The properties of practical importance in the mix are- viscosity, acidity (lactic) and pH, mix stability, specific gravity, surface tension, freezing point, and whipping rate.
A. Viscosity:
This is defined simply as the resistance offered by liquids to flow. Viscosity is considered an important property of the ice cream mix, and a certain amount of it seems essential for proper whipping and the retention of air. Two types of viscosity exist in ice cream mixes- Apparent Viscosity, which is a thickened condition that disappears with agitation and Basic Viscosity, which remains after the Apparent Viscosity disappears.
The viscosity of an ice cream mix is influenced by:
(i) Composition;
(ii) Kind and quality of ingredients;
(iii) Processing and handling of the mix;
(iv) Total solids concentration; and
(v) Temperature.
(i) Composition:
Among the various mix ingredients, viscosity is more influenced by milk fat and stabilizer than the other ingredients. Gelatin affects viscosity to a great extent largely due to gel formation, while fat increases it slightly. Sugar, on the other hand, decreases the viscosity of the ice cream mix.
(ii) Kind and Quality of Ingredients:
The kind of ingredient refers to whether it is a source of fat, serum-solids, sweetening, stabilizer, etc.; the quality indicates the emulsion stability of fat, colloidal stability of protein, etc. Those carrying the fat are especially important. Also, heat and salts (such as calcium, sodium, citrates, etc.) greatly affect the viscosity due to their effect on casein and other proteins. Among stabilizers, gelatin causes a decided increase in viscosity after ageing.
(iii) Processing and Handling of the Mix:
These include:
1. Pasteurization,
2. Homogenization and
3. Ageing.
1. Pasteurization:
At batch-holding temperatures followed by cooling, pasteurization causes a decrease in viscosity, while at higher temperatures it produces an increase in viscosity.
2. Homogenization:
If the globules are small and individually dispersed, the basic viscosity of the mix remains at a minimum. However, the tendency of the sub-divided fat globules to clump or aggregate greatly increases.
3. Ageing:
The ageing of mixes containing gelatin considerably increases their apparent viscosity. The increase is chiefly due to gel formation, but to some extent to the hydration of proteins and the clumping of fat globules. Aside from the increase in apparent viscosity, ageing causes an increase in basic viscosity as a result of a solidification of the butter fat and hydration of the milk proteins.
(iv) Total Solids Concentration:
As the liquid phase is replaced by a solid phase in a mixture such as ice cream mix, the viscosity usually increases. Consequently, as the solids content of the ice cream mix is raised, the viscosity increases although the effect of the different solids in this respect varies with the type and source of the solid.
For example, slight increases in fat content may affect the mix viscosity to only a limited extent, whereas a slight increase in gelatin will affect the mix viscosity greatly. The extent to which the mix solids increase mix viscosity depends largely upon the physical state of these solids. When the proteins are partially coagulated and v hen the fat globules form clumps, the mix viscosity increases on both counts.
(v) Temperature:
With a lowering of temperature, the mix viscosity increases, and vice versa.
Note:
It has not yet been determined how much viscosity is desirable in ice cream mix. A high viscosity was believed essential at one time, but for fast freezing (rapid whipping) in modern equipment, a lower viscosity seems desirable. In general, as viscosity increases, the resistance to melting and the smoothness of body increases, but the rate of whipping decreases.
Viscosity is now considered a phenomenon that frequently accompanies rather than causes good whipping, body and texture. Therefore the mix should be properly balanced (in regard to composition, concentration and quality of ingredients) and then properly processed to produce the desired whipping ability, body and texture. Under these conditions, a desirable viscosity is assured. The basic viscosity of mix ranges from 50 to 300 centipoise.
B. Acidity (Lactic) & pH:
The normal acidity of ice cream mixes is dependent upon the serum solids content, and is calculated by the formula:
% acidity of mix = % acidity of milk × (% serum solids in mix/% serum solids in milk)
A rule of thumb method to determine mix acidity, is to multiply the serum solids by 2 and divide by 100. The normal pH of mix is about 6.3. Acidity and pH are related to the composition of the mix and an increase in milk-solids-not-fat raises the percentage acidity and lowers the pH. If fresh dairy products of excellent quality are used in preparing the mix.
It may be expected to have a normal acidity. When acidity is above normal, it indicates that lactic acid is present in the dairy products used in the mix. A high acidity is undesirable as it contributes to excessive mix viscosity, a decreased whipping rate, an inferior flavour and a less stable mix resulting in ‘cook on’ or possible coagulation during the pasteurizing and processing stages.
If the mix acidity is higher than normal, it can be neutralized to the level of normal acidity. Over-neutralization (below 0.15 per cent titratable acidity) should be avoided as the ice cream will tend to have a flat (or neutralized) flavour and a dull or even greyish colour. When neutralizing, add the sugar at 32°C (90°F) and place all the ingredients in the mixing vat before the acidity test is run.
The neutralizer best suited to ice cream is sodium bicarbonate. Mix the neutralizer (1 kg. neutralizer to 1 kg. acid) with 10 times its v eight of water. Heat the mix to 32°C (90°F), or slightly higher if butter is used. Keeping the agitator moving, add the neutralizer solution slowly so as to distribute it uniformly throughout the mix. The temperature should be maintained at 32°C (90°F) for at least 10 minutes before pasteurizing.
Note:
It should be remembered that good ice cream cannot be made from a highly acidic mix.
C. Mix Stability:
This refers to stability or resistance to separation by the milk proteins in an ice cream mix. Instability results in separation of milk proteins as coagulated or precipitated material in the mix, and the resulting ice cream has a curdled appearance on melting.
This defect is caused by various factors which affect the colloidal stability of the milk proteins, such as high mix acidity, low citrate and phosphate content, a high calcium and magnesium content, high homogenizing pressure, high (pasteurizing) heat-treatment, low ageing time (resulting in poor hydration), destabilizing effect of freezing, etc.
Particle size, charge, and hydration are important factors influencing the stability of an ice cream mix; the most stable mix particle is the hydrophilic suspension because it is charged and hydrated; and the least stable suspension is that wherein the particle is neither hydrated nor carries a charge. It is the latter which results in instable mixes.
D. Specific Gravity:
The specific gravity or density of an ice cream mix varies with its composition and may range from 1.05 to 1.12.
It can be determined by using the following formula:
E. Surface Tension:
This pertains to the attraction between the molecules of a liquid at its surface. The greater the attraction between the molecules, the higher the surface tension, and vice versa. The unit of measurement of surface tension is dyne.
Investigations on the surface tension of ice cream are limited. Studies indicate that increasing the surface tension above that of a freshly-made mix (made from fresh ingredients) is difficult, although it may be readily decreased by the addition of products such as emulsifiers and the like.
Mixes with lower surface tension values (caused by the addition of excessive amounts of emulsifier to the mix) have shown excessive rates of whipping, fluffy short body characteristics, and susceptibility to the shrinkage defect. The normal surface tension values of ice cream mix may range from 48 to 53 dynes sq. cm. (At 20°C/68°F the surface tension of water is 72.75, and of milk, 40 to 60.)
F. Freezing Point:
The freezing point of ice cream is dependent on the soluble constituents and varies with its composition. The mix constituents which affect the freezing point directly are sugar, milk sugar, milk salts, and any other substances that may have been added and are in true solution. Other mix constituents which affect the freezing point indirectly by replacing water are fat, protein and any other constituent not in true solution.
Glucose depresses the freezing point almost twice as much as the same weight of sucrose, since the molecular weight of glucose is about one-half that of sucrose. On the other hand, corn syrup depresses the freezing point less than sucrose. In fruit ice cream, the freezing point will depend on the type of sugar used in fruit preparations (glucose or sucrose) and the extent to which the added sugar has undergone hydrolysis.
An average mix (containing 12 per cent fat, 11 percent serum solids, 15 per cent sugar, 0.3 per cent stabilizer and 61.7 per cent water) has a freezing point of about 27.5°F. Mixes with high sugar and milk-solids-not-fat contents may range to 26.5°F; while high fat, low milk-solids-not-fat or low sugar content mixes may range to 29.5°F.
G. Whipping Rate:
A high whipping rate means the ability to whip rapidly to a high overrun. It is now definitely known that the differences in whipping ability cannot be explained on the basis of viscosity. The present hypothesis is that whipping ability is based on tensile strength and the strength of the lamella (i.e., walls around the air cells). Whipping ability is improved by a high processing temperature, proper homogenization and ageing the mix for 2-4 hours.
Smaller fat globules and less clumping increase whipping ability. Mixes made with butter, butter-oil, or frozen cream have a less satisfactory dispersion of fat and poor whipping ability. Egg yolk solids, regardless of their source, and fresh cream buttermilk solids, improve whipping ability.
The usual variations in concentration of milk- solids-not-fat have no pronounced effect on whipping ability, but qualitative variations in the milk-solids-not-fat are important. Sugar decreases the whipping ability except when added after homogenization, in which case it increases it. Finally, the construction and operation of the freezer itself determine whether the maximum whipping ability of a given mix can be obtained.
The rate of whipping is measured by calculating the overrun at one-minute intervals while the mix is being frozen in a batch freezer. Normally, within 3 to 5 minutes after the freezing process starts, the mix is frozen and within 7 minutes an overrun of 90 per cent is obtained. In mixes which have a rapid whipping rate, 90 per cent overrun may be reached in 5 minutes or less. Mixes requiring 8 minutes or more to reach 90 per cent overrun are considered to have a slow whipping rate.