Sandiness has been a defect that has plagued the ice cream manufacturer for many years. In fact it has been a problem ever since it became known that increasing the solids-not-fat content above that found in milk markedly improved the body of ice cream. About the end of World War I the ice cream industry was expanding rapidly and increasing the milk solids was a convenient and economical method of improving the eating quality. However it soon became apparent that hard, gritty particles developed in this ice cream that seemed as though there was sand in the product; thus "sandy" became the term to describe the defect. Lactose crystals were suggested as the causative agent in 1920, and definitely proven to be the cause in 1921. Since that time many investigators have contributed to our knowledge on the subject, but even today we have no adequate explanation as to why certain ice creams became sandy while others do not.
We know that the problem cannot be solved merely by limiting the lactose below the saturation point, because every ice cream contains a saturated lactose solution at the storage temperatures normally used. As water is frozen into ice the lactose becomes concentrated into a smaller and smaller amount of water so when ice cream storage temperatures are reached, the water is supersaturated with lactose even in ice creams of the lowest solids.
Crystallization is a two step process; nucleation or start of a crystal center followed by crystal growth. The first step, nucleation, usually does not occur and consequently crystals do not develop. Thus ice cream does not normally become sandy because crystal nuclei do not develop. In some ice creams for one reason or another, these crystal nuclei do develop and subsequently grow so that they are detected in the mouth as "sand."
In many cases small particles of other material can act as crystal centers on which lactose is deposited. These foreign particles apparently can be of a widely diverse nature, and explain why some lactose solutions crystallize while others under similar conditions do not. Perhaps one reason why sandiness is less of a problem now is that the ice cream ingredients, particularly the milk products, are of much better quality. the use of more efficient clarifiers and of cleaner raw milk has most likely resulted in fewer foreign nuclei in ice cream to initiate lactose crystallization.
Other foreign nuclei may be added with the flavoring material or the flavoring material may absorb moisture from the surrounding ice cream. Nuts, dehydrated fruit, or candy pieces often cause sandy ice cream in this way. Because of the absorbed moisture, a higher lactose concentration is produced where nuclei are more likely to develop. Other types of powdered flavorings that fail to dissolve completely may promote sandiness either by serving as nuclei or by absorbing moisture form the area surrounding the particles. Undissolved particles of high heat powder can have a similar effect.
Undissolved dry milk particles also can have the opposite effect by actually preventing sandiness. In this case milk powder is added to partially frozen mix in the freezer and unnumerable small crystals are produced when the lactose glass of the milk particles is dampened. This was suggested years ago as a method of producing high solids ice cream that would not go sandy. The explanation given at the time was that the milk powder did not dissolve and therefore the lactose of the powder would not be part of the solution of the ice cream. In reality the powder produced so many small crystals that they could not be detected in the mouth.
More recently seeding methods have been proposed that accomplish the same thing. A small quantity of lactose "seed," such as dried whey powder or "instant" dry milk, is added to the mix as it goes to the freezer. This material contains such large quantities of minute crystals that the entire mass of ice cream develops crystals less than 10 microns long. Such crystals are not detectable in the mouth. Fewer crystals are produced and these grow larger than 10 microns, then they may be detectable, depending on the number. The larger the crystals become the fewer can be tolerated without causing a mealiness or sandiness. This is the problem with foreign nuclei; too few are present to adequately seed the mix and therefore they grow too large and cause sandiness. When adequate nuclei are added the ice cream is fully protected against sandiness regardless of flavor, storage conditions, or age.
Another reason why present day ice cream does not tend to go sandy is due to the action of stabilizers. Apparently most of the stabilizers now in use aid in preventing nucleation; the ice cream is more resistant to crystallization when the stabilizers are present. Stabilizers will not prevent crystallization, however, once nuclei develop or if they are added.
The concern about sandiness has made some ice cream manufacturers less flexible with regard to formulating frozen desserts. Some have been reluctant to use edible whey solids when it would be feasible to do so, because they feel the lactose content would be increased to the danger level for sandiness. This seems unjustified in view of the fact that research shows adding 2% pure lactose to a regular commercial mix (11% SNF) did not encountering any problem with lactose crystallization. This was true even in a standard ice cream cabinet set at 15°F.
Sandiness certainly is not the major defect. A number of factors probable contribute to this fact, such as improved refrigeration to hold the product at lower and more constant emperatures, cleaner products that reduce contamination with foreign nuclei, and modern stabilizers that retard nucleation. When sandiness becomes a problem, for example with special flavors, the other techniques previously mentioned should be investigated as a possible solution to the problem. Sensory testing alone should not be relied upon as the sole means of detecting crystallization. This is satisfactory from a consumer's standpoint, but is not sensitive nor accurate enough to check the susceptibility of your ice cream to sandiness. Inexpensive polaroid filters are available to fit the ordinary microscope that will furnish polarized light. The lactose crystals under the polarized light appear as very bright areas in a dark field and are easily detected. This becomes a useful tool in the quality control of ice cream.
Adapted by John C. Bruhn, Dairy Research and Information Center, UC Davis from writings of Professor T. A. Nickerson.