What causes “lumpiness” in ice cream? How can it be corrected or managed?

What causes “lumpiness” in ice cream? How can it be corrected or managed? Ice cream at the surface of freshly extruded products such as ice cream cakes shows a rough, irregular or “lumpy” appearance that resembles the surface of cottage cheese. Although that appearance remains after hardening, it seems strange that the eating quality of the product is smooth and creamy without any trace of observable “curdiness.” Occasionally, a similar situation can be seen at the surface of a freshly-filled carton before it is closed. In such cases, when the hardened package is opened the irregularity has disappeared and the exposed surface is smooth.

Lumpiness occurs most frequently at low drawing temperatures. Experience has shown that it can be corrected by increasing the size of the line between the freezer and filling equipment. For example, in one case, the problem was corrected by changing the filling line from a two-inch to a three-inch pipe.

As the temperature of the pipe between the freezer and the filling equipment drops, the thermodynamic relationships involved cause a layer of frozen mix to build up on the inner surface of the pipe. Ultimately, this layer restricts the passage of ice cream through the pipe to the point where what was previously laminar flow becomes turbulent flow. Ultimately, the turbulence is enough to break off small portions of frozen mix layer. These portions do not have incorporated air, so they take on a different appearance from the aerated, frozen ice cream. As a result, they are noticeable as a surface irregularity.

In products such as an ice cream cake, where the product surfaces are not modified by contact with packaging material, this lumpy appearance becomes frozen in place. However, in packaged ice cream, the irregularity disappears as a result of contact between the product and the inside surface of the carton closure and cannot be seen when the hardened package is opened. In fact, lumpiness would not be visible if the freshly filled carton were opened before hardening.

The explanation for the fact that the very rough surface appearance does not produce a coarse or icy texture can be speculated as follows. Lumps that contribute the rough appearance are not each a single particle of ice. Rather, they each represent a loosely-bound matrix of individual ice crystals. The ice crystal particle size within that matrix is no different than that in the main body of ice cream. Therefore, the crystals in the lumpy matrix do not detract from the overall smoothness that is characteristic of the low draw temperatures at which this phenomenon is observed.

The effect of increasing the size of the line between the freezer and filler is likely to be explained as reflecting variables of mass flow. When a layer of frozen mix develops next to the pipe, a point of equilibrium is reached between the temperature of the pipe and that of the ice cream where no further buildup of frozen material occurs. In a larger pipe, the reduction of the effective diameter of the line at this equilibrium point is not enough to convert the normal laminar flow to turbulent flow. As a result, there is no erosion of portions of the frozen mix system and the product maintains its normal appearance.

It is not possible to make specific recommendations with regard to the degree of change in filling line diameter that will relieve this situation. The relationships between product temperature and the dynamics of the frozen mix buildup are complex. As a result, some experimentation may be necessary to determine the specific changes that will correct the problem.

For more detailed discussion of ice cream attributes and defects, join Bruce Tharp & Steve Young at Tharp & Young On Ice Cream Technical Short Course, Workshops, & Clinics to be presented in Mexico City, Sept. 18-21 in cooperation with the National University of Mexico, and in Las Vegas, Dec. 5-7. Visit www.onicecream.com  or call 610/975-4424 or 281/596-9603. Information on the Mexico City program can be found at www.alimentos.unam.mx or by emailing Dr. A. Galvez, National University of Mexico, at Galvez@servidor.unam.mx.