Have you ever curious why sushi rice taste sour?

      It is believed that sushi originated from certain countries in Southeast Asia, and was developed from the food preservation effort of farmers dwelling along the riverbanks. People in some Southeast Asian nations, namely Thailand and Laos, living near the Mekong River were keen on fishing and rice farming. As a result, fish and rice were considered people’s most important food cooked for their daily meals. In those days, as food preservation methods had never been advanced, local people preferred to marinate fish and rice together aiming to keep them edible for longer. Later, this traditional way of food preservation spread to Japan, inspired by Chinese culture. However, original Nare sushi and expensive Funa sushi possessed a unique taste similar to Pla-Som (Thai-style fermented fish) a dish greatly enjoyed by people in Southeast Asian countries.

       In the meantime, the sushi cooking methods had been improved for better quality and taste, requiring the addition of vinegar to be mixed with rice instead of using fermented fish. As a perfect blend of rice and fish could guarantee a stronger and more pleasing taste, people generally enjoyed eating sushi with wasabi and pickled ginger, believed to have efficient properties in killing germs and nourishing the balance of the body. Besides, the different tastes of these ingredients were found to be ideal, particularly when they were mixed together.

      Actually, sushi had been developed with several topping ingredients. In the early years that sushi achieved immense popularity worldwide, American people appeared to have no comprehension of Japanese culture in consuming raw fish. Consequently, sushi chefs determined to develop a variety of sushi menus to be compatible with the westerners’ eating styles. For instance, newly evolved sushi menus were California roll, a seaweed rice roll filled with crab meat, cucumber, and avocado and an original seaweed rice roll filled with vegetable and egg. In some states, people preferred to use mango or banana to substitute for the unavailability of avocado. Some sushi menus were even garnished with sesame or Tobiko (flying fish roes), making sushi become highly popular in the US during the 1980s.


Requirements for Sushi Rice Cooking
Ingredients of Sushi Sauce
Cooked Japanese rice                                 4 cups
Japanese vinegar                                        1 cup
Pure water                                                  ¼ cup
Sugar                                                          ½ cup
Salt                                                              2 tablespoons
Mirin                                                            1 tablespoons
Kombu seaweed sliced in 2-inch size         4 pieces

Directions
1. Boil pure water in a cooker. Add Japanese vinegar, sugar, salt, mirin, and Kombu seaweed. Leave the mixture of ingredients until
    they are fully boiled. Lower the heat and stir the well-mixed ingredients for another 3 minutes. Turn the heat off and leave it for a few
    minutes.
2. Use cheesecloth or a colander to filter the sushi sauce obtained from No. 1.
3. Slowly pour 3-4 tablespoons of sushi sauce on cooked Japanese rice. Use a ladle to softly spread the mixture of cooked Japanese
    rice and sushi sauce. Never crush the well-mixed rice as it will make the sushi mushy and unpleasant to eat.
4. Fan the sushi rice for 10 minutes to ensure that excessive water and sourness are sufficiently evaporated. Keep the sushi rice at a
    temperature of 37 degrees Celsius before bringing it to make a sushi roll.

      Basically, Japanese people prefer to keep their prepared sushi rice in Hisue, a deep-bottomed bamboo bucket with cheesecloth. Aside from being one of the main sushi ingredients, this type of rice can be perfectly served with seafood and sweet egg, commonly known as Chirashi or Kaisen Don.

Source: Easy Japanese Cooking
https://en.wikipedia.org/wiki/California_roll