Will Your Slushie Actually Freeze?

🧊 How Slushie Machines Work

A slushie machine works by chilling a barrel to just below the mix's freezing point. A rotating scraper continuously shaves thin ice crystals off the barrel wall into the liquid bulk, keeping the whole thing at a semi-frozen "slush" state.

The magic number is your mix's freezing point. Too high (above around −1.5°C) and the mix freezes solid at the barrel wall. Too low (well below the barrel temperature) and the mix may remain fully liquid during the machine's cycle. The sweet spot for a Ninja-class machine is roughly −1.5°C to −4°C.

🧬 The PAC Method

SlushiCalc uses the PAC (Potere Anti-Congelante) method — the same science used by professional gelato makers. PAC measures how much any dissolved solute depresses the freezing point relative to sucrose (table sugar).

Alcohol (ethanol) has a PAC coefficient of 740 compared to sucrose's 100 — making it nearly 7× more powerful at depressing the freezing point. That's why even a small amount of spirits dramatically affects slush behaviour.

📊 Ideal Ranges

In home machines, crystallisation behaviour depends on total dissolved solids rather than sugar alone. A recipe may sit in the ideal Brix range while still carrying enough glycerol, fruit acids, or liqueur solids to prevent slush formation — check the Effective TDS warning below the ingredient list.

🔢 Component Masses

SlushiCalc calculates the mass of each component (sugar, alcohol, water) from density and Brix data — giving you an accurate per-ingredient breakdown. Water mass is everything that isn't sugar or alcohol, and it's the denominator in all PAC calculations.

🏠 Optimised for Home Machines

SlushiCalc uses a two-condition model tuned for home counter-top machines like the Ninja Slushi. A recipe is only Slushie Ready when it passes both checks:

1. Freeze point (PAC) — the freezing point must fall within the slush formation window (typically −1.5°C to −4°C for home machines). Above −1.5°C and the mix freezes solid at the barrel wall. Below the machine's barrel temperature and it stays liquid entirely.
2. Crystallisation (dissolved solids) — even if the freeze point is perfect, too much dissolved sugar or the presence of fat (e.g. cream of coconut) can prevent ice crystals from forming in the short cycle of a home machine.

These are separate physical effects. PAC thermodynamics answers "will it freeze at all?" The dissolved-solids check answers "will ice crystals actually grow in ~20 minutes?"

Effective TDS is used as a proxy for crystallisation behaviour in home machines. Real Ninja failure can also be driven by glycerol, fruit acids, and liqueur solids that don't appear in sugar Brix — the wine correction partially accounts for this.

Home slush machines operate in short cycles without barrel temperature control. Even mixtures that are thermodynamically able to freeze may fail to crystallise during the machine's run time.

ABV is capped at 11% for the ideal range. Home machines lack barrel temperature control and recovery cycles, so mixes above ~11% ABV tend to stay liquid even when the PAC freeze point looks acceptable.