Bakers use plain flour in delicate cakes, biscuits and pastries. The starchiness of plain flour makes baked goods light and crumbly without falling apart. You can't use plain flour and regular wheat flour interchangeably for most recipes. Wheat flour has a higher protein content that might make your cakes too tough. In a pinch, make a suitable alternative to plain flour by using a reduced amount of all-purpose flour or combining it with corn flour.
All-purpose flour and corn flour
Combine corn flour with all-purpose flour to create a reliable homemade alternative to plain flour. To replace 128 g (1 cup) of plain flour in a recipe, mix 2 tbsp of cornstarch with 96 g (3/4 cup) of all-purpose flour. Make sure to sift this homemade substitution with the rest of your dry ingredients to remove lumps and distribute the corn flour throughout the recipe. The corn flour adds starchiness to the flour that should give a tender, crumbly texture to your baked good similar to the texture yielded by plain flour.
If you must substitute all-purpose flour for plain flour in a recipe, reduce the amount of total flour in the recipe. As a general rule, subtract 2 tbsp of flour for every 128 g (1 cup) of plain flour called for in the recipe. Since all-purpose flour consists of about 11 per cent protein compared to plain flour's 7 per cent, reducing the flour will reduce the amount of protein in the recipe. Avoid this substitution in desserts, such as angel food cake, that rely on lightness for their structure.
All-Purpose flour and arrowroot powder
If you have a corn allergy or other aversion to using corn flour, you can easily replace plain flour with a mixture of all-purpose flour and arrowroot powder. Thought to help sooth stomach aches, arrowroot powder has a bland flavour that easily blends in to cakes, biscuits and desserts. Chefs use arrowroot to thicken sauces and puddings and, like corn flour, its high starch content makes it a suitable addition to all purpose flour when replacing plain flour in a recipe. For every 128 g (1 cup) of plain flour called for, use 1 tbsp plus 1 tsp of arrowroot powder for every 96 g (3/4 cup) of all-purpose flour
Consisting of about 9 per cent protein, pastry flour contains a lower percentage of protein than all-purpose flour, but more than plain flour. While plain flour is bleached to remove more protein, manufacturers leave pastry flour unbleached. Use pastry flour as an alternative to plain flour by reducing the total amount of flour in the recipe by about 1 tbsp per 128 g (1 cup). Pastry flour makes an excellent substitution for plain flour in muffins and cookies with a low ratio of sugar and liquid to flour.