Editor’s note: The theme of Gohan Lab is to help people make simple, tasty “gohan” (meals).

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In ancient times when blocks of ice offered the only form of refrigeration, the accepted wisdom at this time of year to prepare fish so it doesn't go off in the heat was to marinate the flesh in salt and vinegar. This tried-and-tested method still works a treat today.

Horse mackerel and sardines are now fattening up as the rainy season approaches, and a number of shops are selling the tasty fish in fillet form to be enjoyed as sashimi. Now is the best time to prepare “sujime,” which literally translates as “ingredients tightened with vinegar.”

Blueback fish are fatty and rich in flavor, but are soft and perishable. That is where marinating in vinegar comes in. By draining them of water with salt and restraining germs from proliferating with vinegar, the technique flavors the fish while improving its storage properties.

This week’s recipe uses a modest amount of salt to retain the flavor of the horse mackerel. The marinating time in vinegar should be short if the fish is to be eaten as sashimi, but longer to get a textured finish if it will be used in sushi.

Those lucky enough to come by an abundant supply of good horse mackerel would do well to make a batch and freeze it for future enjoyment.

Don't forget to remove the small bones. If a specialized fish boner is not close to hand, a pair of tweezers will do the trick. After all, the fish are hardly large.

SUSHI VINEGAR SWEETER IN WESTERN JAPAN

Ingredients varied as sushi evolved as local food, and the combination of seasonings in the sushi vinegar to make sushi rice consequently differ according to region.

“Nigiri-zushi,” or sushi crafted by hand, uses fresh fish that is supposed to be eaten right away. As such, sushi vinegar that is light on sweetness is used.

Some sushi restaurants that adhere to the “Edo-mae” style, that of Edo, the former name of Tokyo, do not add sugar to the sushi vinegar and use strong-flavored sake lees vinegar.

But sushi rice used to make “bara-zushi” (rice with different slices of seafood “scattered” on top) and “oshi-zushi” (rice and toppings “pressed” in a wooden mold) tends to be sweet. Sugar tends to retain water, so sushi rice remains moist even if used later.

“There are regional characteristics to people’s preference of sushi vinegar,” notes Sho Ishii, who specializes in flavors at Mizkan, a food company based in Aichi Prefecture, and who supervised the cookery science aspect of this recipe. “People in Western Japan choose sweeter ones than in the east. Those in Shikoku and Kyushu regions prefer the sweetest sushi vinegar,” he said.

BASIC COOKING METHOD

(Supervised by Akiko Watanabe in the cooking aspect and Sho Ishii in the cookery science aspect)

* Ingredients (Serves two)

100 grams net horse mackerel (aji) (fillets of a medium-sized fish), 1/3 tsp salt (2 percent of net weight of fish), 1/2 cup vinegar (enough to cover fish when marinated), a piece (about half the size of a thumb) ginger

About 65 kcal and 0.5 grams salt per portion

1. Have two fillets of horse mackerel ready. Sprinkle half of salt on flat container, lay fillets and sprinkle rest on top (PHOTO A). Cool in fridge for 30 minutes to an hour. Feel fillet and if flesh has tightened, rinse off salt on surface and pat dry with a paper towel.

2. Lay fillets in glass or ceramic dish resistant to acid and pour enough vinegar to cover them. Marinate for 10 to 20 minutes until surface turns white (PHOTO B).

3. Lay fillet vertically on cutting board and slice off abdominal bones with tilted kitchen knife. Place fillet horizontally and pull out small bones that remain along the vertebra. A good way is to pinch the bone with a fish boner and pull toward the head. Peel off the thin skin from the head side to the tail.

4. Cut fillet into sashimi slices that are 8 mm to 1 cm thick. To make up for the thinness, tilt the knife to get wider slices (PHOTO C). Serve with grated ginger on the side.

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Akiko Watanabe is a cooking expert specializing in Japanese cuisine.

Sho Ishii works at a section that focuses on tastes at Mizkan.

ARRANGED VERSION

Nigiri-zushi with sardines (Serves two)

Soak 2 sardines (net weight 100 grams) in 1/3 tsp salt and 1/2 cup vinegar as above. To make sushi rice, mix sushi vinegar (1 Tbsp vinegar, 2 tsp sugar, 1/4 tsp salt) with 200 grams rice. Divide into 12 equal parts and shape into oblong balls in plastic wrap. Grate a piece of ginger and chop a stalk of “hosonegi” green onion. Slice a sardine fillet into three pieces with a tilted knife and make a vertical incision in the center. Place on sushi rice and serve with green onion and ginger.

COOKERY SCIENCE

“Sujime” is a process where raw fish is dusted with salt, rinsed and soaked in vinegar. First the salt draws out the water from the surface of the fish through osmotic pressure and also firms up the protein in the fish’s muscle. Then the vinegar also acts on the protein and hardens it. The fish firms up while taking on the flavor of salt and vinegar and retaining the moisture.