With the rack in the middle position, preheat the oven to 350°F (180°C). Butter four 6 x 3-inch (15 x 7.5 cm) loaf pans, each with a capacity of 2 cups (500 ml). Line each pan with parchment paper, letting it hang over both sides.
In a bowl, combine the flour, baking powder and salt. Set aside.
In another bowl, combine the grapefruit juice and milk.
In a third, larger bowl, cream the butter and sugar with an electric mixer. Add 1 egg yolk at a time, the grapefruit zest and rosemary. Mix until smooth. On low speed, add the dry ingredients, alternating with the juice mixture, and continue to mix until the batter is smooth.
In a separate bowl, beat the egg whites until stiff peaks form. Using a whisk, gently fold the egg whites into the batter. Divide the batter between the prepared loaf pans.
Bake for 40 minutes or until a toothpick inserted into the centre of one of the loaves comes out clean. Let cool for 15 minutes before unmoulding. Serve hot or warm. These mini loaves will keep in an airtight container for 3 days at room temperature.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.