Sauce Thickening Calculator (Roux & Slurry)

Sauce Thickening Guide

What should I know about Thickener Ratios?

Standard ratios by method (per 500mL liquid): Roux: light: 25g butter + 25g flour. Medium: 35g + 35g. Thick: 50g + 50g. Cornflour slurry: light 15g, medium 25g, thick 40g. Arrowroot: same as cornflour but produces clearer, glossier sauces (no opacity from starch granules). Beurre manié: equal parts flour and butter mixed cold and added by spoonful at end. Egg yolk liaison: 2-3 yolks per 500mL. Tempering essential — add hot liquid to yolks gradually to prevent scrambling. Each method gives a different final texture and mouthfeel, which is why the right thickener depends as much on the desired result as on simple thickening power.

What do I need to know about Roux Method?

The classical thickener. White roux: cook just 1-2 minutes — for béchamel, velouté. Blonde roux: cook 3-5 minutes — slight nutty flavour. Brown roux: 10-30 minutes for gumbo, classic French brown sauces. Method: melt butter, add flour, cook (stirring constantly) to your desired colour. Add hot liquid slowly, whisking continuously. Bring to simmer — sauce will thicken as starches gelatinise. Common mistake: not cooking the roux long enough (sauce has raw flour taste). Cook the roux for at least 2-3 minutes over gentle heat before adding liquid, giving the raw flour taste time to cook out properly.

What should I know about Cornflour and Slurry?

Cornflour (cornstarch in US) is twice as powerful as flour by weight. Use half the quantity. Always mix with cold water first (the slurry) — never add directly to hot liquid (forms instant lumps). Add slurry to simmering liquid, stir continuously, simmer 1-2 minutes to remove starchy taste. Cornflour-thickened sauces: clearer than roux. Best for Asian sauces, fruit sauces, gravies. Limitations: do not reheat repeatedly (sauce thins). Do not boil vigorously after thickening (breaks the gel). Freezes less well than a roux-based sauce, since the starch structure can break down and separate on thawing.

What do I need to know about Reduction and Finishing?

Reduction: simmer to evaporate water, concentrating flavours and thickening through reduced volume. Time: 10-30 minutes depending on quantity. Stock reductions are central to classical French sauces. The body comes from gelatin in the stock (especially veal stock). Cannot reduce a stock that has not been made properly — gelatin must be present. Finishing techniques: monter au beurre (mounting with butter) — add cold butter at end. Don't boil after — emulsion breaks. Cream finish: add at the end, bringing the sauce back to a gentle simmer only briefly, since boiling cream hard can cause it to split rather than smoothly enrich the sauce.

What's the difference between Thickening Hot and Cold Liquids?

Starch-based thickeners behave differently depending on whether they're added to a hot or cold liquid, and getting this wrong is a common cause of lumpy sauces. Cornflour (cornstarch) and other pure starches should always be mixed first into a small amount of cold liquid to form a smooth slurry before being added to a hot sauce — adding dry starch directly to a hot liquid causes the outer layer of each granule to gelatinise instantly on contact, sealing in dry starch at the centre and creating permanent lumps that won't smooth out no matter how much stirring follows. Once the slurry is made, it should be stirred into the hot liquid gradually while the sauce is simmering, not boiling hard, and allowed to come back to a gentle simmer for a minute or two to fully activate and thicken, since starch needs both heat and time to reach its full thickening power. Flour-based thickeners used as a roux work differently, since the flour is first cooked in fat (which coats the starch granules and helps them disperse evenly) before any liquid is added, which is why a roux can be whisked directly into a hot or cold liquid without the same lumping risk, provided it's added gradually and whisked continuously as the sauce heats through.

What should I know about Gluten-Free Thickening Alternatives?

For gluten-free cooking, several thickeners work well in place of wheat flour, though each has slightly different properties worth knowing. Cornflour (cornstarch) is naturally gluten-free and widely available, giving a glossy, clear finish well suited to fruit sauces, gravies, and stir-fry sauces, but it loses thickening power if overcooked or reheated repeatedly, and breaks down (thins out) if combined with highly acidic ingredients without enough starch to compensate. Arrowroot gives an even glossier, clearer result than cornflour and holds up slightly better to acidic liquids, but it shouldn't be boiled for long once added, since prolonged heat makes it turn stringy. Potato starch thickens at a lower temperature than cornflour and gives a smooth result, useful for sauces that shouldn't be boiled hard. Rice flour and gluten-free flour blends can replace wheat flour in a roux-style thickening method, though they sometimes need slightly more than the equivalent wheat flour amount and can leave a faint graininess if undercooked. As a general substitution guide, cornflour is typically used at about half the quantity of wheat flour for the same thickening effect, since pure starches thicken more powerfully gram-for-gram than flour, which is roughly 75% starch with the rest being protein and other components.

Worked Example: Thickening 500ml of Stock into a Pan Gravy

A classic roux-based gravy uses roughly equal parts fat and flour by weight, typically around 10% of the liquid's weight in flour for a medium-bodied sauce. For 500ml of stock (treated as roughly 500g for this purpose), that's around 50g of flour cooked with about 50g of butter or fat into a roux before the stock is whisked in gradually. A cornflour slurry achieves a similar consistency with less thickener because cornflour has roughly twice the thickening power of plain flour by weight; for the same 500ml of stock, around 25-30g of cornflour mixed with a little cold water into a smooth slurry, then whisked into the simmering stock, gives a comparable medium-bodied result.

Fixing Lumpy or Broken Sauces

Lumps most often come from adding thickener directly to hot liquid rather than dispersing it first; the outside of each clump cooks and seals instantly, trapping dry starch or flour inside where it can't dissolve. Whisking cornflour into cold liquid first to make a smooth slurry, or cooking flour into fat as a roux before any liquid is added, avoids this by making sure every starch granule is coated or hydrated before it hits full heat. A sauce that has gone past thick into gluey, or one that has visibly curdled or separated (common with cream- or egg-based sauces taken too hot, or acidic ingredients added to dairy too quickly), can sometimes be rescued by whisking vigorously off the heat, straining out any solid lumps, or blending briefly, but a badly curdled dairy or egg-based sauce is often best started again rather than pushed further, since re-heating a split sauce tends to make the separation worse, not better.

Why Thickened Sauces Can Thin Out Again on Reheating

Starch-thickened sauces, particularly those using cornflour or arrowroot, can sometimes thin out noticeably when reheated, especially after a second reheating or extended simmering, because the starch molecules that swelled and trapped liquid during initial cooking can partially break down with further heat and time, releasing some of that trapped water back into the sauce. This is more pronounced with cornflour than with a flour-based roux, where the gluten structure holds up somewhat better to repeated reheating. Whisking in a small extra amount of slurry during reheating, rather than assuming the original thickness will hold indefinitely, is a simple fix when a make-ahead sauce has thinned by the time it's served.

Why Acidic Ingredients Can Weaken a Starch Thickener

Strongly acidic ingredients, such as a generous splash of vinegar, wine or citrus juice, can partially break down starch molecules over time and with heat, meaning a sauce thickened with cornflour or flour and then simmered for a long time with acidic ingredients can thin out more than expected compared with a less acidic version of the same recipe. Adding acidic ingredients toward the end of cooking, after the sauce has already thickened and only needs a brief simmer, rather than at the very start of a long cook, generally preserves the thickened texture better than adding them early and simmering the sauce for an extended period afterwards.