This study investigates the effect of agitation speed on the freezing process of ice cream using a batch freezer. The temporal change in apparent viscosity during freezing is measured by the Metzner–Otto method. It was found that the apparent viscosity of ice cream after freezing decreased with an increase in the agitation speed. In addition, the number of bubbles incorporated into the ice cream also increased with the increase in the agitation speed. The bubbles were broken down into smaller bubbles by the higher shear force, that is, the higher agitation speed. Furthermore, a narrower distribution of the bubble size was obtained at higher agitation speeds. This indicated that the rheological properties and the microstructure of ice cream can be controlled by the agitation operation. Practical applications This study clarified the effect of agitation speed on ice cream properties such as the apparent viscosity, the overrun and the size distribution of contained bubbles. Especially, the apparent viscosity is important for both industries and consumers. The Metzner–Otto method was used to directly measure the temporal change in the apparent viscosity during freezing. This temporal change has not been investigated so far. It was found that the apparent viscosity decreased with the increase in the agitation speed. In other words, the rheological properties can be controlled by the agitation operation. In industries, the continuous freezer is more preferable than the batch freezer used in this study. Nevertheless, the knowledge would be quite useful for the development of industrial processes of ice cream.

Using a batch‐type freezer, this study investigated the influence of agitation speed on the time course change in the size of air bubbles and fat globules to establish kinetics of those during freezing. Higher agitation is advantageous not only for mixing enhancement but also for preventing local freezing and undesirable solidification, according to the results of flow visualization and time‐course change in the Reynolds number during freezing. Air bubbles and fat globule size were measured using samples obtained during freezing. For each agitations speed, the time‐course change in air bubble size was clearly different. Because of significant shear force, a smooth reduction in air bubble size was observed at a relatively high agitation speed. However, at the later stage of freezing, the increased collision frequency of the bubbles induced an undesirable increase in size. Aggregation of fat globules occurred immediately after the aqueous phase begin to be frozen. In addition, a greater agitation speed improved fat destabilization and hence increased the degree of aggregation (DA) as measured by the fat globule size distribution. At all agitation speeds, a decrease in DA was observed after a temporary increase in DA. This decrease can be explained by the breakup of the aggregates. At higher agitation speed, more serious decrease in DA was found. It was found that agitation speed significantly affects the kinetics of air bubbles and fat globules during freezing.Practical ApplicationsAfter freezing, air bubbles and fat globules size are important factors in determining ice cream quality. Nevertheless, no time‐course change in size has been constructed from the perspective of kinetics. To the best of our knowledge, this is the first study to investigate the effects of agitation speed on the time‐course change in air bubbles and fat globules size. The kinetics of air bubbles and fat globules are important in determining process parameters such as freezing time and agitation speed in the process design of freezers for ice cream manufacturing. Although this study was conducted using a bath‐type freezer, the kinetics of size change can be applied to process design with a continuous‐type freezer.