Rye bread

In terms of total production (less than 1% of world production) rye is a minor cereal. The world rye production largely takes place in the Northern part of the region from the Nordic Sea to the Ural Mountains. From that point of view Europe can be divided in two main regions: wheat products in the west (France, Italy, Belgium, Spain), rye products in the east (Russia, Poland, Denmark, Finland, Baltic states), while Germany shows a mixed situation. In countries with a high consumption of rye, the per capita consumption is in the range of 10 – 30 kg/year. In contrast to wheat, rye is mostly consumed as whole grain flour in breads and other cereal products, which makes rye products a good source of dietary fibre and micro-nutrients and bioactive compounds such as phenolic compounds, vitamins, trace elements, and minerals.

Rye proteins, unlike wheat protein, cannot form a continuous network and an elastic dough. Rye pentosans (mainly arabinoxylans), however, are able to bind water during mixing to produce a dough that can be baked into bread. The flavour and structure of rye bread are also quite different from those of wheat bread, and they vary depending on flour type, other raw materials and ingredients, process, baking conditions, as well as the size and shape of the bread.

Baking with rye is in many respects different from baking with wheat. The main difference between rye and wheat flour is that rye protein cannot form a continuous network and an elastic dough. Instead, rye pentosans (mainly arabinoxylans) are able to bind water during mixing to produce a dough that can be baked into bread. The most typical rye bread in what I call the "rye" countries is soft whole-grain rye bread made by using a sourdough method. By the way, sourdough on the basis of rye is something totally different from sourdough based on wheat (acidity, flavour etc.). In this method the main ingredients, whole-grain rye flour, water, and starter culture (usually seed from a previous sourdough batch with stable composition of microorganisms) are mixed and fermented for about 8 - 18 hours. During the fermentation period, the lactic acid bacteria and the sourdough yeast grow, and due to the microbial activity and the enzymatic reactions in the sourdough, flavour compounds and flavour precursors such as amino acids are formed. The flow chart below shows a typical procedure to produce rye bread, but as for wheat bread, there are is many recipes and methods, as there are bakers.

The main components formed are lactic acid and acetic acid. After fermentation, more flour, water, and other ingredients are mixed into the sourdough to make a dough. The dough is left to rise for a short period, after which the breads are shaped, left to rise again, and baked. Without sourdough, whole-meal rye or wheat-rye flour mixes are very difficult to process because acids and enzymes formed or activated during fermentation modify protein and pentosan phases, having a positive effect on moisture, porosity, and elasticity of crumb. Sourdough also provides aromatic and pleasing flavour, and improves overall quality and shelf life. The positive influence of the acid present is based on the increased swelling power of the pentosans and mucilages of rye flour and the simultaneous inactivation of some enzymes, particularly amylase (Brummer and Lorenz 2003).

Whole-meal rye doughs contain high amounts of flour particles with a high concentration of cell walls. The cell wall material is largely responsible for the water-binding capacity and rheological properties of rye doughs. The structure of the cell walls and, for example, the effects of pre-harvest sprouting on the composition of the grain play an important role in determining the baking quality of rye. Differences in rye raw material and technological operations cause structural alteration to the dough components. Hydration and degradation of cell walls have been found to have a noticeable effect on dough rheology and change the baking behaviour of the dough and influence the structure of the bread (Fabritius et al. 1997, Autio et al. 1998).

A sourdough process is commonly used in rye bread baking, and the acid conditions of the process have great effect on enzyme activities and starch gelatinisation Both the cell wall degrading enzymes and a-amylase affect the baking properties of rye doughs and the quality of breads. Softening of dough during fermentation has been shown to be partly dependent on the swelling or fragmentation of cell walls induced by xylanase, whereas the softening during baking is mainly due to a-amylase.

The most important methods for determining the baking quality of rye flour are the falling number and the amylogram, which are both related to a-amylase activity. Rye flour with a low falling number gives a soft, sticky dough, and the resulting bread is dough-like. Rye flour with a high falling number yields a rigid, stable dough, but the resulting bread is dense and hard.

Another popular type of rye bread is crisp bread, which has a long shelf life due to its very low water content. For the production of crisp bread, rye with low amylase activity is required. There are three types of rye crisp bread: normal yeast fermented, sourdough fermented, and cold crisp bread (which is baked without the addition of yeast or sourdough).

The third type of rye bread is pumpernickel, which originates from Germany. It is produced from whole-meal rye flour with a sourdough process. Baking temperature is held rather low (150 - 170°C), and baking time range from 10 to 36 hours. Pumpernickel bread has a very dark and dense crumb, a highly aromatic, bitter-sweet taste, and a very long shelf life. The bitter-sweet taste of pumpernickel is due to the acids that are formed in the sour fermentation and the amylolysis of the starch during sourdough fermentation, proofing, and baking that produces glucose, maltose, and dextrins (Seibel and Weipert 2001, Pyler 1988).

It is not uncommon to boil the water and the rye flour and once it's cooled down to add the sourdough and other ingredients. Parboiled flour and yeast are traditionally used to bake rye bread. The procedure of preparing parboiled flour and sourdough is the most important stage in the baking process. This technique also has the advantage that microorganisms originally present in the rye flour are killed and that the inoculation can happen with known strains of lactic acid bacteria.

Sourdough and parboiled flour affect the taste of bread. The taste of rye flour bread can be mildly sour-sweet or sour. "Yeast" in this case, is a system of microorganisms (sourdough), lactic acid and ferments which make the rye flour dough lighter. Qualitative rye flour, water and pure culture are necessary to form the yeast. The quality of the yeast is influenced by the duration of its maturation, temperature, and consistency. To create the yeast in large amounts there is a need for special equipment that carry out such activities as pumping, weighing, mixing, providing fermentation process and  the necessary temperature. Good yeast can be described as “alive” – at first with small bursting bubbles moving in it, later on the yeast “calms down,” matures and becomes still. Yeast which is good and ready to bake has a pleasant sour aroma.

The positive effect of the sourdough on the bread quality resides on the specific metabolic activities of the LAB and yeast which are able to produce different organic acids, exopolysaccharides and enzymes (Arendt et al., 2007) therefore extending the shelf life of sourdough bread (longer mold-free period, prevention of rope in bread, antistaling). One of the main functions of sourdough in rye bread making consists in the inactivation of α-amylase activity. Hansen et al. (2002) showed that the activity of α-amylase in the bread dough prepared with 20 % sourdough was halved after resting (pH 4.5) compared to the initial activity in the flour. Moreover, the rye dough acidification is essential for achieving bread with suitable physical properties in terms of elasticity and extensibility and for conferring the acid flavour notes characteristic to the rye breads (Arendt et al., 2007). LAB are mainly responsible for dough acidification, while yeasts for the production of flavour compounds. Compared to the single starters containing Lactobacillus brevis and baker's yeast, the use of mixed starter cultures allow obtaining more aroma compounds and, as determined by bread sensory analyses.

The second most essential component of rye bread is parboiled flour. It gives to bread a pleasant taste and aroma, as well as prolongs its shelf-life. In order to create parboiled flour, part of flour, malt and spices such as cumin are poured in hot water and mixed well until homogeneous.  Right after the parboiled flour is ready, it has to be crystallised. During the first few hours after the parboiled flour is made, the starch starts to break up into sugars and the amount of sugar increases. Parboiled flour becomes sweet. To accelerate the process, parboiled flour is cooled in special barrels equipped with a cooling system.

The two main ingredients which are prepared separately experience obvious transformation: changing their shape, colour and aroma. However, it is only the beginning. Yeast and parboiled flour meet when the dough kneading takes place. According to a recipe the necessary amounts of both components are transported to the next section where the dough will be kneaded.  Flour, salt, sugar and other ingredients are added to the initial two components when making the black bread dough. Kneading dough for rye bread is comparatively easy.  After kneading the yeast dough must be left to rise. Duration of a rise for the rye dough is approximately 45–60 minutes; this stage is very important, since the dough lightens during the process. If not left to rise, bread would not be puffy and spongy, on the other hand – it would be dense and flat. The risen dough can then move forward to be divided and shaped.

Rye dough is moist and sticky. A special machine is used to divide rye dough. With a little help of another special machine a proper shape of a loaf is acquired. This is how loaves of bread become beautifully oval-shaped. During the dividing and shaping process, part of the gases are forced out of the dough, therefore prior to bake the dough it has to be left to rise again. The oval-shaped loaves of bread are transported along the conveyor belt to the machine where each of them is placed in a special baking pan and put in a proofing cabinet. The proofing cabinet is warm and humid, here the dough rises for the second time and increases in size.

After baking the bread can be cooled and sliced. The cooling time for rye bread is normally longer then for wheat bread. All this time the loaves of bread are resting on shelves in the cooling tower, moving upwards and downwards to gradually cool down.

In general bread quality is evaluated in terms of specific volume, humidity, total titratable acidity, crumb characteristics and sensory profiles. Digital image analysis reveals that rye bread with 40 % sourdough had a considerably denser crumb structure. Rye bread with 20 % sourdough maintained superior texture characteristics over the storage period, while increasing the sourdough content to 40 % had a negative effect on the texture. The sensory profiles of the bread highly depended on the type of starter cultures used for fermentation.

It is not uncommon that rye bread is pasteurised after packaging. In order to do this, the bread is exposed in a cabinet to a hot air (130 - 150°C) flow. It is kept in the cabinet until the bread reaches a temperature of 80 ± 1°C in the centre. It is than stored in plastic crates where some kind of "post pasteurisation" takes place i.e. the breads closely stacked in a crate maintain for quite some time a temperature around 80°C. Important is the type of packaging film. It is not uncommon that on the inside of the pack a layer of water droplets is formed. That water comes obviously from the product, so the danger is that the product gets dry. But on cooling this water (which also constitutes an ideal environment for bacteria to grow in - hence it is important that the product really gets pasteurised) should get absorbed again by the product. In general it is accepted, that polymer films are best to maintain the quality parameters and minimise changes during thermal treatment. Thermo-resistant combined polymer material film is recommended for packaged rye bread pasteurisation and to guarantee maximal bread quality.

NoŽl Haegens