Tharp & Young "On Ice Cream" - Questions & Answers

Mix Processing Variables - Effect on Eating Quality:

Question: What mix processing variables affect the eating quality of ice cream?

Answer: Unit operations up to but not including, whipping and freezing, can be considered mix processing. All mix processing variables have significant and complex influence on the eating quality (body, texture, and flavor) of ice cream. It is possible here to provide only a broad overview of the relationships involved. Many of these have been considered in greater detail in previous columns; all are addressed in detail at Tharp & Young On Ice Cream technical short courses.

Mix ingredients should have maximum stability with regard to flavor, functionality, and, in the case of protein containing ingredients, protein stability. These elements are particularly vulnerable to deterioration due to heat and shear applied during processing.

In dispersing ingredients, the use of high-speed mixers should be limited so that no more agitation than what is required for dispersion is applied; otherwise, protein and fat instability can occur. This can negatively affect mix stability and fat agglomeration in the freezer.

Exceeding pasteurization specifications (time/temperature) can cause quality variations with regard to flavor, color, and protein stability. Flavor effects primarily involve variable levels of cooked flavor which can become a problem when that flavor is not a normal part of the flavor profile. Color can deteriorate as well. Protein can also be damaged, particularly when protein stability has been diminished in one or more dairy ingredients. In the extreme, protein instability can produce serum separation in the package and/or a meltdown that is curdy and shows whey separation.

Following pasteurization, it is important to cool mix to 40 F or lower. In addition to the inhibition of microbiological growth during storage, a low mix temperature is important to achieving the lowest possible exit temperature at the freezer that is essential to the initial texture and heat shock stability of the ice cream.

Homogenization should produce a fat globule size distribution between 1 and 2 microns. Successful homogenization requires all fat components to be liquid, so it should be applied at a temperature above the melting point of all fats present. That is particularly relevant to emulsifiers, some of which have a melting point above 150 F. Pressures applied should follow the guidelines of the homogenizer manufacturer.

It is also important to ensure that the homogenizer valves are maintained properly. Any departure from proper homogenization conditions can have a negative effect on quality related to the desirable fat agglomeration that occurs in the freezer. Effects can include such undesirable characteristics as: buttering; poor handling properties at the freezer; weak, short body; greasy mouthfeel; loss of creaminess and richness; and shrinkage.

Such quality defects can also be associated with the development of fat agglomeration in the liquid mix after homogenization. That can occur through the application of excess shear during mix transfer as a result of the use of oversized or improperly operated centrifugal pumps or in the turbulence produced by sharp piping angles.

Mix aging is required to prepare the fat to achieve the targeted level of agglomeration in the freezer and thus avoid the negative quality effects of insufficient or excess agglomeration in the freezer.

Two critical events occur during aging: complete fat crystallization; and conditioning the fat globule surface. Fat crystallization requires about two hours, regardless of the cooling temperature. Conditioning the fat globule surface can extend the required aging time up to four hours, depending on the amount and type of added emulsifier. Frequently used emulsifiers, ranked in order of increasing requirement for aging time, are: polysorbate 80; polysorbate 65; unsaturated monoglycerides; saturated monoglycerides; and phospholipids (lecithin per se or as introduced via egg yolk or buttermilk).

During mix storage even a slight degree of serum separation can produce variable quality because of changes in the level and nature of the total solids as a non-uniform mix goes to the freezer. Thus, the need for agitation in storage. However, too much agitation can be a negative factor by producing premature fat agglomeration. That can affect air incorporation by increasing mix viscosity and causing the defects associated with excess fat agglomeration. Management of storage agitation may involve the use of intermittent agitation through the use of a timer on agitator motors.

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