Why do we use oils only and not other mediums in this process?
Oils and fats are long molecules which are mainly non-polar and hence the opposite of water which is a polar molecule. Volatile molecules – the molecules that we detect by their smell – are mainly non-polar and therefore soluble in oil. This is one reason why foods with fat often have a different and often better flavor compared with their fat-free counterparts (fat of course also influences mouth feel etc.). Everytime you cook with oil it will actually help extract aroma (or smell flavorants) from the food ingredients and deliver these to your nose. There are several oil extracts used in the kitchen, and the nice thing about them is that the oil extracts aromas and then protects them from the air. This is good as it prevents oxidation of the aroma molecules.
The above excerpt is from http://blog.khymos.org/2008/10/24/wonders-of-extraction-oil/
What fats are best suited for this process and why?
Fats that are locally used in India are based on the region and may range from mustard oil,coconut oil,groundnut oil etc based on the availability.Due to globalization, we have been exposed to global trends like olive oil,canola oil and due to industrialization even to refined oils and we tend to adopt whichever we perceive as the best in our cooking practices.However the best fats to be used in a tadka are the ones that can withstand high heat and will not break into toxic compounds.
The science involved: vegetable oils are mainly unsaturated fats which when exposed to high heat break down easily.This makes olive oil a bad choice for tadka and you are instead adding toxic compounds to your food by the time you add the tadka to the dish.
Ghee on the other hand has saturated fats which is why it doesnt break down when exposed to high heat.
The above information was found on http://www.dawn.com/news/1179918
Does the quantity of oils matter?
Yes. There has to be enough oil to cover all the ingredients that are going to be added so that the essential oils released from spices are protected by a coating of oil from escaping into the air and oxidizing.
A practical test of this will be when you use less oil than required,you will observe the scent of spices assault you to the point of inducing cough as a reaction; where as when appropriate amount of oil is used, only a enjoyable fragrance is emitted.
Why is the tadka pan preferred for this process and a flat pan is not ideal. What has shape got to do with it at all.
A tadka pan is more of a wok shaped deep bowl with a long handle and ones made of cast iron are preferred though other materials can also be used.
Importance of material of a tadka pan:
Cast iron pans are a better choice as they can withstand high heat as opposed to aluminium or stainless steel. They also heats food evenly which are main requirements of the tadka process.
Moreover,the use of a cast iron tadka pan helps leach a small amount of iron into the tadka everytime you use it thus adding to your iron intake. More on this on https://examine.com/nutrition/are-cast-iron-pans-unsafe/
Significance of the deep bowl shape:
the deep bowl shape will require lesser amount of oil to submerge a specific quantity of spices whereas the flat pan will require more oil for the same. As submerging spices in oil is an important step for a perfect capture of chemical compounds, this is an important factor.
Why do mustard seeds splutter in tadka?
Mustard seeds have a hard coating on the outside which is its bran which protects its inner enzymes and oils. When mustard seeds are heated, the oil inside (which is different from the oil outside) heats up and expands, breaking open the hard coat.
Why do chillies pop in tadka?
The aroma and heat of spices are caused by organic compounds that are synthesized by plants. These compounds are usually hydrophobic in nature. For example capsaicin , is one such compound which contributes to the “heat” of chilies/peppers. Capsaicin is essentially hydrophobic(insoluble in water) and has a solubility of only 0.0013 g/100 mL in water. A higher solubility can also be expected in cooking oil. Thus when spices are tempered in oil, these compounds are readily soluble in the oil and the aroma and fragrance is dispersed through the dish along with the oil. Added to this, some spices like mustard seeds or sesame seeds contain oil inside the seeds, when added to hot oil, these seeds pop and splutter, this releases these fragrant oils into the bulk cooking oil.
What kind of ingredients is this method best suited for?
Any kinds of spices and herbs that can withstand high heat.
Is order of adding spices and ingredients to tadka important?
Since tadka involves adding multiple ingredients its wiser to add the ones that take longer time to cook followed by the ones that take lesser time to avoid burning them.
Can tadka be stored and used later?
Tadka aims to bring aroma,medicinal values and flavors to the dish and while tadka once added to the dish that is served hours later will still retain its medicinal values and flavors the element of aroma will be far more subdued by then. Hence it is best to make it freshly and top the dish off just before serving.
Different types of tadka in India:
Each region and dish has its own signature tadka based on local cuisine and taste favored.
Bengalis have thier panch phoran which uses fennel seeds,nigella seeds,cumin seeds,black mustard seeds,fenugreek seeds.
South India mainly uses red chilly,curry leaves,mustard seeds along with other spices.
Benefits and science involved in its use
According to nutritionist Neha Chandna, ‘Herbs have their own healing properties and some (like jeera) are excellent for digestion. So, it makes complete sense to add it your dishes like dals, veggies, etc. It also gives the food a mouth-watering taste and adds a visual appeal to boring, routine food.’
Celebrity nutritionist Rujuta Diwekar in her Indian Food Wisdom and the Art of Eating Right series stresses on the fact that tadka increases the nutrient value of the dish. Phytonutrients present in haldi, curry leaves, jeera, etc are important for our body — they all have medicinal values.
She also busts the myth that the oil or ghee used for tadka only adds to our intake of calories. According to her, the fat in them actually helps us absorb many hidden nutrients in the vegetables present in the dish.
For more info on individual spices and tadka ingredients and their medicinal values: