This is a great question! Lot of misinformation in that reddit post lol. Here's hoping I can set things straight. Most of the following is summarized from Harold McGee's magnum opus "On Food and Cooking". He provides extensive journal references for these claims, so I feel fairly confident believing this:
The term "tempering" is a misnomer. Tempering/blooming (synonyms) and dry-toasting are very similar. Both act with heat to liberate flavor compounds from where plants store them (in specialized oil-storage cells, glands on leaves or channels between cells). Once released, those flavor compounds -- which chemically resemble oils because of their evolutionary history as defensive chemicals -- undergo a bunch of chemical reactions. For each compound, there are a bunch of possible reactions that can happen, and only a fraction of the total initial quantity will react. So heat acts to increase the overall complexity of flavor: a smaller number of initial compounds gets transformed into a larger number of final compounds.
They also become somewhat milder, because some of the initially strong, characteristic compounds are transformed into other chemicals. But why would that transformation tend to lead to milder flavors? Again, because of the evolutionary history of most flavoring compounds. Their primary function is to make the plants that produce them obnoxious and therefore resistant to attack by animals or microbes. The flavors are defensive chemical weapons that are released from plant cells when they're chewed on. Their volatility also serves a function: they can attack through the air, and act as a warning signal to deter some animals by smell alone. So the initial compound is carefully evolved to be an effective, obnoxious chemical weapon. Then we heat it and degrade it into a bunch of somewhat random other compounds, so we should generally its effectiveness as a chemical weapon to go down. This is also why cutting raw onions makes you cry -- they release chemical weapons to deter animals from attacking them -- but cooked onions are milder and more complex. Heat changes those chemical weapons (lachrymators, from the Latin lacrima, for tear) into a bunch of random other compounds, which are way less likely to serve the original evolved function of stimulating mammalian tears.
So everything I said above is common to all techniques that apply heat to dry spices. What's the difference? Doing it in oil means a bunch of the oil-soluble compounds go into the oil, creating a flavored oil. Another consequence is that once dissolved in oil, compounds can react with each other more easily, since in liquid there are more opportunities for different dissolved molecules to bang into one another. Importantly, compounds from *different spices* can interact with each other.
Here's McGee:
"The toasting on a hot pan of whole dry spices, typically mustard, cumin, or fenugreek, for a minute or two until the seeds begin to pop, the point at which their inner moisture has vaporized and they are just beginning to brown. Spices cooked in this way are mellowed, but individually; they retain their own identities.
The frying in oil or ghee of mixed powdered spices, often including turmeric, cumin and coriander. This step allows the different aroma chemicals to react with each other so that the flavors become more integrated [...]"
I remember once learning about blooming spices to increase their flavor, and at first I thought that was the same thing as the tempering you describe here. But some places on the internet say they're opposite things (Reddit says blooming is to increase flavor while tempering is to distribute/make flavors milder? https://www.reddit.com/r/AskCulinary/comments/fpl8h7/toasting_spices_to_improve_flavor_is_a_common/#:~:text=screwyoushadowban-,Toasting%20spices%20to%20improve%20flavor%20is%20a%20common%20culinary%20practice,spices%20(making%20them%20milder).) and others seem to lump them together?
hard to find consistent descriptions but wondering if you knew what the difference is! There are some good comments in that Reddit post too
This is a great question! Lot of misinformation in that reddit post lol. Here's hoping I can set things straight. Most of the following is summarized from Harold McGee's magnum opus "On Food and Cooking". He provides extensive journal references for these claims, so I feel fairly confident believing this:
The term "tempering" is a misnomer. Tempering/blooming (synonyms) and dry-toasting are very similar. Both act with heat to liberate flavor compounds from where plants store them (in specialized oil-storage cells, glands on leaves or channels between cells). Once released, those flavor compounds -- which chemically resemble oils because of their evolutionary history as defensive chemicals -- undergo a bunch of chemical reactions. For each compound, there are a bunch of possible reactions that can happen, and only a fraction of the total initial quantity will react. So heat acts to increase the overall complexity of flavor: a smaller number of initial compounds gets transformed into a larger number of final compounds.
They also become somewhat milder, because some of the initially strong, characteristic compounds are transformed into other chemicals. But why would that transformation tend to lead to milder flavors? Again, because of the evolutionary history of most flavoring compounds. Their primary function is to make the plants that produce them obnoxious and therefore resistant to attack by animals or microbes. The flavors are defensive chemical weapons that are released from plant cells when they're chewed on. Their volatility also serves a function: they can attack through the air, and act as a warning signal to deter some animals by smell alone. So the initial compound is carefully evolved to be an effective, obnoxious chemical weapon. Then we heat it and degrade it into a bunch of somewhat random other compounds, so we should generally its effectiveness as a chemical weapon to go down. This is also why cutting raw onions makes you cry -- they release chemical weapons to deter animals from attacking them -- but cooked onions are milder and more complex. Heat changes those chemical weapons (lachrymators, from the Latin lacrima, for tear) into a bunch of random other compounds, which are way less likely to serve the original evolved function of stimulating mammalian tears.
So everything I said above is common to all techniques that apply heat to dry spices. What's the difference? Doing it in oil means a bunch of the oil-soluble compounds go into the oil, creating a flavored oil. Another consequence is that once dissolved in oil, compounds can react with each other more easily, since in liquid there are more opportunities for different dissolved molecules to bang into one another. Importantly, compounds from *different spices* can interact with each other.
Here's McGee:
"The toasting on a hot pan of whole dry spices, typically mustard, cumin, or fenugreek, for a minute or two until the seeds begin to pop, the point at which their inner moisture has vaporized and they are just beginning to brown. Spices cooked in this way are mellowed, but individually; they retain their own identities.
The frying in oil or ghee of mixed powdered spices, often including turmeric, cumin and coriander. This step allows the different aroma chemicals to react with each other so that the flavors become more integrated [...]"