What effects will variations in flour protein content have on baked product quality How is the property measured
The protein content of flour is probably the single most important property of wheat flour. Perhaps more correctly we should refer to wheat proteins since there is more than one type of protein present. The scheme established by Osborne (1907) is most commonly used for the groups of proteins in wheat, which comprise:
• albumins, soluble in distilled water;
• globulins, soluble in dilute salt solutions;
• prolamines, soluble in 70% aqueous ethanol;
• glutelins, soluble in dilute acid.
The two most important groups for bread and fermented goods are the prolamines and the glutelins. They contain the gluten-forming proteins that give wheat flour its almost unique ability to form a dough capable of retaining gas and increasing in volume under the influence of heat and carbon dioxide gas released by yeast fermentation. The properties of wheat gluten were recognised as long ago as 1729 (Bailey, 1941).
Gliadin and glutentin are the two wheat protein components that give wheat gluten its special properties. These can best be appreciated by making a dough of flour and water and hand kneading it under running water. As time proceeds a milky-white liquid is washed out: this is the starch and other insoluble components. Eventually all that is left is a grey, light brown mass with an extensible but also an elastic character. This is the gluten and its gas retention properties can be shown by placing the mass of gluten in the oven and watching it swell. The quantity of gluten that can be extracted varies with the protein content of the flour.
Bread and other fermented product volumes are directly related to the quantity of protein present: the higher the protein content of the flour, the greater the product volume. This positive relationship has been reported by a large number of observers for many different breadmaking processes and products (e.g. Cauvain et al., 1985). Thus in answer to the question, variation in protein content will result in potential variations in bread and fermented product volume. They will also affect the lift obtained with laminated products but will have no significant effect on the volume of other baked products, though variations in protein content may affect other product attributes, e.g. eating quality in cakes.
Protein absorbs water, 1.3 g of water for each 1 g protein (Stauffer, 1998),and so variations in protein also affect flour water absorption.
Wheat proteins contain nitrogen, and protein measurement methods are based on that basic measurement. For many years the standard 'wet chemistry' method was the Kjeldahl (AACC Method 46-10, 1995). The method involves the acid digestion of the flour using sulphuric acid in the presence of a catalyst. The Kjeldahl nitrogen value so determined is converted to protein using a factor; for wheat this involves multiplying by 5.7. More recently Kjeldahl protein determination has been replaced by the Dumas method based on combustion in the presence of oxygen (AACC Method 46-30, 1995).
Flour protein is also commonly measured using near infrared reflectance (NIR) technology (CCFRA, 1991). This provides a fast and simple to use method which can also be applied to on-line processes in the flour mill. However, it should be noted that NIR protein is calibrated against an accepted 'chemical' method since it does not represent a fundamental measurement of protein.
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