What is the difference between strong white flour and plain white flour?

Answer 1

In some markets, the different available flour varieties are labeled according to the ash mass ("mineral content") that remains after a sample was incinerated in a laboratory oven (typically at 550 °C or 900 °C, see international standards ISO 2171 and ICC 104/1). This is an easily verified indicator for the fraction of the whole grain that ended up in the flour, because the mineral content of the starchy endosperm is much lower than that of the outer parts of the grain. Flour made from all parts of the grain (extraction rate: 100%) leaves about 2 g ash or more per 100 g dry flour. Plain white flour (extraction rate: 50-60%) leaves only about 0.4 g.

German flour type numbers (Mehltyp) indicate the amount of ash (measured in milligrams) obtained from 100 g of the dry mass of this flour. Standard wheat flours (defined in DIN 10355) range from type 405 for normal white wheat flour for baking, to strong bread flour types 550, 650, 812, and the darker types 1050 and 1600 for wholegrain breads.

French flour type numbers (type de farine) are a factor 10 smaller than those used in Germany, because they indicate the ash content (in milligrams) per 10 g flour. Type 55 is the standard, hard-wheat white flour for baking, including puff pastries ("pâte feuilletée"). Type 45 is often called pastry flour, but is generally from a softer wheat. Types 65, 80, and 110 are strong bread flours of increasing darkness, and type 150 is a wholemeal flour.

In the United States and the United Kingdom, no numbered standardized flour types are defined, and the ash mass is only rarely given on the label by flour manufacturers. However, the legally required standard nutrition label specifies the protein content of the flour, which is also a suitable way for comparing the extraction rates of different available flour types.

It is possible to find out ash content from some US manufacturers. However, US measurements are based on wheat with a 14% moisture content. Thus, a US flour with .48 ash would approximate a French Type 55. For US bakers of French pastry seeking an equivalent, for example, they could look at tables published by King Arthur Flour, showing their all-purpose flour is a close equivalent to French Type 55.

In general, as the extraction rate of the flour increases, so do both the protein and the ash content. However, as the extraction rate approaches 100% (whole meal), the protein content drops slightly, while the ash content continues to rise.

source:

http://en.wikipedia.org/wiki/Flour

Answer 2

This is sometimes called "bread flour". It is flour with a higher gluten content. Gluten is essential for the structure of bread; stretching to trap air bubbles as the yeast ferments. Plain flour is sometimes used in breadmaking when a specific gluten content needs to be achieved, but it is rarely used without the addition of strong flour.