Ice cream lovers around the world appreciate the smooth and creamy texture that makes this frozen treat so enjoyable. The key to achieving this desirable texture lies in the size of the ice crystals. While the fat content plays a significant role in creaminess, the average size of the ice crystals also contributes to the overall mouthfeel. In this article, we’ll dive into the factors affecting ice crystal size during the freezing process, along with tips for improving the quality of homemade ice cream.
Ice Crystals in Ice Cream
Ice crystals in ice cream vary in size, ranging from 1 to over 150 μm in diameter. On average, the size hovers around 25 μm. Studies have shown that smaller ice crystals, typically measuring 10 to 20 µm, create a smooth and creamy texture. Conversely, larger ice crystals, exceeding 50 μm, give ice cream a grainy mouthfeel.
To produce ice cream with the smallest possible ice crystals, it’s essential to understand the process of ice formation, also known as crystallization, during freezing.
The Ice Cream Freezing Process
The freezing process of ice cream occurs in two stages. Initially, the mix is frozen in a scraped-surface freezer (SSF), also known as an ice cream machine. During this dynamic freezing stage, the mix is agitated to incorporate air, destabilize the fat, and promote ice crystal formation. As the ice cream exits the SSF, it typically maintains a temperature of approximately -5°C to -6°C (23°F to 21.2°F) and possesses a soft-serve consistency. The ice cream then undergoes static freezing in a freezer without agitation until the core reaches a specified temperature, typically -18°C (-0.4°F).
The dynamic freezing stage is arguably the most critical step in creating ice cream because it is the only stage where crystallization occurs. During dynamic freezing, the ice cream mix is added to the SSF at temperatures between 0°C and 4°C (32°F and 39.2°F). As the refrigerant absorbs heat from the mix, a layer of ice forms on the cold barrel wall, leading to rapid nucleation of small ice crystals. These crystals are then scraped off by the rotating dasher and scraper blades, dispersed into the center of the barrel, and subjected to recrystallization, where some crystals melt while others grow.
To achieve creamy ice cream, a high rate of nucleation is crucial, as it produces numerous small ice crystals. The more ice crystals formed during dynamic freezing, the smaller the average crystal size and the smoother the texture. Insufficient nucleation results in larger crystals, leading to a coarse texture.
Growth and Recrystallization
Dynamic freezing can be divided into two regions: the wall region and the bulk region. In the wall region, nucleation occurs due to the low temperature. In contrast, the bulk region experiences warmer temperatures, fostering crystal growth and recrystallization. The extent of growth and recrystallization in the bulk region directly influences the size of the ice crystals. It is crucial to reduce growth and recrystallization to achieve small ice crystals.
Factors Affecting Nucleation, Growth, and Recrystallization
Several factors contribute to the nucleation, growth, and recrystallization processes during ice cream freezing. Let’s take a closer look at some critical factors:
The amount of time the ice cream spends in the SSF, known as the residence time, significantly impacts the final ice crystal size distribution. Shorter residence times result in smaller ice crystals due to reduced recrystallization. On the other hand, longer residence times prolong exposure to warmer temperatures, leading to more significant recrystallization and larger ice crystals.
Evaporation Temperature of the Refrigerant Fluid
The primary refrigerants used in SSFs, such as liquid ammonia or freon, provide temperatures ranging from -23°C to -29°C (-9.4°F to -20.2°F). Decreasing the refrigerant temperature enhances heat removal at the barrel wall, promoting rapid nucleation and smaller ice crystals.
The barrel wall temperature directly affects the cooling rate of the ice cream mix. Lower wall temperatures result in faster cooling, reducing residence time and improving the ice crystal size distribution.
During dynamic freezing, the rotating scraper blades generate significant heat due to friction at the barrel wall. This heat contributes to the product temperature, which affects ice crystal size. Higher dasher speeds result in a higher ice cream temperature, leading to larger ice crystals. Dasher speeds are typically limited to 100-200 revolutions per minute (rpm) to prevent excessive crystal growth and recrystallization.
The draw temperature, the temperature at which ice cream is extracted from the SSF, plays a crucial role in determining ice crystal size. Lower draw temperatures generally result in smaller ice crystals. Factors influencing draw temperature include the refrigerant temperature, heat transfer, residence time, and overrun.
Tips for Optimizing Domestic Ice Cream Machines
To achieve the best results with domestic ice cream machines, consider the following tips:
- For machines that require the bowl to be frozen, set your freezer’s temperature to the coldest setting possible. Lower freezer temperatures promote higher rates of nucleation and shorter residence times.
- For machines with an in-built compressor, such as the Cuisinart ICE-100 or Breville BCI600XL, switch on the compressor and let it run for about 15 minutes before adding your mix.
The size of ice crystals significantly impacts the texture of ice cream. Smaller ice crystals, achieved through high nucleation rates and minimal growth and recrystallization, contribute to a smooth and creamy mouthfeel. Factors such as residence time, refrigerant temperature, dasher speed, and draw temperature play crucial roles in influencing ice crystal size. With a better understanding of these factors, you can optimize your ice cream-making process to create the creamiest and most enjoyable frozen treats.
For more information on ice crystal formation and other aspects of ice cream science, visit Ice Cream – your go-to resource for all things ice cream.
References available upon request.