The Freezing Process in Ice Cream Production: Methods and Effects

Freezing is the defining moment in ice cream manufacturing. It is the step where a liquid mix transforms into the semi-solid, creamy product we know and love. But freezing ice cream is not as simple as placing a mix in a cold environment. It is a carefully controlled process that involves rapidly lowering temperature, incorporating air, forming tiny ice crystals, and destabilising fat – all happening simultaneously inside specialised equipment. How well these events are managed determines whether the final product is smooth and luscious or coarse and icy.

Why freezing matters so much in ice cream production 🔗

After the ice cream mix has been pasteurised, homogenised, and aged, it enters the freezer as a cold liquid – typically at around 4-5 °C (39-41 °F). The freezing stage must accomplish three things at once: convert a portion of the water in the mix into ice crystals, whip air into the mix (a process called overrun), and partially destabilise the fat emulsion so that fat globules form a network around the air bubbles. The speed and efficiency of this step directly influence texture, body, mouthfeel, and shelf stability of the finished ice cream.

According to food science research published by Springer, freezing is typically accomplished in two stages. The first is dynamic freezing, where the mix is frozen quickly under constant agitation. The second is static freezing (also called hardening), where the partially frozen product is placed in a very cold environment without agitation to complete the freezing process.

Dynamic freezing: where ice cream is born 🔗

Dynamic freezing takes place inside a scraped-surface freezer (SSF) – the core piece of equipment in any ice cream plant. Inside the SSF, the mix contacts a refrigerated barrel wall cooled by ammonia or freon to temperatures between −23 °C and −29 °C. As the barrel wall absorbs heat from the mix, a thin layer of ice forms on its surface. A rotating dasher with scraper blades continuously removes this frozen layer and disperses it into the warmer bulk mix in the centre of the barrel.

This scraping action is critical. It serves multiple purposes: it exposes fresh liquid mix to the cold wall for further freezing, it breaks newly formed ice into small crystals, and it incorporates air into the mix. The ice cream exits the dynamic freezer at a draw temperature of roughly −5 °C to −6 °C, with a consistency similar to soft-serve ice cream.

The role of nucleation 🔗

When water in the mix first begins to crystallise on the cold barrel wall, the process is called nucleation – essentially the birth of new ice crystals. A high rate of nucleation means many small crystals form at once, which is exactly what producers want. Research shows that nucleation happens almost exclusively at the barrel wall, where temperatures are low enough to trigger crystal formation. Once scraped off and moved to the warmer centre, these tiny crystals can grow or merge with each other through a process known as recrystallisation.

The key insight for ice cream manufacturers is straightforward: the more small ice crystals created during dynamic freezing, the more of them survive into the final product, and the smoother the texture will be. Fewer crystals formed at this stage means each one grows larger, producing a coarser, icier mouthfeel.

Why fast freezing produces smoother ice cream 🔗

Speed is everything during dynamic freezing. The faster the mix reaches its draw temperature, the less time ice crystals spend in the warm bulk zone where recrystallisation causes them to enlarge. Studies have demonstrated that residence time – the duration the mix spends inside the scraped-surface freezer – has the single greatest impact on final crystal size. Shorter residence times lead to smaller ice crystals and smoother texture.

Lowering the refrigerant temperature also helps. Colder barrel walls remove heat faster, speed up nucleation, and reduce the time ice cream stays in the freezer. As a general rule in ice cream science, a fast freezing rate generates small ice crystals while a slow freezing rate generates large ones.

Types of freezers used in ice cream production 🔗

The three main types of freezers – batch, continuous, and soft-serve – differ in scale, speed, and the degree of control they offer over freezing and air incorporation.

Batch freezers 🔗

Batch freezers process a fixed quantity of mix per cycle. A measured volume of liquid mix is loaded into the freezing cylinder, frozen while being agitated, and then discharged once the desired texture is reached. These machines are the go-to choice for artisanal ice cream shops, gelato producers, and R&D labs where flexibility and flavour experimentation are priorities.

Freezing time in a batch freezer typically ranges from 10 to 20 minutes, which is considerably longer than in a continuous freezer. Because of this extended residence time, batch freezers tend to produce slightly larger ice crystals. However, the operator has hands-on control over texture, temperature, and the moment of discharge – a significant advantage for craft producers. Overrun in batch freezers generally falls in the range of 30-50 % for traditional gelato-style machines and up to 80-100 % for larger units, resulting in a denser, creamier product.

Continuous freezers 🔗

Continuous freezers are the workhorses of industrial ice cream manufacturing. Instead of processing one batch at a time, they receive a continuous stream of mix from a bulk tank. Air is injected via a separate pump just before the mix enters the freezing barrel, where a high-speed dasher scrapes the barrel wall and disperses the frozen mix.

The entire freezing process in a continuous freezer takes only a few seconds to a couple of minutes, far less than in a batch machine. This rapid throughput offers two major advantages: extremely short residence time (which means smaller ice crystals and smoother texture) and highly consistent output hour after hour. Overrun in continuous freezers can be precisely controlled and typically ranges from 50 % to over 100 %. Up to half the volume of the finished product can be air.

Modern continuous freezers are equipped with microprocessor controls that monitor overrun percentage, product viscosity, cylinder pressure, mix flow rate, and draw temperature in real time. This level of automation makes them essential for large factories where product uniformity across thousands of litres is non-negotiable.

Soft-serve freezers 🔗

Soft-serve freezers are specialised machines found in quick-service restaurants and soft-serve shops. They freeze the mix quickly and dispense it directly to the consumer at a warmer serving temperature of about −6 °C to −7 °C, which gives soft-serve its characteristic smooth, flowing texture. Because the product is served almost immediately from the freezer, there is no separate hardening step.

Soft-serve machines can achieve high overrun levels – often 50-60 % or more – producing a light, airy product. They combine production, storage, and dispensing in a single unit, making them convenient for point-of-sale operations.

Understanding overrun: the role of air 🔗

Overrun is the term used for the amount of air incorporated into the ice cream mix during freezing. It is expressed as a percentage increase in volume. For example, 100 % overrun means that one litre of liquid mix has become two litres of finished ice cream – half of the final volume is air.

Air is not just filler. It is a structural component that affects how ice cream feels in your mouth. The right amount of air makes ice cream light and easy to scoop, enhances flavour release, and slows the rate of melting. Too little air produces a dense, heavy product that is hard to scoop. Too much air makes the product feel insubstantial, melts quickly, and lacks richness.

Premium ice creams typically have lower overrun (around 50-80 %), which gives them a richer, denser character. Economy-grade ice creams may push overrun past 100 %, maximising volume from the same quantity of raw materials. During dynamic freezing, the dasher blades break the injected or entrained air into tiny bubbles, and partially coalesced fat globules form a thin membrane around each bubble to stabilise it.

Ice crystal size: the key to smooth texture 🔗

The single biggest factor determining whether ice cream feels smooth or grainy on the tongue is the size of its ice crystals. According to research compiled by Ice Cream Science, crystals in the range of 10-20 µm give ice cream its desirable smoothness, whereas crystals larger than 50 µm are perceived as coarse and icy.

Ice crystals in commercial ice cream range from about 1 µm to over 150 µm, with an average of approximately 25 µm. The goal of the freezing process is to push as many crystals as possible toward the smaller end of this range. Several factors influence crystal size:

Freezing speed: Faster freezing promotes higher nucleation rates and shorter residence times, both of which favour small crystals.

Barrel wall temperature: Colder refrigerant temperatures at the barrel wall increase the rate of heat removal and nucleation.

Dasher speed: Faster dasher rotation scrapes ice from the wall more frequently, but excessive speed can generate frictional heat that actually melts small crystals and promotes aggregation into larger ones.

Draw temperature: A lower draw temperature means more water is frozen at the time of discharge, but it must be balanced against the risk of overly long residence times if the freezer lacks sufficient cooling capacity.

Static freezing (hardening) 🔗

When ice cream exits the dynamic freezer, only about 50 % of the water is actually frozen. The product at this point resembles soft-serve. To produce hard ice cream, the product is packaged and transferred to a hardening tunnel or blast freezer operating at −30 °C to −40 °C.

During hardening, the remaining free water freezes onto existing ice crystals – no new crystals are formed because the product is not agitated and the undercooling is relieved by growth of existing nuclei. This is why the crystal population created during dynamic freezing is so important: it is essentially locked in at that stage. If few, large crystals were formed during dynamic freezing, they will only get larger during hardening.

Hardening should be completed as quickly as possible. The faster the core temperature of the packaged ice cream drops to −18 °C or below, the less opportunity the existing crystals have to grow through recrystallisation. Packaging should also happen as close to the freezer discharge as possible to minimise heat gain from friction in transfer pipes.

Mix composition and its effect on freezing 🔗

The formulation of the ice cream mix plays a major supporting role in the freezing process. Several ingredients directly influence how water freezes:

Sugars lower the freezing point of the mix through freezing-point depression. This means the mix does not begin to freeze at 0 °C but at a lower temperature, typically around −2 °C to −3 °C depending on the sugar concentration. The practical benefit is that some water remains unfrozen even at serving temperature, keeping the ice cream soft enough to scoop.

Stabilisers such as guar gum, locust bean gum, and carrageenan do not prevent ice crystal formation, but they slow down the rate at which small crystals merge into larger ones during storage. They increase the viscosity of the unfrozen phase surrounding the crystals, physically isolating crystals and inhibiting recrystallisation.

Fat content contributes to creaminess and acts as a structural scaffold around air bubbles. Higher fat mixes tend to produce smoother ice cream partly because fat globules interfere with ice crystal growth and stabilise the foam structure created during freezing.

Nonfat milk solids (proteins and lactose) also help stabilise the air cells incorporated during freezing, contributing to body and texture.

Emerging freezing technologies 🔗

While scraped-surface freezers remain the industry standard, newer approaches are gaining attention. Cryogenic freezing using liquid nitrogen (at −196 °C) can freeze an ice cream mix almost instantly. The ultra-rapid freezing rate produces extremely small ice crystals and an exceptionally smooth texture. Even mixes with high alcohol content – which normally resist freezing – solidify without issues under liquid nitrogen.

Another advancement is low-temperature extrusion, where ice cream is processed at temperatures significantly below the traditional draw temperature. This technology produces finer microstructures and improved resistance to heat shock during distribution. While these methods are still niche, they point toward a future where freezing technology plays an even more central role in product innovation.

Common freezing defects and how to avoid them 🔗

Coarse or icy texture 🔗

This is the most frequent freezing-related defect. It results from large ice crystals caused by slow freezing, long residence time in the SSF, high draw temperature, or inadequate stabiliser levels. The fix is to ensure fast, efficient freezing with properly maintained refrigeration and appropriately formulated mixes.

Excessive overrun 🔗

Too much air makes ice cream feel light, fluffy, and unsatisfying. It melts quickly and may collapse under its own weight. Overrun should be monitored and adjusted to match the product category – premium products typically stay below 80 %, while standard ice cream may go up to 100 %.

Sandy or gritty texture 🔗

This defect can develop during storage if temperature fluctuations cause repeated partial melting and refreezing. Each cycle allows ice crystals to grow larger. Maintaining a consistent storage temperature of −18 °C or colder and minimising exposure to warm air are essential preventive measures.

Key takeaways 🔗

The freezing process is where all the preparation – ingredient selection, pasteurisation, homogenisation, and ageing – comes together to create the final ice cream texture. Batch freezers offer flexibility and control for small-scale, artisanal production. Continuous freezers deliver speed, consistency, and precise overrun control for industrial operations. Soft-serve freezers serve a specialised market by dispensing a ready-to-eat product at point of sale. Regardless of the equipment used, the fundamental principle remains the same: freeze the mix as quickly and uniformly as possible, incorporate the right amount of air, and form as many small ice crystals as you can.

What do you think? How much of a role do you believe freezing speed plays compared to mix formulation in determining the final texture of ice cream? And if you had to choose between artisanal batch-frozen and industrially produced continuous-frozen ice cream, which would you prefer and why?

References

1. https://www.milkfacts.info/Milk%20Processing/Ice%20Cream%20Production.htm
2. https://link.springer.com/chapter/10.1007/978-1-4615-0163-3_7
3. https://www.icecreamscience.com/blog/ice-crystals-in-ice-cream
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC10648627/
5. https://www.gelatomachine.com/what-is-the-difference-between-a-batch-freezer-and-a-continuous-freezer/
6. https://dairyprocessinghandbook.tetrapak.com/chapter/ice-cream
7. https://pregelamerica.com/pmag/continuous-freezers-just-keep-on-churning-along/
8. https://www.euroglaces.eu/ice-cream-technology