Zestfully Plain:The muffins of Spain lean mainly toward the plain. 'Tis certainly true, but a special ingredient perks up the madalenas at Le Moulin European Bakery [5645 Beechnut, (713)779-1618], the Meyerland-area shop that also specializes in French, Austrian, German, Swedish, Italian and kosher-certified breads. It's the zest of Valencia oranges, combined with other citrus juices, that makes the madalena muffins sing. The bakery offers them on a rotating basis, depending on when it delivers a giant supply to The Houstonian Hotel, but regular folk can special-order them anytime for 75 cents each. They're highly recommended for road trips, whether through the plains of Spain or the fields of soccer, since they hold up well for days and stave off hunger for hours.