The Science of Making Stocks

Stocks form the base flavours of many dishes, from sauces all the way to stews, and are a blank sheet of canvas on which different layers of flavours can be painted on. They are the backbone of flavour for many dishes and in my opinion are what separates the quality of taste between home cooking and restaurant cooking.

They usually consists of a herbs, meat and bones slowly simmered over many hours together with a mixture of finely chopped vegetables known as a mirepoix. The type of vegetables typically added to stocks are carrots, onions and celery, while the herbs typically added to stocks include thyme, bay leaf and parsley, with the occasional addition of basil and tarragon. These herbs tied together with twain are collectively known as a bouquet garni.

To traditionally make a stock, the mirepoix is lightly cooked with olive oil without any browning, before the bones, meat and bouquet garni are added in. The pot is then filled up with just enough water to cover the ingredients are allowed to simmer for many hours. As the pot slowly comes to a simmer, bits of coagulated blood and proteins start to raise to the top. These impurities are then skimmed off with a ladle and discarded. If not removed, they will cause the stock to become cloudy. This also explains why stocks are never boiled, because boiling would cause the the impurities to be vigorously mixed back into the stock. After many hours of cooking, the stock is the strain and ready to be used. In most cases, all the vegetables, bones and meat that went into the stock is discarded as it no longer contains any flavours. One way at this point to test if the stock is done is by tasting the ingredients in the stock, which should basically be close to tasteless at this point, as any remaining taste would indicate that not all the flavour has been extracted into the stock. Brown stocks are stocks made from roasted meat and bones while white stocks are stocks made from raw meat and bones.

The science of diffusion and maximum flavour extraction

When diving into the science of flavour extraction, everything points back to Fick’s Law, derived by Adolf Fick in 1855, which some people might be familiar with from high school physics. Instead of explaining Fick’s law however, we can just jump straight to what factors we need to change in order to increase diffusion and thus maximise the rate of flavour extraction. These factors are:

  • Surface Area

  • Temperature

  • Pressure

  • Flavour Concentration

The effect of surface area on flavour extraction

The smaller you cut a piece of food, the larger it’s surface area to volume ratio becomes. Take a carrot cut into tiny pieces for example, as it’s surface area increase, flavour molecules are able to leave the carrot and enter the water faster. The rate of flavour extraction increases based on the square of the size of the food. For example, a carrot half the size will have 4 times the speed of flavour extraction, and a carrot a third of the size will have 9 times the speed of flavour extraction (the math behind this is complicated and not exact but you can sort of round it off this way). This means that slicing your vegetables and meat into smaller pieces, you can greatly reduce the time required to make a stock.

There is a bottom limit to how small you’d want your stock ingredients to be of course. If cut too finely, you might not be able to strain them out, resulting in a not clear broth. Even worse however, is maybe blending up the vegetables and meat into a paste. This not only because it’s harder to strain out, but because blending the vegetables means releasing a whole load of other substances into the stock such as sugars and fibres etc, which might chance the taste and clarity of your stock. Lesson learned- cut up your vegetables finely.

The effect of temperature on flavour extraction

Increasing the temperature of the stock increases the speed at which all the flavour molecules move at, which in turns causes faster flavour extraction. Furthermore, a stock made at a higher temperature will have a different flavour compared to one made at a lower temperature due to the different types of aromatic compounds extracted. At higher temperatures, a wider range of flavour compounds are extracted as there is enough energy to extract low volatile compounds, at low temperatures, only the compounds with a higher volatility will be extracted. This can be both a good and bad effect. For example, a more gentle extraction at lower temperatures work best of light stocks like fish and vegetable stocks, as a higher temperature might start to extract bitter flavours. More heavy and robust stocks like brown stocks however, will benefit from higher temperatures. A good example of this is low temperature chicken stock, where chicken bones or meat is left at 70°C for 10 hours to yield a clear and pure chicken stock, though not the most time efficient.

With all this being said, the fact that stock is made in water also puts limitations on this due to the boiling point of water being 100°C. As the water starts to boil the contents of the stock will be agitated and the stock will start to become cloudy. This brings us on to next factor- pressure.

The effect of pressure on flavour extraction

If cooked in a pressure cooker, a pressure increase of 1 bar (or ~15 psi), increases the boiling point of water to 120°C. This means your stock can reach temperatures above 100°C without boiling and becoming cloudy. At a pressure of 1 bar, the time needed to make a stock can be reduced from 3 hours down to around 45 minutes. When the stock is done cooking in a pressure cooker, the ideal way to finish extracting flavour is by allowing the stock to fully cool down before opening the cooker. This allows the volatile flavour compounds in the steam to condense back into the stock. Opening the cooker immediately after releasing the pressure allows this flavourful steam to escape into the air.

As mentioned above, vegetable and fish stocks tend to do better at low temperature extractions and thus low pressures as well (without a pressure cooker). Anyway, the time required to cook a vegetable or fish stock is so short that there is no time saving benefit of using a pressure cooker.

The effect of flavour concentration on flavour extraction

Molecules move from areas of high concentration to that of low concentration. Think about a drop of ink dropped into water. The ink continues to spread out into parts of the water without ink. In that same way, flavour molecules move out of the vegetables into the water. This continues to occur until the concentration between the mediums are the same, or are at equilibrium. As the concentration between the two mediums gets closer and closer to equilibrium, the rate of diffusion gradually drops as well. At the point of equilibrium, diffusion stops. Because of this, completely flavour extraction from stock ingredients never actually fully occurs because after simmering for hours and hours, the concentration of flavour molecules left inside the stock ingredients will only be the same or less than that of the stock. The only way you could possible get around this problem is by straining out the ingredients before recooking them in plain water again to extract the remaining amount of flavour. However, this second batch of stock from the same ingredient is much more dilute and will need to be reduced down to be used which is why it’s never done in restaurants. For now, we just have to do without fully extracting all the flavour from our stocks.

Final Notes:

The shorter time for cooking a stock that you can achieve by cooking the stock in a pressure cooker whilst cutting the ingredients finer produces a slightly less complex stock. This is because the traditional long simmering that the stock goes through allows the breakdown of compounds into a much larger variety of flavours. This is my opinion, is hardly noticeable in the final product unless a side by side comparison is done.

Below are my recipes for some traditional stocks:

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