Wonders of extraction: Water


Extraction of peppermint leaves with hot water

Water is a polar molecule, meaning that one end has a small negative charge and the other a small positive charge. Because of this water is a very good solvent for other polar molecules and ions. For instance water is the solvent of choice for substances that provide taste, be it salt, sour, sweet or bitter as these are normally quite polar molecules.

A general rule is that the solubility of molecules and ions increases with the temperature of the water. Extractions are therefore faster if the water is boiling. This is the reason why we use hot water to extract tea leaves or ground coffee beans, even if we want to prepare ice tea or ice coffee. But by lowering the temperature and extending the extraction time we can change the relative proportion of what we extract. It therefore makes perfectly sense that different temperatures are recommended for different types of tea. Using different temperatures for the same kind of tea will of course also influence the flavor profile.

Polar molecules are more easily extracted than non-polar molecules. This is evident if we leave a tea bag for a long time in hot water. The bitter taste is due to the slow extraction of large polyphenol molecules which are less soluble in water. If tea is brewed at a lower temperature, less of the bitter tasting substances will be extracted.

Although water is polar, less polar and even non-polar substances can be extracted with water, especially if the water is boiling hot. You do this every day when prepare coffee. If you take a close look at cup of freshly brewed coffee you can notice small pools of oily substances floating on top of the coffee. The more severe conditions used when extracting coffee to make an espresso ensure that even more oily substances are extracted. Other examples of extraction using water in the kitchen include preparation of stock, soups and gravies.

The principle of extraction is simple, but a number of questions remain largely unexplored with regard to flavor: How do ions affect extraction? What role does pH play? How does temperature influence flavor? There is surprisingly little research on this that includes a sensory evalution.

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Filed under: experiments, molecular gastronomy, science, tips & tricks

Comments

  1. Weston Says:

    I know one place were I use to work we used baking soda in the water instead of salt for blanching, that would change the pH wouldn’t it?

  2. Weston Says:

    How Do I Email you anyways
    Found this link on the study of pH levels of blanching done by a canning company
    http://www.aseanfood.info/Articles/11016477.pdf

  3. Martin Lersch Says:

    It is true that baking soda will change the pH, but blanching doesn’t really have anything to do with extraction.

    I can be reached at webmaster (at) khymos dot org.

  4. Colin Says:

    For latte panna cotta I steeped whole coffee beans in heavy cream overnight in the cooler. Made for a very smooth and un bitter coffee flavor. I know you are talking about water but I’m sure fat plays a critical role in many extraction. Also, Does this mean you could make herb consomes in the same manner? I wonder what the time would be? and what is the right temp? Thanks for the new experiment

  5. achemist Says:

    Actually, saying “one end has a small negative charge and the other a small positive charge” for water is not exactly correct. The net dipole is negative on one end and positive on the other. H2O is shaped kind of like a boomerang with the positive charges on the ends and the negative part at the center of the curve.

    “Polar molecules are more easily extracted than non-polar molecules.” Yes, for water. Obviously that changes when you move to more non-polar substances, like ethanol and of course oils or fats.

    With tea, a bit part of the flavour profile is not just the extraction/temperature per se but also the oxidation by oxygen dissolved in the water and by other compounds in there as well. This becomes obvious if you try to ‘reuse’ and reheat previously heated water. I think this is also why traditional Asian tea recipes call for boiling the water and then cooling it slightly before use – it not only lowers the temperature appropriately but gives it a chance to reoxygenate.

  6. lizet Says:

    Coincedence: yesterday, in the archives of a 18th century castle I dug up a recipe ‘a method of making rose oil in Asia’ dating from 1829.
    “Manière de faire l’eau de Rose en Azie. On effeuille les roses dans un vase de bois dans lequel on met de l’eau bien pure; et on les expose pendant quelques jours au soleil; la partie huileuse se separe et surnage l’eau, on ramasse doucement cette huile précieuze avec du coton fin, on l’exprime dans de petites bouteilles qu’ on bouche hermétiquement. Voila tout le sécret de cette essence, qui se vent si cher a Constantinople, et en France quand on peut en avoir.”
    So, it seems that roseleaves steeped in water in a wooden bowl, put in the hot Asian sun will loosen the fragrant oil, which then can be gathered.
    Of course, not for consumption but as a perfume. As in the Netherlands we seldom have temperatures of this kind, woud the oven be a good substitute?

  7. Paul. Says:

    @Colin–the different compounds in cofee (and there are many) dissolve at different temperatures. Fortunately, many flavorful compounds readily dissolve at low temperatures albeit slowly. The compounds that we tend to perceive as bitter in coffee tend to dissolve more readily (i.e., faster) at higher temperatures. I’m sure the fat in the cream dulls the perception of whatever of those bitter compounds have dissolved in your decoction.

    If you wanted to change the rate of dissolution without changing the temperature you might want to experiment with changing the unprotected surface area of your coffee (i.e., grinding the beans). It’s hard to give you precise guidelines because the variables would include the type, freshness and roast of the beans in question as well as the size of the grind.

    Hope this helps.

  8. Colin Says:

    Thanks for the info. I also ground the beans another time and the infusion was more pronounced.

  9. Papin Says:

    I sometimes enjoy making tea by letting the leaves infuse at room temperature. The leaves sit in the water for around ten hours and I find the tea less bitter. In the case of tea, the infusion process seems to be very dependent on the geometry of the leaves: variations of around 20% in size cause fourfold variations in the rates of extraction. (Mike Spiro has published a lot on infusion, see http://dx.doi.org/10.1002/jsfa.2740321115 for example.)

  10. restaurant a paris Says:

    thanks for the info::))

  11. blog.khymos.org » Blog Archive » Wonders of extraction: Oil Says:

    [...] a polar and a non-polar end falls in between oil and water. I’ve covered extractions using water and ethanol previously. That water and oil are opposites is easily observed by the fact that they [...]

  12. blog.khymos.org » Blog Archive » Wonders of extraction: Espresso (part I) Says:

    [...] posts in the extraction series: water, ethanol and [...]