By body, the whole mass of ice cream is referred to - its firmness, its resistance; texture refers to the finer particles of the ice cream.

Effect of fat on texture. High butter fat produces a firm body, for the chilled fat particles are very firm. Too much fat produces a very hard ice cream. Fillers are sometimes used to give a better body. In home-made ice cream when little cream is used, egg may be added to produce a firmer body, and to add flavor.

Effect of proteins on texture. The milk solids not fat and the protein content particularly affect the body of the ice cream. The casein and albumin of milk are found as calcium and magnesium caseinates and albuminates in the milk. As such they imbibe water and swell. With too little protein the body has little resistance, and with too much protein its hydration produces a very soggy, heavy ice cream. Mortensen states that high serum solids give a smooth mix for they absorb the water, but their use can be carried to an extreme. He adds that those who do not use a high percentage of serum solids must pay more attention to the treatment of the mix. Combs and Martin concluded that a certain amount of acidity produces a finished appearance in the mix. They have found that too high acidity causes the ice cream to melt quickly, since the reaction is brought nearer the isoelectric point of casein and at the isoelectric point very little water is held by casein.

Effect of treatment of the mix on texture. Any treatment of the ingredients of the ice cream or of the mix itself that increases the viscosity affects the body and texture of the ice cream. Pasteurization, homogeniza-tion, and aging all affect the viscosity and will be discussed later. Though the texture of the ice cream, like the body, is affected by the ingredients used and their proportions, it is also affected to a greater extent than the body by the freezing process. The texture of ice cream depends largely on the size of the crystals and the amount of air incorporated during freezing.

Incorporating chocolate or fruit. Some special problem studies have indicated that the temperature at which the chocolate is combined with the other ingredients is the most important factor in obtaining a uniform color and not a speckled product. The temperature at which the chocolate is combined, at least with a portion of the ingredients, should be above the melting point of the chocolate. The chocolate may be added to a portion of the milk which has been warmed or made into a sirup. Martin advises that the chocolate be made into a sirup with the sugar and a portion of the milk.

Fruit, such as strawberries, raspberries, and peaches, may be put through a sieve before adding to the mix. If larger pieces of fruit are desired, they can be prevented from freezing into chunks of ice by letting them stand for some time, or over night, in a heavy sirup. The sugar entering the berries or fruit lowers the freezing point of the fruit, so that it does not freeze or is less hard. To keep the fruit as whole as possible, the sirup should be added to the cream mix. Then the fruit is added after the cream is in a soft frozen state.

The freezing process. If the ice-cream mix has not been cooled before it is put in the freezer it is better to turn the freezer slowly until the mix is cooled. If the freezer is turned rapidly while the mix is at a temperature above 4.5°C. or 40°F. the butter fat is clumped together and a buttery product results. If carried far enough, butter may even be churned in the mix. The mix does not hold the air incorporated in it until its temperature is cooled to about 1°C. or 34°F., so that rapid turning is of no advantage from this standpoint until the mix is cooled. With a freezer turned by hand, if the freezer is turned rapidly all the time, one's energy is often expended before the freezing is finished, and it is during the latter part of the freezing period that the turning should be rapid. The size of the crystals formed during freezing depends upon the rate of turning of the freezer and the length of time of the freezing process. Stirring the solution during crystallization increases the number of nuclei formed, and the resulting crystals are smaller than if the mixture is not stirred. If the ice cream is not stirred during freezing, few nuclei are formed, the water crystals join onto each other, and a product with very coarse, large, spiny crystals is the result. On the other hand, if the ice cream is stirred the size of the crystals depends upon the rate of turning. Very slow turning while freezing is in progress results in larger crystals; rapid turning results in smaller crystals. If the freezer is turned very slowly the crystals build onto the crystals already formed; with rapid turning many new crystals are developed. The time of the freezing process also affects the size of the crystals. If the crystals are formed very rapidly, too little air is mixed with the cream, which produces an ice cream without a velvety texture. Since rapid freezing occurs with low temperatures it is better not to have the temperature of the brine too low, thus 1 part of salt to 8 parts of ice produces good results.

Overrun. The addition of air to the ice cream during freezing causes it to swell so that the ice cream increases in bulk. This increase is known as the swell or overrun.

This swell or overrun due to the incorporation of air particles gives a smoother, more velvety texture to the ice cream. Mortensen states that it is preferable to have the air cells in the finished cream small, for the strength of the film of a small air cell is stronger than that of a large one. To obtain small air cells in the ice cream the mix should be viscid. A viscid substance resists incorporation of air, and it will be worked in in finer divided portions. With increased viscosity the air is held in the mix better after it is incorporated. Mortensen includes homogenization as an important factor in increasing the viscosity of the ice-cream mix.

As the swell in freezing improves the texture, it is desirable to obtain it in home-made ice cream. In the home-made ice cream the freezer is often filled so full that there is no room for overrun and no air is beaten into the ice cream during freezing. Filling the freezer two-thirds to three-fourths full gives good results for home-made ice cream. In home freezing the overrun obtained is seldom as great as with commercial ice creams, for the freezing conditions in the home are not controlled so carefully as in the factory.

Too great an overrun produces an ice cream of poor body and quality, for it becomes very frothy and foamy. An overrun of 50 per cent is considered to give a desirable texture.

Percentage overrun. The percentage of overrun depends upon the speed of freezing and rate of turning.

Table 12 Effect of the Speed of the Machine on Overrun (Baer)

Revolutions per minute

Overrun

Time of freezing

Lot I

Lot II

Lot I

Lot II

50

25

33

10

11

50

28

33

11

10

50

25

35

9

12

150

48

43

7

7

150

48

39

8

5

150

50

40

7.5

6

175

45

58

7

6

175

50

50

7.5

6.5

175

52

55

8

6

The above table, from Baer's results, shows the effects of the speed of the machine on the overrun. He concluded that 50 revolutions per minute were too slow, as the overrun was too low, the texture of the ice cream was coarse, and the body weak and spiny.

From the table it will also be noted that increasing the speed of the freezer shortens the freezing time. The first crystals are formed at the edge of the ice-cream mix and against the side of the metal container. The faster speed of turning keeps these crystals scraped away and brings warmer portions of the mix against the can to be frozen. In this way the whole mass is chilled and frozen more rapidly than it would be if the mix were not stirred.

For home freezing it is probably better to over-freeze a little but not too much, if the ice cream is to be served soon after it is frozen. After stirring is stopped the rate of cooling is slower. Therefore it requires a period of standing after the freezing process is completed for the ice cream to harden suitably for serving. Increasing the quantity of salt for packing also increases the rate at which the ice cream hardens.