The fundamental guiding principle behind the preparation of any ingredient is simple: to prepare ingredients in a manner that enables the chosen cooking method to showcase their finest qualities. Whether through the basic peeling and turning of root vegetables, to more elaborate techniques like tying, blanching and oblique cuts.
In this case, it’s important to bear in mind what tempura actually is- a type of deep frying where food is quickly cooked by completely submerging in hot oil. As such, ingredients for tempura should be prepared in a way which allows for the food to be cooked to your desired doneness in a short period of time. Furthermore, as the hot oil transforms the water within the ingredient into steam, any excess internal steam emitted from the food after removal from the oil will cause the tempura to go soggy. Therefore, ingredient preparation for tempura should aim to maximize the dehydration of the ingredients though frying. Balancing these two key ideas is crucial for making good tempura.
The key ideas to bear in mind are to:
- Remove excess moisture or choose ingredients with a low moisture content
- Maximize surface area to volume ratio by cutting ingredients thinly
- Fry starchy ingredients at lower temperatures
- Choose ingredients that fry well and do not absorb oil
- Use starch to help batter to cling onto the ingredients
The first step to preparing any ingredient for tempura is to remove any excess moisture. This is mainly a problem for meat or seafood and less a problem for vegetables (more on that later). Looking at fish as an example, most restaurants will always wrap their fish filets in fish paper to absorb any excess moisture prior to frying. This is why for tempura, you’ll hardly ever see fish fileted right in front of the guest as you would in a sushi restaurant (with the exception being Ayu sweetfish).
The traditional choice of fish used in tempura is also limited to lean thin small fish with a low fat content. Fish that have a thin thickness have an extremely high surface area to volume ratio, allowing them to lose moisture quickly without overcooking at the same time. Good examples of these are Japanese whiting (kisu/鱚), sea eel (anago) and young sea bream (kasugo/春子鯛) which you’ll always find butterflied rather than your typical three piece fileting as it maximizes surface area.
In contrast, you rarely ever see adult sea bream (tai/鯛) used in tempura as the thickness of the filet is unsuitable. As the meat is thick, the appearance of the outer tempura crust can appear perfectly cooked while the inside remains under cooked. Whilst this can be overcome by cutting the filet into thinner pieces, the flesh from older fish tends to harden up and overcook much easiest, giving them a rough and grainy mouthfeel when fried.
Additionally, oily fish with a high fat content are never used to fry tempura. Whilst fish like Kohada might seem like an ideal candidate to fry due to their small and thin size. The oil from the fish easily mixes with the deep frying oil, contaminating the frying oil with off flavors which can then easily transfer to other ingredients. Frying oil mixed with fish oils tend to also go rancid quickly, limiting their ability to be reused. As such, it’s rare to find oily fish such as shima aji, salmon or tuna used in tempura.
The same principles of ingredient preparation also apply to vegetables. Some vegetables naturally already contain a low moisture content and thus do not require as much processing such as fukinoto, mitsuba and shiso leaves. For other vegetables however, the surface area to volume ratio is usually maximized by cutting them thinly. For example, Lotus root and eggplant are cut into large but thin slices to maximize exposure to the oil. You’ll sometimes find baby eggplant cut into the highly recognisable fan shape.
Eggplant is actually a fantastic example of where ingredient choice and preparation can greatly affect frying outcome. In collaboration with farmers, some tempura restaurants specifically request eggplant that is slightly less ripe with greener flesh. Such eggplant has a much lower moisture content and firmer flesh, meaning that the resulting tempura crust will remain crispy for a much longer time, with the inner flesh not having a mushy texture. Some varieties of eggplant can also have a thick skin. In this case, small cuts or slits can be made on the skin to soften it during frying.
The preparation of starchy vegetables is great to contrast how high quality tempura can be made by adjusting ingredient preparation and applying the same principles in different ways.
Even if cut thinly, root vegetables such as sweet potato and kabocha squash will still most likely burn on the outside before the inside is cooked through. And so instead, starch rich ingredients are sometimes cut as thick slices and cooked at a very low temperature. Cooking them at a lower temperature allows the thick starchy vegetables to steam through whilst also concentrating their flavor. It also allows sugars in the vegetables time to caramelize slightly, accentuating their sweetness. This way, enough moisture can be evaporated from the ingredients without it overcooking.
A unique example that falls into this category is asparagus. Whilst the tips of asparagus can be fried as tempura like any normal vegetables, tempura chef Niitome devised a special way to bring out the best flavor in different parts of the asparagus by separately cooking the lower end. As the lower end is tougher and contains more starch, he switches off his stove and cooks the asparagus end with the residual heat in the pot. By using such low heat, the asparagus has time to tenderize and cook though.
Again, frying seafood other than fish is all about controlling the moisture content. Seafood such as shrimp, scallops, clams and oysters are always blotted dry with paper towel or fish paper. This is particularly important for clams and oysters which can have a lot of internal moisture combined, with outer folds that can harbor moisture even after patting dry with paper towel. Seafood in the bivalve class are actually notorious among tempura chefs for causing the frying oil to splash and splutter from the amount of moisture they contain. As such, some chefs choose to apply a thicker than usual coating of potato starch over them before dipping them in batter.
For shrimp, a small cut is also made in the tail before being scraped with the knife to squeeze out excess water before the entire shrimp is dried with fish paper again.
For certain seafood, some chefs deliberately fry it just long enough for the outside to cook, leaving the inside still raw but just warmed through. The most notable example of this are scallops, with the idea being to allow the guest to enjoy the contrasting savory meaty outer layer with the sweet tender raw center. Tempura cooked in this manner can still have a high moisture content and so much be served immediately.
Meat is not traditionally served at tempura restaurants but nowadays you can find it at certain stores. At some lunch-rush establishments, you can occasionally find chicken and pork served. In order to obtain a desirable texture and to increase the surface area to volume ratio, these pieces of meat are typically hammered flat with a meat tenderizer, rather than cut thinly. Cutting the meat thinly tends to lead to a much higher rate of moisture lost during frying, causing the meat to dry out and become tough. As meat naturally already has a very high moisture content, tempura-ed meat should be served as soon as possible.
Other specialty ingredients
There exists a number of specialty ingredients that are prepared in a unique manner for tempura.
The most famous of these are sea urchin (uni). Due to the fact that the part of sea urchin consumed are the fluid filled gonads, it’s basically impossible to fry them without causing a large oil explosion. Instead, the sea urchin is wrapped with shiso leaves that have been briefly dusting with potato starch and then dipped in batter before frying. This preparation is only fried briefly, just long enough for the tempura crust to form whilst keeping the sea urchin raw.
Instead of sea urchin, wagyu beef is sometimes also wrapped in shiso leaves and fried. This is possibly the only tempura preparation that contains beef, and even then is a fairly new invention. Again, it is only fried just long enough for the tempura crust to form, with the beef being served rare.
The last specialty tempura preparation is only done at specialty restaurants and is probably the most shocking to the layman- tempura-ed Ayu sweetfish. These fish are kept alive in water until the very last minute where they are tossed into the batter and straight into the frying hot whilst still alive. Whilst seen as cruel outside of Japan, it is seen as a testament to purity of Japan’s freshwater rivers, so much that the Ayu can be eaten whole, with the bones being soft enough to chew and the guts imparting a gently sweet vegetal and grassy flavor that some associate with celery.
Outside of Japan, one of the more innovative uses of tempura are courgette flowers stuffed with ricotta and parma ham. The flower petals of courgette flowers themselves contain little to no moisture and are therefore perfect for tempura. They are the perfect vessel that can be used to encapsulate ricotta cheese which will melt from their own steam.
Final steps before frying
At high end counter seat restaurants, most ingredients are only prepared in front of the guest moments before they are fried. Typical examples include:
- Oysters and scallops shucked live,
- Sweet potato and lotus root only peeled before frying,
- Anago sea eel is spiked, gutted and fileted live,
- Eggplant sliced at the last minute to prevent oxidation,
- Wheel shrimp are beheaded, peeled and deveined just before frying,
- Ayu sweetfish is tempura-ed alive.
The main exception to this are fish that are sometimes gutted, scaled and fileted long earlier in the day. As per any normal restaurant, ingredients are kept refrigerated to maintain freshness. These can be in wooden or metal boxes that can then be used to showcase the ingredients to the guest upon arrival.
Before being dipped into the batter, some chefs apply a dusting of potato or rice starch to their ingredients. This dusting of starch allows the batter to better cling to the ingredients. They also help absorb a little moisture, creating a barrier between the outer crust and food when fried, allowing the food to steam perfectly without making the crust soggy.
The application of starch should be done right before the ingredients are dipped into the batter as any earlier and any moisture on the surface of the ingredients can hydrate the starch, causing it to become sticky and alter the texture of the final crust.
Whether or not to apply starch to an ingredient before dipping it in batter is a decision made on a case by case basis by the chef. You’ll find that chefs at high end restaurants choose to apply starch selectively. Some apply it on mushrooms or fish, some choose not to. Some chefs choose to completely forgo the starch coating. At lower end restaurants where the emphasis is to wolf down an energy rich bowl of deep fried food rather than to slowly appreciate the seasonality of ingredients, it’s more standard to apply the starch coating to every single piece of ingredient as they’re placing more emphasis on consistency.
Please note that rice or potato scratch is interchangeable with rice or potato flour. We feel that the word ‘starch’ is more accurate as it doesn’t contain gluten. Rice and potato flour also doesn’t contain gluten and is the same product with a different name. If you can’t find these starches where you live, corn flour or corn starch can be used as a substitute.
If any kind of starch is not available, please do not coat ingredients with wheat flour of any kind as the gluten content will interfere with the batter and affect the mouthfeel of the final tempura crust.