Prepare a bowl of acidified water by mixing vinegar, lemon juice, or citric acid into a bowl of cold water until it is slightly sour. Wash and peel the renkon and put the peeled bulbs in the water.
Slice the renkon into thin cross-sections and add them to the acidified water. When you're done slicing, drain and rinse the renkon and then shake off as much water as you can.
Heat a frying pan over medium-high heat and add the vegetable oil, renkon, and sugar. Stir-fry until the sugar starts to caramelize.
Add the soy sauce and sake and continue stir-frying until all the liquid has evaporated.
Toss the kinpira with the toasted sesame seeds to finish and garnish with chili flakes to taste.