Tam Keung, who made a name for himself by rearing pigs on pineapple and ginseng at his farm in Yuen Long, says his finicky tastes were the reason why he began making homemade soy sauce five years ago. 'It's all about control,' he says. 'I prefer a sweeter soy sauce that is slightly less salty.' Tam uses soya beans imported from Canada, which he claims have a higher protein content than those from other regions and yield a sweeter final product.

In the kitchen of his restaurant, Healthy and Tasty Congee, which he opened last year, Tam uses only his own line of soy sauce and soy sauce salt - made from the crystallised salt deposits that form on the top of fermenting soy sauce. They sell for HK$30 for a 180ml bottle of soy sauce and HK$10 for a 50-gram package of soy sauce salt.

Tam's products are expensive compared to others on the market, which can be as cheap as HK$5 per bottle. Still, they are a bargain compared with boutique producer I Ho Yuan (or Yuan's) premium soy sauce. At HK$160 for a 125ml bottle, it is Hong Kong's most expensive version of the sauce.

The 80-year old founder of I Ho Yuan, Tsang Heh-kwan, says there's good reason why her sauces are so costly. 'I make my soy sauce the way old-school soy sauce masters used to in pre-Cultural Revolution China. While most soy sauces in the market today take just three months to make, my premium soy sauce takes more than one year to naturally ferment.' She rolls around a drop of her sauce on a plate to show it is denser and more viscous than others.

Tsang established her 120,000-sq ft factory in Yuen Long in 1974 to support herself and her two sons when her husband was diagnosed with a brain tumour. Today, Tsang's company makes more than 10 sauces, including oyster, plum, black bean and chilli.

Chef Jacky Yu Kin-chi of Xi Yan restaurant recently launched three bottled sauces - XO sauce, chicken sauce and chicken 'ma lah' chilli oil. 'More and more diners were asking to buy the sauces we use so they could recreate some of their favourite dishes at home,' Yu says. So the company leased a kitchen space in Quarry Bay to start larger-scale sauce production.

Yunyan Szechuan Restaurant chef Kenny Chan Kai-tak also began making sauces in response to diners' requests. He launched his Sichuan chilli paste three years ago, and last year added two more, mushroom sauce and piquant chilli sauce, which are sold at the restaurant in Tsim Sha Tsui.

'My father, who was Sichuanese, always made his own sauces at home,' says Chan. When asked if he uses a family recipe, he says with a wink: 'My sauces are far superior to the ones my father made.'

But mastering the art of sauce-making is not easy, Chan says. 'Flavours need to be balanced and each individual ingredient requires separate preparation.' Chan's mushroom sauce contains many ingredients, including five types of mushroom, and is labour intensive to make. It takes two days of preparation: the mushrooms are soaked and sliced before being slowly stir-fried into a sauce.

Tam learned his craft the hard way. He did his own research by reading recipe books, then began to experiment. He says his first batch of soy sauce resulted in a 'rotten mess. Everything had to be thrown away.'

Tsang has dedicated her life to improving her sauce-making skills. A trained chemist, she was the first woman to be employed at the Amoy Soy Sauce factory in 1956, where she conducted research and testing in the laboratories. She continued her research when she established her own company and has been tweaking her recipe ever since.

'Humans only do 10 per cent of the work, the rest is up to the enzymes,' Tsang says. She studied the exact strain of bacteria that develops on the mould under a microscope. 'If you add a little too much flour to the boiled soya beans during the spotting process [when the cooked soya beans develop a layer of mould], then a more aggressive type of bacteria develops, which ferments too quickly. If you add too little flour, then you get another type of bacteria that isn't strong enough. You need to breed the perfect type of bacteria to produce the right types of enzymes.'

Another of her secrets lies in her long fermentation process. She says: 'The different seasons - summer, spring, winter and autumn - bring a different type of fermentation in the beans, and this will develop deeper flavours.'

'Making sauces isn't as intuitive as you might think,' says Xu. 'There are a lot of small steps that vary depending on how much sun, rain or humidity there is. It's a process of trial and error.'

Sauce-making also requires space. 'Soy sauce barrels need to sit in the sun to ferment, so this requires a lot of space. Not just for the barrels, but also because there must be no tall buildings around to cast shadows or block the sunlight,' Tam says. He says he's fortunate to have a 10-hectare farm in Yuen Long, which provides him plenty of space to make his sauces.

Xu is struggling to find extra storage space for her sauces. All five producers make their sauces without preservatives, and Xu refrigerates hers to ensure they don't spoil. 'Even though the sauces are canned properly and there is an air-tight vacuum, it makes me feel safer if they are also chilled,' she says.

Xu's unopened jars last up to one year but, once opened, just a few weeks in the fridge. Yunyan and Xi Yan's sauces have a similar shelf life. Tam and Tsang's soy sauces last a bit longer, up to a few months after opening if refrigerated.

Xu, Yu, Tam and Chan have been using their own sauces primarily for their respective restaurants. 'This is the exact same sauce we use in our kitchens,' says Yu. 'So it is not too much of an added investment for us to make a little extra and see if there's any interest.' Yu plans to make only 1,500 to 2,000 bottles per year, which will be sold through his restaurant and at certain supermarkets, including Wellcome. Chan makes 10kg of each sauce every two weeks, but his output varies on demand. Tam offers just 1,000 bottles of soy sauce a year, but Xu sells even less.

'Sometimes I don't like to sell my sauces,' says Xu, who makes only about 30 bottles of each flavour depending on the availability of the ingredients. 'I have to ensure there are enough sauces left for the restaurant to use.' She has no desire to manufacture her products for sale in supermarkets for fear that the quality would be compromised.

Tsang, who once exported to Europe and Australia, has cut production of her premium soy sauce to just 24,000 bottles a year, sold mostly in Hong Kong. 'These days, I do it for fun,' she says. 'My kids aren't interested in soy sauce-making. It is not glamorous, and so when I die this will all be gone.'