Not all flours are created equal. Using the wrong type of flour for baking a particular item may lead to a not so happy result, and it is partially to do with protein. Protein is directly associated with how much gluten will form in the dough – which affects both structure, and texture. Flours with low protein contain less gluten, and those with high protein contain more. If you want a light-and airy bake, as in the case of a sponge cake, then low protein is the flour to use. A more chewy structure, as is the case in breads, you need high protein. There are basically three core types of (white) flour, all with differing levels of protein (let’s assume wheat flour).
→ bread flour: 14 – 16%
→ all-Purpose (AP) flour: 10 – 12%
→ cake and pastry flour: 7-9%
Calculating protein is not hard, but you do have to watch the nutritional information on the package. Sometimes it’s as simple as 12 grams of protein per 100g, which means the protein level is 12%. Normally though, protein level is expressed in some other manner, such as 4% per 34g. So now we have to divide 100 by 34, and multiply this by 4. Now we get 11.8% protein. Here are two flours I use for baking. The Italian flour on the left, farina tipo is 10% (on the lower end of AP), and the Bob’s Red Mill AP flour on the right is 11.8%. For fine cookies, I would lean towards the lower protein content, whereas for choc-chip cookies, using the AP flour is fine.
Using AP flour for bread is likely not such as good idea, because it won’t develop enough gluten, and the bread may not turn out as well. Again, some types of bread are more forgiving than others. In Canada, Robin Hood makes an, “All-Purpose Unbleached Flour”, which is 4g/30g, which gives it a protein content of 13.3%, which I think is way too high for making cookies (Five Roses All Purpose has the same amount of protein). Better to use Robin Hood’s “Best for Cake & Pastry Flour”, which is only 10% protein. There is no standardized system in Europe, and so each country has its own methodology for designating flour (usually based on ash content): a cake and pastry flour with low protein might be equivalent to a 405 flour in Germany (soft wheat 8-10% protein), and T45 flour in France, and an Italian OO flour.
Baking is as much a science as it is an adventure in experimentation. Knowing how protein works in flour is one of the keys to successful baking!