A comprehensive review on biochemical and technological properties of rye (Secale cereale L.)

ABSTRACT

Rye (Secale cereale L.) is widely grown, especially in areas of Europe and North America, where the soil and temperature are unfavorable for cultivating other cereals. Rye grains are distinguished by having one of the greatest levels of fiber content as compared to the other cereals commonly ingested by people. Rye is one of the best sources of nutritional and bioactive substances, and it is the second most significant raw material for bread and bakery goods after wheat. Rye is also used in many culinary products, including pasta, snack foods, breakfast cereals, and porridge. Several physicochemical processes that affect the composition, characteristics, and availability of nutrients and technologically significant components take place during rye and rye flour preparation. Different Changes impact rye flour’s ability to absorb water, the dough’s viscosity and structure, the bread’s quality and yield, and its glycemic index. A recent trend in the milling and processing of rye is the production of more fiber and wholemeal flour. Wholemeal and dark rye flours with fine granulation are frequently produced using non-traditional, customized mill equipment. Our study aims to provide an in-depth analysis of the most recent findings about rye’s compositional and technological characteristics that significantly impact its use as food. In addition, this evaluation seeks to pinpoint the most recent developments and trends in rye product development.

KEYWORDS:

• Rye
• nutrition
• technological aspects
• food industry

Introduction

Rye (Secale cereale L), closely related to wheat (Triticum aestivum), has been cultivated in Europe for over 1000 BCE (Before the Common Era) and is challenging to grow compared to wheat. Despite the complex chemical structure, rye showed a range of nutritional and technological potentials. Physiologically, rye kernels closely resemble wheat grain, but their species differ from an excellent milieu.^[1] The outer rye kernel layers are the pericarp and testa, which enclose the germ and endosperm consisting of aleurone and the starchy endosperm. Rye is cultivated and used as a food source in Northern Europe, with Russia, Poland, Germany, Belarus, and Ukraine being the most prominent producers, according to FAOSTAT in 2006.^[2]

Epidemiological data have linked the consumption of whole grains with a lower risk of long-term illnesses with persistent conditions, including high blood sugar, cardiovascular ailments, and different malignancies.^[3] Variations in the yearly consumption of rye amongst other countries, such as Poland, Belarus, and Estonia, have been estimated at over 35 kg per capita in comparison to Finland, Denmark, and Sweden, which have a decreased rye consumption of 10–15 kg per capita. This is lower than the global average consumption. Due to this, rye serves as a crucial dietary fiber (DF) provider in several countries. For instance, in Finland and Denmark, rye meals contribute about 40% of DF.^[4]

Some of the most common foods from rye include black and sour bread, rye flour bread, hot bread, porridge flakes, and morning cereals.^[5] Rye is becoming increasingly popular as a snacking and munching ingredient. Consumers increasingly gravitate toward whole grains owing to their nutritional and therapeutic benefits, including gut-friendly metabolism and health-promoting properties attributed to bioactive compounds. However, investigating the bodily functions of health implications of various grain properties at the cellular extent needs to be considered and heavily examined.^[6] This review examines the chemistry of rye grains, their components, and rye-based food sources. The study also discusses the methods used in rye production and explores the bioavailability of several bioactive compounds and their potential health benefits.

Nutritional profile

Food intolerance affects many people in many ways, and these factors are usually caused by an individual’s change in dietary habits or a considerable change in their lifestyle. In developing nations, many individuals cannot maintain a balanced diet due to cellular-level metabolic abnormalities, notably celiac disease. Celiac disease has shockingly had considerable research attention in recent years due to the adverse effects of the disease. Recent scientific literature supports the hypothesis that celiac disease is a genetic disorder that can also spread due to various environmental and immunological factors. After the diagnosis of gluten intolerance, subjects with celiac disease are restricted from consuming gluten and gluten-based food products. The scientific community has made efforts to characterize food that could serve as a substitute for gluten-based food products.^[3]

The feasibility of producing gluten-free rye bread using degrading proteins. They found that incorporating prolyl end peptidase into the sourdough system can reduce gluten intolerance to below 20 mg kg1. However, compensatory measures, such as using gluten-free proteins or hydrocolloids, may be necessary to offset the loss of rye protein activity. It is important to note that rye products may not be appropriate for individuals with irritable bowel syndrome (IBS) due to their high FODMAP levels. Conversely, low-FODMAP diets, often combined with low-fiber diets, can harm digestive health.^[4]

Over the last few years, evidence of significant progress in producing gluten-free foods rich in nutrients has been promising. Researchers have explored different ways of creating gluten-free rye bread alongside proteolysis. They found that prolyl end peptidase in the sourdough system leads to gluten intolerance below 20 mg kg1. However, one can compensate for the loss of rye protein activity by using hydrocolloids or gluten-free proteins. Due to their high FODMAP levels, rye products may not be suitable for individuals with irritable bowel syndrome (IBS). While people commonly combine low-FODMAP diets with low-fiber diets, such diets can harm the gut microbiota. Clinical trials have shown the effectiveness of FODMAP low rye bread in the diets of IBS patients. As a result, scientists are working to develop rye products with lower FODMAP levels, allowing IBS patients to consume nutrient-dense rye.^[5]

Dietary fiber

Cereals are considered an important crop and staple diet due to the presence of major dietary components, mainly starch, dietary fibers (DFs), and phenolic compounds.^[6,7] The complex cell wall matrix of grains gives cereal dietary fibers (DFs) distinct properties compared to other fibers.^[8] Shah et al.,^[9] and Xu et al.,^[10] believe high-fat diets may reduce the risk of chronic diseases while stating that cereal dietary fibers offer various beneficial effects on health.^[11]

Moreover, whole grains that are rich in fiber have been linked to reduced mortality with dietary fiber as a critical protective factor.^[12] A reduction in blood pressure was linked to the intake of ß-glucan, as reported by Bozbulut and Sanlier^[13] in their systematic review and meta-analysis. One European Prospective Investigation into Cancer and Nutrition (EPIC) study suggests that consumption of 10 g of cereal-based dietary fiber per day was associated with an 11% decrease in the risk of colorectal cancer.^[14] Aune,^[15] and Bangar & Kaushik,^[16] found that grain-derived dietary fiber was associated with a decreased risk of breast cancer than fruits and vegetables.

Additionally, they observed that individuals who consume a high amount of wholegrain foods have a lower risk of heart disease, diabetes, obesity, and certain gastrointestinal illnesses. For more than 60 years, there has been much discussion and controversy surrounding the concept and definition of dietary fiber. Most DF in plants comprises polysaccharides and lignin that resist hydrolysis by digestive enzymes.^[17] The Codex Committee on Nutrition and Foods for Special The organization Dietary Uses defines DF as a “carbohydrate polymer consisting of three or more monomeric units that are digestible by endogenous enzymes in the small intestine of humans,” and it classifies them into various groups. These carbohydrate polymers can be obtained from raw foods through physical, enzymatic, or chemical methods and have been demonstrated to have a beneficial impact on health, as demonstrated by scientific evidence generally accepted by competent authorities.^[18]

The group of experts has recommended distinguishing DF from plant cell walls that form part of a plant matrix and are present in a product and pure DF additives added to a product for a specific purpose. Processing can alter the internal/natural DF and the additional DF supplementation, which may affect their physiological and metabolic benefits. Therefore, the food industry faces challenges providing enhanced and DF-compliant meals.^[19]

Processing of grains is necessary for their use in food products. It involves the application of various units, such as mechanical or thermal energy, hydration, and activation of the grain’s inherent biological enzyme system.^[20] These processes lead to macroscopic changes in grain component’s cellular levels, impacting the food product’s nourishing content and technical and physical properties. Although separation results cause an unequal distribution of DF in terms of value and quality in the ensuing fractions, additional changes in the dietary fiber matrix occur during continuous processing while neighboring structures remain unaffected.^[15]

Although the impact of plant origin and processing history on biological responses and technological features of cereal dietary fibers is well-established, further research is needed to establish a reliable and practical understanding of the health benefits of DF. A significant challenge in this research is the need for well-defined sources and features of DF used in human experiments, making it challenging to evaluate DF’s structural relationship function in health-related outcomes. Additionally, precise processing methods and characteristics should be specified when studying the effects of dietary fibers on health. Brownlee et al.^[21] reported that the impact of whole grains and isolated dietary fibers are not the same, while Vinelli et al.^[22] demonstrated that alternative processing methodologies have significantly different effects on health-related structures of dietary fibers. For instance, while the introduction of whole wheat increased the price of microbiota-accessible carbohydrates (MAC), it did not increase the production of short-chain fatty acids (SCFA).^[23] SCFA production increased when they added whole wheat to fermented sourdough foods from the same wheat. Furthermore, although scientists design grain processing to induce specific changes in dietary fibers, oxidation and enzymatic hydrolysis by endogenous or microbial enzymes may cause unexpected changes. These changes can open or limit DF’s physiological and technological value by affecting its melting, extraction, physicochemical structures, and grain matrix re-organization.^[24]

Bioactive compound

The composition of whole grain rye flour, expressed as a percentage of dry matter, includes carbohydrates (56–70%), proteins (8–13%), lipids (2–3%), ash (2%), and total dietary fiber (15–21%). Rye contains more dietary fiber (15–21%) than wheat (11–13%), with around 20% of the fiber being soluble. The primary fibers in rye are arabinoxylans (8–12%), ß-glucan (1.3–2.2%), and cellulose (1–1.7%).^[21]

Fructans (including fructooligosaccharides) are found in varying amounts in the rye, ranging from 4.6 to 6.6%. Because fructans are now recognized in various countries as a component of DF, rye’s DF concentration has increased even more.^[25]

Usually substitute with L-arabinofuranosyl residues at the O-2 position (2mXyl), the O-3 position (3mXyl), or both positions (dXyl) as reported by Kamal-Eldin et al.^[26] Furthermore, AX contains minute quantities of ferulate residues that are esterified to arabinose at its O-5 position.^[27] Studies have found that rye AX contains small amounts of ferulic acid dimers, which suggests that some extractable AX is associated with endosperm walls.^[28]

When treated with peroxidase/H2O2, extractable arabinoxylans (AX) from rye formed more robust gels than wheat ones.^[29] Stronger gels from wheat are due to the to result from the dimerization of ferulic acid substituents, which causes crosslinking of AX chains. Rye contains ferulate residues bound to tyrosine, suggesting that peroxidase can induce a covalent crosslink between arabinoxylans (AX) and proteins. Bonnin et al.^[30] reported that the degree of substitution and the placement of substituents along the xylan backbone impact the physicochemical characteristics of AX. They used the degree of substitution as a criterion for categorizing rye AX.; The solubility of arabinoxylans in water is affected by the ratio of arabinose (A) to xylose (X) residues in the molecule. Arabinoxylans with an A:X ratio of 0.5 were completely soluble in water, whereas xylan with an A:X ratio of 1.4 was only partially soluble in water.^[31] Scientists have classified rye arabinoxylans (AXs) into four groups based on their solubility and structural features. There is some overlap in the solubility of these groups.^[18]

Several factors, including molecular weight, degree of substitution, presence of other substituents, and technical and physiological value, influence the rheological properties of rye arabinoxylans (AX).^[5] Compared to dextran and gum arabic, AX has a viscosity similar to guar gum, with water-extractable AX from different rye cultivars exhibiting increased viscosity due to a higher proportion of high-molecular-weight polymers.^[32] Different milling fractions contain varying levels of AX with different A:X ratios, reflecting differences in substitution patterns throughout the kernel.

The A:X ratio represents different forms of AX found in various structures. For instance, the pH of whole grain rye flour, endosperm, aleurone, and pericarp/testa are 0.63, 0.75, 0.42, and 1.04, respectively. While the water solubility of AX in the endosperm is more than 70%, it is almost insignificant in the aleurone and pericarp/testa.^[33] Rye AXs possess remarkable water-binding and gelling abilities and can generate highly viscous solutions, which is related to the concentration of the highest molecular weight water-extractable AX is associated with anti-nutritive effects in rats and chicks.^[34] Secalins, or rye proteins, are prolamins in different molecular weights, including high molecular weight secalins, S-rich secalins of 75 kDa, S-poor secalins, and S-rich secalins of 40 kDa.^[35,36] Hackauf et al.,^[37] discovered that non-adapted cultivars with high protein concentrations in rye grains contained fewer carbohydrates than adapted hybrids. Starch is present only in the starchy endosperm of rye, and its gelatinization temperature is lower than wheat starch. Although both rye and wheat starches exhibit type A crystallinity, rye starch contains a more significant proportion of A-type granules (85–90% for granules up to 62.5 m in diameter) and a smaller percentage of B-type granules (10–15% for granules ≤9.3 m in diameter).^[38] Moreover, rye starch has larger granules than wheat starch, with a lower proportion of B-type granules and larger A-type granule particle sizes. Unsaturated fatty acids, primarily in the germ, constitute the majority of lipids in rye grain, with oleic, linoleic, and linolenic acids accounting for 81.6% of the total fatty acids.^[39] Rye meal’s total vitamin E content is 1.6 mg tocopherol equivalent/100 g fresh weight, with α-tocopherol content accounting for the majority. Rye is rich in phytosterols/phytosterols, with the bran containing 955 mg/kg. The primary sterols/stanols in stigmasterol are α-sitosterol (50%), α-sitostanol (15%), campesterol (15%), campestanol (10%), and stigmasterol (10%).^[28]

Anti-nutritional impact of rye

Minerals are abundant in cereals and legumes, but their bioavailability is typically limited because of anti-nutritional elements such as phytate, trypsin inhibitors, and polyphenols. The most significant anti-nutrient is phytic acid, present in most grains and potent enough to compound multi-charged metal ions, particularly Zn, Ca, and Fe, rendering them inaccessible for human usage.^[40] The grain has been processed to increase its nutritional value using straightforward, conventional home technologies like roasting, germination and fermentation, heating, and soaking. The amount of phytic acid and the availability of phosphorus can be affected by various factors, including genetics, location, environmental changes, irrigation conditions, type of soil, and fertilizer application. Phytase enzymes break down phytate salt during germination, supplying phosphate for the growing seedling. Due to its potent capacity to combine highly charged metal ions, particularly Zn (II), Ca (II), and Fe (III), phytic acid has long been regarded as an anti-nutrient.^[41] As a result, eating a lot of food with a high phytic acid content may result in a deficiency in the absorption of several dietary minerals.^[42] However, by limiting the use of phytic acid-rich grain products, the harmful effects of phytic acid could be avoided. Sprouts now have superior nutritional value due to the reduction of phytic acid caused by the germination of wheat and legumes.^[42,43] In particular, the fermentation of germinated rye, which also led to a lower pH when compared to native rye flour, increased folates, free phenolic acids, lignans, total phenolic compounds, and alkylresorcinols. The folate level increased up to seven times and that of free phenolic acids by up to 10 times. The bioactive potential of whole meal rye can thus be further increased through fermentation.^[44]

Techno-functional characteristics

Properties related to the quality of grains and the milling process

While grain quality testing in rye correlates to wheat, variations are seen in grain sensitivity, ergot bodies, and test weight.^[45] Grinding rye grains have a similar process to wheat. However, the process has some notable distinctions, such as shorter heating time and using corrugated iron trucks instead of smooth rolling rollers. Due to the fragile rye albuminous seed compared to the hard albuminous seed of wheat, rye flour requires a broad surface area to avoid lumps. However, it is often challenging to distinguish endosperm from the seed coat, particularly grains with a high proportion of non-starchy polysaccharides, resulting in a lower yield rate of rye flour than wheat.^[46] Rye flour is classified based on its ash content, which varies by country. For example, in Germany, eight classes of flour are classified based on ash content ranging from 0.9% to over 1.8%.^[5]

Technological properties of rye flour

Singh et al.,^[47] reported that the scientific community had developed various testing methods to predict the quality of the flour. Scientists have devised quick techniques to measure the quantity of protein (gluten) and the quality of starch, which are the significant components of wheat that affect baking performance. Similarly to wheat starch, non-starch polysaccharides (NSP) and storage proteins impact the final quality of rye flour. The functional properties are influenced mainly by the carbohydrate fractions and the activity of α-amylase. Németh and Tömösközi,^[48] stated that the current technique primarily evaluates the properties of the components regarding making dough and gel-forming processes. It is important to note that, unlike wheat, there are no standardized methods for laboratory manufacture of rye flour, which can lead to variations in the ash content and particle size distribution of tested samples. Additionally, in a lot of cases, the specific sort of rye milling products used isn’t specified by any literature, which in turn restricts comparing data with one another and therefore makes it difficult to speculate a defined conclusion in regards to dough-forming characteristics.^[49]

Dough properties are measured using empirical and fundamental rheological principles, as stated by various researchers. Evaluating the different components/properties of rye doughs made from white flour (with an ash content of 0.5–0.7%) and that are rich in fiber or wholemeal flour (with an ash content of 1.0–1.8%) is done using standard Farinograph and Mixolab measurements.^[9]

Researchers have conducted extensive research in recent years on the practicality of individual rye components, such as proteins, starch, and non-starch polysaccharides, and their interconnections with dough components. The dough properties of rye are unique from those of wheat doughs, with shorter development time alongside stability and a higher degree of softening.^[48] Researchers widely accept that rye storage proteins cannot form a viscoelastic network as strong as wheat gluten proteins due to rye’s lower protein concentration and fundamentally different protein composition. The scientific community widely accepts that rye’s lower protein concentration and fundamentally different protein composition make it unable to create a viscoelastic network as strong as wheat gluten proteins with their storage proteins. However, using transglutaminases (TG) to improve protein aggregation in rye dough (flour type 1,150) causes a solid and flexible protein network, which shows evidence and the potential of the building structure of rye proteins. Non-gluten proteins (albumins) in rye flour (type 1,150) possibly play a vital role in the performance of rye proteins, according to studies on the fractionated composition of rye flour.^[50,51]

Rye grain and flour mainly comprise starch as their primary component, which significantly affects the behavior of rye dough when combined with amylase activity.^[46] Rye flour starch granules have an increased level of damage, usually enzymatic and mechanical. In addition, starch granules also have elevated swelling ability when compared to wheat starch, causing a squashy and adhesive dough. The behavior of rye dough is also influenced by pentose polysaccharides, specifically arabinoxylans (AXs), which rely heavily on their solubility, molecular weight, and crosslinking ability. AXs, particularly water-availability components, can generate highly viscous fluids, which has a significant impression on rye dough behavior.^[48]

Pasting and thermomechanical properties

Researchers have extensively studied carbohydrates’ swelling, pasting, and retrogradation in rye quality testing. Due to the susceptibility of rye to pre-harvest sprouting, it is critical to measure a-amylase activity and characteristics. Researchers commonly use the Falling Number or Stirring Number tests to calculate the amylolytic level of rye grain and flour.^[28] In contrast, viscosimetric methods such as the Amylograph or Rapid ViscoAnalyser provide more comprehensive information on rye flour functionality. Recent studies have revealed that evaluating whole meal bread characteristics and the quality of baking of modern rye cultivars requires more than the conventional testing methods. The Mxolab method, which assesses thermomechanical properties in dough matrices, is more effective in simulating bread-baking conditions. According to the literature data, commonly used testing methods indicate lower viscosity values for pastes of white (ash content: 0.5–0.7%) and wholegrain rye flours, together with peak viscosity, hot paste viscosity alongside ultimate viscosity, when set side by side to wheat flours.^[18] It is due to delicate sprout resistance and increased amylase activity of rye grains, leading to lower falling number values for rye flours.^[37]

Baking quality of rye flour

Milling techniques and flour fractionation

Milling companies primarily use roller flour mills to mill rye, employing a diminishing process similar to wheat milling. The procedure involves conditioning and washing the rye grains, followed by breaking and reduction steps that separate the endosperm from the bran. The resultant grits from the breaking stage are further milled in reduction mills to produce fine flour. This milling process yields multiple flour fractions or mill streams, which are combined to produce flours with various properties.^[32] As the distribution of chemical constituents varies randomly within the kernel, each flour stream has a unique composition. Different milling techniques have multiple merits and demerits on the quality of rye, as elaborated in Table 1.

Table 1. Merit And demerits of different milling techniques of rye cereal.

Type of rye	Milling method	Process	Merit	Demerits	References
Whole rye grain	Classical milling	There are multiple grinding phases (passages) in this operation. Roller grinders are usually employed.	It is possible to get several kinds of white flour and whole rye flour, and the endosperm and bran can be separated effectively. little heat production and dust production while grinding	Costly and complex technologies. There are several different kinds of machines. Very unlikely that bran will break down into fine particles.	Dziki et al.^[Citation52]; Cappelli, Oliva, & Cini^[Citation53]; Probst et al .^[Citation54]
Whole rye grain	Ultrafine grinding	Various mills, including jet mills, colloidal mills, impact classifier mills, ball mills, and superfine impact mills, can be used.	Very small particles are produced, often 1 to 50 µm or smaller. Enhances the bioavailability of bioactive substances. Changes the solubility and dissolving rates of flour particles. Enables simple bran particle reduction.	Extremely high energy input is necessary. Rapid wear of operating components. Limited capability. Frequently, high temperatures are produced. Metal contamination of a product may result from wear and tear.	Dhiman et al.^[Citation55]; Pulivarthi et al.^[Citation56]
Whole rye grain	Impact milling	Frequently carried out by various mills, like pin or hammer mills.	Particularly favored for the manufacturing of whole rye flour. Technology that is relatively easy to apply and mill. Flour and bran may form fine particles.	Grinding can produce a lot of heat, which may negatively affect the quality of the flour. A challenge in separating the endosperm from the bran. It produces a wide range of particle sizes.	Prabhasankar & Haridas^[Citation57]; Probst et al.^[Citation54]

The chemical changes of rye bread during milling significantly impact flour quality and its potential applications through starch characteristics, amylases, and the soluble components in the cell wall. Previous researchers have examined various factors, including color, ash content, protein content, lipid content, pentosans, fiber, enzymatic activity, glutathione, and phenolic content, in wheat milling to assess their rheological qualities and suitability for different formulations. However, rye milling streams have received less attention than wheat milling streams.^[31]

This study investigated the chemical and functional features of millstreams obtained during rye flour manufacturing. They used differential scanning calorimetry and interfacial behavior to characterize the rye streams and assess their nutrient content and bread-tasting profile. Despite the limited research on rye streams, their popularity is increasing due to their beneficial chemical composition in treating disorders such as cardiovascular disease, hypercholesterolemia, and cancer. Therefore, examining the characteristics of various flour streams obtained during rye flour manufacturing is crucial. It was also taken into account how they behaved while mixing and pasting.^[22]

The milling process can significantly impact the microstructure and components of flour. The milling industry frequently utilizes rye as a raw material to produce white and black bread flour as its primary products.^[58,59] In Central Europe, rye flour typically has an ash content ranging from 0.5% for white flour to 1.5% for dark flour. The mill also produces wholemeal rye flour with an ash content of up to 2.0% w/w in the dry matter. The most commonly used rye flour in the Czech Republic is T 930 bread flour, which has an ash content of up to 1.1%.^[8]

Flakes made from rye grain and other processed grains, such as those that have been hydrothermally treated, fermented, or roasted, are produced in addition to flour. Particular bread and cereals commonly use these ingredients. Millers typically mill rye using a method similar to wheat milling^[58] which includes a series of passes for dissolution and separation. Before milling, millers moisten and condition the grains to remove the outer layers and germ from the endosperm. In contrast to wheat, separating the endosperm from the outer layers is less efficient in the rye because of the close attachment between the outer layers and endosperm of rye grains.^[60]

Rye requires a more rigorous milling process involving fewer passes than wheat. An example of a specific milling method is the successful testing of producing finely granulated wholemeal rye flour. This process is taking place in the Czech Republic using a specialized impact mill with a vertically rotating shaft. The mill achieves disintegration by using revolving steel segments called “hammers” arranged in multiple rows above each other and appropriately adjusting the mill’s inner housing.^[34] A screen, whose mesh size can be changed, is located on the upper side of the mill drum, and an air stream carries the material through the disintegration space until the particles pass through the designated screen. As the air stream has the material, it flows upward in the mill chamber. Compared to the usual milling technique, it enables much finer granulation of the outer layers while causing minimal damage to the endosperm’s starch granules. The finely granulated wholemeal flours produced using this method are more manageable and lead to improved sensory characteristics in bread.^[38]

In the Czech Republic, producers traditionally produce two types of rye flour: white (light) rye flour with a low ash content of 0.50–0.65% in dry matter and bread rye flour with a high ash content of as much as 1.10%, which has become relatively dominant in recent decades. The color of white rye flour, which has a 10–30% extraction rate, is similar to wheat flour. Bread flour (formerly classified as T930) possesses features like gray color, and its milling yield differs depending on the level of white flour production, usually falling between 60 and 70%.^[45] In the Czech Republic, the overall extraction rate of rye flour varies between 75 to 80%, and bread rye flour comprises a significant amount of arabinoxylans, with concentrations of up to 10%. Due to the higher amylase activity in rye grains than wheat grains, starch granules are often extensively damaged by enzymatic activity. In addition to amylases, the activity of proteases and xylanases is higher in wheat than in wheat. Amylases play a critical role in developing rye sourdough and wheat’s higher activity of proteases and xylanases.^[30]

The degree of starch damage crucially influences the intensity of the fermentation process and the quality of rye dough and breadcrumb, characterized by high levels of arabinoxylans.^[61] Arabinoxylans are present in high levels in rye dough and breadcrumbs. Enzymatic hydrolysis is more likely to occur in starch granules undergoing thermal and physical damage during milling. The degree of harm inflicted on the starch granules not only affects the behavior of flour sourdough ripening and dough production but also has a significant nutritional effect. More damaged starch granules are rapidly absorbed into the bloodstream as glucose, potentially causing a substantial increase in the glycemic index (GI). This rise in GI is effectively offset when using darker flour with high fiber content, such as wholemeal rye flour.^[62] However, when creating lighter flours, the milling process involves more rigorously eliminating the grain’s outer layers, specifically the bran. It leads to the rapid absorption of glucose into the bloodstream, leading to higher glycemic index (GI) values, which have adverse effects on other nutritional parameters such as a decrease in the content of essential amino acids like lysine, fiber, minerals, and other bioactive compounds.^[59,61]

Utilization of rye

Rye in food products

Rye provides grass for cattle and can also be used as green manure in various crops, as it is a green plant. When rye grain enters maturity, it is reaped and used as ruminant fodder, hay, and waste storage, mainly for horses. Researchers have investigated the use of grain as a component in biofuel production, and chemical factories can utilize chemurgy to process grain pentosans into xylose and furfural.^[45] The rye plant is helpful in a variety of ways. Thatching and building are all done with rye straw. The most crucial aspect of milling flour for baking various bread and other products is that it adds the most value.

• Rye “black bread” with a strong rye flour content is typical in Eastern Europe. The rye flour used has a variety of ash (0.8 to 1.6%).

• People often use rye grain to make crisp bread (Knaeckebrot), which manufacturers make from the grain of rye and without yeast. Because of the manufacturing process, the rye flour used for this purpose requires low alpha-amylase production.^[52]

• The traditional method of making Pumpernickel stands out for using 100% rye meal and a sourdough fermentation process, which takes 18 to 36 hours to bake. As a result, it is dark, with a tonne of dextrin and a bitter aftertaste. The bread has a long shelf life and does not shape a traditional crust. Pumpernickel is a traditional German bread originating in Westphalia, but the recipe and baking process vary greatly in (Eastern) Europe. In North America, for example, medium Pumpernickel (produced with rye-wheat flour and probably using a sourdough method) is made as a substitute for traditional bread.^[24]

• Lighter rye bread varieties and bread rolls are made with gritted rye and wheat flour. American rye bread uses wheat flour as a major component (60 to 80%), with flour as a minor replacement.

• A flour grinder is often used to make sourdough bread, but in this case, “old dough” is mixed in with grist to give it the distinctive “sour” flavor.^[45]

• Of course, these are biscuits. Rye is used to produce several biscuits and crackers instead of fermented bread.

• Cereals are a great way to start the day. Rye is often found in cereal as a breakfast or muesli snack, with rye chips, extrusion rye, and crushed corn as examples.

• Whisky is a form of alcoholic beverage. [If it arrives from the United States or England, the pronunciation is “whiskey” or “whiskey,” depending on whether it comes from Ireland, Australia, or Japan.] Processing and extracting rye into beverages like rye whiskey is a one-of-a-kind use of grain

• Another choice is to use biofuels. Although not identical, there is a connection between the use of rye grain in ethanol production and related biofuels.^[63] The higher levels of -amylase found in molted rye grain are simply an incidental advantage of use. Winter rye, in particular, has recently been designated as a critical biofuel raw material. While corn grain has traditionally been the primary source for producing fuel ethanol from cereal grain, other grain species have recently been studied for this purpose. Additionally, cereal straw has been examined as a potential source of lignocellulosic waste. Rye is a popular choice due to its abundance and low cost compared to other grains or applications, especially in the pet food industry. Furthermore, rye flour finds use in various products such as adhesives, coatings, corrugated packaging, and mats. Wheat starch has an edge in these applications due to the unique properties of rye grain. Manufacturers use it as a solvent in veneer production, such as plywood.^[64]

• Pet food manufacturers occasionally use the rye plant, usually in combination with other grains.

They use offal after rye is ground into flour to make pet food. Rye-based foods are known for their nutritional benefits as they are high in protein, vitamins, minerals, and bioactive phytonutrients. Using whole flour when baking rye-based goods is recommended to maximize the benefits of these nutrients.^[64] Rye also contains bioactive compounds that complement other natural sources, such as grapes. Rye-based products have a higher fiber content than wheat-based products, and the consistency of rye used in these products is usually double that of wheat-based foods.

Additionally, rye-baked products tend to have a lower Glycaemic Index, indicating that some bioactive have extra nutritional benefits. The bran of cereal may have a better antioxidant role in vivo than in vitro studies. The phenolic acids used in rye aid in keeping low-density lipoproteins from degrading. The antioxidant behavior of rye bread, indicating that the Reaction mechanism can help natural antioxidants grow. The nutritional benefits of phenolics (phenolic acids, lignans, and alkylresorcinols) in the human diet were recently investigated.^[32]

Usage of rye in other than food

Rye has become an essential feed grain worldwide primarily because it can grow in areas where other cereals cannot produce acceptable results. The local milling and baking industries need help to keep up with the abundance of rye production, necessitating its use in animal feed. About two-thirds of the rye supply in Germany, renowned for rye production, is used for pet food mixed with other feed supplies. Rye-grain proteins have an enhanced amino-acid content, making them appear to be a better option for pet food than most other cereal grains. However, food producers require more than this consistency to pay a premium for rye grain.^[65] Farmers commonly use rye in animal feed despite its anti-nutritional properties, mainly due to non-starch polysaccharides like pentosans. These properties limit the use of rye in feed formulation, requiring it to be combined with other feed sources. Non-starch polysaccharides, particularly arabinoxylans, cause the listed problems by increasing viscosity. They are problematic for poultry as they have a high water-holding capacity, allowing highly viscous liquids to form in the bird’s or mammal’s intestine, interfering with micronutrient absorption. Poultry fed a diet high in the rye will experience weak weight gain and lower metabolizable energy.^[45] To counteract these adverse effects, pentosan-degrading enzymes such as xylanase-glucanase may be added to the rye diet. These enzymes break down pentosans, reducing viscosity and speeding up development.^[65] Various enzyme compositions of varying quantities are now available in the merged food industries. 5-alkylresorcinols, concentrated in the bran, are often thought to impede pig growth, causing skin inflammation and a condition known as “blood sample case.” Although phenolic chemicals can be found in all cereal grains, their use in pet food rations is restricted for most pigs because they are particularly abundant in rye grain.^[66] The amounts of these compounds vary by variation and are concentrated in the bran.

Utilization of rye flour in products development

In addition to traditional bakery items, there has been an increase in the variety of food products that use rye as a base ingredient (Table 2). These include snacks, crispbreads, breakfast cereals, porridges, and other items, especially in Nordic countries (according to the Nordic Rye Forum). The evolution of these innovative rye-based products is driven by various factors and goals, such as meeting the demands of health-conscious consumers for high-quality products that contain more dietary fiber and bioactive compounds.^[38,63]

Table 2. Role of rye flour in the baking industry.

Raw material	Quantity	Dough type(s)	Microorganism	Mixing	Resting	Dividing/Shaping	Proofing	Baking	Reference
Whole rye flour	1200 g	Sourdough	Dry yeast	Hobart mixer for 9 min	At 30°C for 30 min	Divided and distributed dough in 3 pieces	Temp 37°C for 37 min and RH 89%	Temperature 200°C for 35 mint	Boskov Hansen et al.^[Citation67]
Whole rye flour	792 g	Sourdough with yeast	Yeast	Lab scale mixer	At 30°C for 30 min and RH 100%	Divided into 450 g piece, molded and panning	Temp 30°C for 15 min	Temperature 200°C for 60 mint	Kariluoto et al.^[Citation68]
		Sourdough without yeast
Whole rye flour			Yeast fermentation	Diosna mixer	At 37°C for 90 min	Divided into 100 g piece, molded and panning	Temp 37°C for 60 min	Temperature 230°C for 30 mint	Buksa et al.^[Citation69]
Rye flour	720 g		Yeast fermentation	Diosna mixer 25°C at 100 RPM for 4 mints		Divided into 300 g piece, molded and panning	Temp 30°C for 50 min and RH 80%	Initial steam injection of 0.5 L, for 55 min at 230°C	Doring et al.^[Citation70]
Whole meal rye flour	1000 g	Sourdough fermentation	yeast	Diosna mixer At low speed mixing for 10 mint	10 mint	Divided into 350 g weight, 5 piece, molded and panning	Temp 35°C and RH 75% till dough out of the pan	10 sec steam injection, for 10 min at 260°C and 30 min at 220°C	Stȩpniewska et al.^[Citation71]
Whole rye flour	100 g	Yeast fermentation		3 mint		Divided into piece, molded and panning	12 mint	for 65 min at 160°C	Torbica et al.^[Citation72]
Whole rye flour	800 g	Sourdough fermentation	yeast	Hobart mixer (30°C for 10 mint)	45 mint	Divided into 2 piece, molded and panning	For 45 mint at temp 32°C and RH 78%	for 55 min at 210°C	Oest et al.^[Citation73]

Ongoing research aimed at developing innovative rye-based milling products rich in and bread items with higher fiber and bioactive content is catering to the demand for high-fiber products.^[74] However, increasing the dietary fiber content of rye products can negatively affect their quality. As a result, various research and development efforts are focused on improving rye-based products’ technological and sensory characteristics. One such approach involves using heat treatment to modify the pasting properties of rye flour and improve the quality of baked goods.^[75] Enzymatic methods can also change the rheological properties and baking quality of rye bread dough. For instance, combining xylanase and transglutaminase enzymes can enhance protein networking in rye dough, improving the quality of yeast-leavened rye bread, as highlighted by Doring et al.,^[76] Subjecting the bran section to lactic acid fermentation or particle size reduction can improve the sensory properties of high-fiber rye crispbreads. In an innovative approach, high-protein and high-fiber snacks can be generated by combining milk powder and wholemeal rye flour in an extrusion-based 3D printing process. However, this method is currently challenging for making unique rye-based products.

Rye bread and other baked goods are among the most popular rye-based products, with baking quality being a critical factor in their production, much like wheat-based products. Rice bread is available in various shapes, colors, flavors, and textures and is often blended with wheat flour. As a result, despite a single standard guideline, rye bread testing is less developed globally than wheat bread production, possibly due to the prevalence of national methodologies for evaluating the success of rye baking in rye-producing countries.^[25]

Compared to whole wheat bread, rye bread has a denser and less porous crumb structure with a gel-like fragility. The pace of starch recovery in bread production may be slowed down by various factors such as the appearance of bread crumbs, volume and composition, fermentation products, and the activity of amylases, proteases, and xylanases. All variables in sourdough and bread production, including crucial ingredients such as bread aldehydes, ketones, alcohol, heterocyclic compounds, esters, acids, furans, pyrazines, lactones, other alkanes.^[14]

Furfural aldehyde is one of the distinctive properties of rye bread’s aroma. 3-methyl butanol, hexanol, 3-pentanone, acetaldehyde, isoamyl ester acetic acid, I -furan-2-pentyl, and hexanoic acid significantly enhanced the flavor of sourdough samples.^[17]

The utilization of rye sourdough in bread making has been linked to the unique characteristics of rye sourdoughs and their cultural significance. These characteristics include improvements in sound quality, the presence of yeast, and alterations in the structure and stability of the bread crumbs. On the other hand, sourdough dough, fermentation procedures, and their products are currently being investigated in another vital arena. This category includes sourdough’s diet and nutritional advantages.^[65] Fertilization is crucial in enhancing conventional grain products’ nutritional value, biological activity, and health advantages. The leavening of rye grain components results in various changes, modifications, and adaptations to their structure, content, and biological properties. These changes include the slowing down of starch digestion, enzyme activation, production of other vitamins, preventing the growth of fungi, and inhibition of fungal growth. During fermentation, various substances can be produced, including prebiotic oligo- and polysaccharides (such as dextrans and levans), amino acids like glutamate and ornithine, and their derivatives as peptides.^[48]

In rye sourdough, heterofermentative lactic acid bacteria effectively metabolize maltose and sucrose, as sugar is naturally present in rye flour, and amylase extracts maltose from starch. Additionally, sucrose is essential in generating acetic acid and selective exopolysaccharides (EPS) by these bacteria. When compared to rye-glucan and arabinoxylan, the concentration of EPS during the process of baking bread is relatively consistent. Enzymatic activity leads to the breakdown and dissolution of proteins and dietary fiber components in the rye.^[55]

Changes in the physical characteristics of grain biopolymers, such as molecular weight, viscosity, absorption, and melting properties, can occur during fermentation. These alterations may influence nutrient absorption, recovery, and overall physiological effects. Notably, rye arabinoxylans are highly soluble at low pH and may cause digestive discomfort.^[56] Yeasts and lactic acid bacteria have been shown to possess phytase activity, which can help to decrease phytate levels and enhance the availability of organic matter. The presence of bioactive peptides, amino acids, and their metabolites is influenced by the peptidase activity of lactic acid bacteria under acidic conditions.^[67]

Sourdough can indirectly reduce starch resorption by releasing fiber ingredients, lowering bread’s glycemic index while enhancing gut flora activity. As sour rye bread undergoes ripening, there is an increase in the breakdown rate of dietary fiber components, as well as the level of nicotinamide generated through the microbial process during fermentation.^[68] Lactobacillus plantarum, a type of lactic acid bacteria, was especially effective in decreasing acrylamide levels in rye wheat bread. Additionally, bread made with rye flour elicits a lower postprandial insulin response than whole wheat bread.^[69]

Conclusion and future perspectives

When compared to wheat, rye is a harder cereal. From the food business and industrial point of view, rye production has gradually decreased during the past 20 years. Rye and rye products are a significant source of grain fiber with specified health advantages. Rye is superior to wheat in terms of nutrients, even though it also contains the protein known as gluten, which is harmful to those who have celiac disease. New rye varieties are an attractive choice for commercial application due to the high protein content of rye compared to other grains and the low proportion of fiber content. In terms of carbohydrate and crude protein, rye grain was superior to barley. In comparison to barley, rye grain included less ash, fat, and fiber. However, those who do not have a gluten intolerance can benefit nutritionally from rye. Several physicochemical processes that affect the composition, characteristics, and availability of nutrients and technologically significant components take place during rye and rye flour preparation. Different Changes impact rye flour’s ability to absorb water, the dough’s viscosity and structure, the bread’s quality and yield, and its glycemic index. A recent trend in the milling and processing of rye is the production of more fiber and wholemeal flour. Wholemeal and dark rye flours with fine granulation are frequently produced using non-traditional, customized mill equipment. With this change, the product has a lower degree of starch degradation, a lower glycemic index, easier accessibility of rye fiber and other beneficial compounds, and enough substrate for fermentation. The alteration of the mill products’ chemical composition and functional characteristics is made more accessible by adopting appropriate procedures for grinding and classifying the mill products. Due to its use as a food ingredient and its numerous industrial uses, rye bran receives special attention. It is an excellent source of lignans, phenolic acids, and fiber compared to similar grains. Rye has the highest concentration of arabinoxylans among cereal grains, as well. Additionally, rye grain fermentation and enzymatic hydrolysis improve the amount and bioavailability of phytochemicals while producing various physiologically active substances. These modifications make it possible to use rye bran for multiple purposes, including protecting plants from pathogenic organisms, food, feed, cosmetics, and pharmaceuticals. An essential raw material for the creation of biofuels is rye bran. However, in-depth research is needed to examine further uses for rye by-products in the manufacture of food and non-food items, as well as to create fresh, cutting-edge technologies for the productive processing of rye products.

Acknowledgement

Authors are thankful to government college university faisalabad Pakistan for providing literature collection facility.

Disclosure statement

No potential conflict of interest was reported by the authors.