Wetzel tilts her head, looking thoughtful. "I don't know is the honest answer," she says. "I'm a single mum and I run my own business. For me, the most tricky thing is managing my life: juggling being a good mum with work.
"I always think I would make a brilliant husband," she laughs. "I don't like handbags or clothes, I much prefer cars. I have an ironing lady and a gardener. None of the domestic things I look after. The most difficult thing is being stuck in a meeting somewhere and knowing my son Noah is being chucked out of the childminder's any minute.
"With business you can always overcome any difficulties. It might be a particularly s*** moment, but you can get past that. The hard part is the organisational element. When I have someone flying in from Italy for an important meeting but it's the same day as Noah's school concert. It's things like that I struggle with."
Wetzel smiles softly. "I'm every man's dream – and every husband's nightmare," she adds. "Most blokes would think that a blonde woman who runs a brewery is quite a catch but actually, you wouldn't want me. I work long hours, I'm fiercely independent and have a 'once bitten, twice shy' approach to life. It would take a strong and special man to fit into that set-up."
It's an answer that sums up the fierce tenacity of Wetzel: she doesn't suffer fools gladly ("I call a spade a spade. I hate people who beat around the bush and you never know quite where you stand with them," she asserts).
German-born Wetzel, 38, and her father Herbert, an engineer by trade, famously coined the idea for a Scottish "Bavarian-style" brewery while having a drink in the Ubiquitous Chip in Glasgow in 1994. Something of a beer connoisseur, he ordered a pint of Tennents, took a sip and winced.
"He asked me what the best Glasgow lager was and I realised I didn't know," she recalls. "In Germany every village and town has a good brewery. It's something people are proud of and want to show off to visitors. He couldn't believe that a city like Glasgow didn't have its own brewery."
Almost two decades later, housed in the historic former Templeton Carpet Factory, is the product of that light bulb moment. In 2011, Wetzel announced plans for second premises in the Port Dundas area of the city, known as the Cooperage, a £6.6 million investment which will increase the company's capacity for making beer by a factor of 20. Subject to planning permission ("it's now in the hands of the Glasgow City Council gods"), Wetzel hopes to open to visitors by Easter 2014.
"We have a saying in German 'Ueber seinen eigenen Schatten springen' which means 'to jump over your own shadow,'" she says. "It's about taking yourself out of your comfort zone and that's what I'm doing with the Cooperage. It's a far bigger project than anything I have ever done before."
The eldest of two daughters, Wetzel grew up in Bammersdorf, a rural Bavarian village in the forest lands of Upper Franconia. The nearest big city is Nuremberg, coincidentally twinned with Glasgow.
She describes it as an "idyllic" and a "picture postcard" upbringing. "I always joke that I learned how to ride on a cow," she says. "My friend had a farm but they didn't have any horses. We would climb on the fence and wait until the cows were close enough to fling ourselves onto their backs. We would sit there as they trotted along.
"After school my friends and I would explore the woods. We would each bring a little tool and get nails from our parent's workshops to build tree houses."
At 13, Wetzel came to Kippen, Stirlingshire, on a school exchange to stay with a pen friend ("I still get emotional when I say this, but I feel I melted into Scotland"). Six years later, then 19, she was back – with just two suitcases to her name – to study politics, history of art and management at Glasgow University.
She subsequently won a scholarship to a French business school, but after two years the pull of Scotland proved too strong and in 1998 returned to complete her degree, gaining a first.
After graduating, Wetzel worked for the Glasgow Tourist Board, now the Glasgow City Marketing Bureau, before retraining as a lawyer. But the idea of her own brewery never left her. With some financial help from her father, she and her now ex-husband Gordon Stewart set up WEST in 2006.
As Wetzel worked part-time in law and focused on being a mum to new son Noah, her husband oversaw the running of the fledgling business. Soon after the couple separated in late 2006, Wetzel discovered WEST was floundering. The business went into administration, but Wetzel was determined to try again.
The new company, set up in early 2008 with the name Noah Beers, now has an estimated annual turnover of more than £1m and 40 staff. The first UK brewery to make all of its lagers and wheat beers in accordance with the Reinheitsgebot, or German Purity Law, using only water, malt, hops and yeast, last year WEST produced some 300,000 litres.
Wetzel likes to describe the business as having a "Glaswegian heart and a German head". She has a knack for canny sayings. Her staff have nicknamed her "The Kaiser", which Wetzel takes in good form. "It's because I'm a bit bossy," she admits. "I don't take nonsense from anyone. People like to know where they stand: there is nothing worse than a backstabber. With my first name being Petra you have this expression in English 'petrified', so it's like that except it's 'Petrafied'.
"I'm a hothead. I shout a lot, but I calm down quickly. It's a bit like weathering the 'Petra storm'. I don't like the colour grey: I see things in black and white. But I don't ever bear a grudge, life is far too short for that."
While her business acumen is undoubtedly formidable, it is her seven-year-old son who forms the axis of her world. "Noah is the light of my life and I want to spend as much time with him as I can," she says. Weekends are typically spent walking their golden retriever Heidi in Mugdock Country Park or pottering around the Botanic Gardens. They take golf lessons, play badminton and have a movie night twice a week ("we just watched Escape to Victory – brilliant film," says Wetzel).
Her accent is a curious hybrid of Glaswegian and German – "Gerwegian," she dubs it – the end result not unlike a Hebridean twang. "I do get people asking if I'm from Harris and Lewis but I've never been to either," says Wetzel. "English has since become a language I enjoy speaking more than German," she adds. "I don't like reading books or watching movies in German. I prefer English."
While she has "melted" into her adopted homeland, one thing Wetzel can't quite get her head around is Scotland's drink to excess culture.
"A lot of people in this country have a strange relationship with alcohol," she says. "I don't because I come from a country where alcohol is part of what you do but never drunk to excess.
"I always say I'm the cheapest date in Scotland – two pints and I'm anyone's," she jokes. "I don't have a good alcohol tolerance. For me, it's about drinking and savouring it. It's not about 'Oh, I can drink 10 pints of lager and still be standing.' If I had 10 pints in a line in front of me, that would go against everything I stand for."
While Wetzel is notably cagey on the matter of turnover – "it's millions now, not £2.50 any more" is all she will say – her ingrained desire to succeed is palpable. "I'm a daddy's girl and I wanted to make him proud," she admits.
"That conversation we had about the beer in 1994 was always at the back of my mind. It was a big deal for me. Last summer, he sat here at the bar, looked me right in the eye and said: 'I'm proud of you'".
"When I first started I thought it would be awful: being German – that is, a Johnny Foreigner – a woman and someone who had never done anything with beer before, I thought people would laugh in my face," she adds. "But, you know what, I'm good at what I do. That's because I swotted. I hate looking like a fool so I learned everything I could." Wetzel smiles. "I became a beer anorak."
Life and loves
Winning three gold medals in 2009 from the DLG (Deutsche Landwirtschafts Gesellschaft) for three of our beers ranks highly, as does meeting Hilary Jones, my business partner.
Realising that West Brewing Company Ltd, the company I had founded with my now ex-husband, had to be put into administration in early 2008.
Last book read?
I’m currently reading Life of Pi by Yann Martel with my son Noah.
My grandmother Betti.
Neil Diamond and Dolly Parton, pictured.
Ideal dinner guests?
My two sets of grandparents (sadly no longer alive), plus my parents, my sister and Noah. We never had that four-generation family meal and I think Noah would have loved to meet them.