Flour Quality II (Proteins)

Flour Quality II (Proteins)

After our initial entry where we discussed general aspects of flour quality, such as moisture, particle size, ash content, germination problems, and protein degradation (which you can find at this link), it’s time to delve into the proteins found in cereals.

Protein Quantity

The protein content of flours is a well-known analysis and can be conducted in many places. Many laboratories have equipment for measuring protein, typically using the Kjeldahl method, as protein is one of the most important macronutrients in food. This method can be somewhat tedious, and in most flour mills and grain-processing companies, you’ll find equipment based on near-infrared technology (NIR) calibrated for protein analysis. These instruments can analyse protein content, as well as moisture and other parameters, in just a few seconds, without the need for specialized personnel, but they must be properly calibrated for each type of flour. Calibration must also be specific to each type of flour. For instance, a calibration for white flour will not be suitable for measuring whole wheat flour, and one for wheat flour will not work for rye flour.

Protein content is a frequently used analysis, but it is often given less importance. This is because the baking quality of flour is highly influenced by the quality of the proteins rather than their quantity. However, protein content is still an important analysis because, while protein quality is crucial, quantity matters too. In the case of flours other than wheat, where gluten formation doesn’t occur, or for wheat flour used in preparations where gluten development is inhibited, protein analysis can provide valuable information. Higher protein content generally means lower starch content, which, in turn, affects all flour behaviours related to starch, including gelatinization and retrogradation. While measuring starch content might be more effective, it is a much more complex and labour-intensive analysis, so measuring protein content provides a good indication.

In summary, protein content is an important parameter for defining the quality of gluten-free flours and wheat flour used in preparations where gluten network formation is inhibited, as discussed in the entry about gluten. However, it’s also an important parameter for defining the quality of flours for breadmaking, although it should be accompanied by other analyses related to protein quality.

Nutritional Quality of Proteins

Cereals do not contain proteins of high nutritional quality because most of them are deficient in lysine and have low levels of tryptophan and other essential amino acids. It is well known that the combination of cereals and legumes provides highly complete and nutritionally valuable proteins because they complement each other. Legume proteins are a good source of lysine, while cereal proteins provide sulphur amino acids such as cysteine and methionine, which are deficient in legumes. These proteins are not typically analysed for their nutritional aspects due to their low nutritional quality. In cases where it’s necessary, usually for research purposes, analysis of various amino acids present is conducted, but protein digestibility aspects are also important.

Gluten and Gluten Quality

From this point forward, we’ll discuss gluten and related aspects. Therefore, these sections are only relevant to wheat flours and ancient wheat varieties such as spelt, kamut, etc. While most wheat proteins can form the gluten network, not all of them can. For example, whole grain flours have a higher protein content than white flours due to the higher protein content in the bran compared to the endosperm. However, proteins in the bran cannot form gluten. Since gluten is the protein responsible for the baking quality of these flours, measuring gluten gives us a more accurate idea of this quality compared to simply measuring total protein content.

Analysing gluten content requires the formation of the gluten network. To do this, the flour is mixed with a saline solution (water and salt) and subjected to mechanical work. The resulting gluten must then be washed to remove all soluble substances present in the flour, such as starch and other proteins. Traditionally, this process was done manually, but there are specific devices capable of performing this analysis. The resulting product is wet gluten. If dried, it becomes dry gluten. Generally, dry gluten is one-third of wet gluten, as proteins can absorb two parts of their weight in water.

However, like proteins, it’s not just the quantity of gluten that matters; its quality is also important. To assess gluten quality, a device has been designed that subjects the obtained gluten to centrifugation and analyses the amount of gluten that passes through a metal sieve. Strong gluten won’t pass through these small holes, but weak gluten partially passes through the sieve. The percentage of gluten that does not pass through the sieve is known as the gluten index.

You can find more information about the most used equipment for analysing gluten quantity and quality on the website of the manufacturer, Perten. The equipment is called Glutomatic, and there are also videos on YouTube showing how the analysis is conducted.

Although gluten analysis is a typical procedure in flour mills, it is not usually decisive for the baking quality of flours, and other parameters are often used. Nevertheless, like protein content, it is still a value to consider. Gluten index, on the other hand, has been successful as a predictor of the quality of durum wheat semolina for pasta making but less so as a predictor of the baking quality of wheat flours.

Gliadins and Glutenins

The reserve proteins of wheat, which form the gluten network, consist of gliadins and glutenins, which can be further divided into various types. Traditionally, it was believed that glutenins provided elasticity to the dough, while gliadins contributed to cohesiveness and extensibility. However, this distinction is less clear today, and studies that have removed gliadins in new wheat varieties suggest that the baking quality of these wheats is similar to that of conventional varieties. The interest in removing gliadins arises from the fact that the sequences of amino acids responsible for celiac disease immune response are concentrated in this fraction. Within glutenins, high and low molecular weight glutenins can be distinguished, and in general, high molecular weight glutenins provide more strength to the dough. However, after a more detailed study, it is now known that certain protein bands obtained through electrophoresis are more related to baking quality than others.

Gliadins and glutenins are typically studied using electrophoresis, and the pattern obtained through this analysis is characteristic of each variety. This analysis is used for variety identification, especially by genetic improvers to obtain wheat varieties with higher baking quality. However, it is not commonly used in the industry. Some pasta factories I have known used this analysis to identify contamination of durum wheat semolina with soft wheat semolina. However, at least in Spain, most industries have stopped conducting this analysis. This is partly due to the complexity of the analysis and partly because the batches of semolina available in the market are more reliable. It’s possible that in other countries, with different issues, this analysis is still useful for this purpose.

Electrophoresis of Wheat Proteins

Sedimentation Analysis

To evaluate the quality of wheat flour proteins, it is common to perform some type of sedimentation analysis. In flour mills, the most well-known analysis is the Zeleny index. These analyses are based on the ability of these proteins to swell when they come into contact with lactic acid, to a greater extent when the gluten is stronger. Therefore, after mixing the flour with water and some reagents, including lactic acid and a dye for better visualization, the mixture is subjected to back-and-forth motion inside a burette. After this motion, the mixture is allowed to rest for some time, and the height of the compacted part is measured. Over time, the sample will compact further, and the height will decrease. As the proteins swell, they compact more slowly, so a greater height indicates better baking quality of the flours. Or, more accurately, a stronger gluten network.

These types of analyses are used in genetic improvement and are very common in flour mills. Additionally, the Zeleny index correlates quite well with the ability of dough to retain air and with the volume of the resulting bread. However, it is a complex analysis and requires specific equipment to subject the mixture to uniform back-and-forth motion, both in tilt and speed.

Here is a link where you can see the analysis to obtain the Zeleny index and the frame used.

Equipment Simulating the Baking Process Despite having various analyses that provide insight into the quality of wheat flour proteins and their suitability for the baking process, it is common to use equipment that simulates some phases of baking to evaluate how these flours would behave in the process. Among these devices, those simulating kneading, dough handling, or fermentation are prominent. These devices will be discussed in the next post, but to provide a preview, the farinograph is the most widely used equipment for studying flour behaviour during kneading, while the extensograph provides information on how flours and dough behave when manipulated (stretched or folded). These two devices belong to the Brabender company, but there are also similar-function devices from other manufacturers. The other major manufacturer of such equipment is the French company Chopin. Chopin manufactures the alveograph, which analyses the gas retention capacity and behaviour during stretching, as well as the reofermentometer, which is highly suitable for studying fermentation. In addition to these devices, they have the Mixolab, which in its first phase is similar to the farinograph, while in the second phase, it is more similar to an amylograph or RVA. But we will discuss these devices in detail, along with their advantages and disadvantages, late

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