Recently, I made a blackberry crisp with oatmeal topping but forgot to add thickener to the berries. After I spooned out some of the dessert, I added a cornstarch slurry to the juices collecting in the bottom of the pan. But it turned the filling a cloudy, grimace-purple color. Could I have used something else to preserve the color?
— Brooke H., via email
We gather you were improvising this crisp rather than following a recipe. Otherwise, it would have specified a thickener.
In addition to cornstarch, all-purpose flour, arrowroot, potato and rice starches and quick-cooking tapioca commonly are used in this type of dish. All-purpose flour is least favored by bakers because it turns juices slightly opaque, and it can impart a gritty texture.
Here is a general guideline for adapting recipes that use all-purpose flour to thicken fruit fillings: For every tablespoon of flour, substitute 1 tablespoon quick-cooking tapioca or 1/2 tablespoon cornstarch (or potato, rice or arrowroot starch).
Fillings made with starches typically set clear when fully cooked and cooled. But too much thickener can affect texture. Cornstarch tends to make fillings gummy while tapioca can make them feel dry in the mouth. So Fine Cooking magazine suggests using a mixture of the two.
Almost as important as choosing a thickener is judging a fruit's juiciness before you begin baking. Generally speaking, the riper the fruit, the more juice it will produce during baking, especially stone fruits (peaches, plums and nectarines). Strawberries, raspberries and blackberries are much juicier than blueberries or cherries.
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