In a city obsessed with Ben Franklin, it's no surprise he figures prominently in the tale behind a signature cocktail at the Loews hotel's new bar.

But the historical connection is legit for Secret Knock, the deceptively clear and lemony "milk punch" that Bank & Bourbon serves from apothecary bottles.

It's rooted in a "receipt" Franklin passed on to James Bowdoin in October 1763, and involves curdling whole milk for its whey and combining that with brandy and lemon juice for a hybrid "posset" and "syllabub" thought to hold cold-fighting properties.

I can't speak to its medicinal virtues.

But "bourbon master" Brian Bevilacqua has deftly updated the punch with whiskey and green tea (for tannins) and an overall effect that is lemony, sweet, smooth and refreshing, with winter variations soon on the way (cloves? sherry? coffee?).

The drink's appeal, though, is universal: the crystal clarity that - for a dairy-based drink - is always a surprise. Bevilacqua's secret? Straining through decidedly non-Colonial biodiesel filter bags that, most likely, Ben Franklin cannot take credit for.

- Craig LaBan

The Secret Knock, $12, Bank & Bourbon, Loews Hotel, 1200 Market St.; bankandbourbon.com