture, setting in cold brine, setting in a cold room called a hardening-room. The first two are not advisable for a large ice-cream plant because of the work and space required.

115. Hardening in ice and salt mixture.

—When the ice-cream is hardened in a salt and ice mixture, the cans are placed usually in a large plank box, Fig. 54, so that there will be a space of four to six inches between the cans and the box. The size of the box will be determined by the amount of ice-cream manufactured. If five-gallon cans are used, it is advisable to build the box in compartments which will hold six. This will require a box 26 inches wide by 32 deep by 36 long, outside measurement, with a hinged cover for each compartment. The box should be made of two-inch matched lumber so that it will not leak. There should be a hole in the side near the bottom so the brine can be drawn off. Before beginning to freeze, it is advisable partly to pack the cans in the box. A layer of four to six inches of cracked ice should be placed in the bottom of the box, then the cans placed in position and the box filled about two-thirds the height of the can with cracked ice. Some salt should be sprinkled on the ice and the box filled to the top of the cans with cracked ice. This cools the cans so that there will be no melting when the ice-cream is put in. The ice-cream may be poured directly from the freezer into the pack-cans or first it may be put into some container which is easier to handle and then poured from it into the pack-can. When all the cans in the compartment are full, they should be covered to a depth of four to six inches with ice and salt. These may be mixed before putting in the box or they may be placed in alternate layers. The proportion of salt to ice for hardening is about one part of salt to eighteen or twenty parts of ice by weight. Most of the salt should be placed in the upper third of the ice, because as the ice melts the salt goes into solution and so is carried to the bottom of the box where it comes into contact with ice where there is little or no salt. For packing, a coarse salt is to be preferred as it dissolves more slowly. After standing overnight in the pack-cans, the ice-cream should be hard. It may be shipped out or held until wanted. If shipped the cans may be packed in tubs, observing the same precaution of placing the salt in the upper third of ice. Instead of packing in a tub, the cans may be put in a box or cabinet on the delivery wagon. If the ice-cream is to be held in the hardening-box, it should be repacked twice a day. This is done by jamming down the ice and salt. The cans should then be recovered with ice and salt. The brine which forms should be allowed to escape from time to time so that it will not run into the cans or cause them to float. The following figures on the amount of ice and salt used were obtained by the “Ice-Cream Trade Journal.”⁠[32] They are the average of a number of plants:

116. The slush-box or brine-box method of hardening.

—In this method, the pack-cans instead of being packed in ice and salt are placed in a box or tank of brine. This brine is usually cooled by means of a mechanical refrigerating system, and circulated with a pump. Great care must be exercised or the brine will get into the ice-cream.

117. The hardening-room.

—In the large ice-cream plants, a cold room is maintained in which the pack-cans are set to harden. This room is cooled by mechanical refrigeration and the temperature should be very near 0° F. or below. There are three general types of hardening-rooms, depending on the location of the evaporating coils and the air circulation.

118. The still-air type.

—This type of hardening-room is used in the smaller ice-cream plants. The evaporating coils are placed directly in the hardening-room and usually arranged in such a way that shelves are formed with parts of the coils on which the cans of ice-cream are put to harden. (Fig. 55).

119. The gravity-air type.

—In this system the coils are placed in a bunker-room directly over the hardening-room and designed so the air will circulate in a natural manner.

120. The forced-air type.

—This system locates the coils in a bunker-room usually, directly over the hardening-room, and the air is forced to circulate by means of a fan or blower. (Fig. 56). The forced-air system is considered the most efficient. The effect is the same as the temperature of the air on the body. With the same temperature, the cold is more noticeable and more penetrating on a windy day than when the air is still. The same is true in the hardening of the ice-cream. The objection to the still-air hardening-room is the longer time required to harden the product. The objection to the forced-air room is the danger of losing too much refrigeration when the doors are opened unless the fan causing the air circulation is stopped before opening.

121. Defrosting the coils.

⁠[33]—“One of the most troublesome things to be contended with in ice-cream hardening-rooms, cooled by means of refrigerating machines, is the accumulation of frost or snow on the coils, and up to the present time no thoroughly satisfactory method has been devised which will meet with success under all conditions, taking into consideration, method of operation, design of coils, and the like.

“The most serious objection to this accumulation is the loss in efficiency, which may amount to as much as 50 or 75 per cent, depending on the thickness of the coating and the interference with the air circulation. The other objections are of minor importance and need not be considered here.”

1. Cooper system.

The Cooper system consists of a trough perforated at the bottom and placed directly over each stack of coils, and in which are put lumps of chloride of calcium. This substance, on coming in contact with moisture in the air, will dissolve slowly and drip down over the coils, thereby keeping them practically free of frost at all times. It may be asserted that this is not a defrosting device; however, prevention is better than cure.

This system can be applied to the first and second types of rooms, as in these cases the floor under the coils is made water-tight and arranged to catch the drip from the coils. It is not suitable for use in the third type, however, as the drippings would fall upon the cream-cans and on the floor of the room. It is possible to catch these drippings and by boiling or evaporation recover the calcium, which can be used over again or in strengthening the brine in the tanks.

The objection to this system is the labor involved in placing it in the troughs, for usually the coils are close together and are in a room with little space around and over them for a man to work; also, the rooms being cold, it is a very uncomfortable task. It may be possible, in designing rooms, to provide space in which to work, or to fill the cans outside the room and then place in position.

2. Cold brine drip system.

This system consists of a means of spraying cold brine (calcium or salt) over the coils. Over each stack of coils is placed a trough, slotted or perforated pipe, similar to the water pipe or trough on atmospheric condensers. Brine from the main tank or circulating system is turned into the trough and allowed to drip over the coils either continuously or intermittently as may be required.

In this system the brine must be very strong or it will freeze on the lower coils, especially if the frost is allowed to accumulate to any extent before defrosting, or unless the coils are out of commission entirely during the operation. This brine is sometimes allowed to drain directly back into the brine tank, although this is poor policy as it weakens the brine and has a tendency to make it (in the case of calcium brine) acid, owing to its contact with the air. A better way, if it is possible to do so, is to drain it back into a tank, where it can be boiled and thus brought to its proper strength before returning to the main tank. This system has the advantage of being operated with little labor and under comfortable conditions, providing the controlling valves are placed outside the room and of easy access. In case the frost is allowed to accumulate to any extent, this system will not work as quickly as may be desired. One great advantage, however, is that it will not warm up the hardening-room, and if operated often and properly it will keep the coils free of frost and not require the room to be out of commission. This system is suitable for use in first and second types of rooms.

3. Hot brine system.

This is arranged the same way as the cold brine drip system as far as interior arrangement on coils is concerned, but instead of using brine from the main tank, a special tank with steam coil is provided and the brine is pumped into the trough in a hot condition and allowed to drain directly back into the tank. This system is quick in its results. A heavy deposit of frost can be removed in this way and it is easily operated.

The principal objection is the great amount of moisture it will cause to be deposited on the walls and ceiling of the room. It also requires the room to be out of commission a short time while being operated. However, it does not seem to have much effect on the temperature after it is shut off.

This method can also be made a cold brine system by boiling off after using, then letting it stand until it is cold before utilizing again. However, it will still deposit some moisture and it has been found best to keep the room out of commission while operating. Another tank with refrigerating coil placed so as to cool this brine before using might be beneficial, although in this case it resolves itself into system No. 1. This can be applied to first and second types.

4. Air blast system.

In this system the air is so designed that the bunker-room can be shut off from the hardening-room and openings arranged so that the circulation fan can draw warm air from outside, blow it across the coils and discharge it at the opposite end. This is a very satisfactory and quick method, although it leaves some moisture deposited on the walls of the room and has a tendency to warm things up a little. This system, of course, needs a fan and is suitable for the first type only.

5. Hot gas system.

This is a radical change from those systems considered, as it works from the inside out. The success of this system depends on the design of the coils, headers and connections. The coils are arranged in such a way that they are, by opening and closing a few valves, converted into an ammonia condenser with the hot ammonia gas from the discharge of the compressor entering the top pipe of coils, and, as it is liquefied, running by gravity back to the ammonia receiver. In operating, it is necessary to have the room out of commission unless the coils can be arranged in independent batteries.

This system can be applied to any type of room provided the coils are properly designed and the receiver on a level below the lowest coil. Also, there must be enough additional refrigeration to enable the compressor to continue in operation in order to supply the hot gas.

6. Warm liquid system.

The coils, headers and piping are so arranged that the liquid ammonia on its way to other rooms can be passed through coils which are to be defrosted. It is self-evident that it is necessary to maintain the same pressure on these coils as obtains in the main liquid line, also that a considerable amount of ammonia must be expanding at other points in order to keep a quantity of liquid flowing through the coils.

When conditions are right, this system is very satisfactory, as it can be operated with a minimum of labor and has the advantage of conserving all the energy previously expended in freezing the ice on the coils. In applying, special consideration must be given to the method by which the ammonia is fed into the coils. It is more easily applied to the flooded system, as in this case it is only necessary when defrosting is completed to cut off the flow of hot liquid, open the suction line from coils and allow the liquid remaining in the coils to expand or evaporate to accumulator, without danger of flooding over into the compressor. After this liquid is partly evaporated, the valve on the feed line from accumulator is opened, and the coils are again in full operating condition.

In applying this system to direct expansion coils, care must be exercised to provide means either to drain the liquid back into the ammonia receiver before opening the outlet from coils into the main suction line, or to expand this liquid through other coils; otherwise, the liquid is liable to reach the compressor with disastrous results.

The application of this system should be attempted only by those thoroughly competent to consider all phases of the situation and if properly applied is one of the most efficient and satisfactory methods of defrosting yet devised. It can be applied to practically any type of room providing, as stated, the other conditions are suitable.

It has been the writer’s experience that the coils in forced-air circulating types give the most trouble, due to the heavy frost, partly from the very rapid accumulation and partly for the reason that these coils being out of sight are more likely to be neglected, and after the ice has become very heavy it is exceedingly difficult to get it all off, without keeping the room out of use for a long time.

The gravity system also gives some trouble, but is not affected quite so quickly as the forced-air type, owing partly to the design of the coils which necessitates greater space between pipes. The still-air type causes very little difficulty and some of these have been run a whole season without being defrosted and without having their efficiency materially reduced.

A large percentage of hardening-rooms have no arrangements for defrosting and as a result it is often necessary to shut down and remove the ice. All kinds of methods are used, such as scraping by hand, spraying with water by means of a hose, placing salamanders in the room or simply leaving the doors open and allowing the temperature of the room to rise to such a point that the frost will melt.

122. Time required for hardening.

—The time necessary for hardening varies, but usually twelve hours is sufficient. The time depends on the rate of removal of heat or the amount of cold supplied and the insulation. The refrigerating boxes should be well insulated. This is especially necessary because of the great difference between the temperature of the hardening-room and that of the surrounding atmosphere. Undoubtedly cork makes the best insulation. The thickness varies, but it should be at least six or eight inches thick. Cork should be kept dry or it is not a good insulator. When the ice-cream becomes hard, it should be held at a low enough temperature so that it will not soften or melt. After hardening, if the ice-cream melts or softens it is liable to cause the separation of the ice crystals and so result in a grainy textured product. If it becomes soft, the fat is likely to rise unless the cream has been homogenized. Ice-cream should not be held in the hardening-room for more than seven days.

When an ice and salt mixture is employed to harden the ice-cream, the rate of hardening is determined by the amount of salt used. The coarse, slow dissolving salt is to be preferred for hardening.

123. Effect of hardening on quality.

—The two qualities of ice-cream affected by hardening are the flavor and body and texture. While hardening, the flavors of the different materials used blend to give the desired characteristic flavor. Some flavors, especially vanilla, will freeze out while hardening. The body and texture are affected only through neglect. If the pack-cans are allowed to stand after they are filled, before being placed in the hardening-room, some of the ice-cream next to the sides and bottom will melt, causing a grainy or icy texture. The same may occur if there is water in the bottom of the pack-cans when the ice-cream is put in.

124. Fancy molded ice-cream.

—There are two kinds of fancy molded ice-cream, bricks and molds to represent various objects. The ice-cream is the same with the exception of the form in which it is hardened. Sometimes a little more stabilizer is used to make the cream more firm. The brick offers many possible combinations. Each kind of ice-cream is put into the brick in a layer. Each layer is leveled with a brick trowel. This trowel is square on the end and just the width of the brick. (Fig. 57.) A different flavor and color of ice-cream forms each layer. In some cases the center is a sherbet or pudding. Mortensen suggests a center layer of solid frozen fruit and calls such ice-cream Aufait. The size of the brick varies from a pint to several quarts. (Fig. 58.) It is the customary practice to use sectional bricks (Fig. 58) which are the exact size to hold six to eight quart bricks. These may have a single or double lid. When hard, the ice-cream is taken from the sectional brick and cut into either quart or pint sizes. In a large factory, a special hardening-room is employed for brick ice-cream, which is kept as near 0° F. as possible. (Fig. 59.) The contents are taken from the brick mold by applying cold water until sufficient frost has been drawn from the mold to allow the ice-cream to slide out. When a large number is made, a special brick-cutting machine may be used. This will cut the bricks much faster and more uniform in size than by hand. Sometimes if a knife is run around the sides of the brick it will help loosen the ice-cream. Care should be taken not to melt the ice-cream too much. In some instances the bricks are packed in square, instead of round pack-cans for delivery. In this case a large number of bricks would be delivered to the same place. The bricks are wrapped in parchment paper, put into paper cartons and packed in ice and salt for delivery.

By means of a specially devised mold or brick known as a center mold (Fig. 60), any letter, figure or form of object may be made in the center of the ice-cream. To accomplish this, two different colored ice-creams must be used. The form is produced by having one cover of the mold with a tube of the desired shape to form the center figure. The space around the tube is filled with one colored ice-cream and the tube or center with the color desired in the center. When the brick is sliced, this design is in the center of each piece of ice-cream.

By means of special molds, ice-cream may be hardened to represent almost any object. (Fig. 61.) These molds are hinged pewter metal. They vary in size from one or two quarts to an individual service. These cannot be packed in an ordinary pack-can without jamming. They usually are wrapped separately with waxed paper and hardened and delivered in an ice cave (Fig. 61), which consists either of a round or square pack-can into which a frame with shelves fits. The molds of ice-cream are placed on these shelves.