For the sweet and sour sauce: Mix together the pineapple juice, cornstarch, ketchup, soy sauce, sugar, vinegar and rice wine in a sauce jug.
For the chicken: Mix together the cornstarch and egg whites in a bowl to make a "batter", adding a tablespoon of cold water to thin out if necessary. Toss the chicken pieces into the batter and sprinkle with the salt and white pepper. Mix well and set aside.
Heat the oil in a wok to 350 degrees F. Add the chicken pieces and cook until golden brown (this helps to seal in the chicken juices) and cooked through, 3 minutes. Turn the heat off and be extra careful - pour the chicken and hot oil into a stainless-steel colander set over a heatproof bowl to collect the excess oil. Set aside 1 tablespoon of the oil, and discard the rest. Drain the chicken well and set aside.
Reheat the wok until smoking and add the reserved oil. Add the ginger and garlic and stir for a few seconds. Add the peppers and cook for less than 1 minute, and then add the pineapple. Pour in the sweet and sour sauce and bring to a boil. Cook until the sauce is reduced and sticky, 1 to 2 minutes, and then toss in the cooked chicken pieces and stir well for 1 minute. Transfer to a serving plate and garnish with scallions. Serve with rice.