Rye is the second most widely used cereal in the world for bread making, after wheat. However, rye protein, although not tolerated by the celiac community, is unable to form a three-dimensional network capable of cohering the dough and retaining the gas formed during fermentation.

Rye cultivation is better adapted to adverse climatic conditions (cold) and poorer soils than wheat, so it has traditionally been grown in areas where wheat struggled to adapt. Over 90% of rye produced is cultivated in Northern, Central, and Eastern Europe. In these countries, there is a greater culture of rye and a wider variety of products based on this cereal. In Spain, and in other countries the consumption of rye in bread production is minimal.

The main interest in rye lies in its nutritional properties and its effects on health, especially if the products made are whole grain.

Milling

The milling systems and machinery for rye are very similar to those for wheat milling. In fact, in Spain, rye flour production is carried out in the same companies that mill wheat. The first factor to consider for its milling is its excessively soft nature. This means that smooth rolls  cannot be used, as they tend to flake the endosperm particles instead of breaking them. This problem is also contributed to by the high content of pentosans, fibers with a high water retention capacity. Therefore, rye milling is usually done with lower moisture content than wheat milling. Therefore, fluted or rough rollers are usually used for milling rye, in greater numbers than in wheat milling. This also makes it difficult to properly separate the bran during the milling process. Thus, the so-called white rye flours have a greater contamination of bran particles than wheat flours, and the extraction rate (kilograms of flour obtained from 100 kilograms of wheat) is much lower than in the case of wheat (60 compared to 75). In general, rye flours are marketed with extraction rates of 60 (white), 80 (brown), or 100 (wholemeal). In most countries, rye flours are classified by their ash content (or by their color), a parameter related to the presence of bran in the flour.

The second factor to consider is the high ease of germination of rye grains. This means that in humid and warm climates, it can sprout on the plant (in the field). In this case, the obtained flours would have a high amount of amylases and other enzymes, such as pentosanases, which would negatively affect the baking quality of the flours.

Care must also be taken in rye milling to remove ergot. Rye infection by the fungus Claviceps purpurea is favoured by low temperatures. This fungus causes infertility, and the flowers are susceptible to its attack during flowering. After infection, hard bodies form, replacing rye grains on the upper part of the plant (ergot). These hard bodies are removed in the grain cleaning phase. Their removal is easy due to their lower density compared to rye grains and their darker colour. Ergot, typically associated with rye cultivation, contains toxic and hallucinogenic alkaloids, so its presence in flour would have negative consequences for human health.

Quality of flours for baking

Unlike wheat, the quality of rye proteins is not very important in the bread making process, as they are unable to form a network similar to wheat gluten. In general, the factors to be controlled for the baking quality of rye flours are as follows:

• Content of pentosans or arabinoxylans. These fibres have a high-water absorption capacity and thickening power, thus significantly modifying the dough.
• Content of damaged starch. Depending on the milling process, a different content of starch will be damaged. Damaged starch, besides being more accessible to enzymes, has a higher water absorption capacity than undamaged starch, affecting the characteristics of the dough.
• Enzymatic content. If the grain has undergone germination processes in the plant, it will have a higher enzymatic content. Among the enzymes present in the grain, amylases will stand out, but pentosanases are also important. These enzymes will hydrolyse the components of the flours from kneading, modifying the characteristics of the dough.

To analyse the enzymatic content and detect germination problems in the field, the falling number index is usually used, which gives us an idea of the amylase content, something that is usually related to the pentosanase content. The content of pentosans and damaged starch is more difficult to measure, but we can have an indirect measure through water absorption tests of the flours, a parameter that is related to their influence on the quality of the dough. However, it is more common to analyse a mixture of flour and water with systems that can measure the viscosity of the mixture or its consistency. For this, it is possible to use the farinograph, or similar equipment, or the RVA (rapid visco analyser) or the amylograph. These last two equipment allow analysing the viscosity of the pastes, their evolution over time, if programmed for it, and the changes in a heating and cooling process.

Breadmaking

In most countries, rye flour is usually used in small quantities, usually not exceeding 30%, along with wheat flour. In these cases, the aim is to provide flavour, aroma, and colour, expanding the range of products. In these cases, the possible differences between the different rye flours usually do not affect the final result, as long as there have been no germinations and enzymatic activity is not excessive. To obtain regular products, it is also necessary to determine the extraction rate of the flours or the ash content. The baking quality of mixtures of wheat flour and rye flour will be determined by the quality and quantity of wheat proteins, responsible for developing the gluten network and forming a cohesive dough and retaining the gases formed during fermentation. In these cases, the bread-making process is very similar to that of wheat bread, with its different variations.

In the case of bread made exclusively with rye flour, or in combinations with wheat flour where rye flour accounts for more than 50%, the characteristics of rye flours are more important. These breads are usually made from sourdough and with processes that include long fermentations, where the action of lactic acid bacteria and acetic acid bacteria is favoured. The result of these processes increases the acidity of the dough to a pH of 4.3. The components formed in this process provide the typical acidity of these breads but also generate other aromas and aroma precursors that will influence the taste and aroma of these breads, which are much more intense than in the case of wheat bread. Acidity also prolongs the shelf life of the bread as it minimizes the development of microorganisms. Additionally, the low pH has a positive effect on the swelling of arabinoxylans and their functionality in the bread-making process. Finally, it should be noted that the high acidity reduces enzymatic activity, especially that of amylases and xylanases or pentosanases, and therefore the degradation of flour components will be lower.

This whole process begins with the mixture of rye flour and water, and in some cases salt, and the subsequent development of the microbiota inherent to these ingredients and the environment. Nowadays, it is common to use starters (selected microorganisms) to start this fermentation process. Sometimes the doughs can also be acidified with organic acids, such as lactic or acetic acid, replacing sourdough, to achieve the beneficial effects of low pH.

Rye breads should be kneaded slowly to avoid generating very viscous doughs. In the case of mixtures with wheat or doughs with high enzymatic activity, this requirement is not as important since it reduces the proportion of high molecular weight pentosans. The amount of water in the doughs will be slightly higher than in wheat doughs, and higher in the case of whole grain flours than in white flours, since the former have a higher proportion of arabinoxylans, with a high water absorption capacity. However, an excess of water can generate excessively sticky doughs.

In general, doughs made with rye flour do not have a good gas retention capacity, and although yeasts generate an adequate proportion of carbon dioxide during fermentation, the percentage of retained gas will be lower than in wheat breads. As a consequence of this phenomenon, rye breads are denser than wheat breads. The high water retention capacity of pentosans also makes these breads somewhat moister. The use of flours with a higher extraction rate and long fermentations results in final products that are darker and with more intense flavours.

The baking of these breads is usually more intense, with higher temperatures, which can even reach 300°C, and longer baking times, but these conditions will depend on the size of the pieces and whether they are in a mold or not. The obtained breads also have a longer shelf life if properly packaged, as they tend to dry out less than wheat breads (due to the high water absorption capacity of pentosans), and the development of microorganisms is minimized due to the high acidity.

Special Breads

Among the baked products made with rye flour, toasted or crispy breads and pumpernickel stand out.

Knaeckebrot, or crispy breads, are toasted slices of rye bread. Depending on whether they are made with wholemeal or white flours, and whether they contain mixtures with wheat flours or not, their colour will be more or less dark and their flavour more or less intense.

Pumpernickel is a very dense and dark bread, usually rectangular, with a very intense flavour, marketed in the form of very thin slices. It originates from Germany and is characterized by being made from sourdough and by its long baking time, inside closed molds, which usually lasts 18-36 hours, at low temperatures (between 100 and 170ºC). In this process, a dry crust is not formed due to the low temperatures and the presence of high amounts of water vapor in the oven. Due to the low temperatures, as well as the high acidity and enzymatic activity, many dextrins are generated, in addition to glucose and maltose, due to the hydrolysis of starch. A very dark colour is also generated (caramelization reactions of sugars and Maillard reaction) and a somewhat bitter, as well as acidic, taste. In some cases, molasses are added to enhance the colour of these breads. It is also possible to incorporate cooked grains and ground old bread, although these should not exceed 10% as they reduce the elasticity and cohesiveness of the breads.

Health Effects

Products made from rye flour are reputed to be healthy. This condition is mainly due to its high fibre content (the highest among cereals) and the large number of bioactive substances it contains. However, the nutritional advantages of rye are significantly reduced if the bran is removed and if it is combined with other flours, such as wheat. Therefore, to obtain all the health benefits of rye flour, wholemeal flours should be used, and in the highest possible percentages.

Consumption of rye-based products has been shown to improve intestinal transit (increased stool weight, shorter transit time, and increased frequency of defecation), which is related to its fibre content and its characteristics. In addition, there is evidence that it may reduce the risk of colon cancer.

There are also studies that claim that consumption of rye bread provides a greater feeling of satiety and reduces insulin demand compared to consumption of wheat bread, something that may have an effect on the prevention of diabetes. In turn, the greater feeling of satiety can be used in weight control regimens. The lower glycaemic index of these products, related to their fibre content and polyphenols, can also help reduce the risk of cardiovascular diseases. The high water absorption capacity of arabinoxylans present in rye flour increases the viscosity of the bolus, reducing the absorption of fats and cholesterol in the body, so it can also help reduce cholesterol.

Furthermore, the high content of phenolic compounds and its high antioxidant capacity have been related to the prevention of various types of cancer and various degenerative diseases.

For more information:

Bushuk, W (2001) Rye. Production, Chemistry and Technology. AACC. St Paul. MN (USA)

Deleu, LJ; Lemmens, E; Redant, L; Delcour, JA (2020) The major constituents of rye (Secale cereale L.) flour and their role in the production of rye bread, a food product to which a multitude of health aspects are ascribed. Cereal Chemistry

Lorenz, K (2003) Rye bread. En K. Kulp y K. Lorenz (Ed). Handbook of dough fermentations. Marcel Dekker. New York (USA)

Valjakka, T-T; Kerojoki, H.; Katina, K. (2003) Sourdough bread in Finland and Eastern Europe. En K. Kulp y K. Lorenz (Ed). Handbook of dough fermentations. Marcel Dekker. New York (USA)