Lemon polka dot sugar cookies

The Art.
    I was designing my blog banner the other day, and found this beautiful yellow polka dot background. It is so simple and cheerful, an embodiment of what I want my blog to be. However, I didn't have a photo of baked goods that would match the lemon yellow of the banner. One of my "delightful eccentricities" is that I like things to match. So, I decided to make iced sugar cookies with a yellow polka dot pattern. Lemon flavor is so wonderfully summery as well, I decided I couldn't go wrong! For presentation, I tied the cookies up with yellow and white daisy trim I found in my sewing box.

The Science.
    When I was a child, my grandmother's friends taught me to make royal icing by beating together egg whites and powdered sugar. More recently, royal icing recipes use meringue powder instead of egg whites, and I assume this is because people are worried about bacterial contamination in their eggs. I hypothesized that adding lemon juice to the egg whites and beating them together would lower the acidity and kill any bacterium present. I performed a rough calculation. The average egg white has a volume of 2 tablespoons, and a medium lemon has 3 tablespoons of juice. My royal icing recipe calls for two egg whites and and the juice of one lemon, so the ratio of egg white to lemon is 4:3. The pH of an egg white can increase over time, but is approximately 7.5, while lemon juice has a pH of 2 [1]. Thus, the approximate acidity of my egg/lemon mixture would be pH 6. Not very acidic, is it? Additionally, a group in Liverpool did a study a few years ago and ascertained that a pH below 2.5 is required to kill E. coli [2]. So, my lemon juice will not kill bacterium in my egg whites. There is really no need for concern, however, since pasteurized eggs are available in the U.S., and also because the sugar content in royal icing is so high.

The Recipe.

Sugar cookies:
2 1/2 cups all-purpose flour
3/4 teaspoon salt
3/4 cup salted butter, softened
3/4 cup sugar
1 large egg
1 teaspoon vanilla
zest from one lemon (optional)

    In a small bowl, combine flour and salt. Beat together butter and sugar in a large bowl on medium-high until pale and fluffy. Beat in egg, vanilla, and lemon zest. Reduce speed to low and add flour mixture, mixing until just combined. Cover and chill dough at least one hour.

Put oven rack in middle position and preheat oven to 350°F. To roll out the cookie dough, I prefer to place it between two pieces of waxed paper rather than flour my surfaces, so I don't get flour everywhere and make my cookies grainy.

Roll out dough with rolling pin until it is 1/4" thick. Cut out as many cookies as possible from dough with cookie cutters. I used fluted square cookie cutters, to set off the polka dot pattern.

Gather scraps and re-roll, cutting out more cookies. Transfer cookies to 2 ungreased large baking sheets, arranging cookies about 1 inch apart. Bake cookies until edges are golden, 10 to 12 minutes, then transfer to racks to cool completely [3]. 

Royal Icing:

2 large egg whites

3 tablespoons juice from one lemon

3 to 4 cups powdered sugar

    Beat together egg whites and lemon in large bowl at medium speed until slightly foamy. Gradually beat in sugar on low speed until stiff peaks form. This is your "outlining" icing. Once done outlining your cookies, thin this icing with a few tablespoons of water to make "flooding" icing.

Decorating:

    Once cookies have cooled, dye the outlining icing bright yellow (or your favorite color), reserving some white icing for the polka dots. I used Wilton Icing Colors lemon yellow. Spoon some outlining icing into a plastic sandwich bag, and carefully snip off the tiniest bit of the bottom corner with a pair of scissors. A little goes a long way! Use this as your icing bag and outline the border of your cookies.

    Thin the rest of the icing with a few tablespoons of water, enough so that it will pool and "flood" your cookies out to the outline you made, without being so watery it will run over. Spoon flooding icing onto center of cookies and allow to flood the entire cookie. While the flooded cookies are still wet, fill another bag with the white outlining icing and snip the corner off. Dot the white icing on cookies in a grid pattern to make polka dots. Doing this while the cookies are still wet will make the dots level with the flooding icing and gives it a smooth finish. You can experiment with dot sizes by squeezing more or enlarging the opening in your bag. Allow cookies to dry overnight.

1.  http://food.oregonstate.edu/learn/egg.html
2.  Zhu H. et al., Bacterial killing in gastric juice--effect of pH and pepsin on Escherichia coli and Helicobacter pylori. J Med Microbiology, 2006 Sep;55(Pt 9):1265-70.
3.  Sugar cookie recipe adapted from Gourmet Dec. 2005, available here.