Chicago a cappella presented yuletide carols being sung by a choir (apologies to Mel Torme/Nat King Cole) and chestnuts galore in their holiday concert, last Saturday night at Pilgrim Congregational Church. These chestnuts were time-honored, not roasted-soothing Chanukah and Christmas tunes making their annual returns in the rich and lush musical settings that have become an a cappella calling card.

Mellowing comes with age, and is preferable to a tangy burst that’s green with youth. Now in its 15th anniversary season, Chicago a cappella has ripened nicely. Their singing is tightly knit; they know exactly how to shade and shape close harmonies, and admirably, they do not shy from trying something new.

Newly-appointed Music Director Patrick Sinozich was nowhere to be seen at this program, though presumably his presence as rehearsal coach was behind the evenly-balanced sounds and precision timing of the music. On hand to take the applause from the hearty ranks of a cappella fans were founder Jonathan Miller and Chicago composers Stacy Garrop and Rollo Dilworth, both of whom contributed new works commissioned in celebration of this anniversary season.

“Lo Yisa Goy,” which Garrop calls a Hebrew prayer for peace, premiered Dec. 2 and should be performed many times over, not filed away like so many forgotten commissions. Austere music in a dissonant, yet delicate texture, is woven into klezmer-like passages. The unusual harmonies, singular enough, I think, to identify as Garrop’s tonal language, get your attention like one of those new-fangled fruits. The color is right, but the flavor is a surprise-a peach that tastes like an apple. Her choice of additional text, “They shall beat their swords into plowshares and their spears into pruning-hooks,” is taken from the eighth-century B.C.E. prophet Micah.

Over the years, this ensemble has ventured into enormously complex new works that challenge both singers and listeners, but they are at their best with a slightly jazzy doo-wah accompaniment against a simple solo line. This texture can seem too much of a good thing, as it was Saturday, but they present this good thing so well. Two pieces written especially for the group showed off this strength: “What Sweeter Music?” by Wayland Rogers (1994) and Rollo Dilworth’s friendly and fun setting of the African-American spiritual “Sistah Mary,” which also premiered Dec. 2.

The ensemble also reprised a sensitive rendition of Elizabeth Alexander’s “When the Song of the Angels Has Stilled,” which in its stillness offers pause for thought. Though the audience joined in on the hand-clapping gospel song, “Who Is the Baby?” by Rosephanye Powell, they did not match a cappella’s precise clapping standards. The tenors shined like a searchlight in Powell’s wily barbershop quartet harmonies. The familiar sound of these and other quiet favorites like “I Wonder As I Wander” and “Silent Night” was as comforting as slipping into an old pair of jeans.

In “The Huron Carol” of Eleanor Daley, the wonderfully reverberant bass drones came awfully close to matching organ pipes. At the risk of slighting the remaining slate of the evening’s super-soloists, I single out baritone Michael Boschert, whose luminescent voice in this number warmed like high-end hot chocolate and bloomed like 50-year-old Scotch.

Surprisingly, perhaps purposefully, the prevailing mood of the evening was calm. But the ensemble let loose with a Jingle Bells satire hinting at slippery winter conditions and a few near-misses with the sleigh. Thanks to their sure-footed singing, these voices tripped through multiple keys and sudden harmonic shifts, á la the early 20th-century polytonal experiments of Charles Ives. In their agile vocal navigating, we got the musical equivalent of “Whoops! Hold on, we’re going for a ride!”

The encore, a Nigerian setting of “For Unto Us A Child Is Born,” was culled from last year’s program. But this lovely night of gentle seasonal songs could just have easily ended with the strains of “Chestnuts Roasting on an Open Fire” in lavish a cappella style.

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