It's only 10am when I speak to David Stewart, but after 20 minutes of impassioned chat about whisky I am almost ready to pour myself a wee dram.

Stewart is a malt master for William Grant & Sons, owners of Glenfiddich, Grant's and Balvenie. It's a prestigious job that sees him using his nose to produce exquisite single malt whiskies and maintain the consistency of the company's output, as well as blend new creations. He is also one of the public faces of the company, regularly flying overseas to explain how the firm's whiskies are made with buyers and clients.

"I left school in 1962 aged 17 with five O-levels. My parents told me I had to get a job, so I got three interviews: one at a bank, one at an insurance company and one at William Grant," Stewart says. "I got offered all three but the whisky company sounded more interesting so I joined as a whisky stocks clerk, knowing nothing about whisky or the drinks industry."

Fortuitously, Stewart's boss was the firm's master blender. After he had been doing clerical work for two years the master blender started to teach him how to "nose" whisky. This involves easing your nose into a tulip-shaped glass and testing the drink through smell.

"He taught me what to look for, what should and shouldn't be there, the style of our own products, how whisky matures … Quite quickly I saw that if I could master the nosing aspect I would be one of very few people in the company who could do it. It took me 12 years to master it to the point where I could prepare blends, but it was quite difficult – I'd never tasted whisky at the age of 19 or 20."

In 1974, when Stewart was 29, his boss left and he was offered the chance to take over on a six-month trial. He has never looked back.

His typical day involves a trip to the sample room where he spends at least 90 minutes nosing. He estimates he noses about 30 whisky casks a day, and has nosed more than 400,000 during his 50-year career.

"The rest of my time is spent in meetings, sorting emails, desk work, even a bit of PR. We have customers coming into the sample room and I talk to them about whisky. I've also travelled overseas to meet clients in markets such as the US, Taiwan, Paris, Singapore and Moscow."

As a malt master, Stewart ensures the quality of single malts produced in individual casks, but because he has also acquired the skills of a master blender he can create new whiskies. He has developed award-winning ranges of single malts and blends, but also introduced the technique of "finishing" single malt in the 1980s.

"The Balvenie PortWood, which we launched in 1995, is the one I'm most proud of," he says. "I took whisky that had been aged for 21 years and put it in casks that had previously held port. It added rich, sweet port wine notes, and raisin and sultana flavours to the whisky." It has subsequently won more than 50 awards, but it's not cheap – "even I have to buy a bottle myself," Stewart jokes.

Malt master and blender jobs do not come up often – there are only about a dozen malt masters in the country at any one time – meaning there is no conventional route into the role.

Stewart's own apprentice had a chemistry degree and had been working in the firm's laboratory for three years before he was handpicked for a blending role. "He then had almost 10 years with me learning the ropes, which is rare in most industries. I think that type of science degree is probably a good way in, but when it comes to nosing you just have to learn over many years."

Apart from having a good nose, Stewart says master blenders need to be creative and patient because you have to wait so long for anything to happen. "You can wait two years to see how something turns out only to find it hasn't improved the whisky and you have to start all over again with something else."