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464 CHAPTER 43 FREEZING PROCEDURE The ice cream freezer has to freeze the mix to the desired consistency, and whip in the desired amount of air in a finely divided condition. The usual aim is to conduct the freezing and later hardening in such a manner as to obtain the smoothest possible texture. The factors involved will be discussed. Freezing of an ice cream mix is freezing of a mixed solution. The solutes which determine the freezing point of this solu tion are the lactose and the soluble salts contained in the serum solids, and the sugars added as sweetening agents. The other constituents of the mix affect the freezing point only indirectly, by displacing water and affecting the inwater concentration of the solutes mentioned. Leighton has developed a reliable method for computing the freezing points of ice cream mixes from their known composition. He adds the lactose and sucrose content of the mix, expresses their concen tration in terms of parts of sugar per 100 parts of water, and determines the freezing point depression due to the sugars by reference to published data for sucrose. This is justified since lactose and sucrose have the same molecular weight. % lactose in mix -- % serum solids X 0.545 (% lactose + % sucrose) X 100 parts lactose + sucrose % water in mix per 100 parts water To the freezing point depression caused by these sugars he adds the depression which will be caused by the soluble milk salts. The depression caused by the salts is computed as follows: Freezing point depression caused by serum ,. % serum solids X 2.37 souds in C ---------------:--;------ :------ % water in the mix Table 1 presents the freezing points of various ice creams and a typical sherbet and an ice, as computed by Leighton's method. The freezing point, of course, merely represents the tem perature at which freezing commences. As in the case of all solutions, the unfrozen portion becomes more concentrated as the freezing progresses, and the freezing temperature therefore decreases as tire freezing progresses. In a simple solution, containing only one solute, this trend would progress until the unfrozen portion represents a saturated solution of of the solute, and thereafter the tempertaure would remain constant until the freezing had been completed. This tem perature is known as the cryohyeric point of the solute in ques- Toble 1 .... Freezing Points of Typical Ice Creams, Sherbet and an Ice Comporifloa of ffw Mix, Percent Fot Serum SoOds Sugar StabIDzef Water . .u Point F 8.5 10.5 12.5 14.0 16.0 10.5 11.5 11.0 10.5 9.5 8.5 8.4 15 0.4 15 0.35 15 0.30 15 0.28 15 0.25 0.40 64.6 63.15 61.7 61.22 60.25 04.7 27.59 27.57 27.55 27.68 27.79 27.39 Sherbet 1.2 1.0 d2!} 0.50 67.3 25.97 Ice 0 0 0.50 87.5 25.68 S -- cacme D -- Dcztroee 1962 Guide And Data Book Table 2 .... Freezing Behavior of a Typical Ice Cream Water frana to lee fill Water Frozen to Ice Percent Freezing Pomt of Unfrozen Partloo, F 0 27.55 40 24.40 5 27.35 45 23.63 10 27.05 50 22.62 15 26.78 55 21 .42 20 26.40 60 19.79 25 26.04 70 14.99 30 25-70 80 5.14 35 25.03 90 -22.29 Composition: fit, 12.5; eennn coUds, 10.5; suenr, IS; etahOieer, U30; nto 1.7. tion. In a mixed solution such as ice cream, containing several sugars and a number of salts, no such point can be recognized. On the conkary, the indications are quite definite that the sugars remain in solution in a supersaturated state in the unfrozen portion of the product. This is apparently due to the fact that by the time the saturation point has been reached, the temperature is so low and the viscosity so high that essen tially a glass state exists. It is interesting to note, however, that in a hiitpH solution the temperature required for com plete freezing must be somewhat below the cryohydric point of that solute which has the lowest cryohydric point. In the nase of ice cream, that solute b calcium chloride, contained as a natural component of serum solids. The cryohydric point of calcium chloride b --59.8 F. Therefore, we may say that the ice cream ranges from 0 to 100 percent frozen between the approximate temperature range of from 27.5 to --67 F. In accordance with the above, the temperature to which the ice cream has been frozen becomes a measure of the de gree to which it has been frozen. This b illustrated by Table 2, in which the freezing points of the unfrozen portions have been computed when from 0-90 percent of the original water has been frozen out as ice. For this purpose the third ice cream of Table 1 has been selected as being most typical of normal operations. Refrigeration Requirements Exact calculation of refrigeration requirements b compli cated by the number of factors involved. The specific heat of the mix varies with its composition. According to Siadan the specific heat of food products may be computed by assuming the following specific heats for the chief components: carbo hydrates, 0.34; proteins, 0.37; fate, 0.40; water, 1.00. This does not include salts. Where they are present in significant amounts as in ice cream (9.5 percent of the serum solids), a specific heat of 0-20 will be quite accurate. The value given by Zhadan for fats b apparently for solid fats. For butterfat in a liquid condition Hammer and Johnson found the specific heat to be 0.52. In addition their data clearly show that the latent heat of fusion of fate becomes involved. From their data the latent heat of fusion of milk fat or butterfat appears to be about 35 Btu per lb. The change from liquid to solid fat occurs over a wide temperature range, approximately 80 to 40 F; whereas in changing from solid to liquid fat the range o approximately 50 to 105. F. Thb wide discrepancy between solidifying and mnlting behavior b apparently due to the fact that butterfat, more so than any other fat, b a mixture of glycerides, and mutual solubility of the glycerides b involved. In any case the latent heat of fusion of the fat b involved w cooling the mix from pasteurizing and homogenizing tempers' Ice. Cream 465 tme, down to the usual aging temperature of 38 to 40 F. InjfruA of undertaking detailed calculations, based on such considerations as just presented, it b common practice to asgmne a specific heat of 0.80 for ice cream mix. Thb value b generous for mixes ranging from 36 to 40 percent total solids. Xq calculating the refrigeration required in freezing and hardening, one b hardly justified in speaking of a specific-heat for frozen ice cream. As has been shown in connection with Table 2, any change in temperature in freezing and hardening involves some latent heat of fusion of the water, as well as the sensible heat of the unfrozen mix and the ice. Hie heat units that become involved as latent heat of fusion per deg temp change differ according to whether the temp change b near the initial freezing point or farther along the freezing process. Near the initial freezing point much more latent heat of fusion b involved per degree temperature change than is true in well hardened ice cream, e.g., at --10 to --11 F. For thb reason it is preferable, instead of attempting to use an overall value in terms of specific heat, to compute as follows: (1) Having in mind the temperature to which the freezing is to be carried, determine, by calculations such as those em ployed in connection with Table 2, how much water will be converted to ice. The heat to be removed b the product of the beat of fusion of ice and the pounds of water frozen. (2) To compute the sensible heat that must be removed in the desired temperature change, the problem b treated as though the product were mix, .e., use the specific heat for be cream mix. F temperature changeXlb of product X0h0 bsensible heat to be removed. It is true that in such a calculation the water that b pres ent is treated as though it all remained in a liquid form until the desired temperature has been reached, when as a matter of fact ice was forming progressively. Insofar as ice has a specific heat of 0.492 instead of 1.0 as for water, tins treatment will err in the direction of generous refrigeration. To offset thb there b the fact that in the freezer vigorous agitation b em ployed, which develops.heat of friction. It has been estimated that approximately 80 percent of the energy input in the motor of the freezer b converted to heat in the product. Where the product b frozen to a stiff consistency and power require ments have thereby been increased, an additional allowance should be made for thb factor, even though the discrepancy of the above calculation b involved. Since capacities in the case of ice cream are customarily figured in terms of gallons, it becomes necessary to convert gallons to pounds of product. The weight of a gallon of ice cream mix ranges from 9.0 lb for mixes with a high fat con tent to 9.2 lb for mixes with a low fat content, and a high content of serum solids and sugar. The weight of a gallon of ice cream varies with the mix weight and overrun in accord ance with the following relationship: Percentage overrun = 100 X Wt per gal of mix--Wt per gal of ice cream Wt per gal of ice cream For the purpose of calculating overrun any unit of volume can be used provided that same unit is used throughout. Freezing Ice Cream Two types of ice cream freezers are in general use: the type, which freezes a measured quantity of mix at one "nm, and the continuous type, which takes a continuous flow of be cream mix and discharges a continuous flow of partly frozen ice cream. Both are arranged with a freezer cylinder having either an annular space or coils around the cylinder jritere cooling b accomplished by direct expansion; either in a flooded arrangement with an accumulator or controlled by a thermostatic expansion valve. The freezer cylinder b ar ranged with a dasher, having blades attached, which revolve within the cylinder. The sharpened metal blades scrape the inner surface of the cylinder to remove the frozen film of ice cream as it forma. Some freezers have heaters-built into the dasher to aid the blades in the mixing and whipping action for the introduction of air into the mix as it b frozen, to produce the overrun desired. Batch freezers range in sizes from 2 to 40 quarts of ice cream per batch, the smaller sizes being used for retail or soft ice cream operations, and the 40 quart size used in small com mercial ice cream plants or in huge plants for running small quantities for special orders. Batch freezers larger than the 40 quart size are not used extensively since the development of the continuous freezer, partly because of the limited hourly capacity and because of the labor required for operation. In operation, a measured quantity of mix b placed in the freezer cylinder and the required flavor, fruit, or nuts are added as freezing of the mix progresses. Freezing b continued until the desired consistency b obtained depending on the judgment of the operator or the indication of a meter showing an in crease in the amperage drawn by the motor, as the partly frozen mix becomes stiffer. At the desired point of freezing the refrigeration b cut off from the freezer cylinder, usually by closing the refrigerant suction valve. The dasher continues operating until enough air has been taken into the mix by the whipping action to produce the overrun desired. The over run b checked by taking a sample from the freezer and weigh ing it, or by the reading of the ammeter. When the desired overrun b obtained, the entire batch b discharged from the freezer cylinder into cans or cartons and the mn/binn is then ready for a new batch of mix. The output of a batch type freezer will vary with the sharpness of the blades, refrigeration supplied, and overrun desired. The average maximum output for commercial type batch freezers b eight batches per hour. Thb schedule would allow three to four minutes to freeze, two to three minutes to whip, and about one minute to empty the ice cream and refill with mix. For thb time schedule, it b assumed the ice cream would be drawn from the freezer at not over 100 percent overrun, at a temperature of about 24 F, and a refrigerant temperature around the freezer cylinder of about --15 F. Continuous ice cream freezers are a more recent develop ment and range in sizes from 40-1050 gal per hr. Thb type machine b used almost exclusively in commercial ice cream plants. Where large capacities are required, multiple units are installed with the ice cream discharge from several ma chines connected together to supply the requirements of automatic or semi-automatic packaging or filling maihmew. In operation, the ice cream mix b continuously pumped to the freezer cylinder by a positive dbplaceroent type rotary pump. An air pressure .b maintained within the cylinder ranging from 20 to 50 psig, supplied by either a separate air compressor or drawn in with the mix through the rn>T pump. The mix entering the rear'of the freezer cylinder becomes partly frozen and takes od the overrun due to air pressure and to the agitation of the dasher and freezer blades as it moves to the front of the cylinder and b discharged. The output capacity of most continuous freezers can be varied from 50 to 100 percent rated capacity by regulating the variable speed control supplied for the mir pump. Continuous freezers can be used for nearly every flavor of ice cream, for iced milk, for sherbets, or for ices. Where flavors requiring nuts, whole fruit or candy pellets are run, the flavored mix b run through the continuous freezer and then passed through a fruit feeder, which automatically feeds and mixes the flavor particles into the ice cream. Fig. t shows the arrangement