Why won't stevia or aspartame work in the Ninja Slushi?

They sweeten but don't lower the freezing point. The Slushi needs dissolved solids (sugar or sugar analogues) to slush at −2°C. Stevia, aspartame, and sucralose are hundreds of times sweeter than sugar, so you only need milligrams — nowhere near enough to depress the freezing point.

Is allulose really keto-friendly?

Yes. Allulose has 0.4 kcal/g and doesn't raise blood glucose or insulin in clinical studies. The FDA lets manufacturers deduct it from the 'total sugars' and 'added sugars' lines on nutrition labels. It's the closest thing to a 1:1 sugar replacement that actually freezes.

Can I mix allulose with monk fruit or stevia?

Yes — and it's often better. Allulose is only ~70% as sweet as sugar, so a small dose of monk fruit or stevia extract restores the sweetness without adding bulk. Use 60 g allulose + 1/8 tsp monk fruit extract per litre.

Will it slush?

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Sugar-free & diet Ninja Slushi guide

Sugar is why your slush slushes — not just why it tastes sweet. Cut it out entirely and the machine grinds to a halt. Here's the science, the sweeteners that actually work, and three sugar-free recipes we tested to death.

By SlushiGuide Editorial · Updated July 5, 2026

The 30-second science

The Ninja Slushi chills to about −2°C (28°F) — warmer than a freezer. Water alone freezes solid at that temperature. Dissolved sugar depresses the freezing point enough that ice crystals form but stay tiny and separated, giving you slush. Every drink needs roughly 11% dissolved solids (Brix) to hit that texture.

Artificial sweeteners like sucralose, aspartame, and stevia are hundreds of times sweeter than sugar, so you only add milligrams. They sweeten but don't contribute mass — so a diet soda has effectively the freezing point of water. Result: solid brick.

What actually works

Sweetener	Verdict	Dose / L
Allulose The gold standard. Freezes almost identically to sucrose, dissolves cold, no aftertaste. Slightly less sweet than sugar (about 70%), so pair with a tiny dose of monk fruit if needed.	✅ Slushes	70–90 g
Erythritol Works, but crystallises above ~80 g/L, giving a gritty mouthfeel. Cooling effect on the tongue. Best as a blend, not solo.	⚠️ Blends only	80–100 g
Monk fruit + allulose blend (Lakanto Classic, Wholesome) 1:1 sugar replacement in the machine. Our default recommendation for beginners.	✅ Slushes	60–80 g
Xylitol Freezes well but toxic to dogs — flag this if you have pets. Slight cooling effect.	✅ Slushes	80 g
Sucralose (Splenda, diet sodas) Solo it won't slush. Needs allulose or sugar as bulk.	❌ Won't freeze	—
Aspartame (Diet Coke) Same problem — no freezing-point depression.	❌ Won't freeze	—
Stevia extract Only useful as a flavor booster on top of allulose.	❌ Won't freeze	—

Three sugar-free recipes to try

Sugar-free lemonade slush

• 600 ml water
• 180 ml fresh lemon juice
• 75 g allulose
• 1/8 tsp monk fruit extract

Whisk until dissolved. Slush mode 55 min. ~5 kcal per serving.

Keto strawberry margarita

• 240 ml blanco tequila
• 60 ml triple-sec (Fiorente or a low-carb orange liqueur)
• 180 ml fresh lime juice
• 300 g fresh or frozen strawberries, blended
• 70 g allulose
• 240 ml water

12% ABV, ~11% effective Brix. Frozen Marg mode 60 min. Tajín rim mandatory.

Diet Baja Blast — the fix

• 1 L Baja Blast Zero (or Diet Mtn Dew + splash of lime)
• 150 ml vodka
• 60 g allulose
• 15 ml fresh lime juice

Solves the Reddit-famous 'my diet Baja Blast froze solid' problem. ~9% ABV.

Want to design your own? The slush calculator treats allulose exactly like sugar — enter it as a sugar ingredient and it'll balance the Brix for you.

Ingredient picks

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Related: our soda chart for fixing specific diet sodas, and the full ingredient roundup.