The snacks that the former hawker Tong Kan-pui, now 84, first started selling from a wooden trolley in Wing Kut Street are known the world over, in Taiwan, Singapore, Canada, America and Russia.
Tong's fame grew as he sold the snacks from his mobile Lemon King cart, come rain or shine. Imitators and bureaucracy have failed to stop the business from flourishing, with celebrities and tourists among the customers who mostly comprise professionals. "My customers are like my friends: seeing they are happy, I am happy," he says with a smile.
However, since his health began to fail in 2007, his son, Tong Sung-chiu, has taken over the business. Sung-chiu says an old man who has lived in New York for 28 years visited their stall just to see his father. "When asked where he was going, he answered, 'The airport'," Sung-chiu says. "I couldn't imagine that, after so many years, he could still remember my dad, and come all the way just to see him.
"This is living proof that what my dad offered his customers was not only preserved lemon, or the relationship between a vendor and a customer ... It was also about quality, reliability and - most of all - friendship."
"Of course, the man also bought six packs of preserved-lemon snacks," he says with a smile.
Business wasn't always easy. Soon after Sung-chiu took over the reins, the government said it did not allow hawker licences to be transferred from parent to offspring.
He was arrested and his trolley and products were confiscated. "I was forced to halt the business for 10 months," he says.
Thankfully, help was at hand. Hong Kong food critic and Lemon King customer Chua Lam wrote a letter, highlighting Sung-chiu's plight and urging the government to preserve what was part of the city's precious heritage.
Finally, in 2008, Warner Cheuk Wing-hing, then director of food and environmental hygiene, awarded Sung-chiu a fixed-pitch hawker's licence. Now he is able to continue the family business - from a permanent home. "Without Chua and Cheuk's help, Lemon King might have vanished," he says.
He says much of the popularity of Lemon King is down to the quality of its ingredients.
"The essence of our preserved lemon is the powdered liquorice root and Thai lime. Instead of using saccharin as a sweetener, we use white sugar. People do not feel thirsty after eating the snack and it is healthier."
Lemon King's success has spawned a number of imitators. "Some people think they are buying our products, when they are not," Sung-chiu says.
"We tried to register Lemon King as our trademark but failed because it is a universal food. Nor can we register Wing Kut Street ... the government says the street is the property of the HK government.
"Luckily, Chua suggested that we use my father's picture as the trademark."
So the next time you get a preserved-lemon snack, look for the picture of Tong Kan-pui on the paper bag, so you know you are buying the real Lemon King.
This is the second in the six best Heritage Detective series covers, written by Hong Kong students.
This week's Delta team are 4C students at Sha Tin Government Secondary School.
Team: Anthea Lau, Dilys Chan and Yoyo Li, (writers); Alison Chui (team leader).