Only 98 Americans hold a diploma from the British-based Wine & Spirit Education Trust. It is awarded after a minimum of two years of study followed by extensive written and blind tasting examinations. Many fail the first time around. Success entitles one to use the initials “D.W.S.” (“Diploma in Wine & Spirits”). Overwhelmingly, the graduates come from the world of sommeliers and wine and spirits salespeople.

Then there is Jonathan Grossweiler, D.W.S. since 2006, a powerfully built, shaven-headed ex-Marine from somewhere in New Jersey.

“I’d rather not say which town I live in,” Mr. Grossweiler, 34, told me. “I don’t want the inmates to be privy to that information.” Those inmates are at a prison for sex offenders in Middlesex County, N.J., where he works the 6 a.m. to 2 p.m. shift as a corrections officer.

Why and how did a prison guard embark on a journey to becoming a wine expert? Mr. Grossweiler answered those questions last week in an interview at the International Wine Center in Manhattan, where he got his diploma.

Q: What was your entry point into wine?

A: Certainly not in the blue-collar town where I grew up. We drank milk, and in the summer, ice tea and lemonade. Fast forward to Camp Lejeune, North Carolina in 1993. Another marine, Tyrone Davis, invited me to his home for dinner. It’s so great to get home hospitality when you’re in the service. I felt I should bring something, so I said, “I’ll bring the wine.” I bought a bottle of Carlo Rossi wine at the Post Exchange. We drank it with mac and cheese. I didn’t spit it out, but I remember thinking, something’s missing here. Wine has got to taste better than this.

Q: Did you upgrade on your next bottle?

A: Not for the next few years, while I was stationed in Europe. That was all about downing Buds and Carlsbads. In 1995, back in New Jersey, my mother moved into a new house. For our first Thanksgiving there, I bought a Chateau St. Jean chardonnay, and that was my wine epiphany. I remember swirling the wine around in my mouth and thinking, this is good — a different animal from the Carlo Rossi. But I had no idea why. So I started to read back labels carefully. But what did it mean that half the wine went through malolactic fermentation? Again, I had no idea. In 2002, I coughed up a few dollars and enrolled in Kevin Zraly’s Windows on the World Wine School.

Q: Was that daunting?

A: I came in a little nervous because I assumed wine people were rich and snobby. What helped me get over that was taking my girlfriend to Le Cirque 2000. I hesitated about that, too, but a buddy said, “Your money is as green as anyone else’s. Just go and be yourself.” I ordered a Ravenswood red zinfandel because I already knew white zin. Then I met Kevin Zraly. He had this huge passion without pretension. I loved that course. When it ended, I gave him a hug and thanked him for letting me into the wine world, which wasn’t what I thought it might have been.

Q: Sounds like you gained a lot of confidence from that course.

A: By the sixth class, it had paid off. I saw a Rioja, Marques de Riscal Reserva, on a wine list and knew it had been oak aged for one year, bottle aged for two. It was only $22, but it was all it was supposed to be. So now, I’m really gaining confidence.

Q: You could have stopped there.

A: I knew there was more to know. Kevin had given us handouts with the names of a couple of other more advanced courses. I wrote to them all, and the one that wrote back was International Wine Center. I didn’t know then about the D.W.S. stuff. I just wanted to know more.

Q: Any bumps along the way to graduating?

A: In a perfect world, you could finish in two years. I failed the written exam the first time around. That was in 2005. When something is tough, I always fall back on the Marine Corps. If I could do boot camp and be a jarhead, I can do this. So the next year, I retook the exam and passed.

Q: Is wine just about pleasure, or is there something more to immersing yourself in this culture?

A: I think that wine is one of the great equalizers of the world. Everyone has a nose, a palate, and so you meet all kinds of people and you drink wine together and talk about what you’re tasting.

Q: Do you ever talk to the inmates about wine?

A: Not to the inmates. But I’m the resident wine geek for the other corrections officers. They’ll be lots of questions now that Valentine’s Day is coming, and I’m glad to answer them. Why would you get knowledge and not help people out?