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That the solubility of lactose in aqueous solution is affected by temperature has been demonstrated also by Hudson (8) as follows:
From the above table it is seen that the final solubility of lactose is 10.6 per cent at 0° C., or 32° F. This table shows the importance of keeping the lactose content below 10.6 per cent at 0° C. In icecream making this amount would usually cause sandiness, since ice cream is cooled many degrees below 0° C. when held in the hardening room.
Evans (5) was the first investigator to study the solubility of lactose at low temperatures. The temperatures used by Evans corresponded to temperatures maintained in hardening rooms, namely, - 18° C., or 0° F. Those data are recorded in Table 3.
TABLE 3.-Concentrations of lactose at 18° C. at different ages.
The crystals obtained from the low-temperature study were not sphenoidal in shape, but rather prismatic and smaller. This was described as being due to forced crystallization rather than from temperature effects. Bothell noted that lactose solubility was noticeably greater in lactic-acid solutions than in neutral or alkaline solutions. He also noted that sucrose added to an aqueous solution containing lactose had an inhibiting effect on crystal growth. Evans confirmed the latter statement, which is shown in Table 4.
TABLE 4.--Solubility of lactose in sugar solutions held 144 hours at-18° C.
Per cent. Original concentration of lactose..
15 Final concentration of lactose alone_
12.45 Concentration of lactose with 1 per cent glucose
11.70 Concentration of lactose with 10 per cent glucose.
15.00 Concentration of lactose with 1.7 per cent sucrose_
12.35 Concentration of lactose with 17 per cent sucrose_
Evans duplicated Bothell's experiment with acid solutions, but used only an amount of acid which could be used in actual practice, and found no noticeable inhibiting effect.
In another experiment in which 2 and 4 per cent of glucose and sucrose were used as inhibitory factors, Evans found no pronounced effect on crystal growth at the end of 14 days at -18° C. In fact the lactose solution containing no foreign sugar showed the least crystal growth. Mojonnier (9), in quoting Travis, showed that lactose was less soluble in 14 per cent sucrose solution than in pure water, and more soluble in ice cream than in water, the difference being due due to the possible effect of "some colloid or colloids in the ice-cream mix."
That colloids have no apparent retarding effect on crystal growth of lactose solution was pointed out by Evans and by Zoller and Williams.
Much space has been taken up in this paper in delving into the history of sandy ice cream. It is a lamentable fact that with the exception of a few outstanding papers there has been very little experimental work published on the subject. The object of this paper is to present a few experiments as well as to give a review of the subject of sandy ice cream. Some of these experiments were done at the dairy division, University Farm, St. Paul, while cooperating with R. D. Evans, who at the time (1922) was experimenting on factors influencing the lactose crystallization in the occurrence of sandy ice cream.
PLAN OF EXPERIMENT.
The plan of the experiment was to study factors influencing lactose crystallization in ice cream. The factors studied were grouped as follows:
1. The effect of other sugars on the occurrence of sandiness.
2. The effect of colloids and other complex substances on the occurrence of sandiness.
3. The effect of degree of concentration of lactose on the occurrence of sandi
4. The influence of holding temperature on the occurrence of sandiness. Before beginning the actual experimental work, it was deemed advisable to adopt an experimental mix, a mix in which the lactose content was sufficient to insure the production of sand. The lactose content was figured to contain no more than 10 per cent lactose on the basis of the water in the mix.
The experimental mix consisted of the following:
The experimental mix contained a very small amount of serum solids, so the additional lactose was supplied from pure commercial milk sugar. The added water of the mix was used as a solvent for the lactose and as the water in which to mix the gelatin. This mix contained 6.45 per cent lactose on the basis of the weight of the mix and 9.66 per cent lactose on the basis of the total water of the mix.
Preliminary experiment.—The purpose of the preliminary experiment was to observe the occurrence of sand in the experimental mix, and to note the conditions of manufacture, holding, and packing, so that the processes could be duplicated identically in subsequent experiments.
The mix was made from cream 36 hours old, Pasteurized at 145° F. for 30 minutes, aged 24 hours, frozen, and placed in the hardening room. Two days later two 6-quart containers were packed in salt and ice under cabinet conditions.
The temperature of the hardening room fluctuated considerably at first, due to the small amount of product in storage. The temperature of the hardening room ranged from -10° F. to 10° F. The temperature of the ice cream in the cabinets fluctuated from 5° F. to 20° F., and at times the cream became somewhat soft. Sandiness appeared in these cabinets at the end of 8 days. The crystals were examined microscopically and identified as lactose hydrate. Crystals occurred at the end of 19 days in the hardening room.
EFFECT OF OTHER SUGARS ON THE OCCURRENCE OF SAND.
Dextrose or glucose.--It has been stated often that sucrose, glucose, and corn sirup in the mix would tend to inhibit sandiness. The experimental mix was recalculated to contain dextrose in an amount equal to 25 per cent of the sucrose. This was used to substitute 25 per cent of the sucrose. The dextrose which was used imparted a slight caramel color to the mix but did not alter the flavor any. The mix was prepared and treated as in case of the previous mix, except that this mix and all subsequent mixes were homogenized at 2,500 pounds pressure. Sandiness was noted in the cabinets at the end of 1 week. None appeared in the hardening room. This esperiment was repeated later, and in this experiment sand occurred as usual in the pack and in the hardening room in 15 days.
Corn sirup.—Corn sirup was added to replace 50 per cent of the cane sugar in the experimental mix. The mix contained the same solid ingredients as the experimental mix. Less water was used because of the water in the sirup. The samples of ice cream in the pack or under cabinet conditions became sandy in 7 days and those in the hardening room in 10 days on the average.
Cane sugar.—The sugar or sucrose content of the experimental mix was increased to 14 per cent. The total solid content was consequently increased to 35.26 per cent, thus lowering the amount of water in the mix by an amount equal to the additional sucrose or cane sugar.
The ice cream was made in the manner of all the previous mixes. Sandiness occurred in the cabinet at the end of 10 days and in the samples in the hardening room in 7 days. The experiment was later
repeated and sandiness occurred in 8 days in cabinet and 14 days in the hardening room.
EFFECT OF COLLOIDS AND OTHER COMPLEX SUBSTANCES ON THE OCCUR
RENCE OF SAND.
Mention has been made in the literature of protective colloids anil their effect on sandiness. The colloids chosen for this experiment were gelatin, because of its low gold number and because of its importance in ice-cream making as a stabilizer, and the colloids of milk.
Gelatin.-The amounts of gelatin used in these experiments were 0.5 per cent, 1 per cent, and 2 per cent. Normal mixes usually contain about 0.5 per cent. More than this amount results in a “livery" mix. The mixes were constructed to vary only in their gelatin con tent, which caused the water content to vary accordingly. The occurrences of sand are listed below:
TABLE 5.-Occurrence of sandiness in mires of high gelatin content,
Vilk-serum solids.-One of the reasons for choosing an experimental mix was for the purpose of studying the influence of milk solids not fat which could be supplied by using condensed or powdered milk and which would not alter the total lactose content of the mix. The mix had to be recalculated to contain the same amount of lactose on the basis of water, but to have more serum solids. The following mix was used :
12 Skim powder (97 per cent total solids).
5. 55 Gelatin.
.5 Skim milk.--
21. 95 The lactose content of this mix was 6.27 per cent on the basis of the mix and 9.8 per cent on the basis of water. Sand occurred in the hardening room in 22 days. Unfortunately the sample held in the pack or under cabinet conditions was destroyed before the occurrence of sandiness was noted. When repeated later, sand occurred in 9 days in the pack and 19 days in the hardening room.
Rennet.-Rennet extract was added after the mix was Pasteurized and cooled to 85° F. The mix was later placed in the cool room at 40° F. to age. After 24 hours it was frozen. The amount of rennet used far exceeded any amounts needed to coagulate milk for cheese making. It was added at the rate of 10 cubic centimeters for each
10 pounds of mix. Sandiness occurred in the cream under cabinet conditions in 7 days and in the hardening room in 11 days.
EFFECT OF THE DEGREE OF CONCENTRATION OF LACTOSE ON THE OCCUR
RENCE OF SANDINESS.
So far in this paper the experimental work has been confined to the study of factors influencing the occurrence of sand. Inasmuch as no inhibiting factors were discovered which would offer a practical solution to the problem of sandy ice cream, it was deemed necessary to ascertain about what amount of lactose could be used in practical mixes without danger from sand when the ice cream is held several days under cabinet conditions.
Mixes were constructed containing from 7.2 to 12 per cent serum solids. Previous experiments mentioned showed that it was possible to obtain sandiness in mixes containing 11.5 per cent serum solids. Condensed milk was used to supply the additional lactose or added milk solids not fat. No sandiness was experienced until the serumsolid content was increased to 10.2 per cent of the mix. The sandiness occurred only in the pack in this case. The ice cream held in the hardening room did not develop sandiness after several weeks. It appears from this experiment that 10.2 per cent serum solids would be the maximum to use without severe danger from sand in this particular mix. The per cent of lactose on the basis of water in this mix was 8.7 per cent. The total solid content was 36.5 per
EFFECT OF CONSTANT SERUM SOLIDS OF THE MIX, WITH VARYING TOTAL
SOLIDS, ON THE OCCURRENCE OF SAND.
To set a given amount of serum solids as the maximum to use would be impracticable, inasmuch as different mixes may have the same serum-solid content and the same per cent of lactose on the basis of the mix, but still have a varying lactose content based on the water of the mix. To study this point two mixes were computed. Those mixes are given in Table 1. Powdered skim milk supplied the additional serum solids. Sandiness occurred in mix No. 2 in 8 days in the cabinet, and 15 days in the hardening room. No crystals occurred in the hardening room in mix No. 1 after 5 weeks, at which time the experiment was discontinued. Some indications of sandiness appeared in a sample which was heat shocked several times at the end of 5 weeks.
THE EFFECT OF TEMPERATURE AND TIME OF HOLDING THE ICE CREAM ON
OCCURRENCE OF SANDINESS.
That the temperature at which the ice cream is held has a decided effect on lactose crystallization has been pointed out by Williams (6). Samples of ice creams studied were held at two different temperatures, namely -10° F. to 10° F. in the hardening room, and 5° F. to 20° F. in the pack or cabinet. Table 6 indicates the results of these trials.