The Science of Slush

The two enemies of a good slush

A slushie machine like the Ninja Slushi works by slowly freezing liquid while constantly scraping the walls of the chamber. To form fine, smooth ice crystals (instead of a rock-solid block or a liquid puddle), two variables matter most:

• ABV — alcohol by volume. Alcohol lowers water's freezing point dramatically.
• Brix — sugar content by weight. Sugar also depresses the freezing point, and gives slush its body.

The magic numbers

After thousands of test batches by the home-bartender community using the Ninja Slushi, the consensus is clear:

• ABV: 8–14% (sweet spot ~10–12%)
• Brix: 11–13%

Drift outside those ranges and your drink either freezes solid or stays liquid. It's that narrow.

Freezing point depression, explained simply

Pure water freezes at 0 °C. Dissolve anything in it — alcohol, sugar, salt, citric acid — and the freezing point drops. The phenomenon is called freezing point depression, and the magnitude depends on how many particles of solute are dissolved per kilogram of solvent (the molality of the solution). Alcohol is a small, plentiful molecule, so it depresses the freezing point aggressively: a 10 % ABV solution freezes near −4 °C, a 14 % solution near −6 °C, and a 20 % solution stays liquid below −10 °C. Sucrose is a much larger molecule, so it depresses the freezing point gently — but it contributes most of the body and mouthfeel.

Why above 14% ABV won't freeze

Pure ethanol freezes at −114 °C. The more alcohol in your mix, the lower the freezing point of the whole solution. A standard freezer hits about −18 °C; a slushie machine hovers near −5 °C. Past about 14 % ABV, the solution can't reach its freezing point at slushie-machine temperatures. The auger keeps turning, the compressor keeps running, and you keep getting a very cold liquid.

Why below 8% ABV freezes solid

Too little alcohol means the freezing point stays close to 0 °C and water-rich mixtures form large ice crystals fast — your slush turns into shaved ice or a frozen brick that jams the auger. This is why mocktails need extra sugar (or glycerin) to slush properly: you're using sugar to replace the freezing-point depression that alcohol would normally provide.

The role of sugar (Brix)

Sugar does two jobs: it depresses the freezing point further (helping low-ABV or non-alcoholic drinks slush), and it gives the final texture its silky body. Too little sugar and your slush is icy and bland; too much and it's syrupy and won't freeze. The sweet spot for a Ninja Slushi sits at 11–13 g of sugar per 100 ml, which is roughly the natural Brix of orange juice or a lightly sweetened cocktail.

If your drink tastes balanced but won't freeze, the answer is rarely "more sugar" — it's usually "less alcohol or more water." Conversely, if it freezes too hard, dial sugar back before reducing alcohol.

Carbonation: the silent killer

Bubbles foam aggressively in a moving slushie chamber. Always flatten soda, beer or Prosecco before pouring — or add the bubbly component after slushing, like the classic Aperol Spritz technique. A simple trick: pour the carbonated ingredient into a wide bowl and stir for 30 seconds to knock most of the CO₂ out before measuring.

Dairy, fat and stabilisers

Cream, milk and coconut milk behave differently from sugar-water systems. Fat does not dissolve in water — it sits as droplets — so it does not depress the freezing point, but it dramatically improves mouthfeel and slows crystal growth. For milkshakes and frappés, aim for at least 6 % fat content (whole milk plus a splash of cream works) and use the Milkshake or Frappé mode, which runs at a warmer set-point to avoid curdling.

Commercial slushie syrups often include glycerin or gum arabic as stabilisers; you don't need them at home, but a teaspoon of glycerin per litre keeps a mocktail batch scoopable for hours longer.

Quick reference

Putting it all together

A reliable slush recipe is a balancing act: enough alcohol or sugar to keep the freezing point below the machine's set-point, but not so much that it never freezes; enough water to dilute the flavour into something you'd want to drink a pint of; and zero carbonation until after the slush is done. Once you internalise those rules, you can convert almost any cocktail into a slush version in a single pass.

Stop guessing

Plug any drink into our calculatoror ask the AI assistant — both run on the same engine and will tell you exactly what to add or remove to hit the slush sweet spot. For the practical troubleshooting walk-through, see why didn't my Ninja Slushi slush?