What is Cornstarch? The Ultimate Thickening Agent Guide
Complete guide to cornstarch - how it works, cornstarch vs flour, slurry technique, cooking uses, and gluten-free thickening power
Chef Steven Harty
Professional Chef
30+ years experience
30+ years professional experience, trained under Rick Bayless. Specializes in precision cooking.
Education
Culinary Training under Chef Rick Bayless
Quick Answer
Cornstarch is a fine white powder made from corn kernels, used to thicken sauces, gravies, and soups. It's gluten-free and creates a glossy, clear finish unlike flour's cloudy result.
Understanding Cornstarch
What It's Made From
Pure starch extracted from corn kernels. The endosperm is isolated, dried, and ground into fine white powder. 100% starch with no protein, fat, or fiber.
Thickening Power
Cornstarch has 2x the thickening power of flour. Creates smooth, glossy, translucent sauces. Activates at 203°F, thickens quickly with heat.
Where to Find It
Baking aisle near flour and sugar. Common brands: Argo, Clabber Girl. Comes in 16 oz boxes. Also called "corn flour" in UK/Australia (confusing!).
Cornstarch vs Flour
| Feature | Cornstarch | All-Purpose Flour |
|---|---|---|
| Source | Corn (pure starch) | Wheat (starch + protein) |
| Thickening Power | 2x stronger | Standard |
| Finish | Glossy, clear | Cloudy, opaque |
| Gluten-Free | Yes | No |
| Taste | Neutral | Slight wheat flavor |
| Heat Needed | 203°F | 155°F |
| Best For | Sauces, Asian cooking, pies | Roux, gravy, baking |
| Amount Needed | 1 tbsp per cup liquid | 2 tbsp per cup liquid |
When to Use Cornstarch
- ✓Clear sauces (stir-fries)
- ✓Fruit pies (glossy filling)
- ✓Puddings and custards
- ✓Sweet and sour sauce
- ✓Crispy coating (fried chicken)
- ✓Gluten-free thickening
When to Use Flour
- ✓Gravies (brown color okay)
- ✓Roux-based sauces
- ✓Creamy soups
- ✓Traditional recipes
- ✓Slow-cooked dishes
How to Use Cornstarch - Slurry Method
The Slurry Technique
Mix cornstarch with COLD liquid (water, broth, juice)
Ratio: 1 tbsp cornstarch + 2 tbsp cold liquid
Stir until smooth, no lumps
Use fork or shake in jar
Add slurry to HOT liquid while stirring
Pour slowly, stir constantly
Simmer 1-2 minutes until thickened
Don't boil vigorously
Remove from heat
Sauce thickens more as it cools
⚠️ Critical Rules
- ❌Never add dry cornstarch directly to hot liquid (lumps!)
- ❌Never add slurry to cold liquid (won't activate)
- ❌Never boil vigorously (breaks down, gets thin)
- ✓Always make slurry first
- ✓Always add to simmering liquid
- ✓Always stir constantly
Common Ratios
Like: Light gravy, soup
Like: Stir-fry sauce, pie filling
Like: Pudding, very thick gravy
Common Uses for Cornstarch
Thickening Sauces
- Stir-fry sauces
- Sweet and sour
- Teriyaki
- General Tso's
Makes: Glossy, restaurant-style
Pie Fillings
- Fruit pies (apple, cherry, blueberry)
- Absorbs fruit juices
- Creates sliceable filling
Makes: Glossy appearance
Puddings & Custards
- Chocolate pudding
- Vanilla custard
- Pastry cream
Makes: Smooth, creamy texture
Crispy Coatings
- Fried chicken (mix with flour)
- Tempura batter
- Extra crispy texture
Makes: Stays crispier longer
Cornstarch Substitutes
| Substitute | Ratio | Works For | Doesn't Work For |
|---|---|---|---|
| Arrowroot Powder | 1:1 | Sauces, acidic dishes | Dairy-based sauces |
| Tapioca Starch | 2 tbsp = 1 tbsp cornstarch | Pies, puddings | Clear sauces |
| Potato Starch | 1:1 | Gluten-free baking | High-heat cooking |
| All-Purpose Flour | 2 tbsp = 1 tbsp cornstarch | Gravies, soups | Clear sauces, gluten-free |
| Rice Flour | 2 tbsp = 1 tbsp cornstarch | Asian cooking | Western dishes |
| Xanthan Gum | ¼ tsp = 1 tbsp cornstarch | Small amounts | Large batch cooking |
⭐ Best Substitute: Arrowroot powder - works 1:1, similar properties, gluten-free
Cooking Tips & Common Mistakes
Common Mistakes
❌Adding dry to hot liquid
Fix: Always make slurry first
❌Sauce gets thin after thickening
Why: Overboiling breaks down starch
Fix: Simmer gently, don't boil
❌Sauce is lumpy
Why: Cornstarch clumped before dissolving
Fix: Strain sauce, or start over with proper slurry
❌Sauce looks cloudy
Why: Not enough heat to fully activate
Fix: Bring to full simmer (203°F)
❌Sauce tastes starchy
Why: Didn't cook long enough
Fix: Simmer 1-2 minutes after thickening
Pro Tips
- ✓Make slurry in small jar, shake vigorously
- ✓Add slurry slowly while stirring constantly
- ✓Can always add more (can't remove!)
- ✓Sauce thickens more as it cools
- ✓Reheating thin sauces works, but gentle heat only
Troubleshooting
Too Thick:
Add more liquid slowly while heating gently
Too Thin:
Make more slurry, add gradually while simmering
Lumpy:
Strain through fine mesh or whisk vigorously
Frequently Asked Questions
What is cornstarch used for?
Cornstarch is primarily used to thicken sauces, gravies, soups, and pie fillings. It's also used in baking for tender texture, as a crispy coating for fried foods, and in puddings and custards. It's gluten-free and creates clear, glossy results.
Can I use flour instead of cornstarch?
Yes, but use double the amount (2 tbsp flour = 1 tbsp cornstarch). Flour creates cloudier sauces and has less thickening power. Best for gravies and soups. Not ideal for clear sauces or gluten-free cooking.
How do you make a cornstarch slurry?
Mix 1 tablespoon cornstarch with 2 tablespoons cold water until smooth. Add this slurry to simmering liquid while stirring constantly. Never add dry cornstarch directly to hot liquid or it will clump.
Does cornstarch go bad?
No, cornstarch lasts indefinitely when stored in a cool, dry place in an airtight container. It may absorb moisture and clump over time, but it's still safe to use. If it smells off or has bugs, discard it.
Why did my cornstarch sauce get watery?
Overboiling breaks down cornstarch's thickening power. Also, some acidic ingredients (lemon juice, vinegar) can thin cornstarch sauces over time. Use arrowroot powder for acidic dishes instead.
Chef Steven Harty
Professional Chef | 30+ Years Experience
Chef Harty brings over 30 years of professional culinary experience, having trained under renowned chef Rick Bayless. He specializes in precision cooking and measurement accuracy, ensuring all our cooking conversions and ingredient calculations are tested and verified in real kitchen conditions.
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